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

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 rev...

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Veröffentlicht in:Monthly notices of the Royal Astronomical Society 2020-03, Vol.492 (4), p.5940-5948
Hauptverfasser: Hekker, S, Angelou, G C, Elsworth, Y, Basu, S
Format: Artikel
Sprache:eng
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Zusammenfassung: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.
ISSN:0035-8711
1365-2966
DOI:10.1093/mnras/staa176