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Internal transport processes in stars and the Sun

schedule: Monday, 9:00 (invited)

Ana Palacios
Université de Montpellier / CNRS

Internal transport processes are known to be at play in (cool) stars of all masses, and manifest through their impact on the surface chemical, rotational and magnetic evolution of stars, as well as through their shaping of the chemical and rotation gradients inside the stars as revealed by asteroseismology. In this talk I will give an updated overview of the recent progress made on the front of theory, modeling and observations concerning the transport processes in stellar interiors.

 

A new generation of evolutionary and seismic solar models

schedule: Monday, 9:30

Gaël Buldgen (1) ; Patrick Eggenberger (2)
(1) Université de Genève; (2) Université de Genève

The Sun is the most observed star in the Universe. Thanks to this privileged status, it plays a key calibrator role for stellar physics, acting as a laboratory to test fundamental physical ingredients used in theoretical computations. Therefore, any refinement of the recipe of solar models will impact the ingredients for all models of solar-type stars. Following the revision of the solar abundances by Asplund and collaborators in 2005, confirmed in 2009, 2015 and 2021, the standard recipe of solar models has been put under question regarding both microscopic and macroscopic ingredients. In this talk, we will present results of new generations of both solar evolutionary and seismic models. We will show how evolutionary models taking into account the effects of rotation and magnetic fields can reproduce both the internal rotation, the lithium surface abundance and the helium abundance in the convective zone of the Sun. Furthermore, we will present a new approach to compute seismic solar models from iterative Ledoux discriminant inversions. We will show how such seismic models can be used to gain insights on the temperature gradient close to the base of the convective zone, where the robustness of opacity tables has been questioned. Combining both approaches will thus provide us key constraints on the required revision of physical ingredients to solve the long lasting solar modeling problem that followed the abundance revision in the early 2000s.

 

Solar neutrino fluxes show the signature of planet formation processes

schedule: Monday, 9:45

Masanobu Kunitomo (1) ; Tristan Guillot (2); Gaël Buldgen (3)
(1) Kurume University; (2) Observatoire de la Côte d'Azur; (3) University of Geneva

Solar evolutionary models are thus far unable to reproduce spectroscopic, helioseismic and neutrino constraints consistently, resulting in the so-called solar modeling problem. In parallel, planet formation models predict that the evolving composition of the protosolar disk, and thus, of the accreted gas by the proto-Sun must have been variable. In this talk, we show that solar evolutionary models including a realistic planet formation scenario lead to an increased core metallicity of up to 5%, implying that accurate neutrino flux measurements are sensitive to the initial stages of the formation of the Solar System. We demonstrate that in addition to macroscopic transport and increased opacities at the base of the convective envelope, the formation history of the Solar System constitutes a key element to resolve the current crisis of solar models.

 

Age or activity? Constraining evolutionary models with Li-depletion and rotation

schedule: Monday, 10:00

A. S. Binks (1) ; R. D. Jeffries (2); G. G. Sacco (3); R. J. Jackson (4); L. Cao (5); A. Bayo (6); M. Bergemann (7); R. Bonito (8); G. Gilmore (9); A. Gonneau (10); F. Jiminéz-Esteban (11); L. Morbidelli (12); S. Randich (13); V. Roccatagliata (14); R. Smiljanic (15); S. Zaggia (16)
(1) MIT Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology & Astrophysics Group, School of Chemical and Physical Sciences, Keele University; (2) MIT Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology; (3) INAF – Osservatorio Astrofisico di Arcetri; (4) MIT Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology; (5) 4055 McPherson Laboratory, 140 West 18th Avenue, Columbus, Ohio 43210-1173, USA; (6) Instituto de Física y Astronomía, Fac. de Ciencias, U de Valparaíso; (7) Galaxies & Cosmology Department, Max Planck Institute for Astronomy, Heidelberg & Niels Bohr Institute, University of Copenhagen; (8) INAF – Osservatorio Astronomico di Palermo; (9) Institute of Astronomy, University of Cambridge; (10) Institute of Astronomy, University of Cambridge; (11) Nicolaus Copernicus Astronomical Center, Polish Academy of Sciences; (12) INAF – Osservatorio Astrofisico di Arcetri; (13) INAF – Osservatorio Astrofisico di Arcetri; (14) Dipartimento di Fisica “Enrico Fermi”, Universitá di Pisa; (15) Centro de Astrobiología (INTA-CSIC), Madrid; (16) INAF – Osservatorio Astronomico di Padova

Something's not right! Standard stellar models are clearly at odds with observations of colour-magnitude diagrams (CMDs) and Li-depletion patterns of pre main sequence (PMS) stars in clusters, and mounting evidence is suggesting inflation in low-mass stars - caused by strong dynamo-driven magnetic fields and/or cool starspots - leads to older inferred isochronal ages, in turn delaying the onset of Li-depletion, prompting a new generation of "magnetic models".

In this talk I'll pose a simple question: "Can any model provide good simultaneous fits of the CMD and Li-depletion patterns observed in clusters?"

By kinematically selecting high-probability members of 5 clusters from the Gaia-ESO Survey, with ages between ~5-150 Myr we examine both standard and magnetic models by fitting isochrones and visually assessing the Li-pattern. We find: (1) standard models provide under-luminous fits at low-masses and can't capture the early stages of Li-depletion; (2) magnetic models are consistently 1.5-2 times older and better match Li-depletion; (3) strong degeneracy between magnetic activity and age.

Using TESS 30-min lightcurves we compiled rotation periods. Among the K-stars in the older clusters we find the brightest and least Li-depleted are the fastest rotators, demonstrating the classic “Li-rotation connection” for the first time at $\sim$35 Myr in NGC 2547, and find some evidence that it exists in the early M-stars of NGC 2264 at <10 Myr.

 

Exploring waves properties with multi-dimensional hydrodynamical simulations: from solar-type stars to intermediate-mass stars

schedule: Monday, 10:15

A. Le Saux (1) ; T. Guillet (2); I. Baraffe (3); D. G. Vlaykov (4); T. Constantino (5); J. Pratt (6); T. Goffrey (7); M. Sylvain (8); V. Réville (9); A. S. Brun (10)
(1) Physics and Astronomy, University of Exeter & ENS-Lyon, CRAL, Université de Lyon; (2) Physics and Astronomy, University of Exeter; (3) Physics and Astronomy, University of Exeter & ENS-Lyon, CRAL, Université de Lyon; (4) Physics and Astronomy, University of Exeter; (5) Physics and Astronomy, University of Exeter; (6) Department of Physics and Astronomy, Georgia State University; (7) Centre for Fusion, Space and Astrophysics, Department of Physics, University of Warwick; (8) Physics and Astronomy, University of Exeter; (9) Institut de Recherche en Astrophysique et Planétologie, CNRS, UPS, CNES; (10) AIM, CEA, CNRS, Universités Paris et Paris-Saclay.

The importance of waves propagating in the stars is twofold. First, they offer a great opportunity to unveil stellar internal structure and dynamics thanks to the observation of oscillations modes of stars, this is the field of asteroseismology. Secondly, they have an impact on the internal structure and evolution of stars as they can transport angular momentum, energy and chemical elements between different region of the star. In this presentation I would like to particularly focus on internal gravity waves (IGW). Observation of these waves remain challenging and their properties in stellar interiors remain poorly constrained. These properties of IGW are inherently 3D, non-linear and anisotropic. Consequently, multi-dimensional simulations are needed in order to test theoretical models and guide observations! This is essential for past, present and future missions such as Kepler, TESS or PLATO. In this talk, I will present a study of IGW in solar-like stars based on multi-dimensional stellar structure models preformed with a fully compressible hydrodynamics time implicit code, the MUSIC code. I will discuss how an artificial increase of the stellar luminosity and of the thermal diffusivity by several orders of magnitudes impact the waves properties. Our results suggest that this technique affect the excitation of IGW, because of an impact on convective motions and overshooting, but also their damping. This is of particular importance when studying mixing and stellar rotation. I will also present preliminary results for IGW propagating in the radiative envelope of intermediate mass stars with convective cores.

 

Magneto-rotational evolution of low-mass stars

schedule: Monday, 11:15 (invited)

Alessandro C. Lanzafame
Dipartimento di Fisica e Astronomia, Università di Catania & INAF - Osservatorio Astrofisico di Catania

I will review some of the exciting developments that have occurred in the last few years regarding the evolution of rotation and magnetic field of low-mass stars, from pre-main sequence to late main sequence. I will focus on the evidence of distinct magneto-rotational regimes in the early phase of stellar evolution and of rapid transitions between them. Such evidence challenges a dependence on rotation only and suggests an important role of the rotational history in the early phase of the magneto-rotational evolution. The rotational evolution of one of these regimes, the slow-rotator one, can be described to a satisfactory level of accuracy and I will outline the role of the core-envelope coupling in the first 1 Gyr. Many other issues remain unsolved and puzzling, like the conditions leading to the ultra-fast rotator regime, and the role of magnetic fields in the stellar radius inflation problem and on the lithium depletion pattern. Major contributions to these topics comes from space missions like Gaia, CoRot, Kepler, and TESS, combined with ground-based photometric and spectroscopic surveys and observations, as well as from Zeeman Doppler Imaging investigations. The most recent contributions come from Gaia DR3, which provides rotational periods for some 500 000 stars and chromospheric activity for some 2M stars.

 

New insights into late stages of the evolution of solar-mass stars thanks to intrinsic and extrinsic stars

schedule: Monday, 11:45

Sophie Van Eck (1) ; Shreeya Shetye (2); Drisya Karinkuzhi (3); Stephane Goriely (4); Lionel Siess (5); Alain Jorissen (6)
(1) IAA, Université Libre de Bruxelles; (2) Laboratory of Astrophysics, Ecole Polytechnique Fédérale de Lausane; (3) Indian Institute of Science, Bangalore; (4) IAA, Université Libre de Bruxelles; (5) IAA, Université Libre de Bruxelles; (6) IAA, Université Libre de Bruxelles

The foundations of stellar nucleosynthesis have been established more than 70 years ago and since then, many progresses have taken place, in particular concerning the heavy-element nucleosynthesis in late stages of the evolution of solar-mass stars. Targeting key-elements, including radio-isotopes, in both intrinsic and extrinsic stars, the latter constituting "cold cases" and useful probes of a past nucleosynthesis, allows to better understand chemical element production by stars with masses as low as 1 Msun during their evolved phases. Given their numerical importance, these stars are major contributors to the chemical evolution of the Galaxy.

 

The X-ray activity-rotation-age relation of M dwarfs and its mass dependence

schedule: Monday, 12:00

Enza Magaudda (1) ; Beate Stelzer (2); Stefanie Raetz (3); Mara Salvato (4); Julien Wolf (5)
(1) Institut für Astronomie und Astrophysik, Eberhard-Karls Universitat Tübingen; (2) Institut für Astronomie und Astrophysik, Eberhard-Karls Universitat Tübingen and INAF- Osservatorio Astronomico di Palermo; (3) Institut für Astronomie und Astrophysik, Eberhard-Karls Universitat Tübingen; (4) Max-Planck-Institut für extraterrestrische Physik and Exzellenzcluster ORIGINS, Garching; (5) Max-Planck-Institut für extraterrestrische Physik and Exzellenzcluster ORIGINS, Garching

The activity of the Sun and solar-like stars is driven by an $\alpha\Omega-$dynamo, according to which the combination of differential rotation and convective motions of the outer atmospheric envelope continuously regenerates the magnetic field that manifests itself in the form of powerful optical, UV, and X-ray radiation. The study of the X-ray activity-rotation-age relation therefore probes both the stellar dynamo and spin evolution. Moreover, how the X-ray emission varies with stellar age is of great importance for planetary atmospheres and yields information about how stellar X-rays affect their evolution. M dwarfs are also known to be magnetically active, but the physical mechanism is poorly understood. In this talk, I present the largest uniform sample of M dwarfs (302 stars) consisting of observations taken with XMM-Newton, Chandra, and K2 combined with X-ray and rotation data from the literature. With the use of spin models we compared the evolution of the predicted rotation periods ($P_{\rm rot}$) with our results on the empirical $L_{\rm x} - P_{\rm rot}$ relation to provide an estimate for the age decay of X-ray luminosity and we compare it to the few stars with known age and X-ray emission. Later, new X-ray data from eROSITA and rotation periods extracted from TESS light curves doubled our sample of M dwarfs, allowing us to put a quantitative constraint on the mass dependence of the X-ray emission, and to determine the drop in activity level with respect to pre-main sequence stars. Finally, we quantified the X-ray variability of this large sample by comparing the $L_{\rm x}$ from eROSITA and ROSAT measurements and we examined the sensitivity improvements with eROSITA. In particular, eROSITA is able to detect faint M dwarfs that are visible by ROSAT only during flaring emission and it is also sensitive for slower rotating M dwarfs in the unsaturated regime of the $L{\rm x}-P_{\rm rot}$ relation ($P_{\rm rot}\geq10$ d) that is inaccessible to TESS.

 

Young stars and their disc - A short but complex story

schedule: Monday, 12:15

Louis Amard (1) ; Sean P. Matt (2)
(1) Université Paris-Saclay, Université de Paris, CEA, CNRS, Astrophysique, Instrumentation et Modélisation Paris-Saclay, F-91191, Gif-sur-Yvette; (2) University of Exeter, Physics and Astrophysics dept, Exeter, EX44QL, UK

During their formation, most young stars are surrounded by a protoplanetary disc. The angular momentum evolution of these systems is quite complex but still poorly understood despite a lot of effort and some recent breakthrough. For instance, observations indicate that stars with a disc tend to rotate more slowly even though they accrete angular momentum, and during the first 10 Myr, young low-mass stars do not seem to spin-up while they are expected to contract. To tackle this long-standing problem, I will present state-of-the-art stellar evolution models with accretion which include a self-consistent treatment of angular momentum evolution thanks to the results of dynamical multi-D MHD simulations. We explore the observed range of several parameter, such as the accretion rate history, the composition and the thermodynamics of the accreted material, as well as the large scale magnetic field strength of the star. I will show that the observed spin rate, the long-standing disc-locking problem, as well as other observed properties of very young stars can be explained by the complex interplay of the different processes.

 

Pre Main Sequence stellar and accretion properties in the era of ULLYSES and PENELLOPE

schedule: Monday, 12:30

Carlo F. Manara
ESO

Determining the stellar and accretion properties of young low-mass Pre-Main-Sequence stars is vital to better understand stellar evolution, and the general evolution of protoplanetary disks and planet formation. The last years are seeing a flourishing of possibilities to properly determine these stellar and accretion properties, combining spectroscopic observations from ground-based telescopes (e.g., the large programme with the Very Large Telescope, PENELLOPE) with space-based (HST) spectra, mainly from the ULLYSES public survey, targeting 82 low-mass ($M_\star \lesssim 2 M_\odot$) young (age$<$10 Myr) stars at UV wavelengths. I will present the main results obtained from these programs, and how these are leading to a better understanding of how we can determine stellar and accretion properties of young stars, to better decipher how stars and planets form.

 

The legacy of space-based asteroseismolgy

schedule: Tuesday, 9:00 (invited)

Margarida Cunha
Instituto de Astrofísica e Ciências do Espaço, Universidade do Porto, Portugal

The detection of solar-like oscillations in thousands of cool stars enabled by the ultra-precise photometry of space missions such as CoRoT, Kepler, and TESS, has empowered research in stellar physics and stellar internal dynamics, moving forward our understanding of stellar evolution. This dramatic change has also a direct impact on research fields that depend on the accurate characterisation of stellar properties, such as exoplanet research and Galactic archaeology.

In this talk I will review some of the key advances brought by space-based asteroseismic data to our understanding of cool stars, discuss key issues that are still under debate and briefly look into the future.

 

Cool stars and their planets with the PLATO mission

schedule: Tuesday, 9:30

S. Aigrain (1) for the PLATO Consortium
(1) University of Oxford

PLATO (PLAnetary Transits and Oscillations of stars) is ESA's M3 mission, scheduled for launch in late 2026. Using 26 cameras with 12 cm aperture and partially overlapping fields of view, it will spend at least 4 years performing high-precision, high-cadence, long-baseline photometry of bright stars, simultaneously searching for planetary transits and stellar oscillations. PLATO will provide the first large sample of well-characterised small planets up to intermediate orbital periods, including terrestrial planets in the habitable zone of solar-like stars. Thanks to its integrated approach (transit detection, asteroseismology and ground-based follow-up are all part of the baseline mission), it will determine the masses, radii and ages of both the host stars and their planets to high accuracy. This will enable detailed tests of theoretical models of the formation and evolution of planets and planetary systems. Many of the planets that PLATO will discover will also be well-suited for atmospheric characterisation from the ground and/or space. In this talk I will give a brief overview of the payload, the mission and its expected scientific yield and impact. One of the key challenges for PLATO will be to disentangle the planetary signals of interest from low-level stellar variability and residual instrumental effects. I will discuss how we can exploit PLATO's unique multi-camera strategy to build data-driven models that can help overcome this challenge.

 

Butterfly diagrams in sun-like stars using asteroseismology

schedule: Tuesday, 9:45

M. Bazot (1) ; M. B. Nielsen (2); D. Mary (3); J. Christensen-Dalsgaard (4); O. Benomar (5); P. Petit (6); L. Gizon (7); K. R. Sreenivasan (8); T. R. White (9)
(1) Heidelberg Institute for Theoretical Studies (HITS gGmbH); (2) University of Birmingham; (3) Laboratoire Lagrange, Université Côte d'Azur, Observatoire de la Côte d'Azur, CNRS; (4) Stellar Astrophysics Centre, Department of Physics and Astronomy, Aarhus University; (5) The University of Tokyo and NAOJ, Tokyo; (6) IRAP/Université de Toulouse, CNRS; (7) Max-Planck-Institut/Institut für Astrophysik/New York University Abu Dhabi; (8) New York University; (9) University of Sydney

Stellar activity is a long-standing problem in stellar physics, pertaining to the solar dynamo model, that is the mechanism that generates the stellar magnetic field. In the Sun, the variations of the latitudinal coordinates of the surface active regions with time, the so-called butterfly diagram, are a key observational constraint. They are an important test for benchmarking solar dynamo models. In other stars, although direct imaging does not allow to resolve their surfaces, it is sometimes possible to reconstruct activity maps for certain classes of stars. As a general rule, this is particularly difficult for Sun-like stars, which are slowly rotating and do not often exhibit a strong magnetic field. In this presentation, I explain how to perform such a reconstruction for a Sun-like star using Kepler time series and asteroseismic analysis. This has been done for the first time for HD 173701 (KIC 8006161). The method rests upon combining classical period estimation using the low-frequency signal in the data and asteroseismic measurements of latitudinal differential rotation using asteroseismology. This was accomplished using an array of methods from computational Statistics. They allowed us to invert for a butterfly diagram over the four years of the Kepler mission, with statistically significant variations observed in the latitude of the active regions of this star. I discuss the limitations of the method as well as the important perspective it opens for the characterization of stellar activity. I replace the significance of this finding in the context of the upcoming PLATO mission.

 

Rotational Characterization of TESS Stars with Deep Learning

schedule: Tuesday, 10:00

Zachary R. Claytor ; Jennifer L. van Saders
University of Hawaii

The TESS mission has the potential to probe stellar rotation in millions of stars across the entire sky, but mission systematics—instrumental noise, observing gaps, and changes in detector sensitivity—have prevented recovery of rotation periods longer than 13.7 days. We used deep learning to see through TESS systematics and recover periods from year-long light curves. Our approach uses a training set of synthesized light curves from realistic star spot evolution simulations, with real light curve systematics from quiet TESS stars. Evaluating the network on real TESS data, we recovered periods for 20,000 cool dwarfs. The period distribution resembles the Kepler and K2 distributions, including periods up to 90 days. Using gyrochronology, we estimated masses, ages, and other fundamental stellar parameters for 5,000 TESS stars with APOGEE spectroscopy. We combine this with a similar sample from Kepler and show that we can use rotation-based ages to recover Galactic chemical evolution trends previously seen only in stars more massive or more evolved than the Sun. With rotation periods across the entire sky, we can characterize stars along many more lines of sight than before, enabling detailed study of the Galaxy’s stellar populations.

 

A four-year follow-up campaign of TRAPPIST-1 from the ground

schedule: Tuesday, 10:15

Elsa Ducrot (1) ; Michaël Gillon (2); Eric Agol (3); the SPECULOOS team (4)
(1) CEA Saclay; (2) University of Liège; (3) University of Washington; (4) University of Liège

In this talk, we present the results from an extensive four-year long follow-up campaign of the TRAPPIST-1 system led from the ground with the SPECULOOS and Liverpool Telescopes. This represents 285 nights of observations, 228 new transits of the seven planets, and includes 3 months of daily monitoring of the star to study its photometric variability. First of all, to try to understand the origin of the existing inconsistency between K2 and Spitzer photometric variability, we derive the rotation period of the host star in the I+z band and use it to propose a spot variability model that would agree with the observations in each band and at the same time provide insights on the nature (proportion and temperature) of the photospheric heterogeneities on the surface of TRAPPIST-1, in a similar way as in Morris et al. (2018). From those outcomes we discuss the expected impact of stellar contamination on the planetary spectra via the transit light source effect. In parallel, we present our statistics of spot-like and faculae-like crossing events on all observed transits and relate them to the photometric modulations. In addition, we analyze 269 new transits in order to (1) refine the planets' parameters using individual and global analyses and (2) derive precise transit timing variations for the seven planets. We show that recent timings (most recent ones from Nov 2021) of planet h seem to slightly deviate from the predictions by Agol et al. (2021), implying either that planet h's timings have an excess of outliers or that a 7-planet model is no longer a good fit, which could suggest the existence of a putative eighth planet. To figure this out, we show the results of new optimization runs with 7-planet and 8-planet models and including periodic orbit (P-O) - families of solutions to the N-body problem - constraints. Indeed, near-resonant planetary system such as TRAPPIST-1 are expected to reside in the dynamical neighborhood of stable P-Os (Antoniadou et al. (2020)) and such a configuration can be used to yield better constraints on the orbital elements. Finally, we look at TRAPPIST-1's flaring activity. On one hand, we compute flare occurrence rates and energies to compute flare frequency distribution and complement the work initiated by Ducrot et al. (2020) on placing the TRAPPIST-1 planets relative to the abiogenesis zone introduced by Rimer et al. (2018). On the other hand, we seek correlations between flaring events and periodic photometric variability to state whether the observation of Morris et al (2018) claiming that visible flares seem to occur preferentially when the star is bright, and when the brightness is increasing most rapidly, is confirmed or not.

 

Revealing the Surfaces of Stars with Interferometric Imaging

schedule: Tuesday, 11:15 (invited)

Rachael Roettenbacher
Yale University

Since the first stellar diameters were resolved with interferometry in the early twentieth century, the technique has revealed the angular extent of hundreds of stars. Knowing the size of a star on the sky and its distance from Earth has allowed for straight-forward measurements of fundamental stellar parameters. Measuring a stellar diameter can be achieved with just two telescopes which make a single baseline, but today's interferometric arrays allow for multiple baselines to be observed at once. Combining the light from three telescopes at once allows for the observations of closure phases, which highlight asymmetries of the target. By increasing the number of telescopes and observations and the size of baselines, sufficient data can be obtained to not just resolve the size of stars, but image details on their surfaces. In recent years, a number of studies have shown that interferometric imaging is capable of capturing the surface details of a wide variety of stars as they appear on the sky. I will present a review of the images obtained to date, particularly noting those of cool stars, like asymptotic giant, supergiant, and magnetically active stars. I will also detail the advantages of the method compared to other imaging techniques and of utilizing stellar surface images.

 

Sub-AU study of CI Tau with VLTI/GRAVITY: The key to link stellar activities, disks and planets.

schedule: Tuesday, 11:45

A. Soulain ; J. Bouvier; K. Perraut
Univ. Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France

For a few million years after the gravitational collapse that led to their formation; young stellar systems remain surrounded by a circumstellar disk from which planets form. ALMA and VLT/SPHERE have provided spectacular images of the planet-forming disks on a scale of a few 10 au (Garufi et al. 2017) and; even more recently; led to the direct detection of forming planets within the disk (Benisty et al. 2021). In contrast; the exploration of the innermost disk regions ($\lesssim$ 1 au) has remained quite challenging so far. Moreover; the Kepler satellite has revealed that the most ubiquitous planetary systems consist of compact strings of super-Earth and mini-Neptunes orbiting close to their host star (Porb $\lesssim$ 100 days; e.g.; Otegi et al. 2022;). It is; therefore; crucial to investigate the physics of the star-planet-inner disk interaction in young stars; not only for the role it plays in the early evolution of solar-type stars (e.g.; accretion/ejection; angular momentum; etc.) but also to determine the environmental conditions that prevail at the time of planetary formation. This is a necessary step towards understanding the formation of the plethora of compact inner low-mass planetary systems observed across the Galaxy. CI Tau is so far the only pre-main sequence star still accreting from its surrounding disk (classical T-Tauri star) claimed to host a hot Jupiter planet. However; the periodic radial velocity variation could result from magnetospheric accretion onto the star rather than from an orbiting body (Donati et al. 2020). The most interesting aspect of CI Tau regards its extreme magnetic field (3.7 kG) that disrupts the inner gaseous disk and generates accretion funnel flows down to the stellar surface. We propose to present our investigation about the inner region of CI Tau; aiming at reconnecting the different spatial scales of the system down to a few stellar radii ($\lesssim$ 0.1 AU).

Method: We investigated this puzzling question using the long-baseline interferometry technique; the only way to probe the inner region of the system at sub-AU precision. Thanks to the high spectral resolution of VLTI/GRAVITY (R=4000); we are both sensitive to the emitting dusty part of the inner rim (K-band continuum); and the magnetosphere itself traced by the Br$\gamma$ emission line. Results: In the continuum; we characterise the disk's inner rim; which appeared to be disconnected from the outer disk with a large misalignment both in inclination and position angle. We report an internal rim position at 0.17 $\pm$ 0.02 AU; remarkably more significant than the theoretical sublimation radius of 0.03-0.06 AU. Such difference could infer the presence of a planet carving the inner part of the disk; as recently supported by hydrodynamical simulation (Muley et al. 2021). The non-zero closure phases measured by GRAVITY suggest an important asymmetry in the disk: the southwest side appears brighter than the northeast. Such difference argues in favour of an inclined disk where the brilliant (and farthest) part is seen from the bottom (distant observer point of view). We confirmed such behaviours using radiative transfer modelling with RADMC3d. In the Br$\gamma$ line (2.1661 $\mu$m), our model suggests a bright but smaller emitting region than the thermal emission with a radius of 0.04 $\pm$ 0.01 AU (4 times smaller than the inner rim). Such characteristic is strongly supported by the magnetosphere accreting models developed in our team and will be presented for comparison.  Conclusion: With GRAVITY; we characterise the inner disk of CI Tau with a sub-AU precision; allowing a direct comparison with the standard YSO's characteristic sizes such as sublimation; co-rotation or magnetic truncation radii. The existence of a larger than expected central cavity could be an observational signature of the well known 11.6 Mj planet (CI Tau b). The substantially detected misalignment between inner and outer disks constitutes a challenge for the modelling efforts and should be carefully investigated in the future.

 

Pre-main sequence stars in the time domain: insights from high-precision, space-borne photometry

schedule: Tuesday, 12:00

Laura Venuti ; Ann Marie Cody; K2 team
SETI Institute

High-precision time series photometry provides a unique window into the dynamics of the inner disk regions (< 1 AU) around young stars (< 5-10 Myr). Exquisite surveys carried out with CoRoT, Kepler, and TESS have revealed a huge variety of photometric behaviors for young stellar objects, driven by variable mass accretion, stellar magnetic activity, or rapidly evolving circumstellar dust structures. This variety of behaviors, observed in each young cluster and star-forming region investigated from space, points to a coexistence of distinct paradigms of star-disk interaction that may govern different stages of pre-main sequence evolution. Thanks to the K2 mission, accurate time domain data for hundreds of young stars are now available for virtually every stage of protoplanetary disk lifetimes, across different stellar mass regimes from cool stars and beyond: surveyed regions range from rho Ophiuchi and the Lagoon Nebula (~2 Myr), to NGC 2264 (3-5 Myr, monitored earlier with CoRoT), to Upper Scorpius (5-10 Myr). In this contribution, we discuss the insights provided by those campaigns into the time evolution of inner disks around young stars, and what these observations teach us regarding the stellar and circumstellar dynamics as a function of stellar mass and environmental conditions.

 

A Demographic TESS View of M-dwarf Activity: Flares, Rotation, and Ages

schedule: Tuesday, 12:15

Maximilian N. Günther (1) ; TESS Science Team (2)
(1) European Space Agency (ESA); (2) others

While TESS searches for temperate cool star worlds, it also provides an unprecedented survey of their host's stellar activity. Especially stellar flares and coronal mass ejections (CMEs) can be a double-edged sword for M-dwarf exoplanets. On the one hand, these energetic outbursts are capable of shaping or stripping off planetary atmospheres; on the other hand, they might deliver the necessary trigger energy for origin-of-life chemistry. Here, I will highlight our study of all stellar flares from the TESS primary mission's 2 min cadence data of 230,000 stars, driven by a convolutional neural network. I will discuss our new demographic insights on M-dwarf flaring as a function of stellar type, age, rotation, spot coverage, and other factors. With an eye on exoplanets, I will link our findings to ozone sterilisation and prebiotic chemistry, identifying which worlds might lie in the "flaring sweet spot" for life. With future extended missions and 20 s cadence, M-dwarf activity studies can guide us further in understanding cool star vs. planet interactions.

 

Atmospheric retrievals of T dwarfs: clouds, composition, and clues to formation

schedule: Tuesday, 12:30

Josefine Gaarn (1) ; Ben Burningham (2); Jacqueline Faherty (3); Channon Visscher (4); Mark Marley (5); Eileen Gonzales (6); Emily Calamari (7); Daniella Bardalez Gagliuffi (8)
(1) University of Hertfordshire; (2) University of Hertfordshire; (3) American Museum of Natural History; (4) Dordt University & Space Science Institute; (5) University of Arizona; (6) Cornell University & American Museum of Natural History; (7) Graduate Center CUNY & American Museum of Natural History; (8) American Museum of Natural History

Atmospheric retrievals of T dwarfs: clouds, composition, and clues to formation Brown dwarfs are often described as the low mass extension of the star formation process. In this scenario, we would expect the compositions of brown dwarfs to follow those of the wider stellar population. At the lowest masses, however, it is reasonable to ask whether some fraction of the brown dwarf population arises from similar processes to those that make planets. We will summarise progress and early results of a large retrieval study of T dwarfs comparing isolated objects and companions to stars, measuring atmospheric abundances with a view to constraining their compositions. We also report the frequency of cloudy atmospheres across a broad range of spectral types, with clouds getting more common towards later types. Finally, we will highlight the case of Ross-458c, potentially planetary mass T8 dwarf in orbit around a M0.5+M7 pair. In agreement with previous studies, we find the atmosphere of Ross-458c to best be described by a cloudy model. We find a very high CH4/H2O ratio of 1.97 $M_\pm$ 0.13, which is challenging to understand in terms of equilibrium chemistry and plausible C/O ratios. This value may be consistent with a C/O ratio ~1.4 if the ${\rm CH_4}$ and ${\rm H_{2}O}$ abundances are quenched at the T=2000 K level, which would itself require vigorous vertical mixing. We explore the implications of these results for the origin of Ross-458c.

 

Doppler cross-correlation spectroscopy as a path to the detection of Earth-like planets - From CORAVEL to ESPRESSO via ELODIE  -

schedule: Tuesday, 18:15 (invited)

Michel Mayor
Geneva Observatory

 

Exoplanets and life in the Universe

schedule: Tuesday, 19:00 (invited)

Didier Queloz
University of Cambridge and ETH Zürich

The richness and diversity of planetary systems that have now been detected have modified our perspective on planet formation and our place in the Universe. They also represent an historical opportunity of perspectives and a compelling call to look for signs of life on these new worlds and to reflect on the origin of life in the Solar System. I will introduce the audience to the challenges and recent advances in this field, in the context of the new research centres set up at Cambridge and ETH-Z and how they address the origins of life on Earth and its prevalence in the Universe. 

 

Magnetic fields of late-type stars - an observational overview

schedule: Wednesday, 9:00 (invited)

Jean-Francois Donati
CNRS / Université de Toulouse

My talk will outline the latest observational advances in detecting and characterizing magnetic fields of late-type stars, from young pre-main-sequence (PMS) to mature and evolved objects. I will focus in particular on the latest magnetic measurements, for instance those of weakly-active M dwarfs and PMS stars collected over the last few years with the infrared spectropolarimeter SPIRou at the Canada-France-Hawaii Telescope, in the framework of the SPIRou Legacy Survey.

 

What makes a stellar surface preferentially facular or spot dominated?

schedule: Wednesday, 9:30

Eliana M. Amazo-Gomez ; Katja Poppenhaeger
Leibniz Institute for Astrophysics Potsdam AIP

The photosphere of Sun-like stars have been found to display a smooth transition between being dominated by spots or by facular features. Our analysis indicates that the Sun lies in the transition between the spot and faculae domination regime. Some hypotheses have been explored suggesting that such surface manifestations may correlate with different magnetic dynamo modes. By using a recently developed method based on the Gradient of the Power Spectra (GPS), we quantified the ratio between faculae to spots signature based on solar and stellar light curves. We characterized a sample of 30 Sun-like stars which we have identified to have different levels of spot- or faculae-dominance on their surface. We analyzed the longitudinal magnetic field, additional activity indicators such as the S-Index from the calcium H&K core line, H-alpha, and the Ca triplet in the near-infrared. We interpret the differences between having spot versus faculae dominated stellar surfaces.

 

Influence of Magnetic Cycles on Stellar Prominences and their Mass Loss Rates

schedule: Wednesday, 9:45

Sarah J. Faller ; Moira M. Jardine
University of St Andrews

Observations of rapidly-rotating cool stars often show coronal “slingshot” prominences that remove mass and angular momentum when they are ejected. The derived masses of these prominences show a scatter of some two orders of magnitude. In order to investigate if this scatter could be intrinsic, we use a full magnetic cycle of solar magnetograms to model the coronal structure and prominence distribution in a young Sun, where we scale the field strength in the magnetograms according to the scaling law $B\propto\Omega^{-1.32}$. Both the observed prominence masses and their scatter are reproduced. We show that both the field strength and the field geometry contribute to the prominence masses that can be supported and to the rate at which it is ejected. Predicted prominence masses follow the magnetic cycle, but with half the period, showing a large peak at cycle maximum and a secondary peak at cycle minimum. We show that mass loss rates in prominences are less than those predicted for the stellar wind, though may make significant contributions to the total mass-loss at cycle maximum. We also investigate the role of small-scale field that may be unresolved in typical stellar magnetograms. This provides only a small reduction in the predicted total prominence mass, principally by reducing the number of large magnetic loops that can support slingshot prominences. We conclude that the observed scatter in prominence masses can be explained by underlying magnetic cycles.

 

First unified modelling of large- and small-scale magnetic fields on active cool stars

schedule: Wednesday, 10:00

Oleg Kochukhov (1) ; Denis Shulyak (2)
(1) Department of Astronomy and Space Physics, Uppsala University; (2) Instituto de Astrofísica de Andalucía

Characteristics of the surface magnetic fields of cool stars are usually established with the help of two complementary approaches. The strength and topology of the global magnetic field are determined from time series polarization measurements, typically employing mean polarization profiles produced from thousands of spectral lines. On the other hand, an estimate of the total magnetic field strength is obtained from modelling Zeeman broadening of the intensity profiles of individual magnetically sensitive lines. For decades these two techniques were applied discordantly, using different types of observational data and yielding vastly different estimates of the global and total magnetic field strengths. The latter discrepancy, which is particularly prominent for active M dwarfs, is attributed to the presence of complex magnetic fields which cancel out in polarization observables but contribute to Zeeman broadening. Until now, the reality of this multi-scale nature of cool-star magnetic fields has not been established through a direct, self-consistent analysis. Here we present a unified magnetic field modelling framework that enables one to simultaneously derive characteristics of both organized global and complex small-scale magnetic field components from the intensity and polarization profiles of individual spectral lines. We present results of the application of this new magnetic retrieval approach to three active M dwarfs (AD Leo, WX UMa, and GJ 51) possessing strong axisymmetric dipolar fields. Our analysis yields mean field strengths consistent with previous Zeeman broadening studies but reveals 2-4 times stronger global magnetic fields compared to previous mean-line polarization analyses of the same stars. These findings have important implications for our understanding of M-dwarf dynamos, magnetospheres and their interaction with potentially habitable exoplanets.

 

Internal magnetic fields detected and measured using asteroseismology in red giants

schedule: Wednesday, 10:15

Gang Li ; Sébastien Deheuvels; Jérôme Ballot; François Lignières
IRAP, Université de Toulouse, CNRS, CNES, UPS, Toulouse, France

Magnetic fields affect stars throughout their evolution. They are thought to be generated in the convective regions of stars or inherited from the stellar formation process. Due to the opacity of stellar interiors, magnetic field measurements have been so far restricted to stellar surfaces. Here we report the unambiguous detection of magnetic fields in the core of three red giant stars using asteroseismology. Magnetic fields induce shifts in the frequencies of oscillation modes and break the symmetry of dipole mode multiplets. We detect such features and find that they closely follow the predictions of magnetic perturbations to oscillation modes. We rule out several other mechanisms that can also lead to asymmetric multiplets. The measured field strengths range from $\sim$\,30 to $\sim$\,100~kG in the vicinity of the hydrogen-burning shell, and we obtain information on the field topology. This result reshapes the current stellar evolution and angular momentum transport theories by providing precise constraints on magnetic fields in stellar radiative cores. We anticipate that this will open a new era in magneto-asteroseismology.

 

The role of dynamo instabilities in the dynamics of solar-like cool stars

schedule: Wednesday, 11:15 (invited)

Maarit Korpi-Lagg
Department of Computer Science, Aalto University, Finland and MPS Göttingen, Germany

For many decades, several research groups have developed sophisticated numerical models of the dynamics and magnetism of solar-like stars. The Sun being our central star, explaining, understanding, and predicting its magnetic activity is a very important goal, as the effects on our hi-tech civilisation can be very harmful. Despite of all these efforts, reproducing the solar non-uniform rotation and its observed cyclic magnetic field evolution correctly in one and the same model are still challenging, and all efforts seem to fail in some respect or another. Arguably, the numerical experiments are far removed from the parameter regime of the Sun, and no clear asymptotic behaviour has been found yet in any of them. We are, however, just approaching the regime, where both of the relevant dynamo instabilities, responsible for generating the different constituents of the solar magnetic field, are captured by the numerical models. These are the large-scale dynamo, generating the global solar magnetic field, and the small-scale dynamo, generating a fluctuating field, the latter of which requires extremely high-resolution simulations to be properly captured in global-scale models. Fluctuations are also generated from the tangling of the global magnetic field by turbulent motions, ubiquitous in the convection zone of the Sun, hence distinguishing between these two mechanisms observationally, and also numerically, is very challenging. In this contribution, I will review the results from the state-of-the-art numerical models, capable of simultaneously capturing these two dynamo instabilities under solar-like conditions, and present one possible interpretation of the results gathered.

 

Solar-like dynamos on other stars

schedule: Wednesday, 11:45

S. V. Jeffers (1) ; R. H. Cameron (2); S. C. Marsden (3); S. Boro Saikia (4); C. P. Folsom (5); M. M. Jardine (6); J. Morin (7); P. Petit (8); V. See (9); A. A. Vidotto (10); U. Wolter (11); M. Mittag (12)
(1) Max-Planck-Institut für Sonnensystemforschung; (2) Max-Planck-Institut für Sonnensystemforschung; (3) University of Southern Queensland; (4) University of Vienna; (5) University of Tartu; (6) University of St Andrews; (7) Laboratoire Univers et Particules de Montpellier; (8) Institut de Recherche en Astrophysique et Planétologie/Université de Toulouse; (9) European Space Agency/University of Exeter; (10) Trinity College Dublin/Leiden Observatory; (11) Hamburger Sternwarte; (12) Hamburger Sternwarte

Cool main-sequence stars, such as the Sun, have magnetic fields which are generated by an internal dynamo mechanism. In the Sun, the dynamo mechanism is a balance between the amounts of magnetic flux generated and lost over the Sun's 11-year activity cycle and is visible in the Sun's magnetic maps and different atmospheric layers using multi-wavelength observations. We use the same observational diagnostics, including a unique data set of magnetic maps that span decades, to probe the emergence of magnetic flux on the two close-by, active and low-mass K dwarfs: 61 Cygni A and Epsilon Eridani.  In this talk I will  discuss the balance between the amounts of magnetic flux generated and lost in these two K dwarfs compared to the Sun, and  if the Solar-dynamo mechanism works in a more extreme region of the stellar-mass and rotation parameter space.  Our results, when also applied to F and G dwarfs, will revolutionise our understanding the nature of the magnetic dynamo in stars other than the Sun.

 

Confirmation of a Magnetic Morphology Shift in Old Solar Analogs

schedule: Wednesday, 12:00

Travis Metcalfe (1) ; Adam Finley (2); Oleg Kochukhov (3); Victor See (4); Thomas Ayres (5); Keivan Stassun (6); Jennifer van Saders (7); Catherine Clark (8); Diego Godoy-Rivera (9); Ilya Ilyin (10); Marc Pinsonneault (11); Klaus Strassmeier (12); Pascal Petit (13)
(1) White Dwarf Research Corporation; (2) CEA-Saclay; (3) Uppsala University; (4) University of Exeter & European Space Agency; (5) University of Colorado Boulder; (6) Vanderbilt University; (7) University of Hawaii; (8) Northern Arizona University & Lowell Observatory; (9) The Ohio State University & IAC; (10) Leibniz-Institut fur Astrophysik Potsdam; (11) The Ohio State University; (12) Leibniz-Institut fur Astrophysik Potsdam; (13) Universite de Toulouse

The rotation rates of main-sequence stars slow over time as they gradually lose angular momentum to their magnetized stellar winds. The rate of angular momentum loss depends on the strength and morphology of the magnetic field, the mass-loss rate, and the stellar rotation period, mass, and radius. Previous observations revealed a shift in magnetic morphology between two F-type stars with comparable rotation rates but very different ages. We confirm a similar transition in several well-characterized solar analogs with ages between 2 and 7 Gyr. We present new spectropolarimetry of 18~Sco and 16~Cyg A & B from the Large Binocular Telescope, and we reanalyze previously published Zeeman Doppler images of HD~76151 and 18~Sco to confirm a shift in magnetic morphology near the middle of main-sequence lifetimes. We combine archival X-ray observations with updated distances from Gaia to estimate mass-loss rates, and we adopt precise stellar properties from asteroseismology and other sources. We then calculate the wind braking torque for each star in the evolutionary sequence, and we assess the uncertainties that arise from errors in the observational inputs. We conclude that the shift in magnetic morphology occurs before the age of the Sun, reinforcing the notion that the solar dynamo may be in a transitional evolutionary phase. We suggest that this magnetic transition may represent a disruption of the global dynamo arising from weaker differential rotation, and we outline our plans to probe this behavior in additional stars spanning a wide range of spectral types.

 

Dynamo action in solar-type stars: from fast to slow rotators

schedule: Wednesday, 12:15

Quentin Noraz ; Allan Sacha Brun; Antoine Strugarek
Département d'Astrophysique/AIM, CEA/IRFU, CNRS/INSU, Univ. Paris-Saclay, Univ. de Paris, 91191 Gif-sur-Yvette, France

The magnetic field of solar-type stars is generated and sustained through an internal dynamo process. This process is mostly determined by the combined action of turbulent convective motions and differential rotation. It can sometimes lead to magnetic cyclic variabilities, like the 11-years solar cycle. Evidence of magnetic cycles have been detected for other solar-type stars as well, ranging from a few years to a few tens of years. How are these cycles controlled? Observations and stellar evolution models show that solar-like stars spin-down during their main-sequence. In parallel, numerical simulations of these stars show that different regimes of differential rotation can be reached and are characterized with the Rossby number. In particular, anti-solar differential rotation (fast poles, slow equator) may exist for high Rossby numbers (slow rotators), which grows when the rotation spins-down. If this regime appears during the main sequence, we may wonder how the dynamo process will be impacted. More generally, can slowly rotating stars have magnetic cycles ?

We performed a numerical multi-D parametric study with the STELEM and ASH codes to understand the magnetic field generation of solar-type stars under various differential rotation regimes. We particularly focused on the existence of magnetic cycles for different stages of the main sequence. We find that short cycles are favoured for small Rossby numbers (fast rotators), and long cycles for intermediate (solar-like) Rossby numbers. Slow rotators (high Rossby number) are found to produce statistically steady dynamo with no cyclic activity in most cases considered. We further assess the energy transfers in these stellar dynamos and quantify that up to 3% of the stellar luminosity can be diverted into sustaining dynamos, and ultimately powering surface eruptive events.

 

Bridging the gap between the Sun and Sun-like stars: Universal atmospheric heating mechanism and empirical reproduction of XUV spectra

schedule: Wednesday, 12:30

Shin Toriumi (1) ; Kosuke Namekata (2); Vladimir S. Airapetian (3); Yuta Notsu (4)
(1) ISAS/JAXA; (2) NAOJ; (3) NASA GSFC, American University; (4) CU Boulder, NSO, Titech

The Sun and Sun-like stars commonly host the multi-million-K corona and the 10,000-K chromosphere. Because these extremely hot gases generate X-ray and extreme ultraviolet (XUV) emissions that may impact the erosion and chemistry of (exo)planetary atmospheres, influencing the climate and conditions for habitability, it is of crucial importance in solar and stellar physics to understand the responsible heating mechanisms. While, for the Sun, the magnetic field is thought to play a pivotal role in driving and transporting the energy from the surface upwards, it is not clear whether such a magnetically driven heating is commonly at work on other stars. Here we present the analysis of 10 years of multi-wavelength synoptic observations of the Sun and comparison with stellar data, providing the critical clues to the common nature of the heating mechanisms of coronae, transition regions, and chromospheres of the Sun and Sun-like stars. First, by analyzing the sequences of solar images of sunspot transit events, we derive the means to understand the magnetic and thermal environments of starspot magnetic fields that cannot be spatially resolved. Specifically, it is found that the surface magnetic flux can be inferred from the chromospheric light curves and that the thermal structures around the starspots can be inspected from the sub-MK UV light curves. Second, by investigating the power-law relationships between the surface magnetic flux and the luminosity of various emission lines with the formation temperatures from the corona to the chromosphere of the Sun, we discover that, in any temperature ranges, the solar scaling laws can be extended to the Sun-like stars with the ages of 50 Myr to 4.5 Gyr. This suggests that the magnetically-driven heating of the atmospheres is universal among the Sun and Sun-like stars, regardless of age or activity. Furthermore, by deriving the scaling laws between the magnetic flux and XUV spectrum, it is now possible to empirically reproduce the XUV spectra for various solar-type stars given the observationally-obtained magnetic fluxes. It is expected that the reproduced XUV spectra are used for photochemical calculations of (exo)planets of various solar-type stars, including the young Sun, and estimation of atmospheric formation and habitability of surface life.

 

Chasing CMEs and SEPs in the era of Parker Solar Probe and Solar Orbiter

schedule: Thursday, 9:00 (invited)

Erika Palmerio
Predictive Science Inc., San Diego, USA

Coronal mass ejections (CMEs) and solar energetic particles (SEPs) are results of the dynamic and explosive nature of the solar activity and important drivers of space weather effects. Historically, these phenomena have been studied mainly between the Sun and Earth, because of the availability of both remote-sensing and in-situ observations from a single viewpoint. Nonetheless, the increased amount of heliospheric and planetary missions launched in the past ~15 years has provided new opportunities for CME and SEP measurements at other locations in the solar system. In particular, the launch of Parker Solar Probe and Solar Orbiter, respectively in 2018 and 2020, represented the first times that dedicated heliophysics missions were sent to explore the solar wind environment between the Sun and Earth since the twin Helios spacecraft in the 70's. These probes have already enabled multiple novel observations and discoveries, and are expected to keep providing important data to advance our understanding of the Sun and its environment during their primary mission lifetime. In addition, the presence of several spacecraft scattered throughout the heliosphere at the same time has enabled multi-point studies of the same CME and/or SEP event at different positions that are well separated in both heliocentric distance and longitude. In this presentation, we will review the main discoveries in CME and SEP research that have resulted from Parker Solar Probe and Solar Orbiter observations, providing a few examples of events that have been studied in detail especially thanks to these two novel missions and/or via multi-spacecraft measurements.

 

Stirring the Base of the Solar Wind

schedule: Thursday, 9:30

Adam J. Finley (1) for the WholeSun ERC Team
(1) CEA Paris-Saclay, France

Current models of the solar wind must approximate (or ignore) the small-scale dynamics within the solar atmosphere, however these are likely important in shaping the emerging wave-turbulence spectrum and ultimately heating/accelerating the coronal plasma. In this talk, I will make connections between small-scale vortex motions at the base of the solar wind and the resulting heating/acceleration of coronal plasma. We apply the Bifrost RMHD code to produce realistic simulations of the solar atmosphere that facilitate the analysis of spatial and temporal scales which are currently at, or beyond, the limit of modern solar telescopes. The simulation is configured to represent the solar atmosphere in a coronal hole region, from which the fast solar wind emerges. The simulation extends from the upper-convection zone (2.5Mm below the photosphere) to the low-corona (14.5Mm above the photosphere), with a horizontal extent of 24Mm x 24Mm. Photospheric flows are found to efficiently twisted the coronal magnetic field, with Poynting fluxes of up to 2-4kW/m$^2$ commonly observed inside the twisted structures. Stronger whirlpool-like flows in the convection, concurrent with magnetic concentrations, launch torsional Alfv\'en waves up through the magnetic funnel network, which are expected to enhance the turbulent generation of magnetic switchbacks in the solar wind. Temperature and density contrasts form between regions with active stirring motions and those without. Therefore, stirring motions in the low-corona represent one possible explanation for the patchy nature of switchbacks in the solar wind, observed by Parker Solar Probe.

 

Space weather for cool stars

schedule: Thursday, 9:45

D. Rodgers-Lee (1) ; A. A. Vidotto (2); A. L. Mesquita (3); A. M. Taylor (4); T. P. Downes (5)
(1) Trinity College Dublin & Dublin Institute for Advanced Studies; (2) Leiden University; (3) Leiden University; (4) DESY; (5) Dublin City University

Energetic particles, such as stellar energetic particles and Galactic cosmic rays, are an important part of space weather for exoplanets orbiting cool stars and the young Earth. Energetic particles bombard exoplanetary atmospheres, leading to unique chemical effects that may be detectable with the James Webb Space Telescope (JWST). The flux of energetic particles reaching an exoplanet depends on the stellar wind properties which vary with stellar age. This means it is important to constrain the stellar wind properties of other stars. Young stars are also very magnetically active and should accelerate stellar energetic particles to higher energies than the present-day Sun.

I will present our results which modelled the energetic particle flux reaching Earth at different ages, such as when life is thought to have begun (approximately 3.8Gyr ago). I will discuss how, at this time, our model shows that stellar energetic particles dominated over Galactic cosmic rays up to GeV energies. At these energies, energetic particles can cause particle showers in the planet atmosphere that can reach the planet's surface. At the same time, to connect with upcoming observations we need to consider exoplanets orbiting stars with well-constrained stellar winds. Thus, I will also discuss our recent results for the Galactic cosmic ray fluxes reaching the habitable zone and exoplanets of a number of nearby stars. Finally, I will discuss our ongoing efforts to connect these energetic particle fluxes closely to upcoming JWST observations.

 

Forward modelling of solar flare emissions in the Solar Orbiter era

schedule: Thursday, 10:00

Rui F. Pinto (1) ; Antoine Strugarek (2); Allan Sacha Brun (3); Bahaeddine Gannouni (4)
(1) Département d'Astrophysique/AIM, CEA/IRFU, CNRS/INSU, Univ. Paris-Saclay; (2) Département d'Astrophysique/AIM, CEA/IRFU, CNRS/INSU, Univ. Paris-Saclay; (3) Département d'Astrophysique/AIM, CEA/IRFU, CNRS/INSU, Univ. Paris-Saclay; (4) Institut de Recherche en Astrophysique et Planétologie, CNRS/OMP, U. Toulouse

Solar flares consist of episodes of intense EUV and X-ray emission that follow from quick releases of energy stored in coronal structrures with complex magnetic fields. Twisted magnetic flux-ropes are likely to play a central role in the triggering and evolution of solar flares, as they are susceptible to develop instabilities leading to quick energy releases in the form of strong coronal plasma heating and of particle acceleration. The interdependence between the large scale topology of the magnetic field and its small scale dynamics determines to a great extent the outcome of such processes (occurrence conditions, amplitude, plasma and particle ejection). Detailed and energetically consistent numerical simulations are thus required to determine the physical links between the magnetic field, the bulk plasma thermodynamics, the charged particle motions, and the corresponding observable electromagnetic signatures. We will present recent simulations focusing on impulsive plasma heating and particle acceleration in modelled solar flares triggered in twisted coronal loops. We use a hybrid approach based on 3D MHD, test-particle and Particle-In-Cell (PIC) techniques. We discuss the outcomes of the simulations in terms of the morphological and spectral properties of the forward-modelled emission in the context of the Solar Orbiter mission, and of the STIX instrument.

 

Surprises in X-ray and EUV emission of cool stars as exoplanet hosts

schedule: Thursday, 10:15

Katja Poppenhaeger
AIP/Potsdam University

The extreme-UV (EUV) emission of cool stars has become very fashionable in recent years, since it is thought to be the most important driver for exoplanet evaporation. Since no observatories have been active at this wavelength range for a long time, scaling laws are typically used to infer the stellar EUV fluxes. However, stellar coronae and transition regions are exciting places with interesting physics that still host some surprises. I will present results from our new catalog of exoplanet X-ray irradiation, showing that high-energy environments can be much more extreme than what is found for the well-studied Hot Jupiters. I will also show that some EUV assumptions made in exoplanet atmosphere studies are off by an order of magnitude, and that certain nondetections of atmospheres are actually unsurprising when one takes into account the physics of emission lines in the stellar corona and transition region.

 

Short and long term effects of interactions in compact exosystems

schedule: Thursday, 11:15 (invited)

Antoine Strugarek
AIM, CEA Paris-Saclay, France

Over the last two decades, a large population of close-in planets has been detected around a wide variety of host stars. Such planets are thought to strongly interact with their host star by means of the irradiation they receive, the tidal forces they induce, as well as their interaction with the ambient magnetised stellar atmosphere and wind they orbit in. Can we use these interactions to better constrain these planets and their hosting stars? And where does this population of hot planets originate from?

The properties of the host star determine the key ingredients at the heart of these interactions. The stellar spectrum is the primary driver of the interaction in the upper atmosphere of exoplanets. The stellar structure determines its response to the tidal forcing from his hot exoplanet. Finally, the stellar global magnetic field is at the heart of star-planet magnetic interaction: its strength sets the magnetic energy available for the interaction, its shape determines the connection path between the star and the planet, and its temporal modulation (e.g. magnetic cycles) is at the source of an on/off behavior of the magnetic interaction.

I will give an overview of our understanding of star-planet interactions. I will focus on describing short term —intra-orbit— and long term —secular— effects of these interactions. I will reflect on our present capability to detect and characterise them, both in individual systems and in the hot exoplanets population. When detected, star-planet interactions indeed provide a fantastic opportunity to better understand the environment of the host star, as well as the properties of the exoplanet triggering them.

 

Diving into the AU Mic system: Planet masses, stellar activity and star-planet interactions

schedule: Thursday, 11:45

Baptiste Klein (1) ; Norbert Zicher (2); Oscar Barragán (3); Suzanne Aigrain (4); Robert D. Kavanagh (5); Louise D. Nielsen (6); James E. Owen (7); Aline A. Vidotto (8); Anne-Marie Lagrange (9); Davide Gandolfi (10); Luisa Maria Serrano (11); Laurel Kaye (12); Vinesh M. Rajpaul (13); Antoine Strugarek (14); Belinda Nicholson (15); Antoine Grandjean (16); Jean-François Donati (17); Jérôme Bouvier (18); Elisa Goffo (19)
(1) Department of Physics, University of Oxford; (2) Department of Physics, University of Oxford; (3) Department of Physics, University of Oxford; (4) Department of Physics, University of Oxford; (5) Leiden observatory, Leiden University; (6) Department of Physics, University of Oxford/European Southern Observatory; (7) Blackett Laboratory, Imperial College London; (8) Leiden observatory, Leiden University; (9) CNRS/Université Grenoble-Alpes; (10) Università degli Studi di Torino; (11) Università degli Studi di Torino; (12) Department of Physics, University of Oxford; (13) Cavendish Laboratory, University of Cambridge; (14) CEA/CNRS/Université Paris-Saclay/Université Paris-Diderot; (15) Department of Physics, University of Oxford/University of Southern Queensland; (16) CNRS/Université Grenoble-Alpes; (17) CNRS/Université de Toulouse; (18) CNRS/Université Grenoble-Alpes; (19) Università degli Studi di Torino/Thüringer Landessternwarte Tautenburg

Close-in planets orbiting low-mass pre-main-sequence stars are primordial targets, not only to understand the formation and evolution of planetary systems, but also to search for signatures of magnetic star-planet interactions, which offer a direct window on planetary magnetic fields. However, such stars exhibit intense magnetic activity inducing spectroscopic signals that overshadow potential planet signatures. As a result, only a handful of planetary systems younger than 25 Myr have been revealed so far without any detection of star-planet interaction in these systems. In this presentation, we propose to dive into the 22-Myr-old planet-hosting system AU Mic, intensively monitored with the high-resolution spectrograph HARPS over the past few years. We used a state-of-the-art multi-dimensional Gaussian process to disentangle the 600-m/s peak-to-peak stellar activity radial velocity signals from the $\sim$10-m/s signatures of the two recently-unveiled transiting planets. This allowed us to provide statistically-reliable mass measurements for AU Mic b and c. Surprisingly, planet c is found significantly denser than planet b, despite orbiting at larger orbital distance and, thereby, being less sensitive to stellar irradiation flux. We will discuss potential planet formation and evolution scenarios that could explain these observations. Additionally, we detect a significant modulation of AU Mic's non-radiative chromospheric flux at the period of AU Mic b. Using magneto-hydrodynamical simulations, we show that magnetic interactions between this planet and its host star's wind can reproduce the observed modulated emission power. Although more observations are needed, this could constitute the first direct detection of star-planet interactions in a young system.

 

Space Weather-driven Variations in Ly alpha Absorption Signatures of Exoplanet Atmospheric Escape: MHD Simulations and the Case of AU Mic

schedule: Thursday, 12:00

Ofer Cohen (1) ; Julian Alvarado-Gómez (2); Jeremy Drake (3); Laura Harbach (4); Cecilia Garraffo (5); Federico Fraschetti (6)
(1) University of Massachusetts Lowell; (2) Leibniz Institute for Astrophysics Potsdam; (3) Harvard-Smithsonian CfA; (4) Imperial College London; (5) Harvard-Smithsonian CfA; (6) Harvard-Smithsonian CfA & University of Arizona

We simulate the space environment around AU Microscopii b and the interaction between the magnetized stellar wind with a planetary atmospheric outflow for ambient stellar wind conditions and Coronal Mass Ejection (CME) conditions. We also calculate synthetic Ly$\alpha$ absorption due to neutral hydrogen in the ambient and the escaping planetary atmosphere affected by this interaction. We find that the Ly$\alpha$ absorption is highly variable due to the highly-varying stellar wind conditions. A strong Doppler blue-shift component is observed in the Ly$\alpha$ profile, in contradiction to the actual escape velocity observed in the simulations themselves. This result suggest that the strong Doppler blue-shift is likely attributed to the stellar wind, not the escaping neutral atmosphere, either through its advection of neutral planetary gas, or through the creation of a fast neutral flow via charge exchange between the stellar wind ions and the planetary neutrals. Indeed, our CME simulations indicate a strong stripping of magnetospheric material from the planet, including some of the neutral escaping atmosphere. Our simulations show that the pressure around close-in exoplanets is not much lower, and may be even higher, than the pressure at the top of the planetary atmosphere. Thus, the neutral atmosphere is hydrodynamically escaping with a very small velocity ($<15$ km s$^{-1}$). Moreover, our simulations show that an MHD treatment is essential in order to properly capture the coupled magnetized stellar wind and the escaping atmosphere, despite of the atmosphere being neutral. This coupling should be considered when interpreting Ly$\alpha$observations in the context of exoplanets atmospheric escape.

 

Evidence for tidal star-planet interaction in planet-hosting wide binaries

schedule: Thursday, 12:15

Nikoleta Ilic ; Katja Poppenhaeger; S. Marzieh Hosseini
Leibniz Institute for Astrophysics Potsdam, Germany and University of Potsdam, Germany

The evolutionary path of single stars is mostly governed by angular momentum loss in the process of magnetic braking. However, if a star has a close-in stellar or planetary companion, tidal interaction may alter the stellar rotation and activity evolution. We explored if exoplanetary systems display observational evidence for star-planet tidal interaction in terms of such altered stellar rotation and activity level. Determining ages and therefore the expected rotational states of single field stars is very challenging. We therefore used a sample of planet-hosting stars that are accompanied by wide stellar companions. Without needing knowledge about the absolute ages of the stars, we test for relative differences in activity and rotation of the planet hosts and their co-eval stellar companions, using X-ray observations to measure the stellar activity levels. Employing three different tidal interaction models, we find that host stars with strongly tidally interacting planets display elevated activity levels compared to their companion stars. We also find that those activity levels and stellar rotation periods follow the usual rotation-activity relationships, implying that the effect is indeed caused by a long-term tidal interaction and not a purely magnetic interaction.

 

Signatures of star-planet interactions across the electromagnetic spectrum

schedule: Thursday, 12:30

Robert D. Kavanagh (1) ; Aline A. Vidotto (2); Harish K. Vedantham (3); Baptiste Klein (4); Moira Jardine (5); Joseph R. Callingham (6); Julien Morin (7)
(1) Leiden Observatory; (2) Leiden Observatory; (3) ASTRON; (4) University of Oxford; (5) University of St Andrews; (6) Leiden Observatory; (7) LUPM/Université de Montpellier

Cool stars can interact magnetically with close-in planets that they are host to, producing a variety of observable signatures at different wavelengths. Knowledge of the stellar wind plasma environment is crucial to determining if and when such signatures could be visible. While the winds of low-mass stars have only been indirectly measured in a handful of cases, the coupling of sophisticated magnetohydrodynamic models with observationally-derived surface stellar magnetic field maps and mass-loss rate constraints can provide realistic snapshots of the stellar wind environment. In this presentation, I will discuss how this approach allows us to predict and interpret hints of star-planet interactions at both radio and optical wavelengths. I will then illustrate how obtaining near-simultaneous observations at these wavelengths is one of our best bets for benchmarking these magnetohydrodynamic models.

 

Star Clusters, the Milky Way, and the Gaia Revolution

schedule: Friday, 9:00 (invited)

Tristan Cantat-Gaudin
Max-Planck-Institut für Astronomie, Heidelberg, Germany

The census of stellar structures and star clusters in the Milky Way has been vastly improved in the recent years, largely owing to the Gaia mission. On a large scale, our ability to estimate more reliable ages and distances to clusters allows us to refine our understanding of the structure of our Galaxy and the mechanisms that drive its evolution. The study of young aggregates in the Solar neighbourhood and their internal kinematics has given us important insight on the conditions of star formation. Despite its paradigm-changing impact, Gaia leaves us with many open questions which will be tackled in combination with observations from asteroseismology and ground-base spectroscopy.

 

A gravitational and dynamical model of star formation in Orion

schedule: Friday, 9:30

Marina Kounkel (1) ; Keivan Stassun (2); Kevin Covey (3); Lee Hartmann (4); Jonathan Bird (5)
(1) Vanderbit University; (2) Vanderbit University; (3) Western Washington University; (4) University of Michigan; (5) Vanderbit University

The Orion Nebula Cluster (ONC) is the most massive region of active star formation within a kpc of the Sun. Using Gaia EDR3 parallaxes and proper motions, we examine the bulk motions of stars radially and tangentially relative to the cluster center. We find an age gradient with distance to the stars in the ONC, from 385 pc for the oldest stars, to 395 pc for the younger stars, indicating that the star forming front is propagating into the cloud. We find an organized signature of rotation of the central cluster, but it is present only in stars younger than 2 Myr. We also observe a net infall of young stars into the center of the ONC's deep gravitational potential well. The infalling sources lie preferentially along the filament, on the other hand, outflowing sources are distributed spherically around the cluster, and they have larger velocity dispersion. We further propose a solution to a long-standing question of why the ONC shows a weak signature of expansion even though the cluster is likely bound: much of this expansion is likely driven by unstable N-body interactions among stars, resulting in low-velocity ejections. Finally we observe a significant infall of stars in various low-mass star-forming regions towards the Orion Complex at distances as far away as 200 pc, presumably due to a strong gravitational potential of Orion. Though analyzing signatures imprinted on stellar dynamics across different spatial scales, these observation shed new light on the signatures of formation and evolution of young clusters.

 

Investigating the early evolution of star clusters via their 6D kinematics

schedule: Friday, 9:45

Joseph Armstrong ; Jonathan Tan
Chalmers University of Technology

Most stars form in clusters or associations but only a small number of these groups remain bound for longer than ~ 50 Myr. Once star formation has ended and the molecular gas around young stellar objects has been expelled via feedback processes, most initially bound young clusters lose most of their binding mass and begin to disperse into the galactic field. Other processes, such as ejection of cluster members via dynamical interactions and tidal shearing, can also influence the subsequent evolution of groups of young stars. Using Gaia eDR3 5-parameter astrometry in combination with radial velocities from large scale surveys such as Gaia, APOGEE, GALAH, LAMOST, Gaia-ESO and RAVE we analyze the 6D kinematics of a large sample of nearby young clusters, calculating velocity dispersions, rates of expansion and kinematic ages to probe their formation history and identify the key processes responsible for their subsequent evolution. In particular, we find significantly anisotropic expansion trends for more than 50 clusters younger than ~100 Myr within 1kpc and we determine the direction of maximum expansion for each cluster.

 

The SPYGLASS program: Mapping the Extensive Star Formation History of the Solar Neighborhood from Young Associations to Large-scale Patterns

schedule: Friday, 10:00

Ronan Kerr ; Adam Kraus
The University of Texas at Austin

Young associations hold a star formation record that can persist for millions of years, revealing the progression of star formation long after the dispersal of the natal cloud. Through the SPYGLASS program we are expanding our grasp of the local star formation record by mapping the extensive and often sparsely characterized network of clusters and associations in the solar neighborhood. Our recently published Gaia DR2-based study has already revealed dozens of new associations and subgroups with many features worthy of further investigation, including an age gradient in Sco-Cen indicative of sequential star formation, and time-separated populations in Perseus and Cepheus Far North that provide evidence for temporary disruption of star formation through stellar feedback. Over a dozen new populations and features have already been targeted with spectroscopic observations, providing measurements of youth indicators and radial velocities covering thousands of candidate members. These new observations provide regional age estimates and kinematic traceback, uncovering the star formation history of many nearby associations in even more exquisite detail, with two such analyses approaching completion. Countless new discoveries remain on the horizon, as early Gaia EDR3 results reveal over 100 new populations within 1 kpc. In this presentation, we provide new developments from the SPYGLASS project, including both the large scale and statistical view of the solar neighborhood provided by our top-level Gaia analyses and targeted investigations revealing smaller-scale patterns supported by robust spectroscopic measurements and Gaia astrometry. Together, these investigations are providing an unprecedented view of the processes shaping local star formation.

 

The spin evolution of stars born in clusters

schedule: Friday, 10:15

Julia Roquette (1) ; Sean P. Matt (2); Andrew J. Winter (3); Louis Amard (4); Sophia Stasevic (5)
(1) University of Geneva; (2) University of Exeter; (3) Heidelberg University; (4) CEA - Paris; (5) Paris Observatory

While most spin evolution models consider stars that form and evolve in isolation, the vast majority of observed young stars are located in clustered environments. Towards reducing this discrepancy, in this talk, we will show the results of a spin evolution model that considers the influence of open clusters' early environments on the spin evolution of low mass stars. In particular, we looked at how stellar density and the presence of massive stars shape the local far-ultraviolet radiation fields of clusters, influencing the dissipation timescales of circumstellar disks due to external photoevaporation. Environmentally dependent disk-dissipation timescales directly impact the duration of the star-disk-interaction phase, during which stars are expected to exchange angular momentum with their disk. By modelling the spin evolution of the low mass population of entire clusters with diverse environments, we will illustrate how these environments can contribute to shaping clusters' rotational distributions, with the feedback from massive stars playing a crucial role in explaining the mass dependence of rotation observed in the period-mass distributions of young regions like NGC 2264 and Upper Sco. By inducing an earlier start of the pre-main-sequence spin-up phase in stars evolving under high far-ultraviolet environments, the presence of high-mass stars can skew the spin-rate distribution of surrounding stars towards fast-rotation, explaining the excess of fast-rotating stars in the open cluster h Per. Environmental fingerprints on cluster's period-mass distributions should be observable at ages as early as 3 Myrs and remain visible until the stars' spin starts following the Skumanich spin-down. This suggests that the rotation of stars prior to the Skumanich spin-down phase could be used to trace their primordial ultraviolet irradiation, offering a potential method to connect planetary systems around stars with early-MS ages to their birth environment.

 

Multiple stellar populations in globular clusters

schedule: Friday, 11:15 (invited)

Anna Fabiola Marino
INAF - Osservatorio Astronomico di Padova

The presence of more than one stellar population in globular clusters (GCs) is now an established fact. Yet, the phenomenon remains an enigma. From an observational perspective, the multiple stellar populations are mostly studied in the ancient Milky Way GCs. For these clusters, the properties of the different populations of stars are constrained by chemical abundances coupled with the nicknamed "Chromosome Map" photometric diagram. One of the most intriguing results is that a new class of GCs, which includes Omega Centauri, harbors stellar populations with different metallicity, and this phenomenon could be much more widespread than previously believed. Having retained the material ejected by SNe, these GCs could have been substantially more massive at birth. I will present our latest observational results on how to read the properties of stellar populations on the Chromosome Maps, including the features that are proxies of metallicity variations. Furthermore, I will discuss future observational perspectives for the understanding of this puzzling phenomenon.

 

Survey of multiple populations in globular clusters among very low-mass stars.

schedule: Friday, 11:45

Emanuele Dondoglio (1) ; Antonino P. Milone (2); Alvio Renzini (3); Enrico Vesperini (4); Edoardo P. Lagioia (5); Anna F. Marino (6); Andrea Bellini (7); Marília Carlos (8); C. Cordoni (9); Sohee Jang (10); Maria Vittoria Legnardi (11); Mattia Libralato (12); Anjana Mohandasan (13); Francesca D'Antona (14); Marco Martorano (15); Fabrizio Muratore (16); Marco Tailo (17)
(1) Dipartimento di Fisica e Astronomia “Galileo Galilei”, Università di Padova; (2) Dipartimento di Fisica e Astronomia “Galileo Galilei”, Università di Padova; (3) Istituto Nazionale di Astrofisica - Osservatorio Astronomico di Padova; (4) Department of Astronomy, Indiana University; (5) Dipartimento di Fisica e Astronomia “Galileo Galilei”, Università di Padova; (6) Istituto Nazionale di Astrofisica - Osservatorio Astronomico di Padova; (7) Space Telescope Science Institute; (8) Dipartimento di Fisica e Astronomia “Galileo Galilei”, Università di Padova; (9) Dipartimento di Fisica e Astronomia “Galileo Galilei”, Università di Padova; (10) Dipartimento di Fisica e Astronomia “Galileo Galilei”, Università di Padova; (11) Dipartimento di Fisica e Astronomia “Galileo Galilei”, Università di Padova; (12) AURA for the European Space Agency (ESA); (13) Dipartimento di Fisica e Astronomia “Galileo Galilei”, Università di Padova; (14) INAF - Osservatorio Astronomico di Roma; (15) Dipartimento di Fisica e Astronomia “Galileo Galilei”, Università di Padova; (16) Dipartimento di Fisica e Astronomia “Galileo Galilei”, Università di Padova; (17) Dipartimento di Fisica e Astronomia “Galileo Galilei”, Università di Padova

Recent work has shown that NIR Hubble Space Telescope (HST) photometry allows us to disentangle multiple populations (MPs) among M dwarfs of globular clusters (GCs) and investigate this phenomenon in very low-mass (VLM) stars. Here, we present the color-magnitude diagrams (CMDs) of nine GCs and the open cluster NGC 6791 in the F110W and F160W bands of HST, showing that the main sequences (MSs) below the knee are either broadened or split thus providing evidence of MPs among VLM stars. In contrast, the MS of NGC 6791 is consistent with a single population. The color distribution of M-dwarfs dramatically changes between different GCs and the color width correlates with the cluster mass. We conclude that the MP ubiquity, variety, and dependence on GC mass are properties common to VLM and more-massive stars.

We combined UV, optical, and NIR observations of NGC 2808 and NGC 6121 (M 4) to identify MPs along with a wide range of stellar masses ($\sim0.2-0.8 \mathcal{M}_{\odot}$), from the MS turn off to the VLM regime, and measured, for the first time, their mass functions (MFs). We find that the fraction of MPs does not depend on the stellar mass and that their MFs have similar slopes. These findings indicate that the properties of MPs do not depend on stellar mass. In a scenario where the second generations formed in higher-density environments than the first generations, the possibility that the MPs formed with the same initial MF would suggest that it does not depend on the environment.

 

High-precision masses of red giants in a globular cluster : asteroseismology of M4

schedule: Friday, 12:00

Marco Tailo (1) for the Asterochronometry team
(1) Univesità degli studi di Bologna - Dipartimento di Fisica e Astronomia

The amount of mass lost by stars during the red-giant branch (RGB) phase is one of the main parameters needed to fully understand later stages of stellar evolution and, from a wider point of view, understand the fate of planetary systems, including our own. In spite of its importance, a fully-comprehensive physical understanding of this phenomenon is still missing, and we, rely mostly on empirical and semi-empirical formulations. Galactic Globular Clusters are ideal targets to derive such formulations, but, until recently, the presence of multiple populations has been a major challenge in constraining both stellar mass and RGB mass loss.

In this talk I will report the detection of solar-like oscillations in K2 data of red giants stars and horizontal branch stars belonging to M4. The final sample of stars where solar-like oscillations have been detected is the largest up to date for asteroseismic GC studies. Our results about mass, distance and mass loss will be compared with those expected from well-constrained, independent estimates. The implication for benchmarking asteroseismology in the low-metallicity regime and the insights about mass loss as a physical phenomenon will also be discussed.

 

Lithium in field clump giants, with an eye on stellar mass

schedule: Friday, 12:15

Julio Chanamé (1) ; Marc Pinsonneault (2); Claudia Aguilera-Gómez (3); Joel Zinn (4)
(1) Pontificia Universidad Católica de Chile; (2) The Ohio State University; (3) Universidad Diego Portales; (4) American Museum of Natural History

The existence of a small percentage of lithium (Li)-rich giants, defined as those showing A(Li) > 1.5 dex in their surfaces, is a decades old problem that challenges standard stellar evolution. More recently, based on Li data for large samples of evolved stars delivered by large spectroscopic surveys, the case has been made for Li-rich giants on a much more generalized scale, suggesting that all low-mass clump giants in the field, with typical levels 0 < A(Li) < 1.0 dex, are also enriched in Li beyond what is to be expected from standard stellar evolution. This has prompted works that suggest new production channels of Li inside low-mass stars, particularly associated to the helium flash. In this talk, I will argue that no new production channels of Li are needed, and that the Li observations are naturally explained when properly accounting for the properties of the progenitor stars of today's clump giants, particularly their mass and their Li content before they evolved off the main sequence.

 

Young stars in the Galactic bulge? Revised age determinations of micro-lensed subdwarfs with modern isochrones and statistical techniques

schedule: Friday, 12:30

Meridith Joyce (1) ; Christian Johnson (2); Tommaso Marchetti (3); Michael R Rich (4); Iulia Simion (5)
(1) Space Telescope Science Institute; (2) Space Telescope Science Institute; (3) European Southern Observatory; (4) UCLA; (5) Shanghai Key Lab for Astrophysics

In 2017, Benbsy et al. presented ages for a sample of 91 subdwarf stars in the Galactic bulge whose physical parameters (surface gravities, effective temperatures, and metallicities) were determined spectroscopically and without reliance on distance thanks to the unique circumstances of microlensing events. Their analysis finds a large constituency of young stars (<= 2.5 Gyr) in the bulge: a prominent result that has called into question astronomers' understanding of the formation history of the Galaxy. However, a re-determination of these stellar ages using Bensby et al.'s own measurements tells a different story. Using modern isochrones from the MIST (MESA Isochrones and Stellar Tracks) database, carefully applied statistical techniques, and taking into account the effects of alpha-element enhancement in metallicity, we find an age distribution notably lacking in very young stars. While our age distribution does include a tail consisting of some young stars, we do not reproduce the overabundance at 4 Gyr reported by Bensby et al., 2017. Further, we find a mean age for this population about 2-3 Gyr higher than the mean age reported in the initial study. In this seminar, I will discuss the age determination techniques used in this study, emphasizing in particular the role of uncertainties, and the implications our age distribution carries for Galactic formation scenarios.