Probing the mid-layer structure of red giants

Belkacem, K.; Pinçon, C.; Goupil, M. J.

doi:10.1051/0004-6361/201936864

Cited by 29 publications

(46 citation statements)

References 58 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…The period spacings from the optimisation are also in agreement with the results of Deheuvels et al (2014). The increase in the coupling factor for subgiant stars is due to a very thin zone where mixed modes are evanescent (Pinçon et al 2020). This is the strong coupling assumption that leads to high transmission between the gravity mode cavity and that of the pressure mode cavity (Takata 2016).…”

Section: Results From Optimisation: Period Spacing and Coupling Factorsupporting

confidence: 79%

On attempting to automate the identification of mixed dipole modes for subgiant stars

Appourchaux

2020

A&A

View full text Add to dashboard Cite

Context. The existence of mixed modes in stars is a marker of stellar evolution. Their detection serves for a better determination of stellar age. Aims. The goal of this paper is to identify the dipole modes in an automatic manner without human intervention. Methods. I used the power spectra obtained by the Kepler mission for the application of the method. I computed asymptotic dipole mode frequencies as a function of the coupling factor and dipole period spacing, as well as other parameters. For each star, I collapsed the power in an echelle diagramme aligned onto the monopole and dipole mixed modes. The power at the null frequency was used as a figure of merit. Using a genetic algorithm, I then optimised the figure of merit by adjusting the location of the dipole frequencies in the power spectrum. Using published frequencies, I compared the asymptotic dipole mode frequencies with published frequencies. I also used published frequencies to derive the coupling factor and dipole period spacing using a non-linear least squares fit. I used Monte-Carlo simulations of the non-linear least square fit to derive error bars for each parameter. Results. From the 44 subgiants studied, the automatic identification allows one to retrieve within 3 μHz, at least 80% of the modes for 32 stars, and within 6 μHz, at least 90% of the modes for 37 stars. The optimised and fitted gravity-mode period spacing and coupling factor are in agreement with previous measurements. Random errors for the mixed-mode parameters deduced from the Monte-Carlo simulation are about 30−50 times smaller than previously determined errors, which are in fact systematic errors. Conclusions. The period spacing and coupling factors of mixed modes in subgiants are confirmed. The current automated procedure will need to be improved upon using a more accurate asymptotic model and/or proper statistical tests.

show abstract

Section: Results From Optimisation: Period Spacing and Coupling Factorsupporting

confidence: 79%

On attempting to automate the identification of mixed dipole modes for subgiant stars

Appourchaux

2020

A&A

View full text Add to dashboard Cite

show abstract

“…Eq. ( 5), we can crudely estimate that ∆π 1 is about inversely proportional to the maximum of the Brunt-Väisälä frequency in the radiative region, which was shown by Pinçon et al (2020) to be approximately proportional to the square root of the helium core density. The evolution of the helium core density as a function of its mass is plotted in Fig.…”

Section: Period Spacing ∆πmentioning

confidence: 89%

“…This matches the observations for the pressure offset, indicating that the pressure offset holds an information about the density contrast and the structure in the evanescent region. Indeed, Pinçon et al (2020) showed that the structure of the intermediate evanescent region behaves as power laws of the radius when the density contrast between the core and the evanescent region is large, independently of the stellar mass. This also goes for the Brunt-Väisälä and Lamb frequencies, In contrast, the structure deviates from such a configuration for lower core-envelope density contrast as observed in subgiant stars (see also discussion in Sect.…”

Section: Pressure Offset Pmentioning

confidence: 99%

“…These studies showed that the decrease observed in the value of q during the evolution along the red giant branch is correlated with the increase in the size of the evanescent region. Pinçon et al (2020) demonstrated by means of analytical models that the thickening of this region on the red giant branch actually results from its migration to the radiative core towards the base of the convective envelope. This fact also explains the variations observed in the measurement of the gravity offset (Pinçon et al 2019).…”

Section: Introductionmentioning

confidence: 99%

See 1 more Smart Citation

Asteroseismology of evolved stars with EGGMiMoSA I. Theoretical mixed-mode patterns from the subgiant to the RGB phase

Farnir,

Pinçon,

Dupret

et al. 2021

Preprint

Self Cite

View full text Add to dashboard Cite

Context. In the context of an ever increasing amount of highly precise data, thanks to the numerous space-borne missions, came a revolution in stellar physics. This data allowed asteroseismology to thrive and improve our general knowledge of stars. Important results were obtained about giant stars owing to the presence of 'mixed modes' in their oscillation spectra. These modes carry information about the whole stellar interior, enabling the comprehensive characterisation of their structure. Aims. The current study is part of a series of papers that provide a technique to coherently and robustly analyse the mixed-modes frequency spectra and characterise the stellar structure of stars on both the subgiant branch and red-giant branch (RGB). In this paper we aim at defining seismic indicators, relevant of the stellar structure, as well as studying their evolution along a grid of models. Methods. The proposed method, EGGMiMoSA, relies on the asymptotic description of mixed modes. It defines appropriate initial guesses for the parameters of the asymptotic formulation and uses a Levenberg-Marquardt minimisation scheme in order to adjust the complex mixed-modes pattern in a fast and robust way. Results. We are able to follow the evolution of the mixed-modes parameters along a grid of models from the subgiant phase to the RGB bump, therefore extending previous works. We show the impact of the stellar mass and composition on the evolution of these parameters. We observe that the evolution of the period spacing ∆π1, pressure offset p, gravity offset g , and coupling factor q as a function of the large frequency separation ∆ν is little affected by the chemical composition and that it follows two different regimes depending on the evolutionary stage. On the subgiant branch, the stellar models display a moderate core-envelope density contrast. Therefore, the evolution of ∆π1, p, g , and q significantly changes with the stellar mass. Furthermore, we demonstrate that, for a given metallicity and with proper measurements of the period spacing ∆π1 and large frequency separation ∆ν, we may unambiguously constrain the stellar mass, radius and age of a subgiant star. Conversely, as the star reaches the red-giant branch, the core-envelope density contrast becomes very large. Consequently, the evolution of p, g and q as a function of ∆ν becomes independent of the stellar mass. This is also true for ∆π1 in stars with masses 1.8M because of core electron degeneracy. This degeneracy in ∆π1 is lifted for higher masses, again allowing for a precise measurement of the stellar age. Overall, our computations qualitatively agree with previous observed and theoretical studies. Conclusions. The method provides automated measurements of the adjusted parameters along a grid of models and opens the way to the precise seismic characterisation of both subgiants and red giants. In the following papers of the series, we will explore further refinements to the technique as well as its application to observed stars.

show abstract

“…-The mean core rotational splitting, δν rot,core , is a proxy of the mean core rotation rate and was measured for almost 900 stars on the RGB (Gehan et al 2018) and 200 stars in the red clump (Mosser et al 2012b). -The coupling factor, q, characterises the strength of the coupling in the evanescent region between p and g waves and was measured for about 5000 RGB and red clump stars (Mosser et al 2017a;Pinçon et al 2020). -The gravity offset, ε g , provides information on the stratification occurring in the radiative region and was measured for almost 400 stars both on the RGB and in the red clump (Mosser et al 2018;Pinçon et al 2019).…”

Section: Introductionmentioning

confidence: 99%

Automated approach to measure stellar inclinations: validation through large-scale measurements on the red giant branch

Gehan

Mosser

Michel

et al. 2021

A&A

View full text Add to dashboard Cite

Context. Measuring stellar inclinations is fundamental to understanding planetary formation and dynamics as well as the physical conditions during star formation. Oscillation spectra of red giant stars exhibit mixed modes that have both a gravity component from the radiative interior and a pressure component from the convective envelope. Gravity-dominated (g-m) mixed modes split by rotation are well separated inside frequency spectra, allowing accurate measurement of stellar inclinations. Aims. We aim to develop an automated and general approach to measuring stellar inclinations that can be applied to any solar-type pulsator for which oscillation modes are identified. We also aim to validate this approach using red giant branch stars observed by Kepler. Methods. Stellar inclination impacts the visibility of oscillation modes with azimuthal orders m = {−1, 0, +1}. We used the mean height-to-background ratio of dipole mixed modes with different azimuthal orders to measure stellar inclinations. We recovered the underlying statistical distribution of inclinations in an unbiased way using a probability density function for the stellar inclination angle. Results. We derive stellar inclination measurements for 1139 stars on the red giant branch for which Gehan et al. (2018) identified the azimuthal order of dipole g-m mixed modes. Raw measured inclinations exhibit strong deviation with respect to isotropy which is expected for random inclinations over the sky. When taking uncertainties into account, the reconstructed distribution of inclinations actually follows the expected isotropic distribution of the rotational axis. Conclusions. This work highlights the biases that affect inclination measurements and provides a way to infer their underlying statistical distribution. When a star is seen either pole on or equator on, measurements are challenging and result in a biased distribution. Correcting biases that appear in low-and high-inclination regimes allows us to recover the underlying inclination distribution.

show abstract

Probing the mid-layer structure of red giants

Cited by 29 publications

References 58 publications

On attempting to automate the identification of mixed dipole modes for subgiant stars

On attempting to automate the identification of mixed dipole modes for subgiant stars

Asteroseismology of evolved stars with EGGMiMoSA I. Theoretical mixed-mode patterns from the subgiant to the RGB phase

Automated approach to measure stellar inclinations: validation through large-scale measurements on the red giant branch

Contact Info

Product

Resources

About