scholarly journals Mixed Modes and Asteroseismic Surface Effects. II. Subgiant Systematics

2021 ◽  
Vol 922 (1) ◽  
pp. 18
Author(s):  
J. M. Joel Ong ◽  
Sarbani Basu ◽  
Mikkel N. Lund ◽  
Allyson Bieryla ◽  
Lucas S. Viani ◽  
...  
Keyword(s):  
Mode Coupling ◽  
Asymptotic Relation ◽  
Stellar Structure ◽  
Order Effects ◽  
Surface Term ◽  
Giant Stars ◽  
Fundamental Properties ◽  
Mixed Modes ◽  
Red Giant ◽  
Stellar Masses

Abstract Models of solar-like oscillators yield acoustic modes at different frequencies than would be seen in actual stars possessing identical interior structure, due to modeling error near the surface. This asteroseismic “surface term” must be corrected when mode frequencies are used to infer stellar structure. Subgiants exhibit oscillations of mixed acoustic (p-mode) and gravity (g-mode) character, which defy description by the traditional p-mode asymptotic relation. Since nonparametric diagnostics of the surface term rely on this description, they cannot be applied to subgiants directly. In Paper I, we generalized such nonparametric methods to mixed modes, and showed that traditional surface-term corrections only account for mixed-mode coupling to, at best, first order in a perturbative expansion. Here, we apply those results, modeling subgiants using asteroseismic data. We demonstrate that, for grid-based inference of subgiant properties using individual mode frequencies, neglecting higher-order effects of mode coupling in the surface term results in significant systematic differences in the inferred stellar masses, and measurable systematics in other fundamental properties. While these systematics are smaller than those resulting from other choices of model construction, they persist for both parametric and nonparametric formulations of the surface term. This suggests that mode coupling should be fully accounted for when correcting for the surface term in seismic modeling with mixed modes, irrespective of the choice of correction used. The inferred properties of subgiants, in particular masses and ages, also depend on the choice of surface-term correction, in a different manner from those of both main-sequence and red giant stars.

scholarly journals Effect of a strong magnetic field on gravity-mode period spacings in red giant stars

2020 ◽  
Vol 496 (3) ◽  
pp. 3829-3840
Author(s):  
Shyeh Tjing Loi
Keyword(s):  
Magnetic Field ◽  
Strong Magnetic Field ◽  
Strong Field ◽  
Average Density ◽  
Stellar Structure ◽  
Frequency Interval ◽  
Giant Stars ◽  
Red Giant Stars ◽  
Red Giant ◽  
Gravity Mode

ABSTRACT When a star evolves into a red giant, the enhanced coupling between core-based gravity modes and envelope-based pressure modes forms mixed modes, allowing its deep interior to be probed by asteroseismology. The ability to obtain information about stellar interiors is important for constraining theories of stellar structure and evolution, for which the origin of various discrepancies between prediction and observation is still under debate. Ongoing speculation surrounds the possibility that some red giant stars may harbour strong (dynamically significant) magnetic fields in their cores, but interpretation of the observational data remains controversial. In part, this is tied to shortfalls in our understanding of the effects of strong fields on the seismic properties of gravity modes, which lies beyond the regime of standard perturbative methods. Here, we seek to investigate the effect of a strong magnetic field on the asymptotic period spacings of gravity modes. We use a Hamiltonian ray approach to measure the volume of phase space occupied by mode-forming rays, this being roughly proportional to the average density of modes (number of modes per unit frequency interval). A strong field appears to systematically increase this by about 10 per cent, which predicts a ∼10 per cent smaller period spacing. Evidence of near integrability in the ray dynamics hints that the gravity-mode spectrum may still exhibit pseudo-regularities under a strong field.

scholarly journals Solar-like oscillations in subgiant and red-giant stars: mixed modes

2013 ◽  
Vol 9 (S301) ◽  
pp. 325-331 ◽  
Author(s):  
S. Hekker ◽  
A. Mazumdar
Keyword(s):  
High Resolution ◽  
Outer Part ◽  
Space Missions ◽  
Giant Stars ◽  
Internal Structures ◽  
Different Depths ◽  
Mixed Modes ◽  
Red Giant Stars ◽  
Red Giant

AbstractThanks to significant improvements in high-resolution spectrographs and the launch of dedicated space missions MOST, CoRoT and Kepler, the number of subgiants and red-giant stars with detected oscillations has increased significantly over the last decade. The amount of detail that can now be resolved in the oscillation patterns does allow for in-depth investigations of the internal structures of these stars. One phenomenon that plays an important role in such studies are mixed modes. These are modes that carry information of the inner radiative region as well as from the convective outer part of the star allowing to probe different depths of the stars.Here, we describe mixed modes and highlight some recent results obtained using mixed modes observed in subgiants and red-giant stars.

scholarly journals Eclipsing binaries in the OGLE variable star catalogue: long-period evolved systems

2020 ◽  
Vol 496 (1) ◽  
pp. 550-563
Author(s):  
Barış Hoyman ◽  
Sara Bulut ◽  
Orkun Özdarcan ◽  
Ömür Çakırlı
Keyword(s):  
Stellar Evolution ◽  
Binary Stars ◽  
Eclipsing Binaries ◽  
Parameter Determination ◽  
Atmospheric Parameter ◽  
Physical Parameters ◽  
Giant Stars ◽  
Red Giant ◽  
Type Giant ◽  
Stellar Masses

ABSTRACT Red giant stars are proving to be an exceptional source of information for testing models of stellar evolution, as photometric and spectroscopic analysis has opened up a window into their interiors, providing an exciting chance to develop highly constrained stellar models. In this study, we present a determination of precise fundamental physical parameters belonging to five detached, double-lined, eclipsing binary stars in the Large and Small Magellanic Clouds containing G- or early K-type giant stars with extended envelopes. We also derived the distances to the systems by using a temperature–colour relation and compared these distances with the measurements provided in the literature. The measured stellar masses are in the range 1.8–3.0 M⊙ and comparison with the PAdova and TRieste Stellar Evolution Code (PARSEC) isochrones gives ages between 0.4 and 1.1 Gyr. The derived uncertainties for individual masses and radii of components are better than 3 and 7 per cent, respectively, for these systems. Additionally, we performed atmospheric parameter determination and [M/H] analysis for each, where we disentangled the spectra.

scholarly journals Variations of the mixing character of dipolar mixed modes in red giant stars

2020 ◽  
Vol 495 (1) ◽  
pp. 621-636 ◽  
Author(s):  
C Jiang ◽  
M Cunha ◽  
J Christensen-Dalsgaard ◽  
QS Zhang
Keyword(s):  
Asymptotic Expansion ◽  
Stellar Evolution ◽  
Coupling Strength ◽  
Stellar Model ◽  
Quality Data ◽  
Chemical Abundances ◽  
Giant Stars ◽  
Mixing Character ◽  
Mixed Modes ◽  
Red Giant

ABSTRACT Because of the high-quality data of space missions, the detection of mixed modes has become possible in numerous stars. In this work, we investigate how the mixing character of dipolar mixed modes changes with stellar evolution, as well as with frequency within each stellar model. This is achieved by monitoring the variations in the coupling strength and the period spacing of dipolar mixed modes in red-giant models. These parameters are measured by fitting the asymptotic expansion of mixed modes to the model frequencies of a grid of red-giant models with masses between 1.0 and 2.0 M⊙ and three different chemical abundances. The coupling strength and the period spacing decrease with stellar evolution. We find that the slopes of their decreasing trends depend on the radial order of the pressure mode component. A non-negligible increase of the coupling strength with frequency by up to around 40 per cent is found in the observable frequency range for a set of red-giant models. On the contrary, no significant changes of the period spacing with frequency are found. The changes in the mixing character of the modes are in most cases affected by the model mass and metallicity. Buoyancy glitches also have an impact on the mixing character. Significant fluctuations in the estimated coupling strength and period spacing are found for models approaching the luminosity bump, if the glitch impact of the frequencies is not considered in the applied asymptotic expansion.

scholarly journals Solar-Like Oscillators in the Kepler Era: A Review

2021 ◽  
Vol 7 ◽  
Author(s):  
Jason Jackiewicz
Keyword(s):  
Theoretical Work ◽  
Stellar Structure ◽  
Structure Evolution ◽  
Mixing Processes ◽  
Precision Data ◽  
Stellar Oscillations ◽  
Giant Stars ◽  
Standard Models ◽  
Red Giant ◽  
Complex Phenomena

Many late-type stars across the Milky Way exhibit observable pulsations similar to our Sun that open up a window into stellar interiors. The NASA Kepler mission, a space-based photometric telescope, measured the micro-magnitude luminosity fluctuations caused by solar-like oscillations of tens of thousands of stars for almost 10 years. Detailed stellar structure, evolution, and oscillation theoretical work established in the decades before, such as predictions about mode mixing in the interior of red-giant stars, among many others, now had voluminous precision data against which it could be tested. The overwhelming result is the general validation of the theory of stellar oscillations as well as stellar-structure models; however, important gaps in our understanding of interior physics was also revealed by Kepler. For example, interior rotation, convection, and mixing processes are complex phenomena not fully captured by standard models. This review explores some of the important impacts Kepler observations of solar-like oscillations across the cool end of the H-R diagram has had on stellar astrophysics through the use of asteroseismology.

scholarly journals New population synthesis approach: The golden path to constrain stellar and galactic physics

2019 ◽  
Vol 15 (S357) ◽  
pp. 184-187
Author(s):  
Nadège Lagarde ◽  
Céline Reylé
Keyword(s):  
White Dwarfs ◽  
European Space Agency ◽  
Chemical Properties ◽  
Population Synthesis ◽  
Chemical Abundances ◽  
Giant Stars ◽  
Red Giant ◽  
Stellar Masses ◽  
First Time ◽  
Synthesis Approach

AbstractThe cornerstone mission of the European Space Agency, Gaia, has revealed properties of 260 000 white dwarfs in the Galaxy, allowing us for the first time to constrain the evolution of white dwarfs with a large sample. Complementary surveys (CoRoT, Kepler, K2, APOGEE and Gaia-ESO), will revolutionize our understanding of the formation and history of our Galaxy, providing accurate stellar masses, radii, ages, distances, and chemical properties for very large samples of stars across different Galactic stellar populations. To exploit the potential of the combination of spectroscopic, seismic and astrometric observations, the population synthesis approach is a very crucial and efficient tool. We develop the Besançon Galaxy model (BGM, Lagarde et al.2017) for which stellar evolution predictions are included, providing the global asteroseismic properties and the surface chemical abundances along the evolution of low- and intermediate-mass stars. For the first time, the BGM can explore the effects of an extra-mixing occurring in red-giant stars Lagarde et al.2019, changing their stellar properties. The next step is to model a consistent treatment of giant stars and their remnants (e.g., white dwarfs). This kind of improvement would help us to constrain stellar and Galactic evolutions.

scholarly journals Precise radial velocities of giant stars

2018 ◽  
Vol 616 ◽  
pp. A33 ◽  
Author(s):  
Stephan Stock ◽  
Sabine Reffert ◽  
Andreas Quirrenbach
Keyword(s):  
Test Sample ◽  
Evolutionary Stage ◽  
Evolutionary Models ◽  
Planetary Evolution ◽  
Giant Stars ◽  
Small Test ◽  
Red Giant ◽  
Stellar Masses ◽  
Host Stars

Context. The determination of accurate stellar parameters of giant stars is essential for our understanding of such stars in general and as exoplanet host stars in particular. Precise stellar masses are vital for determining the lower mass limit of potential substellar companions with the radial velocity method, but also for dynamical modeling of multiplanetary systems and the analysis of planetary evolution. Aims. Our goal is to determine stellar parameters, including mass, radius, age, surface gravity, effective temperature and luminosity, for the sample of giants observed by the Lick planet search. Furthermore, we want to derive the probability of these stars being on the horizontal branch (HB) or red giant branch (RGB), respectively. Methods. We compare spectroscopic, photometric and astrometric observables to grids of stellar evolutionary models using Bayesian inference. Results. We provide tables of stellar parameters, probabilities for the current post-main sequence evolutionary stage, and probability density functions for 372 giants from the Lick planet search. We find that 81% of the stars in our sample are more probably on the HB. In particular, this is the case for 15 of the 16 planet host stars in the sample. We tested the reliability of our methodology by comparing our stellar parameters to literature values and find very good agreement. Furthermore, we created a small test sample of 26 giants with available asteroseismic masses and evolutionary stages and compared these to our estimates. The mean difference of the stellar masses for the 24 stars with the same evolutionary stages by both methods is only ΔM = 〈Mtrk. − MAst.〉 = 0.01 ± 0.20 M⊙. Conclusions. We do not find any evidence for large systematic differences between our results and estimates of stellar parameters based on other methods. In particular we find no significant systematic offset between stellar masses provided by asteroseismology to our Bayesian estimates based on evolutionary models.

scholarly journals Fast core rotation in red-giant stars as revealed by gravity-dominated mixed modes

Nature ◽  
2011 ◽  
Vol 481 (7379) ◽  
pp. 55-57 ◽  
Author(s):  
Paul G. Beck ◽  
Josefina Montalban ◽  
Thomas Kallinger ◽  
Joris De Ridder ◽  
Conny Aerts ◽  
...  
Keyword(s):  
Giant Stars ◽  
Mixed Modes ◽  
Red Giant Stars ◽  
Red Giant ◽  
Core Rotation

Can we really use chemical properties of red-giant stars as age indicators?

2017 ◽  
Vol 13 (S334) ◽  
pp. 325-326
Author(s):  
Nadège Lagarde ◽  
A. C. Robin ◽  
C. Reylé
Keyword(s):  
Stellar Evolution ◽  
European Space Agency ◽  
Chemical Properties ◽  
Transport Processes ◽  
Giant Stars ◽  
Space Agency ◽  
Stellar Photosphere ◽  
Red Giant Stars ◽  
Red Giant ◽  
Stellar Masses

AbstractThe cornerstone mission of the European Space Agency, Gaia, together with forthcoming complementary surveys (CoRoT, Kepler, K2, APOGEE and Gaia-ESO), will revolutionize our understanding of the formation and history of our Galaxy, providing accurate stellar masses, radii, ages, distances, as well as chemical properties for a very large sample of stars across different Galactic stellar populations. Using improved population synthesis approach and new stellar evolution models we attempt to evaluate the possibility to derive ages of clump stars from their chemical properties. A new version of the Besançon Galaxy models (BGM) is used in which new stellar evolutionary tracks are computed from the stellar evolution code STAREVOL. The effects of mixing on chemical composition of the stellar photosphere has a significant impact on the determined stellar age from the observed [C/N] ratio. We clearly show that transport processes occurring in red-giant stars should be taken into account in the determination of ages for future Galactic archaeology studies.

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