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Hydrogen and helium shell burning during white dwarf accretion

Research in Astronomy and Astrophysics 2018年第5期18卷 111-118页

作者： Xiao Cui Xiang-Cun Meng Zhan-Wen Han Yunnan Observatories Chinese Academy of Sciences Kunming 650216 China Key Laboratory for the Structure and evolution of Celestial Objects Chinese Academy of Sciences Kunming 650216 China University of Chinese Academy of Sciences Beijing 100049 China

Type Ia supernovae（SNe Ia） are believed to be thermonuclear explosions of carbon oxygen（CO） white dwarfs（WDs） with masses close to the Chandrasekhar mass limit. How a CO WD accretes matter and grows in mass to this limit is not well understood, hindering our understanding of SN Ia explosions and the reliability of using SNe Ia as a cosmological distance indicator. In this work, we employed the stellar evolution code MESA to simulate the accretion process of hydrogen-rich material onto a 1.0 M⊙CO WD at a high rate（over the Eddington limit） of 4.3 × 10^-7 M⊙yr^-1. The simulation demonstrates the characteristics of the double shell burning on top of the WD, with a hydrogen shell burning on top of a helium burning shell. The results show that helium shell burning is not steady（*** flashes）. Flashes from the helium shell are weaker than those in the case of accretion of helium-rich material onto a CO WD. The carbon to oxygen mass ratio resulting from the helium shell burning is higher than what was previously thought. Interestingly, the CO WD growing due to accretion has an outer part containing a small fraction of helium in addition to carbon and oxygen. The flashes become weaker and weaker as the accretion continues.

关键词： stars evolution - supernovae general - white dwarfs

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Distribution of ^(56)Ni Yields of Type Ia supernovae and its Implication for Progenitors

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Chinese Journal of Astronomy and Astrophysics 2008年第1期8卷 71-80页

作者： Bo Wang Xiang-Cun Meng Xiao-Feng Wang Zhan-Wen Han National Astronomical Observatories / Yunnan Observatory Chinese Academy of Sciences Kunming 650011 Graduate School of Chinese Academy of Sciences Beijing 100049 Tsinghua Center for Astrophysics （THCA） and Department of Physics Tsinghua University Beijing 100084 Astronomy Department University of California at Berkeley CA 94720 USA

The amount of 56↑Ni produced in Type Ia supernova （SN Ia） explosion is probably the most important physical parameter underlying the observed correlation of SN Ia luminosities with their light curves. Based on an empirical relation between the 56↑Ni mass and the light curve parameter △m15, we obtained rough estimates of the 56↑Ni mass for a large sample of nearby SNe Ia with the aim of exploring the diversity in SN Ia. We found that the derived 56↑Ni masses for different SNe Ia could vary by a factor of ten （e.g., MNi = 0.1 - 1.3 M⊙）, which cannot be explained in terms of the standard Chandrasekhar-mass model （with a 56↑Ni mass production of 0.4 - 0.8 M⊙）. Different explosion and/or progenitor models are clearly required for various SNe Ia, in particular, for those extremely nickel-poor and nickel-rich producers. The nickel-rich （with MNi 〉 0.8 M⊙） SNe Ia are very luminous and may have massive progenitors exceeding the Chandrasekhar-mass limit since extra progenitor fuel is required to produce more 56↑Ni to power the light curve. This is also consistent with the finding that the intrinsically bright SNe Ia prefer to occur in stellar environments of young and massive stars. For example, 75% SNe Ia in spirals have △ml5 〈 1.2 while this ratio is only 18% in E/S0 galaxies. The nickel-poor SNe Ia （with MNi 〈 0.2 M⊙） may invoke the sub- Chandrasekhar model, as most of them were found in early-type E/S0 galaxies dominated by the older and low-mass stellar populations. This indicates that SNe Ia in spiral and E/S0 galaxies have progenitors of different properties.

关键词： stars evolution - supernovae general - white dwarfs

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