Are sdAs helium core stars?

Open Astronomy

卷:26

Koester Detlev¹  Kepler S. O.²  Pelisoli Ingrid² 

[1] Institut für Theoretische Physik und Astrophysik, Kiel, Schleswig-Holstein, 24098, Germany;

[2] Instituto de Física, Universidade Federal do Rio Grande do Sul, Porto Alegre, Rio Grande do Sul, 91501-900, Brazil;

关键词: white dwarfs;    subdwarfs;    binaries;   

DOI  :  10.1515/astro-2017-0433

来源: DOAJ

【 摘 要 】

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 0.5 M⊙. 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 8 000-10 000 K. Several thousand have significant proper motions, indicative of distances smaller than 1 kpc. 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.

【 授权许可】

Unknown