Pulsation and rotation in NGC 6811: the Kepler short-cadence stars

ABSTRACT We have analysed a selected sample of 36 Kepler short-cadence stars in the field of NGC 6811. The results reveal that all the targets are variable: two red giant stars with solar-like oscillations, 21 main-sequence pulsators (16 δ Scuti and five γ Doradus stars), and 13 rotating variables. Three new γ Doradus (γ Dor) variables (one is a hot γ Dor star) are detected in this work together with five new rotating variables. An in-depth frequency analysis of the δ Scuti (δ Sct) and γ Dor stars in the sample shows that the frequency spectra are very rich, in particular for the δ Sct-type variables. They present very dense frequency distributions and wide diversity in frequency patterns, even for stars being members of the cluster and with very similar location in the Hertzsprung–Russell (H–R) diagram. Rotational modulation is found for a high percentage of these main-sequence pulsating stars, which is an indication of stellar activity being common on the surfaces of these hot stars. In some cases, activity dominates the luminosity variations. Significant amplitude variability is also detected in at least some of the pulsation modes of highest amplitude in almost all the δ Sct stars. One of the δ Sct pulsators is a member of a binary system with the pulsating component tidally distorted. This system also shows strong stellar activity, including several flares that probably originate in the δ Sct component.

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Ages for large samples of stars - caveats and uncertainties

Proceedings of the International Astronomical Union ◽

10.1017/s1743921317007116 ◽

2017 ◽

Vol 13 (S334) ◽

pp. 147-152

Author(s):

Arlette Noels-Grötsch

Keyword(s):

Main Sequence ◽

Planetary Systems ◽

Foreseeable Future ◽

Time Line ◽

Giant Stars ◽

Large Samples ◽

Formation And Evolution ◽

Stellar Models ◽

Red Giant ◽

Age Determinations

AbstractAlthough a stellar age accuracy of about 10 % seems to be a reasonable requirement to draw a time line in the evolution of our Galaxy as well as in the formation and evolution of exo-planetary systems, theoretical stellar models are at present still too imperfect to really achieve this goal. Asteroseismic observations are definitely of invaluable assistance, especially if individual pulsation frequencies are available, which is still far from common. Large stellar samples are now in the spotlight with two different lines of attack, spectroscopic and photometric surveys as well as asteroseismic missions. I shall review the problems arising from stellar physics in the context of large stellar samples of main sequence and red giant stars, and I shall raise some alarm bells but also highlight some positive news for a drastic improvement in stellar age determinations below the limit of 10 % in a foreseeable future.

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Chemical Abundances of Main-sequence, Turnoff, Subgiant, and Red Giant Stars from APOGEE Spectra. II. Atomic Diffusion in M67 Stars

The Astrophysical Journal ◽

10.3847/1538-4357/ab0b43 ◽

2019 ◽

Vol 874 (1) ◽

pp. 97 ◽

Cited By ~ 19

Author(s):

Diogo Souto ◽

C. Allende Prieto ◽

Katia Cunha ◽

Marc Pinsonneault ◽

Verne V. Smith ◽

...

Keyword(s):

Main Sequence ◽

Atomic Diffusion ◽

Chemical Abundances ◽

Giant Stars ◽

Red Giant Stars ◽

Red Giant

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Asteroseismology of evolved stars to constrain the internal transport of angular momentum

Astronomy and Astrophysics ◽

10.1051/0004-6361/201833447 ◽

2019 ◽

Vol 621 ◽

pp. A66 ◽

Cited By ~ 14

Author(s):

P. Eggenberger ◽

S. Deheuvels ◽

A. Miglio ◽

S. Ekström ◽

C. Georgy ◽

...

Keyword(s):

Angular Momentum ◽

Transport Process ◽

Main Sequence ◽

Red Giants ◽

Rotation Rates ◽

Giant Stars ◽

Evolved Stars ◽

Surface Rotation ◽

Red Giant ◽

Internal Transport

Context. The observations of solar-like oscillations in evolved stars have brought important constraints on their internal rotation rates. To correctly reproduce these data, an efficient transport mechanism is needed in addition to the transport of angular momentum by meridional circulation and shear instability. The efficiency of this undetermined process is found to increase both with the mass and the evolutionary stage during the red giant phase. Aims. We study the efficiency of the transport of angular momentum during the subgiant phase. Methods. The efficiency of the unknown transport mechanism is determined during the subgiant phase by comparing rotating models computed with an additional corresponding viscosity to the asteroseismic measurements of both core and surface-rotation rates for six subgiants observed by the Kepler spacecraft. We then investigate the change in the efficiency of this transport of angular momentum with stellar mass and evolution during the subgiant phase. Results. The precise asteroseismic measurements of both core and surface-rotation rates available for the six Kepler targets enable a precise determination of the efficiency of the transport of angular momentum needed for each of these subgiants. These results are found to be insensitive to all the uncertainties related to the modelling of rotational effects before the post-main sequence (poMS) phase. An interesting exception in this context is the case of young subgiants (typical values of log(g) close to 4), because their rotational properties are sensitive to the degree of radial differential rotation on the main sequence (MS). These young subgiants constitute therefore perfect targets to constrain the transport of angular momentum on the MS from asteroseismic observations of evolved stars. As for red giants, we find that the efficiency of the additional transport process increases with the mass of the star during the subgiant phase. However, the efficiency of this undetermined mechanism decreases with evolution during the subgiant phase, contrary to what is found for red giants. Consequently, a transport process with an efficiency that increases with the degree of radial differential rotation cannot account for the core-rotation rates of subgiants, while it correctly reproduces the rotation rates of red giant stars. This suggests that the physical nature of the additional mechanism needed for the internal transport of angular momentum may be different in subgiant and red giant stars.

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The Distance Modulus of LMC

Symposium - International Astronomical Union ◽

10.1017/s0074180900149484 ◽

1986 ◽

Vol 116 ◽

pp. 513-514

Author(s):

Cesare Chiosi ◽

Luisa Pigatto

Keyword(s):

Luminosity Function ◽

Main Sequence ◽

Bimodal Distribution ◽

Distance Modulus ◽

Red Giants ◽

Ccd Photometry ◽

Giant Stars ◽

Red Giant Clump ◽

Classical Models ◽

Red Giant

Deep CCD photometry of the star clusters NGC2162 and NGC2190 in LMC presented by Schommer et al. (1984) is used togheter with new evolutionary models computed by Bertelli et al. (1985a) which take into account overshooting from convective cores, to derive the clusters ages and the distance modulus of LMC. A preliminary analysis of the two clusters indicates that NGC 2162 and NGC 2190 belong to the same class of clusters discussed by Barbaro and Pigatto (1984). In fact, for the turn-off mass estimated by means of classical models (<2.2m⊙) these clusters should possess an extended red giant branch and a bimodal distribution of red stars (cifr. Fig.2). On the contrary they show a clump of red stars. This means that ages and other properties derived from classical models for this range of masses, may not correspond to reality. With the new models, stars of mass as low as 1.6 m⊙, ignite helium in non degenerate conditions, avoid the long lived RG phase, and burn helium as more massive stars. As consequence of it, a clump of red giants is expected. In Fig.1, we show new isochrones (Bertelli et al. 1985b) derived from models with overshooting, overlaid to the CM diagram of NGC 2162. Theoretical luminosities and Teff's are converted into Mv:(B-V)o plane by means of Teff:(B-V):BC scales based on models atmospheres collected from several authors (Chiosi, 1985). At any given age, the new isochrones run brighter than those of Ciardullo and Demarque (1977). By means of the luminosity function, a method more objective (Paczsynski, 1984) than the standard one of ZAMS and/or isochrone fitting, with a reddening of E(B-V)=0.06 and chemical composition X=0.700 and Z=0.02, we find ages of 1 109yr and a true distance modulus of (m-M)O=18.6 instead of 18.2±0.2 mag given by Schommer et al.(1984). Fig.2 shows the theoretical luminosity function at age 1 109yr, (age preliminarly assigned to the clusters by isochrone fitting) for main sequence and red giant stars obtained with Salpeter's IMF (top panel), compares it with the correspondent one of Ciardullo and Demarque(1977), and finally shows the observational LF we derive from stars counts(bottom panel) for NGC 2162. By imposing coincidence between theoretical and observational LF's at the side of main sequence fall-off and rising of the red giant clump, we derive the distance modulus (m-M)O=18.6. In conclusions, models with overshooting not only interpret the morphology of this class of clusters, but assigne LMC a distance modulus in agreement with other independent determinations (Walker, 1984; Visvanathan, 1985).

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γ Doradus stars as a test of angular momentum transport models

Astronomy and Astrophysics ◽

10.1051/0004-6361/201832607 ◽

2019 ◽

Vol 626 ◽

pp. A121 ◽

Cited By ~ 19

Author(s):

R.-M. Ouazzani ◽

J. P. Marques ◽

M.-J. Goupil ◽

S. Christophe ◽

V. Antoci ◽

...

Keyword(s):

Angular Momentum ◽

Main Sequence ◽

Evolutionary Models ◽

Intermediate Mass ◽

Angular Momentum Transport ◽

Rotation Rates ◽

Giant Stars ◽

Intermediate Mass Stars ◽

Red Giant ◽

Core Rotation

Helioseismology and asteroseismology of red giant stars have shown that distribution of angular momentum in stellar interiors, and the evolution of this distribution with time remains an open issue in stellar physics. Owing to the unprecedented quality and long baseline of Kepler photometry, we are able to seismically infer internal rotation rates in γ Doradus stars, which provide the main-sequence counterpart to the red-giants puzzle. Here, we confront these internal rotation rates to stellar evolution models which account for rotationally induced transport of angular momentum, in order to test angular momentum transport mechanisms. On the one hand, we used a stellar model-independent method developed by our team in order to obtain accurate, seismically inferred, buoyancy radii and near-core rotation for 37 γ Doradus stars observed by Kepler. We show that the stellar buoyancy radius can be used as a reliable evolution indicator for field stars on the main sequence. On the other hand, we computed rotating evolutionary models of intermediate-mass stars including internal transport of angular momentum in radiative zones, following the formalism developed in the series of papers started by Zahn (1992, A&A, 265, 115), with the CESTAM code. This code calculates the rotational history of stars from the birth line to the tip of the RGB. The initial angular momentum content has to be set initially, which is done here by fitting rotation periods in young stellar clusters. We show a clear disagreement between the near-core rotation rates measured in the sample and the rotation rates obtained from the evolutionary models including rotationally induced transport of angular momentum following Zahn’s prescriptions. These results show a disagreement similar to that of the Sun and red giant stars in the considered mass range. This suggests the existence of missing mechanisms responsible for the braking of the core before and along the main sequence. The efficiency of the missing mechanisms is investigated. The transport of angular momentum as formalized by Zahn and Maeder cannot explain the measurements of near-core rotation in main-sequence intermediate-mass stars we have at hand.

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Asteroseismic fingerprints of stellar mergers

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/stab2528 ◽

2021 ◽

Author(s):

Nicholas Z Rui ◽

Jim Fuller

Keyword(s):

Stellar Evolution ◽

Total Mass ◽

Main Sequence ◽

Red Giants ◽

Giant Stars ◽

Evolution Dynamics ◽

Red Giant ◽

Stellar Mergers ◽

Gravity Mode ◽

Spacing Measurement

Abstract Stellar mergers are important processes in stellar evolution, dynamics, and transient science. However, it is difficult to identify merger remnant stars because they cannot easily be distinguished from single stars based on their surface properties. We demonstrate that merger remnants can potentially be identified through asteroseismology of red giant stars using measurements of the gravity mode period spacing together with the asteroseismic mass. For mergers that occur after the formation of a degenerate core, remnant stars have over-massive envelopes relative to their cores, which is manifested asteroseismically by a g mode period spacing smaller than expected for the star’s mass. Remnants of mergers which occur when the primary is still on the main sequence or whose total mass is less than ≈2 M⊙ are much harder to distinguish from single stars. Using the red giant asteroseismic catalogs of Vrard et al. (2016) and Yu et al. (2018), we identify 24 promising candidates for merger remnant stars. In some cases, merger remnants could also be detectable using only their temperature, luminosity, and asteroseismic mass, a technique that could be applied to a larger population of red giants without a reliable period spacing measurement.

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The Distance Modulus of LMC

Symposium - International Astronomical Union ◽

10.1017/s0074180900149575 ◽

1986 ◽

Vol 116 ◽

pp. 513-514

Author(s):

Cesare Chiosi ◽

Luisa Pigatto

Keyword(s):

Luminosity Function ◽

Main Sequence ◽

Bimodal Distribution ◽

Distance Modulus ◽

Red Giants ◽

Ccd Photometry ◽

Giant Stars ◽

Red Giant Clump ◽

Classical Models ◽

Red Giant

Deep CCD photometry of the star clusters NGC2162 and NGC2190 in LMC presented by Schommer et al. (1984) is used togheter with new evolutionary models computed by Bertelli et al. (1985a) which take into account overshooting from convective cores, to derive the clusters ages and the distance modulus of LMC. A preliminary analysis of the two clusters indicates that NGC 2162 and NGC 2190 belong to the same class of clusters discussed by Barbaro and Pigatto (1984). In fact, for the turn-off mass estimated by means of classical models (<2.2mO) these clusters should possess an extended red giant branch and a bimodal distribution of red stars (cifr. Fig.2). On the contrary they show a clump of red stars. This means that ages and other properties derived from classical models for this range of masses, may not correspond to reality. With the new models, stars of mass as low as 1.6 mO, ignite helium in non degenerate conditions, avoid the long lived RG phase, and burn helium as more massive stars. As consequence of it, a clump of red giants is expected. In Fig.1, we show new isochrones (Bertelli et al. 1985b) derived from models with overshooting, overlaid to the CM diagram of NGC 2162. Theoretical luminosities and Teff's are converted into Mv:(B-V)o plane by means of Teff:(B-V):BC scales based on models atmospheres collected from several authors (Chiosi, 1985). At any given age, the new isochrones run brighter than those of Ciardullo and Demarque (1977). By means of the luminosity function, a method more objective (Paczsynski, 1984) than the standard one of ZAMS and/or isochrone fitting, with a reddening of E(B-V)=0.06 and chemical composition X=0.700 and Z=0.02, we find ages of 1 109yr and a true distance modulus of (m-M)O=18.6 instead of 18.2±0.2 mag given by Schommer et al.(1984). Fig.2 shows the theoretical luminosity function at age 1 109yr, (age preliminarly assigned to the clusters by isochrone fitting) for main sequence and red giant stars obtained with Salpeter's IMF (top panel), compares it with the correspondent one of Ciardullo and Demarque(1977), and finally shows the observational LF we derive from stars counts(bottom panel) for NGC 2162. By imposing coincidence between theoretical and observational LF's at the side of main sequence fall-off and rising of the red giant clump, we derive the distance modulus (m-M)O=18.6. In conclusions, models with overshooting not only interpret the morphology of this class of clusters, but assigne LMC a distance modulus in agreement with other independent determinations (Walker, 1984; Visvanathan, 1985).

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A correlation between GALEX FUV magnitude and chromospheric activity among red giant stars

Publications of the Astronomical Society of Australia ◽

10.1017/pasa.2018.45 ◽

2018 ◽

Vol 35 ◽

Cited By ~ 2

Author(s):

Graeme H. Smith

Keyword(s):

Stellar Evolution ◽

Main Sequence ◽

Giant Stars ◽

Chromospheric Activity ◽

Chromospheric Emission ◽

Red Giant Stars ◽

Red Giant ◽

Far Ultraviolet

AbstractIt is shown that upon combining GALEX far-ultraviolet and Johnson B magnitudes a resultant FUV–B colour can be obtained that for red giant stars of luminosity classes III and II correlates well with chromospheric emission in the cores of the Mg iih and k lines. Giant stars throughout the colour range 0.8 ≤ B – V ≤ 1.6 exhibit such a phenomenon. The main result of this paper is to show that GALEX far-ultraviolet photometry can provide information about the degree of chromospheric activity among red giant stars, and as such may offer a tool for surveying the evolution of chromospheric activity from the main sequence into the red giant phases of stellar evolution.

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Measuring the core rotation of red giant stars

EAS Publications Series ◽

10.1051/eas/1982021 ◽

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.

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