Helium-shell flashing in low-mass stars and period changes in mira variables

Wood, P. R.; Zarro, D. M.

doi:10.1086/159032

Cited by 181 publications

(167 citation statements)

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“…A definite classification should consider additional criteria such as A11, page 14 of 16 luminosity and surface gravity. Using the relation of Wood & Zarro (1981) and the mass obtained for this object, we estimate log L/L = 3.5 for LSE 148, just above the threshold value for the subdwarfs. The log g value obtained for LSE 148 (3.83 dex) is not consistent with the typical log g for hot subdwarfs (however, some sdO stars such as the object KS 292 (Rauch et al 1991) may show some parameters similar to those of LSE 148).…”

Section: Lse 148mentioning

confidence: 83%

Chemical abundances of hot post-AGB stars

Mello

Daflon

Pereira

et al. 2012

A&A

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Context. Hot post-AGB stars are luminous objects of low-and intermediate mass (0.8-8 M ) in the final stage of evolution, moving between the asymptotic giant branch (AGB) stars and planetary nebulae. The absorption lines observed in their spectra are typical of early-type stars and their abundance pattern may be associated with the occurence of one or more mixing processes during the previous AGB phase. Aims. To better constrain their observed chemical pattern and evolutionary status, we determined chemical abundances for a sample of hot post-AGB stars selected according to spectroscopic criteria. Methods. The observational data are high-resolution spectra obtained with the FEROS spectrograph. The stellar parameters and chemical composition were obtained from fully consistent non-LTE synthesis.Results. The general abundance pattern reveals relevant nitrogen enrichment, slight depletion in carbon and sulfur and mild excess in helium for most of the objects. One notable exception is LSE 148, with Z = 0.001, which is likely to be a metal-poor object at high galactic latitude. The atmospheric parameters and chemical abundances obtained are discussed in the context of evolutionary models. Mixing processes like the second/third dredged-up and "hot bottom-burning" are invoked to explain the obtained results.

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Section: Lse 148mentioning

confidence: 83%

Chemical abundances of hot post-AGB stars

Mello

Daflon

Pereira

et al. 2012

A&A

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“…This has been accompanied by a decrease in amplitude of about 1 mag . The shrinking of the period is attributed to a recent thermal pulse (Wood & Zarro 1981). R Aql is listed in the Washington Visual Double Star Catalog (WDS Mason et al 2001) as triple system.…”

Section: R Aquilaementioning

confidence: 99%

“…The O-rich variable star R Hya is well known for its declining period (Wood & Zarro 1981;Zijlstra et al 2002) and the presence of a detached shell. In the past few centuries, the period of R Hya has declined by over a hundred days and has remained constant since 1950 (Zijlstra et al 2002).…”

Section: R Hydraementioning

confidence: 99%

“…In the past few centuries, the period of R Hya has declined by over a hundred days and has remained constant since 1950 (Zijlstra et al 2002). The steady decrease in the period can be possibly attributed to a recent thermal pulse (Wood & Zarro 1981;Zijlstra et al 2002). In the post thermal-pulse evolution the decline in luminosity causes a reduction in stellar radius, which in turn, causes the period to become shorter.…”

Section: R Hydraementioning

confidence: 99%

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Dust and molecular shells in asymptotic giant branch stars

Zhao-Geisler

Quirrenbach

Köhler

et al. 2012

A&A

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Context. Asymptotic giant branch (AGB) stars are one of the largest distributors of dust into the interstellar medium. However, the wind formation mechanism and dust condensation sequence leading to the observed high mass-loss rates have not yet been constrained well observationally, in particular for oxygen-rich AGB stars. Aims. The immediate objective in this work is to identify molecules and dust species which are present in the layers above the photosphere, and which have emission and absorption features in the mid-infrared (IR), causing the diameter to vary across the N-band, and are potentially relevant for the wind formation. Methods. Mid-IR (8-13 μm) interferometric data of four oxygen-rich AGB stars (R Aql, R Aqr, R Hya, and W Hya) and one carbonrich AGB star (V Hya) were obtained with MIDI/VLTI between April 2007 and September 2009. The spectrally dispersed visibility data are analyzed by fitting a circular fully limb-darkened disk (FDD). Results. The FDD diameter as function of wavelength is similar for all oxygen-rich stars. The apparent size is almost constant between 8 and 10 μm and gradually increases at wavelengths longer than 10 μm. The apparent FDD diameter in the carbon-rich star V Hya essentially decreases from 8 to 12 μm. The FDD diameters are about 2.2 times larger than the photospheric diameters estimated from K-band observations found in the literature. The silicate dust shells of R Aql, R Hya and W Hya are located fairly far away from the star, while the silicate dust shell of R Aqr and the amorphous carbon (AMC) and SiC dust shell of V Hya are found to be closer to the star at around 8 photospheric radii. Phase-to-phase variations of the diameters of the oxygen-rich stars could be measured and are on the order of 15% but with large uncertainties. Conclusions. From a comparison of the diameter trend with the trends in RR Sco and S Ori it can be concluded that in oxygen-rich stars the overall larger diameter originates from a warm molecular layer of H 2 O, and the gradual increase longward of 10 μm can be most likely attributed to the contribution of a close Al 2 O 3 dust shell. The chromatic trend of the Gaussian FWHM in V Hya can be explained with the presence of AMC and SiC dust. The observations suggest that the formation of amorphous Al 2 O 3 in oxygenrich stars occurs mainly around or after visual minimum. However, no firm conclusions can be drawn concerning the mass-loss mechanism. Future modeling with hydrostatic and self-consistent dynamical stellar atmospheric models will be required for a more certain understanding.

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“…1 to 3) was computed by Simon W. Campbell using the Monash/Mount Stromlo stellar evolution code (MONSTAR) (Campbell & Lattanzio 2008;Wood & Zarro 1981). It corresponds to a metal-free Pop III star with a mass of 0.85 M near the peak of the core helium flash.…”

Section: Initial Stellar Model Scmentioning

confidence: 99%

The core helium flash revisited

Mocák¹,

Campbell

Müeller

et al. 2010

A&A

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Context. Degenerate ignition of helium in low-mass stars at the end of the red giant branch phase leads to dynamic convection in their helium cores. One-dimensional (1D) stellar modeling of this intrinsically multi-dimensional dynamic event is likely to be inadequate. Previous hydrodynamic simulations imply that the single convection zone in the helium core of metal-rich Pop I stars grows during the flash on a dynamic timescale. This may lead to hydrogen injection into the core and to a double convection zone structure as known from one-dimensional core helium flash simulations of low-mass Pop III stars. Aims. We perform hydrodynamic simulations of the core helium flash in two and three dimensions to better constrain the nature of these events. To this end we study the hydrodynamics of convection within the helium cores of a 1.25 M metal-rich Pop I star (Z = 0.02), and, for the first time, a 0.85 M metal-free Pop III star (Z = 0) near the peak of the flash. These models possess single and double convection zones, respectively. Methods. We use 1D stellar models of the core helium flash computed with state-of-the-art stellar evolution codes as initial models for our multidimensional hydrodynamic study, and simulate the evolution of these models with the Riemann solver based hydrodynamics code Herakles, which integrates the Euler equations coupled with source terms corresponding to gravity and nuclear burning. Results. The hydrodynamic simulation of the Pop I model involving a single convection zone covers 27 h of stellar evolution, while the hydrodynamic simulations of a double convection zone, in the Pop III model, span 1.8 h of stellar life. We find differences between the predictions of mixing length theory and our hydrodynamic simulations. The simulation of the single convection zone in the Pop I model shows a strong growth of the size of the convection zone due to turbulent entrainment. We therefore predict that for the Pop I model a hydrogen injection phase (i.e., hydrogen injection into the helium core) will commence after about 23 days, which should eventually lead to a double convection zone structure known from 1D stellar modeling of low-mass Pop III stars. Our two and threedimensional hydrodynamic simulations of the double (Pop III) convection zone model show that the velocity field in the convection zones is different from that given by stellar evolutionary calculations. The simulations suggest that the double convection zone decays quickly, the flow eventually being dominated by internal gravity waves. The decay could be an artefact caused by the mapping of the initial stellar model to the numerical grid of our hydrodynamics code.

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Helium-shell flashing in low-mass stars and period changes in mira variables

Cited by 181 publications

References 0 publications

Chemical abundances of hot post-AGB stars

Chemical abundances of hot post-AGB stars

Dust and molecular shells in asymptotic giant branch stars

The core helium flash revisited

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