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 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 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 Which physics determines the location of the mean molecular weight minimum in red giants?

2014 ◽  
Vol 443 (2) ◽  
pp. 977-984 ◽  
Author(s):  
Ross P. Church ◽  
John Lattanzio ◽  
George Angelou ◽  
Christopher A. Tout ◽  
Richard J. Stancliffe
Keyword(s):  
Molecular Weight ◽  
Red Giants ◽  
The Mean ◽  
Mean Molecular Weight

scholarly journals Period spacings in red giants

2018 ◽  
Vol 618 ◽  
pp. A109 ◽  
Author(s):  
B. Mosser ◽  
C. Gehan ◽  
K. Belkacem ◽  
R. Samadi ◽  
E. Michel ◽  
...  
Keyword(s):  
Asymptotic Expansion ◽  
Stellar Evolution ◽  
Mixed Mode ◽  
Frequency Resolution ◽  
Red Giants ◽  
Large Set ◽  
Low Mass Stars ◽  
Mode Pattern ◽  
Low Mass ◽  
Mixed Modes

Context. Oscillation modes with a mixed character, as observed in evolved low-mass stars, are highly sensitive to the physical properties of the innermost regions. Measuring their properties is therefore extremely important to probe the core, but requires some care, due to the complexity of the mixed-mode pattern. Aims. The aim of this work is to provide a consistent description of the mixed-mode pattern of low-mass stars, based on the asymptotic expansion. We also study the variation of the gravity offset εg with stellar evolution. Methods. We revisit previous works about mixed modes in red giants and empirically test how period spacings, rotational splittings, mixed-mode widths, and heights can be estimated in a consistent view, based on the properties of the mode inertia ratios. Results. From the asymptotic fit of the mixed-mode pattern of a large set of red giants at various evolutionary stages, we derive unbiased and precise asymptotic parameters. As the asymptotic expansion of gravity modes is verified with a precision close to the frequency resolution for stars on the red giant branch (10−4 in relative values), we can derive accurate values of the asymptotic parameters. We decipher the complex pattern in a rapidly rotating star, and explain how asymmetrical splittings can be inferred. We also revisit the stellar inclinations in two open clusters, NGC 6819 and NGC 6791: our results show that the stellar inclinations in these clusters do not have privileged orientation in the sky. The variation of the asymptotic gravity offset with stellar evolution is investigated in detail. We also derive generic properties that explain under which conditions mixed modes can be observed.

scholarly journals Systematic search for stellar pulsators in the eclipsing binaries observed by Kepler

2019 ◽  
Vol 630 ◽  
pp. A106 ◽  
Author(s):  
Patrick Gaulme ◽  
Joyce A. Guzik
Keyword(s):  
Stellar Evolution ◽  
Percent Level ◽  
Eclipsing Binaries ◽  
Light Curves ◽  
Systematic Search ◽  
Red Giants ◽  
Low Mass Stars ◽  
Orbital Periods ◽  
Red Giant ◽  
Stellar Properties

Eclipsing binaries (EBs) are unique targets for measuring precise stellar properties and can be used to constrain stellar evolution models. In particular, it is possible to measure masses and radii of both components of a double-lined spectroscopic EB at the percent level. Since the advent of high-precision photometric space missions (MOST, CoRoT, Kepler, BRITE, TESS), the use of stellar pulsation properties to infer stellar interiors and dynamics constitutes a revolution for studies of low-mass stars. The Kepler mission has led to the discovery of thousands of classical pulsators such as δ Scuti and solar-like oscillators (main sequence and evolved), but also almost 3000 EBs with orbital periods shorter than 1100 days. We report the first systematic search for stellar pulsators in the entire Kepler EB catalog. The focus is mainly aimed at discovering δ Scuti, γ Doradus, red giant, and tidally excited pulsators. We developed a data inspection tool (DIT) that automatically produces a series of plots from the Kepler light curves that allows us to visually identify whether stellar oscillations are present in a given time series. We applied the DIT to the whole Kepler EB database and identified 303 systems whose light curves display oscillations, including 163 new discoveries. A total of 149 stars are flagged as δ Scuti (100 from this paper), 115 as γ Doradus (69 new), 85 as red giants (27 new), and 59 as tidally excited oscillators (29 new). There is some overlap among these groups, as some display several types of oscillations. Despite the likelihood that many of these systems are false positives, for example, when an EB light curve is blended with a pulsator, this catalog gathers a vast sample of systems that are valuable for a better understanding of stellar evolution.

scholarly journals The Aarhus red giants challenge

2020 ◽  
Vol 635 ◽  
pp. A164 ◽  
Author(s):  
V. Silva Aguirre ◽  
J. Christensen-Dalsgaard ◽  
S. Cassisi ◽  
M. Miller Bertolami ◽  
A. Serenelli ◽  
...  
Keyword(s):  
Stellar Evolution ◽  
Main Sequence ◽  
Energy Generation ◽  
Abundance Ratio ◽  
Solar Radius ◽  
Stellar Mass ◽  
Stellar Structure ◽  
Red Giants ◽  
Numerical Implementation ◽  
Red Giant

Context. With the advent of space-based asteroseismology, determining accurate properties of red-giant stars using their observed oscillations has become the focus of many investigations due to their implications in a variety of fields in astrophysics. Stellar models are fundamental in predicting quantities such as stellar age, and their reliability critically depends on the numerical implementation of the physics at play in this evolutionary phase. Aims. We introduce the Aarhus red giants challenge, a series of detailed comparisons between widely used stellar evolution and oscillation codes that aim to establish the minimum level of uncertainties in properties of red giants arising solely from numerical implementations. We present the first set of results focusing on stellar evolution tracks and structures in the red-giant-branch (RGB) phase. Methods. Using nine state-of-the-art stellar evolution codes, we defined a set of input physics and physical constants for our calculations and calibrated the convective efficiency to a specific point on the main sequence. We produced evolutionary tracks and stellar structure models at a fixed radius along the red-giant branch for masses of 1.0 M⊙, 1.5 M⊙, 2.0 M⊙, and 2.5 M⊙, and compared the predicted stellar properties. Results. Once models have been calibrated on the main sequence, we find a residual spread in the predicted effective temperatures across all codes of ∼20 K at solar radius and ∼30–40 K in the RGB regardless of the considered stellar mass. The predicted ages show variations of 2–5% (increasing with stellar mass), which we attribute to differences in the numerical implementation of energy generation. The luminosity of the RGB-bump shows a spread of about 10% for the considered codes, which translates into magnitude differences of ∼0.1 mag in the optical V-band. We also compare the predicted [C/N] abundance ratio and find a spread of 0.1 dex or more for all considered masses. Conclusions. Our comparisons show that differences at the level of a few percent still remain in evolutionary calculations of red giants branch stars despite the use of the same input physics. These are mostly due to differences in the energy generation routines and interpolation across opacities, and they call for further investigation on these matters in the context of using properties of red giants as benchmarks for astrophysical studies.

scholarly journals On the corpuscular emission theory of stellar evolution

1959 ◽  
Vol 10 ◽  
pp. 115-119
Author(s):  
V. G. Fessenkov ◽  
G. M. Idlis
Keyword(s):  
Molecular Weight ◽  
Chemical Composition ◽  
Stellar Evolution ◽  
Main Sequence ◽  
Heavy Elements ◽  
Main Sequence Stars ◽  
Evolutionary Path ◽  
The Mean ◽  
Image Position ◽  
Mean Molecular Weight

Considerations regarding the evolutionary path of the main sequence stars depend essentially on the theorem of Vogt-Russell. According to this theorem the structure of stars with thermonuclear sources of energy is determined uniquely by their masses and chemical composition, as characterised by the mean molecular weight X, Y, Z being the relative amounts of hydrogen, helium and of the mixture of the heavy elements.

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.

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