Active red giants: Close binaries versus single rapid rotators

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.

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Red giant stars: from mixed modes to angular momentum

EAS Publications Series ◽

10.1051/eas/1982019 ◽

2019 ◽

Vol 82 ◽

pp. 189-211

Author(s):

K. Belkacem

Keyword(s):

Angular Momentum ◽

Current Knowledge ◽

Red Giants ◽

Low Mass Stars ◽

Giant Stars ◽

Physical Mechanisms ◽

Star Evolution ◽

Low Mass ◽

Mixed Modes ◽

Red Giant

Solar-like oscillations are ubiquitous to low-mass stars from the main-sequence to the red-giant branch as demonstrated by the space-borne missions CoRoT and Kepler. Understanding the physical mechanisms governing their amplitudes as well as their behavior along with the star evolution is a prerequisite for interpreting the wealth of seismic data and for inferring stellar internal structure. In this paper, I discuss our current knowledge of mode amplitudes with particular emphasis on non-radial modes in red giants (hereafter mixed modes). Then, I will show how these modes permit to unveil the rotation of the inner-most layers of low-mass stars and how they put stringent constraints on the redistribution of angular momentum.

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

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/staa1981 ◽

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.

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The Aarhus red giants challenge

Astronomy and Astrophysics ◽

10.1051/0004-6361/201936766 ◽

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.

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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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Asteroseismology of red giants to constrain angular momentum transport

Proceedings of the International Astronomical Union ◽

10.1017/s1743921314006656 ◽

2014 ◽

Vol 9 (S307) ◽

pp. 165-170

Author(s):

P. Eggenberger

Keyword(s):

Angular Momentum ◽

Rotational Frequency ◽

Momentum Transport ◽

Red Giants ◽

Angular Momentum Transport ◽

Rotation Rates ◽

Giant Stars ◽

Red Giant ◽

Core Rotation

AbstractAsteroseismic data obtained by theKeplerspacecraft have led to the recent detection and characterization of rotational frequency splittings of mixed modes in red-giant stars. This has opened the way to the determination of the core rotation rates for these stars, which is of prime importance to progress in our understanding of internal angular momentum transport. In this contribution, we discuss which constraints can be brought by these asteroseismic measurements on the modelling of angular momentum transport in stellar radiative zones.

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Observations of Lithium in red giant stars

Proceedings of the International Astronomical Union ◽

10.1017/s1743921310004278 ◽

2009 ◽

Vol 5 (S268) ◽

pp. 301-309

Author(s):

Verne V. Smith

Keyword(s):

Stellar Evolution ◽

Asymptotic Giant Branch ◽

Red Giants ◽

Asymptotic Giant Branch Stars ◽

Multiple Sources ◽

Lithium Abundance ◽

Giant Stars ◽

Red Giant ◽

First Time ◽

Giant Branch Stars

AbstractConnections between observations of the lithium abundance in various types of red giants and stellar evolution are discussed here. The emphasis is on three main topics; 1) the depletion of Li as stars ascend the red giant branch for the first time, 2) the synthesis of 7Li in luminous and massive asymptotic giant branch stars via the mechanism of hot-bottom burning, and 3) the possible multiple sources of excess Li abundances found in a tiny fraction of various types of G and K giants.

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Constraining angular momentum transport processes in stellar interiors with red-giant stars in the open cluster NGC6819

EPJ Web of Conferences ◽

10.1051/epjconf/201510106042 ◽

2015 ◽

Vol 101 ◽

pp. 06042

Author(s):

N Lagarde, ◽

A. Miglio, ◽

P. Eggenberger, ◽

J Montalbán, ◽

D Bossini,

Keyword(s):

Angular Momentum ◽

Open Cluster ◽

Transport Processes ◽

Momentum Transport ◽

Angular Momentum Transport ◽

Giant Stars ◽

Stellar Interiors ◽

Red Giant Stars ◽

Red Giant

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Excitation of internal gravity waves by penetrative convection

EAS Publications Series ◽

10.1051/eas/1982024 ◽

2019 ◽

Vol 82 ◽

pp. 247-251

Author(s):

C. Pinçon ◽

K. Belkacem ◽

M.J. Goupil

Keyword(s):

Angular Momentum ◽

Gravity Waves ◽

Internal Gravity Waves ◽

Penetrative Convection ◽

Low Mass Stars ◽

Low Mass ◽

Red Giant ◽

Excitation Model ◽

Giant Branch Stars ◽

Do So

We investigate the ability of internal gravity waves that are generated by penetrative convection to redistribute angular momentum in the internal radiative zone of evolved low-mass stars. To do so, we use the semianalytical excitation model recently proposed by Pinçon et al. 2016. We briefly report the preliminary results of the study focusing on the subgiant and red giant branch stars.

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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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