Follow-up observations of X-ray emitting hot subdwarf stars: the compact He-poor sdO star Feige 34

We report on results obtained with the XMM-Newton observation of Feige 34 carried out in April 2018. This is the first spectroscopic X-ray observation of a compact and helium-poor hot subdwarf star. The source was detected at a flux level f_X = 3.4 × 10^-14 erg cm^-2 s^-1 in the energy range 0.2-3 keV, which implies an X-ray-to-bolometric flux ratio f_X/f_bol ≃ 10^-6.5. The source spectrum can be described with the sum of two thermal-plasma components with subsolar abundances at temperatures of ≃0.3 and 1.1 keV. These properties are similar to what is observed in early-type main-sequence stars, where the X-ray emission is attributed to turbulence and shocks in the stellar wind. Therefore, the same phenomenon could explain the X-ray properties of Feige 34. However, it is not possible to reproduce the observed spectrum with a thermal-plasma model if the elemental abundances are fixed at the values obtained from the optical and UV spectroscopy. Moreover, we show that the X-ray luminosity and spectrum are consistent with those expected from a young main-sequence star of late spectral type. Therefore, we discuss the possibility that the observed X-ray emission is due to the companion star of M0 spectral type, whose presence is suggested by the IR excess in the spectral energy distribution of Feige 34.

Based on observations obtained with XMM-Newton, an ESA science mission with instruments and contributions directly funded by ESA Member States and NASA.