On the red-giant luminosity bump

Christensen‐Dalsgaard, J.

doi:10.1093/mnras/stv1656

Cited by 41 publications

(28 citation statements)

References 17 publications

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“…The star regains a pivot when the hydrogen-burning shell reaches the mean molecular weight discontinuity. In line with Christensen-Dalsgaard (2015), who has already shown that the encounter of the hydrogen-burning shell with the mean molecular weight discontinuity coincides with the luminosity minimum of the bump.…”

Section: Discussionsupporting

confidence: 87%

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

Hekker

Angelou

Elsworth

et al. 2020

Monthly Notices of the Royal Astronomical Society

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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 expandsa 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.

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Section: Discussionsupporting

confidence: 87%

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

Hekker

Angelou

Elsworth

et al. 2020

Monthly Notices of the Royal Astronomical Society

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“…The same phenomenon is responsible for the occurrence of the luminosity bump in red-giant stars (e.g. Thomas 1967;Christensen-Dalsgaard 2015), first discussed in the context of temporary Roche-lobe detachment in LMXBs in Tauris & Savonije (1999). The interruption of the mass transfer can be a temporary effect if the envelope is massive enough, such that when the burning shell has passed through the discontinuity, the star still has enough material to burn and can therefore resume its mass transfer.…”

Section: Effect Of Metallicitymentioning

confidence: 85%

Models of low-mass helium white dwarfs including gravitational settling, thermal and chemical diffusion, and rotational mixing

Istrate

Marchant

Tauris

et al. 2016

A&A

193

321

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A large number of extremely low-mass helium white dwarfs (ELM WDs) have been discovered in recent years. The majority of them are found in close binary systems suggesting they are formed either through a common-envelope phase or via stable mass transfer in a low-mass X-ray binary (LMXB) or a cataclysmic variable (CV) system. Here, we investigate the formation of these objects through the LMXB channel with emphasis on the proto-WD evolution in environments with different metallicities. We study for the first time the combined effects of rotational mixing and element diffusion (e.g. gravitational settling, thermal and chemical diffusion) on the evolution of proto-WDs and on the cooling properties of the resulting WDs. We present state-of-the-art binary stellar evolution models computed with MESA for metallicities of Z = 0.02, 0.01, 0.001 and 0.0002, producing WDs with masses between ∼0.16−0.45 M . Our results confirm that element diffusion plays a significant role in the evolution of proto-WDs that experience hydrogen shell flashes. The occurrence of these flashes produces a clear dichotomy in the cooling timescales of ELM WDs, which has important consequences e.g. for the age determination of binary millisecond pulsars. In addition, we confirm that the threshold mass at which this dichotomy occurs depends on metallicity. Rotational mixing is found to counteract the effect of gravitational settling in the surface layers of young, bloated ELM proto-WDs and therefore plays a key role in determining their surface chemical abundances, i.e. the observed presence of metals in their atmospheres. We predict that these proto-WDs have helium-rich envelopes through a significant part of their lifetime. This is of great importance as helium is a crucial ingredient in the driving of the κ-mechanism suggested for the newly observed ELM proto-WD pulsators. However, we find that the number of hydrogen shell flashes and, as a result, the hydrogen envelope mass at the beginning of the cooling track, are not influenced significantly by rotational mixing. In addition to being dependent on proto-WD mass and metallicity, the hydrogen envelope mass of the newly formed proto-WDs depends on whether or not the donor star experiences a temporary contraction when the H-burning shell crosses the hydrogen discontinuity left behind by the convective envelope. The hydrogen envelope at detachment, although small compared to the total mass of the WD, contains enough angular momentum such that the spin frequency of the resulting WD on the cooling track is well above the orbital frequency.

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“…In fact, the situation is more complex with the luminosity beginning to decrease prior to the shell burning through the discontinuity. As discussed by Christensen-Dalsgaard (2015) the reason is that the decrease in µ above the discontinuity starts affecting the hydrostatic structure, and hence the temperature, within and above the hydrogenburning shell before it reaches the discontinuity. This causes the decrease in the luminosity.…”

Section: Hydrogen Shell Burning Phase: Subgiants and Red-giant Branchmentioning

confidence: 99%

Giant star seismology

Hekker

Christensen‐Dalsgaard

2017

Astron Astrophys Rev

168

133

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The internal properties of stars in the red-giant phase undergo significant changes on relatively short timescales. Long near-uninterrupted high-precision photometric timeseries observations from dedicated space missions such as CoRoT and Kepler have provided seismic inferences of the global and internal properties of a large number of evolved stars, including red giants. These inferences are confronted with predictions from theoretical models to improve our understanding of stellar structure and evolution. Our knowledge and understanding of red giants have indeed increased tremendously using these seismic inferences, and we anticipate that more information is still hidden in the data. Unraveling this will further improve our understanding of stellar evolution. This will also have significant impact on our knowledge of the Milky Way Galaxy as well as on exo-planet host stars. The latter is important for our understanding of the formation and structure of planetary systems.

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On the red-giant luminosity bump

Cited by 41 publications

References 17 publications

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

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

Models of low-mass helium white dwarfs including gravitational settling, thermal and chemical diffusion, and rotational mixing

Giant star seismology

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