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

Hekker, S.; Angelou, George C.; Elsworth, Y.; Basu, Sarbani

doi:10.1093/mnras/staa176

“…From a pedagogical perspective, we believe that having a simple physical model to explain how stars become luminous red giants will improve our teaching of stellar evolution, and also our interpretation of results from detailed numerical models. In future works we will apply the model presented here to the explanation of other properties of stellar models like the red giant bump (Christensen-Dalsgaard 2015; Hekker et al 2020) or the critical mass at which stars depart from the red giant branch (Soker 2008).…”

Section: Conclusion and Final Remarksmentioning

confidence: 99%

A Red Giants’ Toy Story

Bertolami

¹

2022

ApJ

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In spite of the spectacular progress accomplished by stellar evolution theory, some simple questions remain unanswered. One of these questions is “Why do stars become red giants?”. Here we present a relatively simple analytical answer to this question. We validate our analysis by constructing a quantitative toy model of a red giant and comparing its predictions to full stellar evolutionary models. We find that the envelope forces the value of ∇ = d ln T / d ln P at, and above, the burning shell into a very narrow range of possible values. Together with the fact that the stellar material at the burning shell both provides and transports most of the stellar luminosity, this leads to tight relations between the thermodynamic variables at the burning shell and the mass and radius of the core—T s (M c , R s ), P s (M c , R s ), and ρ s (M c , R s ). When complemented by typical mass–radius relations of the helium cores, this implies that for all stellar masses the evolution of the core dictates the values of T s , P s , and ρ s . We show that for all stellar masses evolution leads to an increase in the pressure and density contrasts between the shell and the core, forcing a huge expansion of the layers on top of the burning shell. Besides explaining why stars become red giants our analysis also offers a mathematical demonstration of the so-called shell homology relations, and provides simple quantitative answers to some properties of low-mass red giants.

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“…Bu yasanın ihlal edildiği bir durumun olup olmadığı merak ediliyordu. Hekker et al (2020) yaptıkları çalışmada, kırmızı devler bölgesinde ışıma gücü çıkıntısının (luminosity bump) olduğu zamanda yasanın işlemediğini gösterdiler.…”

Section: Kabuk Yakmaunclassified

Yıldızlarda Neler Oluyor? Kırmızı Budak Yıldızların Evrimine İlişkin Çıkarımlar

Yıldız

¹

2022

Turkish Journal of Astronomy and Astrophysics

0

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The big data age of astronomy provides high quality and huge amount of data that makes all researchers in this field feel lucky. Planet, galaxy, and black hole studies present both data and new problems for stellar astrophysics. Fundamental parameters of thousands of red giant (RG) and red clump (RC) stars can be calculated using their precise astrometric, asteroseismic, spectral and photometric data. In this study, we compute and compare the masses and radii of the RC stars, which we calculated from the asteroseismic and astrometric methods. We also compare fundamental properties of RCs with the values of RGs we calculated before. From this comparison, we deduce that an average of 0.1 M⊙ mass is lost during the transition from the RG to the RC phase. There are also very important implications about the origin of RC stars. Few of the stars with a mass greater than 1.5 M⊙ reach their current location by passing through the RG region.

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“…We note that while the effective temperature of the bump is shifted by the entropy calibration, its log and luminosity are essentially unaffected. Indeed, the luminosity and log at which the bump occurs are essentially controlled by the evolution of the interior of the model (see, e.g., Cassisi et al 2002;Hekker et al 2020 for detailed discussions), on which the entropy calibration has a modest impact.…”

Section: Radius and Effective Temperature Calibration Of Red Giantsmentioning

confidence: 99%

Stellar evolution models with entropy-calibrated mixing-length parameter: application to red giants

Spada,

Demarque,

Kupka

2021

Preprint

0

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We present evolutionary models for solar-like stars with an improved treatment of convection that results in a more accurate estimate of the radius and effective temperature. This is achieved by improving the calibration of the mixing-length parameter, which sets the length scale in the 1D convection model implemented in the stellar evolution code. Our calibration relies on the results of 2D and 3D radiation hydrodynamics simulations of convection to specify the value of the adiabatic specific entropy at the bottom of the convective envelope in stars as a function of their effective temperature, surface gravity and metallicity. For the first time, this calibration is fully integrated within the flow of a stellar evolution code, with the mixinglength parameter being continuously updated at run-time. This approach replaces the more common, but questionable, procedure of calibrating the length scale parameter on the Sun, and then applying the solar-calibrated value in modeling other stars, regardless of their mass, composition and evolutionary status. The internal consistency of our current implementation makes it suitable for application to evolved stars, in particular to red giants. We show that the entropy calibrated models yield a revised position of the red giant branch that is in better agreement with observational constraints than that of standard models.

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

Cited by 10 publications

References 24 publications

A Red Giants’ Toy Story

A Red Giants’ Toy Story

Yıldızlarda Neler Oluyor? Kırmızı Budak Yıldızların Evrimine İlişkin Çıkarımlar

Stellar evolution models with entropy-calibrated mixing-length parameter: application to red giants

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