Origins of low-mass helium stars

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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.