Massive stars at low metallicity: an empirical investigation

随着2015年9月14日首次探测到引力波，科学兴趣的焦点转移到了高质量的恒星。在发射引力波时合并的两个黑洞每个大约有30个太阳质量，并且必须由最初质量更大的恒星崩溃形成。有人认为这些恒星一定是在低金属含量的环境中形成的，因为低金属含量导致较弱的恒星风;因此，在恒星演化结束时，留下较大的质量，坍缩成黑洞。大质量恒星仍然没有得到很好的理解。大多数现有的大质量恒星的研究都涉及我们银河系中的物体。与理论模拟其演变的协议是不令人满意的。造成这种差异的主要原因可能是我们对两个重要效应的认识不足，即恒星风引起的质量损失和恒星旋转引起的内部混合过程。我们的银河系具有相对较高的金属含量。低金属量的星系，被认为是引力波祖先的诞生地，也被发现在我们的宇宙近邻，即小麦哲伦云。它足够近，可以单独研究相对明亮的大质量恒星。这个矮星系中大约一半的大质量恒星属于相对年轻的星系团NGC 346。因此，该星团为研究低金属量恒星爆发中完整的大质量星星群体提供了一个理想的实验室。在可见光下，我们使用了欧洲南方天文台（ESO）甚大望远镜（VLT）的积分场摄谱仪MUSE。该仪器同时为视场中的90000个像素中的每个像素获取光谱。第一次检查的数据显示200-300恒星源。由于全光谱分析还需要紫外光谱范围，我们还申请了哈勃太空望远镜的观测，这也得到了批准。利用长缝光谱仪STIS，星系团的主要部分将以马赛克的形式被覆盖。在这里提出的项目中，这些新的和极好的数据将在适当的时候被分析。科学开发将解决当前天体物理学中高度关注的各种问题。我们将确定这个种群在哪些方面不同于那些金属丰度较高的种群。在这么低的金属丰度下，质量损失有多小？它们质量损失的微小程度如何影响恒星的演化？恒星质量的统计分布是怎样的？我们独特的新观测也将使我们能够对这个年轻的贫金属恒星团进行整体研究。我们将模拟混合恒星辐射和风对扩散物质的反馈，即对被嵌入恒星电离的巨大区域的反馈。

With the first detection of a gravitational wave on September 14, 2015, the focus of scientific interest moved to stars with a high mass. The two Black Holes which merged when emitting this gravitational wave had about 30 solar masses each, and must have formed by the collapse of stars which were initially even much more massive. It has been suggested that these stars must have been formed in a low-metallicity environment, because a low metal content leads to weaker winds; hence, at the end of the stellar evolution, a larger mass is left that collapses into the Black Hole.Massive stars are still not well understood. Most of the existing investigations of massive stars concern objects in our own Galaxy. The agreement with theoretical simulations of their evolution is not satisfactory. Major reasons for the discrepancies are presumably our poor knowledge of two important effects, the mass loss by stellar winds, and the interior mixing processes induced by stellar rotation.Our Milky Way has a relatively high metal content. Galaxies with low metallicity, as invoked as the birth place of the gravitational wave progenitors, are also found in our immediate cosmic neighborhood, namely the Small Magellanic Cloud. It is close enough that the relatively bright massive stars can be studied individually. About half of all massive stars of this dwarf galaxy belong to the relatively young cluster NGC346. This cluster thus provides an ideal laboratory to study a complete massive star population in a starburst at low metallicity.To this purpose, we have secured the spectra of the massive stars in NGC346. In the visual light, we employed the integral-field spectrograph MUSE at the Very Large Telescope (VLT) of the European Southern Observatory (ESO). This instrument takes spectra simultaneously for each of the 90000 pixels in the field-of-view. First inspection of the data show 200-300 stellar sources. Since the full spectral analysis also requires the ultraviolet spectral range, we have additionally applied for observations with the Hubble Space Telescope, which have been approved as well. With the long-slit spectrograph STIS, major parts of the cluster will be covered in form of a mosaic.Within the project proposed here, these new and superb data shall be analyzed in due time. The scientific exploitation will address various questions of high interest in current astrophysics. We will establish in which respects this population differs from those at higher metallicity. How small is the mass loss at this low metallicity? How does the smallness of their mass loss affect the evolution of the stars? How are the stellar masses statistically distributed?Our unique new observations will also allow to investigate this young, metal-poor stellar cluster as a whole. We will model the feedback of the combined stellar radiation and winds on the diffuse matter, i.e. on the giant region that is ionized by the embedded stars.