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arXiv 0710.2397astro-ph
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不同金属丰度恒星的初终质量关系

Initial-Final Mass Relationship for Stars of Different Metallicities

X. Meng, X. Chen, Z. Han

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14 pages, 8 figures, accepted for publication in A&A
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摘要：

根据Paczyrobski & Ziólkowski（1968）和Han et al.（1994），我们假设当包层的结合能等于零（$ΔW=0$）时，渐近巨星分支（AGB）或第一巨星分支（FGB）星的包层丢失，并将AGB星或FGB星在点（$ΔW=0$）处的核质量作为最终质量。利用这一假设，我们计算了不同金属丰度的恒星的IFMR，发现IFMR强烈地依赖于金属丰度，即$Z=0.0001, 0.0003, 0.001, 0.004, 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.08$和0.1。根据$Z=0.04$，具有给定初始质量的恒星的最终质量随着金属丰度的增加或减少而增加。由于金属性导致的最终质量的差异可高达0.4 $M_{\odot}$。NGC 2099（M37）的初始-最终质量关系的线性拟合显示了金属丰度对IFMR影响的潜在证据。本文得到的$Z=0.02$型恒星的IFMR与银河系中观测推断的结果吻合得很好。对于$Z\geq 0.02$，从$M_{\rm i}\leq 1.0 M_{\odot}$的恒星中获得了氦WD，并且在NGC 6791中发现了许多低质量WD，这是一个富含金属的古老疏散星团。利用本文得到的$Z=0.02$型恒星的IFMR，我们再现了Sloan DR 4中除部分超大质量白矮星外的DA WD的质量分布。WDs平均质量随有效温度降低的趋势可能源于恒星初始金属丰度的增加。我们预测富含金属的低质量恒星可能会变成质量不足的白矮星。

英文摘要：

Following Paczyński & Ziólkowski (1968) and Han et al. (1994), we assume that the envelope of an asymptotic giant branch (AGB) or a first giant branch (FGB) star is lost when the binding energy of the envelope is equal to zero ($ΔW=0$) and the core mass of the AGB star or the FGB star at the point ($ΔW=0$) is taken as the final mass. Using this assumption, we calculate the IFMRs for stars of different metallicities.We find that the IFMRs depends strongly on the metallicity, i.e. $Z=0.0001, 0.0003, 0.001, 0.004, 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.08$ and 0.1. From $Z=0.04$, the final mass of the stars with a given initial mass increases with increasing or decreasing metallicity. The difference of the final mass due to the metallicity may be up to 0.4 $M_{\odot}$. A linear fit of the initial-final mass relationship in NGC 2099 (M37) shows a potential evidence of the effect of metallicity on the IFMR. The IFMR for stars of $Z=0.02$ obtained in the paper matches well with those inferred observationally in the Galaxy. For $Z\geq 0.02$, helium WDs are obtained from the stars of $M_{\rm i}\leq 1.0 M_{\odot}$ and this result is upheld by the discovery of numerous low-mass WDs in NGC 6791 which is a metal-rich old open cluster. Using the IFMR for stars of $Z=0.02$ obtained in the paper, we have reproduced the mass distribution of DA WDs in Sloan DR4 except for some ultra-massive white dwarfs. The trend that the mean mass of WDs decreases with effective temperature may originate from the increase of the initial metallicities of stars. We predict that metal-rich low-mass stars may become under-massive white dwarfs.

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