Seismic signature of electron degeneracy in the core of red giants: hints for mass transfer between close red-giant companions

Ba II stars are red giants showing an enhancement of carbon and s-process elements. The elucidation of their nature seems to require a mass transfer, either by wind or Roche lobe overflow, during their past evolution. Were it really the case, all Ba II stars would be binaries with a white dwarf as companion. To better understand the exact role of their binarity, more orbits are definitely needed. They can be obtained by monitoring the radial velocity variations of those stars. However, a quicker way to find new Ba II stars with orbital elements would be to search for their existence among known spectroscopic binaries. This would also crucially test whether mass transfer is a necessary and sufficient condition to explain Ba II stars. If it is indeed the case, then all spectroscopic binaries, made of a giant and a white dwarf, in a reasonable range of periods, would exhibit the Ba II pecularity. However, the discovery of a peculiar giant+main sequence binary system would imply a revision of our ideas about Ba II stars. To this end have we begun a systematic spectral survey of spectroscopic binaries with orbital periods in the range characteristic of known Ba II stars and containing a red giant. The realization that some stars of the catalogue we compiled were already identified as semibariium stars encourages us to pursue our investigation. Coude spectra were taken with the 152 cm telescope, at a dispersion of 12 Å mm−1 . Until now, 2 stars out of a sample of 31 present a slight enhancement of s-process elements (their anomaly being in the range Ba 0.3 to 0.5), and 2 more appear to be good candidates. The study of a larger sample is currently in progress. A discussion of the nature of the companion to the 2 newly discovered semibarium stars is presented on grounds of their mass function and photometric indices.

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Near-degeneracy effects on the frequencies of rotationally-split mixed modes in red giants

Astronomy and Astrophysics ◽

10.1051/0004-6361/201730786 ◽

2017 ◽

Vol 605 ◽

pp. A75 ◽

Cited By ~ 14

Author(s):

S. Deheuvels ◽

R. M. Ouazzani ◽

S. Basu

Keyword(s):

Internal Rotation ◽

Space Mission ◽

Red Giants ◽

Perturbative Approach ◽

The Core ◽

Mixed Modes ◽

Red Giant ◽

Effects Of Rotation ◽

Near Degeneracy ◽

Mode Mixing

Context. The Kepler space mission has made it possible to measure the rotational splittings of mixed modes in red giants, thereby providing an unprecedented opportunity to probe the internal rotation of these stars. Aims. Asymmetries have been detected in the rotational multiplets of several red giants. This is unexpected since all the red giants whose rotation profiles have been measured thus far are found to rotate slowly, and low rotation, in principle, produces symmetrical multiplets. Our aim here is to explain these asymmetries and find a way of exploiting them to probe the internal rotation of red giants. Methods. We show that in the cases where asymmetrical multiplets were detected, near-degeneracy effects are expected to occur, because of the combined effects of rotation and mode mixing. Such effects have not been taken into account so far. By using both perturbative and non-perturbative approaches, we show that near-degeneracy effects produce multiplet asymmetries that are very similar to the observations. We then propose and validate a method based on the perturbative approach to probe the internal rotation of red giants using multiplet asymmetries. Results. We successfully apply our method to the asymmetrical l = 2 multiplets of the Kepler young red giant KIC 7341231 and obtain precise estimates of its mean rotation in the core and the envelope. The observed asymmetries are reproduced with a good statistical agreement, which confirms that near-degeneracy effects are very likely the cause of the detected multiplet asymmetries. Conclusions. We expect near-degeneracy effects to be important for l = 2 mixed modes all along the red giant branch (RGB). For l = 1 modes, these effects can be neglected only at the base of the RGB. They must therefore be taken into account when interpreting rotational splittings and as shown here, they can bring valuable information about the internal rotation of red giants.

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The Effect of Coulomb Interactions on the Helium Flash

International Astronomical Union Colloquium ◽

10.1017/s0252921100099322 ◽

1989 ◽

Vol 114 ◽

pp. 81-84

Author(s):

A. Harpaz ◽

A. Kovetz

Keyword(s):

Mass Loss ◽

Charged Particles ◽

Red Giants ◽

Coulomb Interactions ◽

Core Mass ◽

The Core ◽

Low Mass ◽

Red Giant ◽

Recent Claim

AbstractDetailed evolutionary calculations show that Coulomb interactions between the charged particles of a stellar plasma reduce the core mass at which a low mass red giant undergoes the helium flash (contrary to a recent claim). This has implications for the determination of the rate of mass loss from red giants.

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Criterion for Dynamical Instability of Mass Transfer in Binary Evolution

International Astronomical Union Colloquium ◽

10.1017/s0252921100001524 ◽

2002 ◽

Vol 187 ◽

pp. 297-302

Author(s):

Zhanwen Han ◽

Philipp Podsiadlowski ◽

Christopher A. Tout

Keyword(s):

Mass Transfer ◽

Angular Momentum ◽

Binary Systems ◽

Main Sequence ◽

Critical Mass ◽

Dynamical Instability ◽

Mass Ratio ◽

The Core ◽

Binary Evolution ◽

Red Giant

AbstractUsing Eggleton’s code, we performed a series of binary evolution calculations in order to investigate the criterion for dynamical instability of mass transfer in binaries. In these calculations, we took the donor’s mass on the zero-age main sequence (ZAMS) from 0.8 to 1.9 M⊙. For each mass, we systematically varied the mass of the core at the beginning of mass transfer and the mass of the companion star. We assumed that mass transfer was completely non-conservative and that all the mass that was lost from the system carried with it the orbital angular momentum of the accreting component. We found that the critical mass ratio, above which mass transfer is dynamically unstable, is from 1.1 to 1.3 in these red-giant binary systems.

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Seismic evidence for near solid-body rotation in two Kepler subgiants and implications for angular momentum transport

Astronomy and Astrophysics ◽

10.1051/0004-6361/202038578 ◽

2020 ◽

Vol 641 ◽

pp. A117 ◽

Cited By ~ 1

Author(s):

S. Deheuvels ◽

J. Ballot ◽

P. Eggenberger ◽

F. Spada ◽

A. Noll ◽

...

Keyword(s):

Angular Momentum ◽

Internal Rotation ◽

Open Clusters ◽

Momentum Transport ◽

Inversion Method ◽

Red Giants ◽

Data Set ◽

Angular Momentum Transport ◽

The Core ◽

Red Giant

Context. Asteroseismic measurements of the internal rotation of subgiants and red giants all show the need for invoking a more efficient transport of angular momentum than theoretically predicted. Constraints on the core rotation rate are available starting from the base of the red giant branch (RGB) and we are still lacking information on the internal rotation of less evolved subgiants. Aims. We identify two young Kepler subgiants, KIC 8524425 and KIC 5955122, whose mixed modes are clearly split by rotation. We aim to probe their internal rotation profile and assess the efficiency of the angular momentum transport during this phase of the evolution. Methods. Using the full Kepler data set, we extracted the mode frequencies and rotational splittings for the two stars using a Bayesian approach. We then performed a detailed seismic modeling of both targets and used the rotational kernels to invert their internal rotation profiles using the MOLA inversion method. We thus obtained estimates of the average rotation rates in the g-mode cavity (⟨Ω⟩g) and in the p-mode cavity (⟨Ω⟩p). Results. We found that both stars are rotating nearly as solid bodies, with core-envelope contrasts of ⟨Ω⟩g/⟨Ω⟩p = 0.68 ± 0.47 for KIC 8524425 and ⟨Ω⟩g/⟨Ω⟩p = 0.72 ± 0.37 for KIC 5955122. This result shows that the internal transport of angular momentum has to occur faster than the timescale at which differential rotation is forced in these stars (between 300 Myr and 600 Myr). By modeling the additional transport of angular momentum as a diffusive process with a constant viscosity νadd, we found that values of νadd > 5 × 104 cm2 s−1 are required to account for the internal rotation of KIC 8524425, and νadd > 1.5 × 105 cm2 s−1 for KIC 5955122. These values are lower than or comparable to the efficiency of the core-envelope coupling during the main sequence, as given by the surface rotation of stars in open clusters. On the other hand, they are higher than the viscosity needed to reproduce the rotation of subgiants near the base of the RGB. Conclusions. Our results yield further evidence that the efficiency of the internal redistribution of angular momentum decreases during the subgiant phase. We thus bring new constraints that will need to be accounted for by mechanisms that are proposed as candidates for angular momentum transport in subgiants and red giants.

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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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Surface Abundance Anomalies of Main Sequence Stars in Globular Clusters — A Probe into Interactions between Stars

Symposium - International Astronomical Union ◽

10.1017/s0074180900207687 ◽

2003 ◽

Vol 208 ◽

pp. 445-446 ◽

Cited By ~ 2

Author(s):

Masaaki Shimada ◽

Masayuki Y. Fujimoto ◽

Shimako Yamada ◽

Daiichiro Sugimoto

Keyword(s):

Mass Transfer ◽

Globular Clusters ◽

Main Sequence ◽

Red Giants ◽

Main Sequence Stars ◽

Giant Stars ◽

Close Encounters ◽

Dense Cores ◽

Abundance Anomalies ◽

Red Giant

In most of globular clusters, surface abundance anomalies are observed not only from red giant stars but also from main sequence stars. We discuss the possibility that the latter anomalies can be explained in terms the pollution due to mass transfer during close encounters with red giants, the latter of which have already developed the anomalies through their internal processes. If this is the case, the main sequence stars with the abundance anomalies may serve as a probe into the star-star interactions in dense cores of globular clusters.

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Core magnetic field imprint in the non-radial oscillations of red giant stars

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/staa1585 ◽

2020 ◽

Vol 496 (1) ◽

pp. 620-628

Author(s):

Pedro Gomes ◽

Ilídio Lopes

Keyword(s):

Magnetic Field ◽

Magnetic Fields ◽

Red Giants ◽

Perturbative Approach ◽

Frequency Splitting ◽

Giant Stars ◽

The Core ◽

Oscillation Modes ◽

Red Giant Stars ◽

Red Giant

ABSTRACT Magnetic fields in red giant stars remain a poorly understood topic, particularly in what concerns their intensity in regions far below the surface. In this work, we propose that gravity-dominated mixed modes of high absolute radial order and low angular degree can be used to probe the magnetic field in their radiative cores. Using two poloidal, axisymmetric configurations for the field in the core and the classical perturbative approach, we derive an analytical expression for the magnetic frequency splitting of these oscillation modes. Considering three distinct red giant models, with masses of 1.3, 1.6, and 2.0 M⊙, we find that a field strength of 105 G is necessary in the core of these stars to induce a frequency splitting of the order of a μHz in dipole and quadrupole oscillation modes. Moreover, taking into account observational limits, we estimate that magnetic fields in the cores of red giants that do not present observable magnetic splittings cannot exceed 104 G. Given the general absence of observable splittings in the oscillation spectra of these stars, and assuming that present mode suppression mechanisms are not biased towards certain azimuthal orders and retain all peaks in each multiplet, our results lead us to conclude that internal fields with the considered configurations and strengths above 104 G are not prevalent in red giants.

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Asteroseismic sensitivity to internal rotation along the red-giant branch

Astronomy and Astrophysics ◽

10.1051/0004-6361/201936947 ◽

2020 ◽

Vol 639 ◽

pp. A98 ◽

Cited By ~ 2

Author(s):

F. Ahlborn ◽

E. P. Bellinger ◽

S. Hekker ◽

S. Basu ◽

G. C. Angelou

Keyword(s):

Angular Momentum ◽

Internal Rotation ◽

Inversion Method ◽

Red Giants ◽

Rotation Rates ◽

The Core ◽

Surface Rotation ◽

Mixed Modes ◽

Red Giant ◽

Core Rotation

Context. Transport of angular momentum in stellar interiors is currently not well understood. Asteroseismology can provide us with estimates of internal rotation of stars and thereby advances our understanding of angular momentum transport. Aims. We can measure core-rotation rates in red-giant stars and we can place upper bounds on surface-rotation rates using measurements of dipole (l = 1) modes. Here, we aim to determine the theoretical sensitivity of modes of different spherical degree towards the surface rotation. Additionally, we aim to identify modes that can potentially add sensitivity at intermediate radii. Methods. We used asteroseismic rotational inversions to probe the internal stellar rotation profiles in red-giant models from the base of the red-giant branch up to the luminosity bump. We used the inversion method of multiplicative optimally localised averages to assess how well internal and surface rotation rates can be recovered from different mode sets and different synthetic rotation profiles. Results. We confirm that dipole mixed modes are sufficient to set constraints on the average core-rotation rates in red giants. However, surface-rotation rates estimated with only dipole mixed modes are contaminated by the core rotation. We show that the sensitivity to surface rotation decreases from the base of the red-giant branch until it reaches a minimum at 60–80% of the bump luminosity due to a glitch in the buoyancy frequency. Thereafter, a narrow range of increased surface sensitivity just below the bump luminosity exists. Quadrupole and octopole modes have more sensitivity in the outer parts of the star. To obtain accurate estimates of rotation rates at intermediate radii (i.e. a fractional radius of ∼0.4), acoustic oscillation modes with a spherical degree of l ≈ 10 are needed. Conclusions. We find a minimum and subsequent maximum in the sensitivity to the surface rotation rate in red giants below the luminosity bump. Furthermore, we show that, if observed, quadrupole and octopole modes enable us to distinguish between differential and solid body rotation in the convection zone. This will be important when investigating the transport of angular momentum between the core and the envelope.

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Population I Helium Burning Red Giants. Theory

Publications of the Astronomical Society of Australia ◽

10.1017/s1323358000012637 ◽

1971 ◽

Vol 2 (1) ◽

pp. 26-27 ◽

Cited By ~ 1

Author(s):

D. J. Faulkner

Keyword(s):

Preceding Paper ◽

Observational Evidence ◽

Red Giants ◽

Helium Burning ◽

Hydrogen Burning ◽

Preliminary Results ◽

The Core ◽

Galactic Clusters ◽

Red Giant

In the preceding paper, Cannon has outlined the observational evidence for the existence of a distinct concentration of stars near the base of the red giant branch in intermediate-age galactic clusters, which he tentatively identifies with the core helium burning phase of evolution occurring after the helium flash. This paper reports preliminary results of evolutionary calculations to test this identification.Since the computation of hydrogen shell-burning evolution up the red giant branch is extremely time-consuming, the present calculations have been commenced at the stage immediately following the helium flash. It is assumed that no overall mixing occurs at the flash, so that the composition discontinuity at the hydrogen-burning shell remains sharp. The initial stellar composition was set at (X, Y, Z) = (0.68, 0.30, 0.02), corresponding to Population I material.

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