scholarly journals An analysis of the TZ Fornacis binary system

2018 ◽  
Vol 617 ◽  
pp. A36 ◽  
Author(s):  
J. Higl ◽  
L. Siess ◽  
A. Weiss ◽  
H. Ritter
Keyword(s):  
Binary System ◽  
Stellar Evolution ◽  
Main Sequence ◽  
Strong Impact ◽  
Evolution Theory ◽  
Stellar Ages ◽  
Self Consistent ◽  
Convective Core ◽  
Red Giant ◽  
System Properties

Context. TZ Fornacis (TZ For) is an evolved detached binary system that is difficult to model and interpret, but very useful for testing stellar evolution theory and physics. Aims. We aim to search for solutions that are self-consistent and to determine the necessary stellar physics input. We also check solutions found previously for their internal consistency and for reproducibility. Methods. We use both a single and a binary stellar evolution code, and take into account all known system properties. We determine the physical stellar parameters by imposing that the models match the known radii for identical stellar ages. The evolution has to be consistent with a binary system in classical Roche geometry. Results. We obtained two different solutions to model TZ For successfully. Both depend on avoiding a long evolution on the first giant branch and imply a sufficiently large convective core on the main sequence. TZ For can be modelled consistently as a detached binary system by invoking either a substantial amount of core overshooting or a tidally enhanced wind mass loss along the red giant branch. An evolution with Roche-lobe overflow can definitely be excluded. Conclusions. A comparison of our results with previous studies also reveals that in addition to uncertainties associated with the input physics, the modelling of overshooting by different algorithms can have a strong impact.

scholarly journals Probing core overshooting using subgiant asteroseismology: The case of KIC10273246

2021 ◽  
Vol 647 ◽  
pp. A187
Author(s):  
A. Noll ◽  
S. Deheuvels ◽  
J. Ballot
Keyword(s):  
Stellar Evolution ◽  
Main Sequence ◽  
Seismic Modeling ◽  
General Study ◽  
Data Set ◽  
The Core ◽  
Stellar Ages ◽  
Oscillation Modes ◽  
Mixed Modes

Context. The size of convective cores remains uncertain, despite their substantial influence on stellar evolution, and thus on stellar ages. The seismic modeling of young subgiants can be used to obtain indirect constraints on the core structure during main sequence, thanks to the high probing potential of mixed modes. Aims. We selected the young subgiant KIC10273246, observed by Kepler, based on its mixed-mode properties. We thoroughly modeled this star, with the aim of placing constraints on the size of its main-sequence convective core. A corollary goal of this study is to elaborate a modeling technique that is suitable for subgiants and can later be applied to a larger number of targets. Methods. We first extracted the parameters of the oscillation modes of the star using the full Kepler data set. To overcome the challenges posed by the seismic modeling of subgiants, we propose a method that is specifically tailored to subgiants with mixed modes and uses nested optimization. We then applied this method to perform a detailed seismic modeling of KIC10273246. Results. We obtain models that show good statistical agreements with the observations, both seismic and non-seismic. We show that including core overshooting in the models significantly improves the quality of the seismic fit, optimal models being found for αov = 0.15. Higher amounts of core overshooting strongly worsen the agreement with the observations and are thus firmly ruled out. We also find that having access to two g-dominated mixed modes in young subgiants allows us to place stronger constraints on the gradient of molecular weight in the core and on the central density. Conclusions. This study confirms the high potential of young subgiants with mixed modes to investigate the size of main-sequence convective cores. It paves the way for a more general study including the subgiants observed with Kepler, TESS, and eventually PLATO.

scholarly journals New prescriptions of turbulent transport from local numerical simulations

2014 ◽  
Vol 9 (S307) ◽  
pp. 70-75
Author(s):  
V. Prat ◽  
F. Lignières ◽  
G. Lesur
Keyword(s):  
Numerical Simulations ◽  
Stellar Evolution ◽  
Turbulent Mixing ◽  
Massive Stars ◽  
Turbulent Transport ◽  
Strong Impact ◽  
Evolution Theory ◽  
Transport Models ◽  
Chemical Stratification ◽  
The Impact

AbstractMassive stars often experience fast rotation, which is known to induce turbulent mixing with a strong impact on the evolution of these stars. Local direct numerical simulations of turbulent transport in stellar radiative zones are a promising way to constrain phenomenological transport models currently used in many stellar evolution codes. We present here the results of such simulations of stably-stratified sheared turbulence taking notably into account the effects of thermal diffusion and chemical stratification. We also discuss the impact of theses results on stellar evolution theory.

scholarly journals Asteroseismic fingerprints of stellar mergers

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.

scholarly journals Li Abundance in Evolved Stars of NGC 6397

2000 ◽  
Vol 198 ◽  
pp. 354-355
Author(s):  
D.M. Allen ◽  
B.V. Castilho ◽  
L. Pasquini ◽  
B. Barbuy ◽  
P. Molaro
Keyword(s):  
Stellar Evolution ◽  
Globular Cluster ◽  
Main Sequence ◽  
Evolved Stars ◽  
Evolution Stage ◽  
Red Giant

Five giants and 11 subgiants of the metal-poor globular cluster NGC 6397 are analysed. In this Poster we present the lithium abundances derived. The present Li abundances and those of turnoff stars by Pasquini & Molaro (1996) are complementary in terms of stellar evolution stage, and show the Li abundances decreasing off the main sequence along the red giant branch.

scholarly journals Massive star evolution: rotation, winds, and overshooting vectors in the mass-luminosity plane

2019 ◽  
Vol 622 ◽  
pp. A50 ◽  
Author(s):  
Erin R. Higgins ◽  
Jorick S. Vink
Keyword(s):  
Binary System ◽  
Mass Loss ◽  
Stellar Evolution ◽  
Massive Star ◽  
Main Sequence ◽  
Test Bed ◽  
Star Evolution ◽  
Red Supergiants ◽  
Nitrogen Abundance ◽  
Massive Star Evolution

Context. Massive star evolution is dominated by various physical effects, including mass loss, overshooting, and rotation, but the prescriptions of their effects are poorly constrained and even affect our understanding of the main sequence. Aims. We aim to constrain massive star evolution models using the unique test-bed eclipsing binary HD 166734 with new grids of MESA stellar evolution models, adopting calibrated prescriptions of overshooting, mass loss, and rotation. Methods. We introduce a novel tool, called the mass-luminosity plane or M−L plane, as an equivalent to the traditional HR diagram, utilising it to reproduce the test-bed binary HD 166734 with newly calibrated MESA stellar evolution models for single stars. Results. We can only reproduce the Galactic binary system with an enhanced amount of core overshooting (αov = 0.5), mass loss, and rotational mixing. We can utilise the gradient in the M−L plane to constrain the amount of mass loss to 0.5–1.5 times the standard prescription test-bed, and we can exclude extreme reduction or multiplication factors. The extent of the vectors in the M−L plane leads us to conclude that the amount of core overshooting is larger than is normally adopted in contemporary massive star evolution models. We furthermore conclude that rotational mixing is mandatory to obtain the correct nitrogen abundance ratios between the primary and secondary components (3:1) in our test-bed binary system. Conclusions. Our calibrated grid of models, alongside our new M−L plane approach, present the possibility of a widened main sequence due to an increased demand for core overshooting. The increased amount of core overshooting is not only needed to explain the extended main sequence, but the enhanced overshooting is also needed to explain the location of the upper-luminosity limit of the red supergiants. Finally, the increased amount of core overshooting has – via the compactness parameter – implications for supernova explodability.

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.

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

2018 ◽  
Vol 35 ◽  
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.

scholarly journals Survey of Some Recent Stellar-Evolution Calculations

1978 ◽  
Vol 80 ◽  
pp. 333-344
Author(s):  
Allen V. Sweigart
Keyword(s):  
Stellar Evolution ◽  
Main Sequence ◽  
Quantitative Information ◽  
Systematic Investigation ◽  
Evolution Theory ◽  
Numerical Techniques ◽  
Physical Processes ◽  
Helium Burning ◽  
Basic Objective ◽  
Physical Effects

A basic objective of stellar-evolution theory is to provide detailed quantitative information on the evolutionary characteristics of stars with different compositions and masses. For the evolutionary phases from the zero-age main sequence (ZAMS) through core-helium burning the important physical processes are sufficiently well understood to justify a systematic investigation, and, indeed, extensive grids of evolutionary sequences have been constructed for these phases by many researchers. However, several difficulties are sometimes encountered when using the available calculations. For example, differences in numerical techniques and physical assumptions can give rise to systematic differences among the results of various investigations. Furthermore, the available calculations do not always explore the full ranges of the input parameters, and in some cases they neglect physical effects that are now believed to be important.

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).

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