Asymmetries of frequency splittings of dipolar mixed modes: a window on the topology of deep magnetic fields

Space asteroseismology is revolutionizing our knowledge of the internal structure and dynamics of stars. A breakthrough is ongoing with the recent discoveries of signatures of strong magnetic fields in the core of red giant stars. The key signature for such a detection is the asymmetry these fields induce in the frequency splittings of observed dipolar mixed gravito-acoustic modes. We investigate the ability of the observed asymmetries of the frequency splittings of dipolar mixed modes to constrain the geometrical properties of deep magnetic fields. We use the powerful analytical Racah-Wigner algebra used in Quantum Mechanics to characterize the geometrical couplings of dipolar mixed oscillation modes with various possible realistic fossil magnetic fields' topologies and compute the induced perturbation of their frequencies. First, in the case of an oblique magnetic dipole, we provide the exact analytical expression of the asymmetry as a function of the angle between the rotation and magnetic axes. Its value provides a direct measure of this angle. Second, considering a combination of axisymmetric dipolar and quadrupolar fields, we show how the asymmetry is blind to unravel the relative strength and sign of each component. Finally, in the case of a given multipole, we show that a negative asymmetry is a signature of non-axisymmetric topologies. Therefore, asymmetries of dipolar mixed modes provide key but only partial information on the geometrical topology of deep fossil magnetic fields. Asteroseismic constraints should therefore be combined with spectropolarimetric observations and numerical simulations, which aim to predict the more probable stable large-scale geometries.Comment: 10 pages, 3 figures, Letter accepted for publication in Astronomy & Astrophysic

Mode Mixing and Rotational Splittings: I. Near-Degeneracy Effects Revisited

Rotation is typically assumed to induce strictly symmetric rotational splitting into the rotational multiplets of pure p- and g-modes. However, for evolved stars exhibiting mixed modes, avoided crossings between different multiplet components are known to yield asymmetric rotational splitting, particularly for near-degenerate mixed-mode pairs, where notional pure p-modes are fortuitiously in resonance with pure g-modes. These near-degeneracy effects have been described in subgiants, but their consequences for the characterisation of internal rotation in red giants has not previously been investigated in detail, in part owing to theoretical intractability. We employ new developments in the analytic theory of mixed-mode coupling to study these near-resonance phenomena. In the vicinity of the most p-dominated mixed modes, the near-degenerate intrinsic asymmetry from pure rotational splitting increases dramatically over the course of stellar evolution, and depends strongly on the mode mixing fraction $\zeta$. We also find that a linear treatment of rotation remains viable for describing the underlying p- and g-modes, even when it does not for the resulting mixed modes undergoing these avoided crossings. We explore observational consequences for potential measurements of asymmetric mixed-mode splitting, which has been proposed as a magnetic-field diagnostic. Finally, we propose improved measurement techniques for rotational characterisation, exploiting the linearity of rotational effects on the underlying p/g modes, while still accounting for these mixed-mode coupling effects.Comment: 21 pages, 13 figures. Accepted to Ap

Can we detect deep axisymmetric toroidal magnetic fields in stars?

One of the major discoveries of asteroseismology is the signature of a strong extraction of angular momentum (AM) in the radiative zones of stars across the entire Hertzsprung-Russell diagram, resulting in weak core-to-surface rotation contrasts. Despite all efforts, a consistent AM transport theory, which reproduces both the internal rotation and mixing probed thanks to the seismology of stars, remains one of the major open problems in modern stellar astrophysics. A possible key ingredient to figure out this puzzle is magnetic field with its various possible topologies. Among them, strong axisymmetric toroidal fields, which are subject to the so-called Tayler MHD instability, could play a major role. They could trigger a dynamo action in radiative layers while the resulting magnetic torque allows an efficient transport of AM. But is it possible to detect signatures of these deep toroidal magnetic fields? The only way to answer this question is asteroseismology and the best laboratories of study are intermediate-mass and massive stars because of their external radiative envelope. Since most of these are rapid rotators during their main-sequence, we have to study stellar pulsations propagating in stably stratified, rotating, and potentially strongly magnetised radiative zones. For that, we generalise the traditional approximation of rotation, which provides in its classic version a flexible treatment of the adiabatic propagation of gravito-inertial modes, by taking simultaneously general axisymmetric differential rotation and toroidal magnetic fields into account. Using this new non-perturbative formalism, we derive the asymptotic properties of magneto-gravito-inertial modes and we explore the different possible field configurations. We found that the magnetic effects should be detectable for equatorial fields using high-precision asteroseismic data.Comment: 4 pages, 2 figures. Proceeding of the Annual meeting of the French Society of Astronomy and Astrophysics (SF2A 2022

Asteroseismology with the Roman Galactic Bulge Time-Domain Survey

Asteroseismology has transformed stellar astrophysics. Red giant asteroseismology is a prime example, with oscillation periods and amplitudes that are readily detectable with time-domain space-based telescopes. These oscillations can be used to infer masses, ages and radii for large numbers of stars, providing unique constraints on stellar populations in our galaxy. The cadence, duration, and spatial resolution of the Roman galactic bulge time-domain survey (GBTDS) are well-suited for asteroseismology and will probe an important population not studied by prior missions. We identify photometric precision as a key requirement for realizing the potential of asteroseismology with Roman. A precision of 1 mmag per 15-min cadence or better for saturated stars will enable detections of the populous red clump star population in the Galactic bulge. If the survey efficiency is better than expected, we argue for repeat observations of the same fields to improve photometric precision, or covering additional fields to expand the stellar population reach if the photometric precision for saturated stars is better than 1 mmag. Asteroseismology is relatively insensitive to the timing of the observations during the mission, and the prime red clump targets can be observed in a single 70 day campaign in any given field. Complementary stellar characterization, particularly astrometry tied to the Gaia system, will also dramatically expand the diagnostic power of asteroseismology. We also highlight synergies to Roman GBTDS exoplanet science using transits and microlensing.Comment: Roman Core Community Survey White Paper, 3 pages, 4 figure

Asteroseismology with the Roman Galactic Bulge Time-Domain Survey

Asteroseismology has transformed stellar astrophysics. Red giant asteroseismology is a prime example, with oscillation periods and amplitudes that are readily detectable with time-domain space-based telescopes. These oscillations can be used to infer masses, ages and radii for large numbers of stars, providing unique constraints on stellar populations in our galaxy. The cadence, duration, and spatial resolution of the Roman galactic bulge time-domain survey (GBTDS) are well-suited for asteroseismology and will probe an important population not studied by prior missions. We identify photometric precision as a key requirement for realizing the potential of asteroseismology with Roman. A precision of 1 mmag per 15-min cadence or better for saturated stars will enable detections of the populous red clump star population in the Galactic bulge. If the survey efficiency is better than expected, we argue for repeat observations of the same fields to improve photometric precision, or covering additional fields to expand the stellar population reach if the photometric precision for saturated stars is better than 1 mmag. Asteroseismology is relatively insensitive to the timing of the observations during the mission, and the prime red clump targets can be observed in a single 70 day campaign in any given field. Complementary stellar characterization, particularly astrometry tied to the Gaia system, will also dramatically expand the diagnostic power of asteroseismology. We also highlight synergies to Roman GBTDS exoplanet science using transits and microlensing

The K2 Galactic Archaeology Program Data Release 3: Age-abundance Patterns in C1–C8 and C10–C18

© 2022. The Author(s). Published by the American Astronomical Society. Content from this work may be used under the terms of the Creative Commons Attribution 4.0 license. https://creativecommons.org/licenses/by/4.0/Abstract: We present the third and final data release of the K2 Galactic Archaeology Program (K2 GAP) for Campaigns C1–C8 and C10–C18. We provide asteroseismic radius and mass coefficients, κ R and κ M , for ∼19,000 red giant stars, which translate directly to radius and mass given a temperature. As such, K2 GAP DR3 represents the largest asteroseismic sample in the literature to date. K2 GAP DR3 stellar parameters are calibrated to be on an absolute parallactic scale based on Gaia DR2, with red giant branch and red clump evolutionary state classifications provided via a machine-learning approach. Combining these stellar parameters with GALAH DR3 spectroscopy, we determine asteroseismic ages with precisions of ∼20%–30% and compare age-abundance relations to Galactic chemical evolution models among both low- and high-α populations for α, light, iron-peak, and neutron-capture elements. We confirm recent indications in the literature of both increased Ba production at late Galactic times as well as significant contributions to r-process enrichment from prompt sources associated with, e.g., core-collapse supernovae. With an eye toward other Galactic archeology applications, we characterize K2 GAP DR3 uncertainties and completeness using injection tests, suggesting that K2 GAP DR3 is largely unbiased in mass/age, with uncertainties of 2.9% (stat.) ± 0.1% (syst.) and 6.7% (stat.) ± 0.3% (syst.) in κ R and κ M for red giant branch stars and 4.7% (stat.) ± 0.3% (syst.) and 11% (stat.) ± 0.9% (syst.) for red clump stars. We also identify percent-level asteroseismic systematics, which are likely related to the time baseline of the underlying data, and which therefore should be considered in TESS asteroseismic analysis.Peer reviewedFinal Published versio

Age dating of an early Milky Way merger via asteroseismology of the naked-eye star ν Indi

Over the course of its history, the Milky Way has ingested multiple smaller satellite galaxies1. Although these accreted stellar populations can be forensically identified as kinematically distinct structures within the Galaxy, it is difficult in general to date precisely the age at which any one merger occurred. Recent results have revealed a population of stars that were accreted via the collision of a dwarf galaxy, called Gaia–Enceladus1, leading to substantial pollution of the chemical and dynamical properties of the Milky Way. Here we identify the very bright, naked-eye star ν Indi as an indicator of the age of the early in situ population of the Galaxy. We combine asteroseismic, spectroscopic, astrometric and kinematic observations to show that this metal-poor, alpha-element-rich star was an indigenous member of the halo, and we measure its age to be 11.0±0.7 (stat) ±0.8 (sys) billion years. The star bears hallmarks consistent with having been kinematically heated by the Gaia–Enceladus collision. Its age implies that the earliest the merger could have begun was 11.6 and 13.2 billion years ago, at 68% and 95% confidence, respectively. Computations based on hierarchical cosmological models slightly reduce the above limits

Detection and Characterization of Oscillating Red Giants: First Results from the TESS Satellite

Since the onset of the "space revolution" of high-precision high-cadence photometry, asteroseismology has been demonstrated as a powerful tool for informing Galactic archeology investigations. The launch of the NASA Transiting Exoplanet Survey Satellite (TESS) mission has enabled seismic-based inferences to go full sky—providing a clear advantage for large ensemble studies of the different Milky Way components. Here we demonstrate its potential for investigating the Galaxy by carrying out the first asteroseismic ensemble study of red giant stars observed by TESS. We use a sample of 25 stars for which we measure their global asteroseimic observables and estimate their fundamental stellar properties, such as radius, mass, and age. Significant improvements are seen in the uncertainties of our estimates when combining seismic observables from TESS with astrometric measurements from the Gaia mission compared to when the seismology and astrometry are applied separately. Specifically, when combined we show that stellar radii can be determined to a precision of a few percent, masses to 5%-10%, and ages to the 20% level. This is comparable to the precision typically obtained using end-of-mission Kepler data

Characterization of solar-type stars and study of their internal magnetic fields along the evolution : Machine learning for asteroseismology and theoretical constraints for internal magnetic fields

Les missions spatiales passées CoRoT et Kepler, actuellement en vol TESS, et en préparation PLATO permettent l'observation de millions d'étoiles. Dans cette thèse, nous nous intéressons en particulier aux géantes rouges (étoiles de type solaire évoluées). Grâce aux missions spatiales précédemment citées, il a été observé que le cœur de ces étoiles tourne environ dix fois plus rapidement que leur surface. Cependant, les modèles numériques d'évolution stellaire actuels prédisent une rotation beaucoup plus rapide: le cœur devrait tourner environ cent fois plus vite que la surface de l'étoile. Nous devons donc identifier le ou les mécanismes permettant d'évacuer efficacement le moment cinétique du cœur des étoiles de type solaire évoluées, une problématique qui est partagée par les étoiles de tout type pour tous les stades évolutifs. Dans ce contexte, des mesures très précises des paramètres stellaires globaux (comme la gravité de surface ou la masse) sont nécessaires pour contraindre au mieux les modèles d'évolution stellaires. L'étude des modes propres d'oscillation des étoiles (astérosismologie) joue un rôle clé car cette méthode permet l'estimation des paramètres stellaires avec une excellente précision. En revanche, cette précision n'est obtenue que lorsque les données sont peu bruitées et dans lesquelles les signaux des oscillations des étoiles sont bien visibles. En combinant les techniques classiques d'analyse photométrique et des algorithmes d'intelligence artificielle, nous obtenons dans le premier volet de cette thèse une meilleure estimation de la gravité de surface des étoiles de type solaire. Nous apportons en particulier des estimations précises de la gravité de surface lorsque l'astérosismologie standard atteint ses limites. Les étoiles de type solaire sont extraites de l'ensemble des données du satellite Kepler par un premier algorithme d'intelligence artificielle, et caractérisées par un second. Nous obtenons finalement les valeurs de gravité de surface des étoiles de type solaire avec une précision de 0.04 à 0.1 dex, entre la séquence principale (fusion de l'hydrogène dans le cœur) et les phases de géantes (fusion de l'hydrogène en couche et/ou de l'hélium dans le cœur), et ce directement à partir de la densité de puissance lumineuse moyenne émise par l'étoile. Ces étoiles bien caractérisées peuvent ensuite être étudiées plus en détail pour améliorer notre compréhension du transport de moment cinétique interne dans les étoiles de type solaire le long de leur évolution. Nous recherchons ensuite d'éventuelles signatures du phénomène de transport de moment cinétique actif dans les intérieurs stellaires dans les fréquences d'oscillation des étoiles de type solaire évoluées. Les champs magnétiques font partie des candidats les plus prometteurs pour expliquer le transport efficace observé. Les étoiles de masse intermédiaire (entre 1.1 et 3 masses solaires) sont connues pour développer un cœur convectif durant la séquence principale, qui peut générer un champ magnétique dynamo, relaxant dans l'intérieur radiatif sous forme d'un champ fossile et persistant durant le reste de l'évolution de l'étoile. Un tel champ magnétique devrait affecter les fréquences des modes d'oscillation mixtes acoustiques et de gravité des étoiles de masse intermédiaire évoluées étudiées dans cette thèse. Nous développons théoriquement la signature d'une telle configuration réaliste de champ magnétique fossile sur les fréquences des oscillations, en fonction de la masse, de la métallicité et de l'âge de l'étoile. Nous dérivons également une étude asymptotique complète, rendant possible l'inversion des données astérosismiques pour l'estimation des amplitudes des champs magnétiques internes. Nous estimons finalement l'efficacité d'un tel champ magnétique pour transporter le moment cinétique à l'intérieur des étoiles de type solaire.Previous space missions CoRoT and Kepler, current mission TESS, and mission in preparation PLATO allow the observation of millions of stars. In this thesis, we are particularly interested in red giants (evolved solar-type stars). One of the legacy results of the Kepler mission is the interestingly low rotation rate of the core of subgiant (SG) and red giant (RG) stars, which is about 10 times lower than predicted with the current theory for the transport of angular momentum by purely hydrodynamical mechanisms. This discrepancy points out an order of magnitude issue concerning the understanding of the evolution of the stellar angular momentum in evolved Solar-like stars, a very ubiquitous problem shared by stars of all types and ages. In this context, high-precision in the observables (such as surface gravity) is needed for stellar models results to be reliable. High-precision asteroseismology is a key player because it provides us with global stellar parameters with unprecedented precision and accuracy. However, asteroseismology is only efficient for stars that present a good signal-to-noise ratio with visible oscillation modes in their power spectrum density. By combining classical asteroseismology and innovative tools such as machine learning, we first focus on the better characterisation of the surface gravity for solar-type pulsators, including those that do not present detectable oscillations in their spectra. With Random Forest machine learning algorithms, solar-type stars observed by Kepler and TESS are classified among the different type of pulsating stars, and surface gravities of solar-type pulsators observed by Kepler ranging from 0.1 to 4.6 dex are estimated directly from the global power of the granulation with very small uncertainties of 0.04 to 0.1 dex. With the sample of well-characterised evolved Solar-like stars it will then be possible to conduct precise asteroseismic analyses to improve our understanding of angular momentum transport along the stellar evolution. To lead this work, we first theoretically seek for a missing process taking place inside the core of RG stars to efficiently extract angular momentum from the core to the surface. Internal magnetic fields are one amongst the most serious candidates that are currently studied to solve the problem. Stars more massive than ~1.1 Ms are known to develop a convective core during the main-sequence: the dynamo process due to this convection could be the origin of a strong magnetic field, trapped inside the core of the star for the rest of its evolution. Such magnetic fields should impact mixed modes inside the core of RG stars, and their signature should be visible in asteroseismic data. To unravel which constraints can be obtained from these observations, we theoretically investigate the effects of a plausible mixed axisymmetric magnetic field with various amplitudes on the mixed-mode frequencies of red giants. Applying a perturbative method, we estimate the magnetic splitting of the frequencies of simulated mixed dipolar modes that depends on the magnetic field strength and its configuration. A complete asymptotic analysis is derived, showing the potential of asteroseismology to probe the magnetism at each depth as this is done for stellar rotation. The effects of the mass and the metallicity of the stars are also explored. Finally, we infer an upper limit for the strength of the field and the associated lower limit for the timescale of its action to redistribute angular momentum in stellar interiors