Abstract
We review the X-ray observations of hot subdwarf stars. While no X-ray emission has been detected yet from binaries containing B-type subdwarfs, interesting results have been obtained in the case of the two luminous O-type subdwarfs HD 49798 and BD +37 442. Both of them are members of binary systems in which the X-ray luminosity is powered by accretion onto a compact object: a rapidly spinning (13.2 s) and massive (1.28 M_sun) white dwarf in the case of HD 49798 and most likely a neutron star, spinning at 19.2 s, in the case of BD +37 442. Their study can shed light on the poorly known processes taking place during common envelope evolutionary phases and on the properties of wind mass loss from hot subdwarfs.
Highlights
Hot subdwarfs are evolved low-mass stars that lost most of their hydrogen envelopes and are in the stage of helium core burning
We review the X-ray observations of hot subdwarf stars
Both of them are members of binary systems in which the X-ray luminosity is powered by accretion onto a compact object: a rapidly spinning (13.2 s) and massive (1.28 M ) white dwarf in the case of HD 49798 and most likely a neutron star, spinning at 19.2 s, in the case of BD +37◦ 442
Summary
Hot subdwarfs are evolved low-mass stars that lost most of their hydrogen envelopes and are in the stage of helium core burning. Many hot subdwarfs are members of binary systems [18, 22], supporting the idea that non-conservative mass transfer played a role in the formation of these stars. X-rays can be used to identify hot subdwarfs with compact companions: they can originate from surface thermal emission of neutron stars or sufficiently hot white dwarfs or can be produced if the compact object accretes mass from the subdwarf at a sufficiently high rate. This is well illustrated by our recent results on the sdO binaries discussed below: HD 49798 and BD +37◦ 442. Accreting compact objects can be used as probes to investigate the poorly known properties of the stellar winds of hot subdwarfs
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