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arXiv 2001.04476astro-ph.SR
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为什么二元相互作用不一定在低金属丰度下主导Wolf-Rayet星的形成

Why binary interaction does not necessarily dominate the formation of Wolf-Rayet stars at low metallicity

T. Shenar, A. Gilkis, J. S. Vink, H. Sana, A. A. C. Sander

备注：
10 pages, 5 figures, accepted to A&A on the 12.01.2020. Comments welcome!
发表信息：
A&A 634, A79 (2020)
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摘要：

经典的沃尔夫-拉叶星（WR）是大质量、氢耗尽的后主序星，具有发射线支配的光谱。对于给定的金属丰度Z，超过一定初始质量M_single（Z）的恒星可以通过内禀质量损失（单星通道）到达WR相。质量较低的恒星可以通过双星质量转移（双星通道）达到WR相。通常认为，在低Z的环境中，如SMC和LMC，双星通道主导WR星的形成。然而，他们报告的WR双星分数为30-40%，与银河系相当。在这里，我们通过考虑剥离产物作为WR星出现所需的最小初始质量M_spec（Z）来解释这个明显的矛盾。我们用云和银河系中亮度最低的WR星校准M_spec（Z）。使用各种演化代码探索M_single（Z）值的范围。我们通过考虑区间[M_spec（Z），M_single（Z）]来估计双星通道的额外贡献，该区间表征了双星可以形成额外WR星的初始质量范围。结果如下：WR现象在银河系、大麦哲伦星云和小麦哲伦星云的光度低于logL = 4.9、5.25和5.6 [Lsun]时停止，这意味着氦星的质量为7.5、11、17 Msun，初始质量为M_spec = 18、23、37 Msun。在各自星系中具有较低初始质量的剥离星往往不会出现WR星。对于Galaxy、LMC和SMC，M_single位于20-30、30-60和> 40 Msun的范围内。我们发现，二进制通道的额外贡献是Z的非平凡函数，不能最终声称是单调增加的Z减少。因此，我们不应该先验地期望双星相互作用在低Z下形成WR星的过程中变得越来越重要，或者WR双星的比例随着Z的减小而增加。

英文摘要：

Classical Wolf-Rayet (WR) stars are massive, hydrogen depleted, post main-sequence stars that exhibit emission-line dominated spectra. For a given metallicity Z, stars exceeding a certain initial mass M_single(Z) can reach the WR phase through intrinsic mass-loss (single-star channel). Stars of lower masses can reach the WR phase via binary mass transfer (binary channel). It is commonly assumed that the binary channel dominates the formation of WR stars in environments with low Z such as the SMC and LMC. However, their reported WR binary fractions of 30-40% are comparable to that of the Galaxy. Here, we explain this apparent contradiction by considering the minimum initial mass M_spec(Z) needed for the stripped product to appear as a WR star. We calibrate M_spec(Z) using the lowest-luminosity WR stars in the Clouds and the Galaxy. A range of M_single(Z) values are explored using various evolution codes. We estimate the additional contribution of the binary channel by considering the interval [M_spec(Z), M_single(Z)], which characterises the initial-mass range in which binaries can form additional WR stars. Results: The WR-phenomenon ceases below luminosities of logL = 4.9, 5.25, and 5.6 [Lsun] in the Galaxy, LMC, and SMC, which translates to He-star masses of 7.5, 11, 17 Msun and initial masses of M_spec = 18, 23, 37 Msun. Stripped stars with lower initial masses in the respective galaxies would tend to not appear as WR stars. M_single lies in the range 20-30, 30-60, and > 40 Msun for the Galaxy, LMC, and SMC. We find that that the additional contribution of the binary channel is a non-trivial function of Z that cannot be conclusively claimed to be monotonically increasing with decreasing Z. Hence, one should not a-priori expect that binary interactions become increasingly important in forming WR stars at low Z, or that the WR binary fraction grows with decreasing Z.

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