scholarly journals On using dipolar modes to constrain the helium glitch in red giant stars

2020 ◽  
Vol 497 (1) ◽  
pp. 1008-1014
Author(s):  
G Dréau ◽  
M S Cunha ◽  
M Vrard ◽  
P P Avelino
Keyword(s):  
Acoustic Waves ◽  
Acoustic Mode ◽  
Stellar Model ◽  
Red Giants ◽  
Dipole Mode ◽  
Structural Variations ◽  
Giant Stars ◽  
Red Giant Stars ◽  
Red Giant ◽  
Additional Constraints

ABSTRACT The space-borne missions CoRoT and Kepler have revealed numerous mixed modes in red giant stars. These modes carry a wealth of information about red giant cores, but are of limited use when constraining rapid structural variations in their envelopes. This limitation can be circumvented if we have access to the frequencies of the pure acoustic dipolar modes in red giants, i.e. the dipole modes that would exist in the absence of coupling between gravity and acoustic waves. We present a pilot study aimed at evaluating the implications of using these pure acoustic mode frequencies in seismic studies of the helium structural variation in red giants. The study is based on artificial seismic data for a red giant branch stellar model, bracketing seven acoustic dipole radial orders around νmax. The pure acoustic dipole-mode frequencies are derived from a fit to the mixed-mode period spacings and then used to compute the pure acoustic dipole-mode second differences. The pure acoustic dipole-mode second differences inferred through this procedure follow the same oscillatory function as the radial-mode second differences. The additional constraints brought by the dipolar modes allow us to adopt a more complete description of the glitch signature when performing the fit to the second differences. The amplitude of the glitch retrieved from this fit is 15${{\ \rm per\ cent}}$ smaller than that from the fit based on the radial modes alone. Also, we find that thanks to the additional constraints, a bias in the inferred glitch location, found when adopting the simpler description of the glitch, is avoided.

scholarly journals The Aarhus red giants challenge

2020 ◽  
Vol 635 ◽  
pp. A165
Author(s):  
J. Christensen-Dalsgaard ◽  
V. Silva Aguirre ◽  
S. Cassisi ◽  
M. Miller Bertolami ◽  
A. Serenelli ◽  
...  
Keyword(s):  
Careful Consideration ◽  
Red Giants ◽  
Giant Stars ◽  
Observational Accuracy ◽  
Low Degree ◽  
Red Giant Stars ◽  
Red Giant ◽  
Internal Properties ◽  
Oscillation Frequencies ◽  
Oscillation Properties

Contact. The large quantity of high-quality asteroseismic data that have been obtained from space-based photometric missions and the accuracy of the resulting frequencies motivate a careful consideration of the accuracy of computed oscillation frequencies of stellar models, when applied as diagnostics of the model properties. Aims. Based on models of red-giant stars that have been independently calculated using different stellar evolution codes, we investigate the extent to which the differences in the model calculation affect the model oscillation frequencies and other asteroseismic diagnostics. Methods. For each of the models, which cover four different masses and different evolution stages on the red-giant branch, we computed full sets of low-degree oscillation frequencies using a single pulsation code and, from these frequencies, typical asteroseismic diagnostics. In addition, we carried out preliminary analyses to relate differences in the oscillation properties to the corresponding model differences. Results. In general, the differences in asteroseismic properties between the different models greatly exceed the observational precision of these properties. This is particularly true for the nonradial modes whose mixed acoustic and gravity-wave character makes them sensitive to the structure of the deep stellar interior and, hence, to details of their evolution. In some cases, identifying these differences led to improvements in the final models presented here and in Paper I; here we illustrate particular examples of this. Conclusions. Further improvements in stellar modelling are required in order fully to utilise the observational accuracy to probe intrinsic limitations in the modelling and improve our understanding of stellar internal physics. However, our analysis of the frequency differences and their relation to stellar internal properties provides a striking illustration of the potential, in particular, of the mixed modes of red-giant stars for the diagnostics of stellar interiors.

scholarly journals Active red giants: Close binaries versus single rapid rotators

2020 ◽  
Vol 639 ◽  
pp. A63
Author(s):  
Patrick Gaulme ◽  
Jason Jackiewicz ◽  
Federico Spada ◽  
Drew Chojnowski ◽  
Benoît Mosser ◽  
...  
Keyword(s):  
Angular Momentum ◽  
Close Binaries ◽  
Red Giants ◽  
Intermediate Mass ◽  
Giant Stars ◽  
Rotational Modulation ◽  
Low Mass ◽  
Red Giant Stars ◽  
Red Giant ◽  
Giant Branch Stars

Oscillating red-giant stars have provided a wealth of asteroseismic information regarding their interiors and evolutionary states, which enables detailed studies of the Milky Way. The objective of this work is to determine what fraction of red-giant stars shows photometric rotational modulation, and understand its origin. One of the underlying questions is the role of close binarity in this population, which relies on the fact that red giants in short-period binary systems (less than 150 days or so) have been observed to display strong rotational modulation. We selected a sample of about 4500 relatively bright red giants observed by Kepler, and show that about 370 of them (∼8%) display rotational modulation. Almost all have oscillation amplitudes below the median of the sample, while 30 of them are not oscillating at all. Of the 85 of these red giants with rotational modulation chosen for follow-up radial-velocity observation and analysis, 34 show clear evidence of spectroscopic binarity. Surprisingly, 26 of the 30 nonoscillators are in this group of binaries. On the contrary, about 85% of the active red giants with detectable oscillations are not part of close binaries. With the help of the stellar masses and evolutionary states computed from the oscillation properties, we shed light on the origin of their activity. It appears that low-mass red-giant branch stars tend to be magnetically inactive, while intermediate-mass ones tend to be highly active. The opposite trends are true for helium-core burning (red clump) stars, whereby the lower-mass clump stars are comparatively more active and the higher-mass ones are less active. In other words, we find that low-mass red-giant branch stars gain angular momentum as they evolve to clump stars, while higher-mass ones lose angular momentum. The trend observed with low-mass stars leads to possible scenarios of planet engulfment or other merging events during the shell-burning phase. Regarding intermediate-mass stars, the rotation periods that we measured are long with respect to theoretical expectations reported in the literature, which reinforces the existence of an unidentified sink of angular momentum after the main sequence. This article establishes strong links between rotational modulation, tidal interactions, (surface) magnetic fields, and oscillation suppression. There is a wealth of physics to be studied in these targets that is not available in the Sun.

scholarly journals Model Atmospheres for Normal and Peculiar Red Giant Stars

2000 ◽  
Vol 177 ◽  
pp. 71-80
Author(s):  
Bertrand Plez
Keyword(s):  
Recent Progress ◽  
Atmospheric Modeling ◽  
Current Status ◽  
Red Giants ◽  
Cool Star ◽  
Giant Stars ◽  
Red Giant Stars ◽  
Red Giant

I review the current status of model atmospheres for red giants, with special emphasis on recent progress and newer grids. I draw attention to some specific problems regarding opacity sources and present current and forthcoming efforts in cool-star atmospheric modeling.

scholarly journals Lithium in metal-poor red giants

2009 ◽  
Vol 5 (S268) ◽  
pp. 361-362
Author(s):  
Laimons Začs ◽  
Arturs Barzdis
Keyword(s):  
High Resolution ◽  
Asymptotic Giant Branch ◽  
Red Giants ◽  
Lithium Abundance ◽  
Giant Stars ◽  
Synthetic Spectra ◽  
Lithium Depletion ◽  
Red Giant Stars ◽  
Red Giant

AbstractThe lithium abundance was calculated for five metal-poor red giant stars from Li i doublet at 6707 Å by fitting the observed high-resolution spectra with synthetic spectra. The lithium abundance was found to be low in all stars, logϵ(Li) ≤ 1.8, confirming lithium depletion on the red giant and asymptotic giant branch.

scholarly journals Models of red giants in the CoRoT asteroseismology fields combining asteroseismic and spectroscopic constraints

2015 ◽  
Vol 11 (A29B) ◽  
pp. 525-528
Author(s):  
Lagarde Nadège
Keyword(s):  
Spectroscopic Study ◽  
Stellar Evolution ◽  
Open Cluster ◽  
Stellar Populations ◽  
Red Giants ◽  
Seismic Properties ◽  
Giant Stars ◽  
Statistical Studies ◽  
Red Giant Stars ◽  
Red Giant

AbstractThe availability of asteroseismic constraints for a large sample of red-giant stars from the CoRoT and Kepler missions paves the way for various statistical studies of the seismic properties of stellar populations. We use a detailed spectroscopic study of 19 CoRoT red-giant stars (Morel et al. 2014) to compare theoretical stellar evolution models to observations of the open cluster NGC 6633 and field stars. This study is already published in Lagarde et al. (2015)

scholarly journals Magnetic Topology and the Heating of Extended Chromospheres

1983 ◽  
Vol 102 ◽  
pp. 445-448
Author(s):  
Robert E. Stencel
Keyword(s):  
Mass Loss ◽  
Compressional Wave ◽  
Red Giants ◽  
Magnetic Topology ◽  
Giant Stars ◽  
Wave Heating ◽  
Red Giant Stars ◽  
Red Giant ◽  
Warm Region ◽  
Rapid Mass

New data indicate that red giants are surrounded by geometrically thick chromospheres of several stellar radii extent. Such chromospheres occur among stars which apparently lack coronae. Maintenance of this extended warm region may require non-compressional wave heating of a magnetic character, and this may provide a crucial clue to the mechanism of rapid mass loss from red giant stars.

scholarly journals A seismic scaling relation for stellar age II: the red giant branch

2019 ◽  
Vol 492 (1) ◽  
pp. L50-L55 ◽  
Author(s):  
Earl Patrick Bellinger
Keyword(s):  
Scaling Relations ◽  
Red Giants ◽  
Giant Stars ◽  
Stellar Ages ◽  
History Of ◽  
Red Giant Stars ◽  
Red Giant ◽  
Computationally Intensive ◽  
Age Relations ◽  
Grid Based

ABSTRACT Owing to their simplicity and ease of application, seismic scaling relations are widely used to determine the properties of stars exhibiting solar-like oscillations, such as solar twins and red giants. So far, no seismic scaling relations for determining the ages of red giant stars have been developed. Such relations would be desirable for Galactic Archaeology, which uses stellar ages to map the history of the Milky Way. The ages of red giants must instead be estimated with reference to grids of theoretical stellar models, which can be computationally intensive. Here, I present an exhaustive search for scaling age relations involving different combinations of observable quantities. The candidate scaling relations are calibrated and tested using more than 1000 red giant stars whose ages were obtained via grid-based modelling. I report multiple high-quality scaling relations for red giant branch stars, the best of which are shown to be approximately as accurate as grid-based modelling with typical uncertainties of 15 per cent. Additionally, I present new scaling mass and radius relations for red giants as well.

scholarly journals Stellar loci IV. red giant stars

2022 ◽  
Vol 21 (12) ◽  
pp. 319
Author(s):  
Ruo-Yi Zhang ◽  
Hai-Bo Yuan ◽  
Xiao-Wei Liu ◽  
Mao-Sheng Xiang ◽  
Yang Huang ◽  
...  
Keyword(s):  
Galactic Halo ◽  
Signal To Noise Ratio ◽  
Sloan Digital Sky Survey ◽  
Red Giants ◽  
Giant Stars ◽  
Sky Survey ◽  
History Of ◽  
Red Giant Stars ◽  
Red Giant ◽  
A New Technique

Abstract In the fourth paper of this series, we present the metallicity-dependent Sloan Digital Sky Survey (SDSS) stellar color loci of red giant stars, using a spectroscopic sample of red giants in the SDSS Stripe 82 region. The stars span a range of 0.55 – 1.2 mag in color g – i, –0.3 – –2.5 in metallicity [Fe/H], and have values of surface gravity log g smaller than 3.5 dex. As in the case of main-sequence (MS) stars, the intrinsic widths of loci of red giants are also found to be quite narrow, a few mmag at maximum. There are however systematic differences between the metallicity-dependent stellar loci of red giants and MS stars. The colors of red giants are less sensitive to metallicity than those of MS stars. With good photometry, photometric metallicities of red giants can be reliably determined by fitting the u – g, g – r, r – i, and i – z colors simultaneously to an accuracy of 0.2 – 0.25 dex, comparable to the precision achievable with low-resolution spectroscopy for a signal-to-noise ratio of 10. By comparing fitting results to the stellar loci of red giants and MS stars, we propose a new technique to discriminate between red giants and MS stars based on the SDSS photometry. The technique achieves completeness of ∼70 per cent and efficiency of ∼80 per cent in selecting metal-poor red giant stars of [Fe/H] ≤ –1.2. It thus provides an important tool to probe the structure and assemblage history of the Galactic halo using red giant stars.

scholarly journals Metallicity effect on stellar granulation detected from oscillating red giants in open clusters

2017 ◽  
Vol 605 ◽  
pp. A3 ◽  
Author(s):  
E. Corsaro ◽  
S. Mathur ◽  
R. A. García ◽  
P. Gaulme ◽  
M. Pinsonneault ◽  
...  
Keyword(s):  
Time Scales ◽  
Model Comparison ◽  
Open Clusters ◽  
Stellar Atmospheres ◽  
Surface Gravity ◽  
Scaling Relations ◽  
Red Giants ◽  
Giant Stars ◽  
Red Giant Stars ◽  
Red Giant

Context. The effect of metallicity on the granulation activity in stars, and hence on the convective motions in general, is still poorly understood. Available spectroscopic parameters from the updated APOGEE-Kepler catalog, coupled with high-precision photometric observations from NASA’s Kepler mission spanning more than four years of observation, make oscillating red giant stars in open clusters crucial testbeds. Aims. We aim to determine the role of metallicity on the stellar granulation activity by discriminating its effect from that of different stellar properties such as surface gravity, mass, and temperature. We analyze 60 known red giant stars belonging to the open clusters NGC 6791, NGC 6819, and NGC 6811, spanning a metallicity range from [Fe/H] ≃ − 0.09 to 0.32. The parameters describing the granulation activity of these stars and their frequency of maximum oscillation power, νmax, are studied while taking into account different masses, metallicities, and stellar evolutionary stages. We derive new scaling relations for the granulation activity, re-calibrate existing ones, and identify the best scaling relations from the available set of observations. Methods. We adopted the Bayesian code Diamonds for the analysis of the background signal in the Fourier spectra of the stars. We performed a Bayesian parameter estimation and model comparison to test the different model hypotheses proposed in this work and in the literature. Results. Metallicity causes a statistically significant change in the amplitude of the granulation activity, with a dependency stronger than that induced by both stellar mass and surface gravity. We also find that the metallicity has a significant impact on the corresponding time scales of the phenomenon. The effect of metallicity on the time scale is stronger than that of mass. Conclusions. A higher metallicity increases the amplitude of granulation and meso-granulation signals and slows down their characteristic time scales toward longer periods. The trend in amplitude is in qualitative agreement with predictions from existing 3D hydrodynamical simulations of stellar atmospheres from main sequence to red giant stars. We confirm that the granulation activity is not sensitive to changes in the stellar core and that it only depends on the atmospheric parameters of stars.

scholarly journals Measuring the core rotation of red giant stars

2019 ◽  
Vol 82 ◽  
pp. 225-232
Author(s):  
C. Gehan ◽  
B. Mosser ◽  
E. Michel
Keyword(s):  
Red Giants ◽  
Frequency Spectra ◽  
Convective Envelope ◽  
Giant Stars ◽  
Physical Conditions ◽  
The Core ◽  
Long Duration ◽  
Red Giant Stars ◽  
Red Giant ◽  
Core Rotation

Red giant stars present mixed modes, which behave as pressure modes in the convective envelope and as gravity modes in the radiative interior. This mixed character allows to probe the physical conditions in their core. With the advent of long-duration time series from space-borne missions such as CoRoT and Kepler, it becomes possible to study the red giant core rotation. As more than 15 000 red giant light curves have been recorded, it is crucial to develop a robust and efficient method to measure this rotation. Such measurements of thousands of mean core rotation would open the way to a deeper understanding of the physical mechanisms that are able to transport angular momentum from the core to the envelope in red giants. In this work, we detail the principle of the method we developed to obtain automatic measurements of the red giant mean core rotation. This method is based on the stretching of the oscillation spectra and on the use of the so-called Hough transform. We finally validate this method for stars on the red giant branch, where overlapping rotational splittings and mixed-mode spacings produce complicated frequency spectra.

Sign in / Sign up

Export Citation Format

Share Document