scholarly journals Mirror principle and the red-giant bump: the battle of entropy in low-mass stars

2020 ◽  
Vol 492 (4) ◽  
pp. 5940-5948 ◽  
Author(s):  
S Hekker ◽  
G C Angelou ◽  
Y Elsworth ◽  
S Basu
Keyword(s):  
Red Giants ◽  
Low Mass Stars ◽  
Hydrogen Burning ◽  
Specific Entropy ◽  
Low Mass ◽  
The Mean ◽  
Red Giant ◽  
Mirror Principle ◽  
Physical Reasons ◽  
Mean Molecular Weight

ABSTRACT The evolution of low-mass stars into red giants is still poorly understood. During this evolution the core of the star contracts and, simultaneously, the envelope expands – a process known as the ‘mirror’. Additionally, there is a short phase where the trend for increasing luminosity is reversed. This is known as the red giant branch bump. We explore the underlying physical reasons for these two phenomena by considering the specific entropy distribution in the star and its temporal changes. We find that between the luminosity maximum and luminosity minimum of the bump there is no mirror present and the star is fully contracting. The contraction is halted and the star regains its mirror when the hydrogen-burning shell reaches the mean molecular weight discontinuity. This marks the luminosity minimum of the bump.

scholarly journals Why Do Low-Mass Stars Become Red Giants?

2009 ◽  
Vol 26 (3) ◽  
pp. 203-208 ◽  
Author(s):  
Richard J. Stancliffe ◽  
Alessandro Chieffi ◽  
John C. Lattanzio ◽  
Ross P. Church
Keyword(s):  
Molecular Weight ◽  
Stellar Evolution ◽  
Energy Generation ◽  
Red Giants ◽  
Low Mass Stars ◽  
Change In The Mean ◽  
Low Mass ◽  
The Mean ◽  
Red Giant ◽  
Mean Molecular Weight

AbstractWe revisit the problem of why stars become red giants. We modify the physics of a standard stellar evolution code in order to determine what does and what does not contribute to a star becoming a red giant. In particular, we have run tests to try to separate the effects of changes in the mean molecular weight and in the energy generation. The implications for why stars become red giants are discussed. We find that while a change in the mean molecular weight is necessary (but not sufficient) for a 1-M⊙ star to become a red giant, this is not the case in a star of 5 M⊙. It therefore seems that there may be more than one way to make a giant.

scholarly journals The destruction of 3He by Rayleigh-Taylor instability on the first giant branch

2006 ◽  
Vol 2 (S239) ◽  
pp. 286-293
Author(s):  
Peter P. Eggleton ◽  
David S. P. Dearborn ◽  
John C. Lattanzio
Keyword(s):  
Local Minimum ◽  
Big Bang ◽  
Taylor Instability ◽  
Big Bang Nucleosynthesis ◽  
Low Mass Stars ◽  
Hydrogen Burning ◽  
Convective Envelope ◽  
Low Mass ◽  
Rayleigh Taylor Instability ◽  
Red Giant

AbstractLow-mass stars, ∼ 1–2 solar masses, near the Main Sequence are efficient at producing 3He, which they mix into the convective envelope on the giant branch and distribute into the Galaxy by way of envelope loss. This process is so efficient that it is difficult to reconcile the observed cosmic abundance of 3He with the predictions of Big Bang nucleosynthesis. In this paper we find, by modeling a red giant with a fully three-dimensional hydrodynamic code and a full nucleosynthetic network, that mixing arises in the supposedly stable and radiative zone between the hydrogen-burning shell and the base of the convective envelope. This mixing is due to Rayleigh-Taylor instability within a zone just above the hydrogen-burning shell. In this zone the burning of the 3He left behind by the retreating convective envelope is predominantly by the reaction 3He + 3He → 4He + 1H + 1H, a reaction which, untypically for stellar nuclear reactions, lowers the mean molecular weight, leading to a local minimum. This local minimum leads to Rayleigh-Taylor instability, and turbulent motion is generated which will continue ultimately up into the normal convective envelope. Consequently material from the envelope is dragged down sufficiently close to the burning shell that the He in it is progressively destroyed. Thus we are able to remove the threat that He production in low-mass stars poses to the Big Bang nucleosynthesis of 3He.Some slow mixing mechanism has long been suspected, that connects the convective envelope of a red giant to the burning shell. It appears to be necessary to account for progressive changes in the C/C and N/C ratios on the First Giant Branch. We suggest that these phenomena are also due to the Rayleigh-Taylor-unstable character of the He-burning region.

scholarly journals Stellar evolution with turbulent diffusion mixing in low mass stars and 12C/13C ratio in giants of the first ascending branch

1984 ◽  
Vol 105 ◽  
pp. 525-527
Author(s):  
O. Bienaymé ◽  
A. Maeder ◽  
E. Schatzman
Keyword(s):  
Molecular Weight ◽  
Stellar Evolution ◽  
Main Sequence ◽  
Turbulent Transport ◽  
Chemical Species ◽  
Life Time ◽  
Low Mass Stars ◽  
Low Mass ◽  
Red Giant ◽  
Mean Molecular Weight

We consider stellar evolution in low mass stars (1–3 Mo) near the main sequence with the hypothesis that mild turbulence is present within the all star. Turbulent transport of the elements is modeled by diffusion equations where the diffusion coefficient is chosen to be D = R✶eν where ν is the kinematical viscosity and R✶e is a Reynolds number. We consider the effects of the growth of the gradient of the mean molecular weight on turbulence. The main consequences of diffusion on stellar evolution are (1) an increase of the life time near the main sequence and (2) a change of the radial distributions of chemical species (12C, 13C, 14N, 160) (figure 1). The inhibition of the turbulence, when the gradient of mean molecular weight reaches a certain critical value, allows the evolution towards the red giant branch. When stars evolve towards the giant branch, chemical species are dredged up to the surface. At this stage models with and without diffusion, predict substantially different surface abundances (in particular the 12C/13C and C/N ratios). Comparison between models and the available data on giants during the first dredge-up show that abundance anomalies can be explained if turbulent mixing is present during the main sequence phase (figure 2).

scholarly journals Red giant stars: from mixed modes to angular momentum

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.

scholarly journals Thermohaline mixing in low-mass giants

2008 ◽  
Vol 4 (S252) ◽  
pp. 103-109 ◽  
Author(s):  
M. Cantiello ◽  
N. Langer
Keyword(s):  
Differential Rotation ◽  
Main Sequence ◽  
Red Giants ◽  
Helium Burning ◽  
Low Mass Stars ◽  
Mixing Processes ◽  
Low Mass ◽  
Magnetic Mixing ◽  
Red Giant

AbstractThermohaline mixing has recently been proposed to occur in low mass red giants, with large consequences for the chemical yields of low mass stars. We investigate the role of thermohaline mixing during the evolution of stars between 1 M⊙ and 3 M⊙, in comparison to other mixing processes acting in these stars. We confirm that thermohaline mixing has the potential to destroy most of the 3He which is produced earlier on the main sequence during the red giant stage. In our models we find that this process is working only in stars with initial mass M ≲ 1.5 M⊙. Moreover, we report that thermohaline mixing can be present during core helium burning and beyond in stars which still have a 3He reservoir. While rotational and magnetic mixing is negligible compared to the thermohaline mixing in the relevant layers, the interaction of thermohaline motions with differential rotation and magnetic fields may be essential to establish the time scale of thermohaline mixing in red giants.

scholarly journals Extra-Mixing in Luminous Cool Red Giants: Hints from Evolved Stars With and Without Li

2009 ◽  
Vol 26 (3) ◽  
pp. 168-175 ◽  
Author(s):  
R. Guandalini ◽  
S. Palmerini ◽  
M. Busso ◽  
S. Uttenthaler
Keyword(s):  
Asymptotic Giant Branch ◽  
Evolutionary Status ◽  
Red Giants ◽  
Low Mass Stars ◽  
Magnetic Buoyancy ◽  
Low Mass ◽  
Red Giant ◽  
Mass Circulation ◽  
Short Time

AbstractWe present an analysis of Li abundances in low mass stars (LMS) during the Red Giant Branch (RGB) and Asymptotic Giant Branch (AGB) stages, based on a new determination of their luminosities and evolutionary status. By applying recently suggested models for extra-mixing, induced by magnetic buoyancy, we show that both Li-rich and Li-poor stars can be accounted for. The simplest scenario implies the development of fast instabilities on the RGB, where Li is produced. When the fields increase in strength, buoyancy slows down and Li is destroyed. 3He is consumed, at variable rates. The process continues on the AGB, where however moderate mass circulation rates have little effect on Li due to the short time available. O-rich and C-rich stars show different histories of Li production/destruction, possibly indicative of different masses. More complex transport schemes are allowed by magnetic buoyancy, with larger effects on Li, but most normal LMS seem to show only the range of Li variation discussed here.

scholarly journals Core rotation braking on the red giant branch for various mass ranges

2018 ◽  
Vol 616 ◽  
pp. A24 ◽  
Author(s):  
C Gehan ◽  
B. Mosser ◽  
E. Michel ◽  
R. Samadi ◽  
T. Kallinger
Keyword(s):  
Angular Momentum ◽  
Red Giants ◽  
Regular Pattern ◽  
Convective Envelope ◽  
Large Sample ◽  
The Mean ◽  
Mixed Modes ◽  
Red Giant ◽  
Core Rotation ◽  
Dipole Gravity

Context. Asteroseismology allows us to probe stellar interiors. In the case of red giant stars, conditions in the stellar interior are such as to allow for the existence of mixed modes, consisting in a coupling between gravity waves in the radiative interior and pressure waves in the convective envelope. Mixed modes can thus be used to probe the physical conditions in red giant cores. However, we still need to identify the physical mechanisms that transport angular momentum inside red giants, leading to the slow-down observed for red giant core rotation. Thus large-scale measurements of red giant core rotation are of prime importance to obtain tighter constraints on the efficiency of the internal angular momentum transport, and to study how this efficiency changes with stellar parameters. Aims. This work aims at identifying the components of the rotational multiplets for dipole mixed modes in a large number of red giant oscillation spectra observed by Kepler. Such identification provides us with a direct measurement of the red giant mean core rotation. Methods. We compute stretched spectra that mimic the regular pattern of pure dipole gravity modes. Mixed modes with the same azimuthal order are expected to be almost equally spaced in stretched period, with a spacing equal to the pure dipole gravity mode period spacing. The departure from this regular pattern allows us to disentangle the various rotational components and therefore to determine the mean core rotation rates of red giants. Results. We automatically identify the rotational multiplet components of 1183 stars on the red giant branch with a success rate of 69% with respect to our initial sample. As no information on the internal rotation can be deduced for stars seen pole-on, we obtain mean core rotation measurements for 875 red giant branch stars. This large sample includes stars with a mass as large as 2.5 M⊙, allowing us to test the dependence of the core slow-down rate on the stellar mass. Conclusions. Disentangling rotational splittings from mixed modes is now possible in an automated way for stars on the red giant branch, even for the most complicated cases, where the rotational splittings exceed half the mixed-mode spacing. This work on a large sample allows us to refine previous measurements of the evolution of the mean core rotation on the red giant branch. Rather than a slight slow-down, our results suggest rotation is constant along the red giant branch, with values independent of the mass.

scholarly journals Observations of low mass stars in clusters: Some constraints and puzzles for stellar evolution theory

1984 ◽  
Vol 105 ◽  
pp. 123-138
Author(s):  
R.D. Cannon
Keyword(s):  
Globular Clusters ◽  
Main Sequence ◽  
Star Clusters ◽  
Variable Stars ◽  
Open Clusters ◽  
Low Mass Stars ◽  
Theory Of Evolution ◽  
Theoretical Predictions ◽  
Low Mass ◽  
Red Giant

This review will attempt to do two things: (i) discuss some of the data which are available for testing the theory of evolution of low mass stars, and (ii) point out some problem areas where observations and theory do not seem to agree very well. This is of course too vast a field of research to be covered in one brief review, so I shall concentrate on one particular aspect, namely the study of star clusters and especially their colour-magnitude (CM) diagrams. Star clusters provide large samples of stars at the same distance and with the same age, and the CM diagram gives the easiest way of comparing theoretical predictions with observations, although crucial evidence is also provided by spectroscopic abundance analyses and studies of variable stars. Since this is primarily a review of observational data it is natural to divide it into two parts: (i) galactic globular clusters, and (ii) old and intermediate-age open clusters. Some additional evidence comes from Local Group galaxies, especially now that CM diagrams which reach the old main sequence are becoming available. For each class of cluster I shall consider successive stages of evolution from the main sequence, up the hydrogen-burning red giant branch, and through the helium-burning giant phase.

scholarly journals Core-hydrogen-burning RSGs in the early globular clusters

2015 ◽  
Vol 11 (A29B) ◽  
pp. 473-473
Author(s):  
Dorottya Szécsi ◽  
Jonathan Mackey ◽  
Norbert Langer
Keyword(s):  
Globular Clusters ◽  
Stellar Population ◽  
Massive Stars ◽  
Shell Formation ◽  
Low Mass Stars ◽  
Hydrogen Burning ◽  
The Core ◽  
Anomalous Surface ◽  
Low Metallicity ◽  
Low Mass

AbstractThe first stellar generation in galactic globular clusters contained massive low-metallicity stars (Charbonnel et al. 2014). We modelled the evolution of this massive stellar population and found that such stars with masses 100-600 M⊙ evolve into cool RSGs (Szécsi et al. 2015). These RSGs spend not only the core-He-burning phase but even the last few 105 years of the core-H-burning phase on the SG branch. Due to the presence of hot massive stars in the cluster at the same time, we show that the RSG wind is trapped into photoionization confined shells (Mackey et al. 2014). We simulated the shell formation around such RSGs and find them to become gravitationally unstable (Szécsi et al. 2016). We propose a scenario in which these shells are responsible for the formation of the second generation low-mass stars in globular clusters with anomalous surface abundances.

The early Solar System and the rotation of the Sun

1984 ◽  
Vol 313 (1524) ◽  
pp. 39-45 ◽  
Keyword(s):  
Angular Momentum ◽  
Solar System ◽  
Magnetic Coupling ◽  
Central Star ◽  
The Sun ◽  
Early Solar System ◽  
Low Mass Stars ◽  
Hydrogen Burning ◽  
Low Mass ◽  
Solid Bodies

In most discussions of the formation of the Solar System, the early Sun is assumed to have possessed the bulk of the angular momentum of the system, and a closely surrounding disc of gas was spun out, which, through magnetic coupling, acquired a progressively larger proportion of the total angular momentum. There are difficulties with this model in accounting for the inclined axis of the Sun, the magnitude of the magnetic coupling required, and the nucleogenetic variations recently observed in the Solar System. Another possibility exists, namely that of a slowly contracting disc of interstellar material, leading to the formation of both a central star and a protoplanetary disc. In this model one can better account for the tilt of the Sun’s axis and the lack of mixing necessary to account for the nucleogenetic evidence. The low angular momentum of the Sun and of other low mass stars is then seen as resulting from a slow build-up as a degenerate dwarf, acquiring orbital material at a low specific angular momentum. When the internal temperature reaches the threshold for hydrogen burning, the star expands to the Main Sequence and is now a slow rotator. More massive stars would spin quickly because they had to acquire orbiting material after the expansion, and therefore at a high specific angular momentum. A process of gradual inward spiralling may also allow materials derived from different sources to accumulate into solid bodies, and be placed on a great variety of orbits in the outer reaches of the system, setting up the cometary cloud of uneven nucleogenetic composition.

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