Origins of low-mass helium stars

低质量氦恒星（太阳质量或更小）是早期型星系中重要的演化恒星群，因为它们对星系的紫外线有很强的影响。它们既存在于巨星中，也存在于炽热的亚光恒星中。虽然我们很清楚大质量氦恒星是如何通过强烈的质量损失形成的；目前还不清楚那些低质量的恒星是如何做到的。已经提出了几种可能的情况：例如，延迟氦核闪光，或者由双星成分合并导致白矮星合并。由于光球模型的严重缺陷，以前的研究无法从光谱观测中约束地层模型。我们开发了复杂的NLTE大气模型，该模型可用于对50多颗恒星的ESO-YLT高质量光谱进行定量分析。由于低质量氦星在其表面暴露了核燃烧的最终产物，为直接研究低质量氦星的核合成提供了可能。低质量氦恒星也可以作为恒星大气理论的重要试验台，特别是用于预测热恒星的近红外光谱。这个波长范围尚未在热演化恒星中得到很好的校准，但对于研究非常拥挤和/或高度模糊的恒星至关重要。未来的主要天文台（例如JWST）将在近红外区内运作。由于它们极其特殊的表面组成，低质量氦恒星可以校准合成氦、碳和氮的谱线，然后可以用于分析模糊区域或其他星系中的正常恒星。

Low mass helium stars (of solar mass or less) are an important population of evolved stars in early type galaxies, because they have an strong impact on the galaxies UV light. They can be found among giants as well as among hot subluminous stars. While it is well understood how massive helium stars form through strong mass loss; it remains unclear how those of low mass do. Several scenarios have been suggested: e.g. delayed helium core flashes, or merging of white dwarfs by coalescence of binary components. Due to severe shortcomings in photospheric models previous studies were unable to constrain formation models from spectral observations. We developed sophisticated NLTE model atmospheres, which shall be used for quantitative analyses of high-quality spectra taken with the ESO-YLT of more than 50 stars. As the low mass helium stars expose the end products for nuclear burning on their surface, they offer the possibility to study nucleosynthesis in low mass stars directly. Low mass helium stars can also be used as important testbeds for stellar atmosphere theory, in particular to predict the near infrared spectra of hot stars. This wavelength range has not yet been well calibrated in hot evolved stars but is of crucial importance for the study of stars in very crowded and/or highly obscured fields. Future major observatories (e.g. the JWST) will operate in the NIR. Due to their extremely peculiar surface composition, low mass helium stars allow to calibrate synthetic helium, carbon and nitrogen line spectra, which can then be used for analyses of normal stars in obscured fields or other galaxies.