3 years ago

Are sdAs helium core stars?.

D. Koester, S. O. Kepler, Ingrid Pelisoli

Evolved stars with a helium core can be formed by non-conservative mass exchange interaction with a companion or by strong mass loss. Their masses are smaller than 0.5M$_{\odot}$. In the database of the Sloan Digital Sky Survey (SDSS), there are several thousand stars which were classified by the pipeline as dwarf O, B and A stars. Considering the lifetimes of these classes on the main sequence, and their distance modulus at the SDSS bright saturation, if these were common main sequence stars, there would be a considerable population of young stars very far from the galactic disk. Their spectra are dominated by Balmer lines which suggest effective temperatures around 8000-10000K. Several thousand have significant proper motions, indicative of distances larger than 1kpc. Many show surface gravity in intermediate values between main sequence and white dwarf, $4.75 < \log g < 6.5$, hence they have been called sdA stars. Their physical nature and evolutionary history remains a puzzle. We propose they are not H-core main sequence stars, but helium core stars and the outcomes of binary evolution. We report the discovery of two new extremely-low mass white dwarfs among the sdAs to support this statement.

Publisher URL: http://arxiv.org/abs/1710.09882

DOI: arXiv:1710.09882v1

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