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

¹ INAF, Istituto di Astrofisica Spaziale e Fisica Cosmica – Milano, Via A. Corti 12, 20133 Milano, Italy
e-mail: nicola.lapalombara@inaf.it
² IUSS-Istituto Universitario di Studi Superiori, piazza della Vittoria 15, 27100 Pavia, Italy
³ Istituto Nazionale di Fisica Nucleare, Sezione di Pavia, Via A. Bassi 6, 27100 Pavia, Italy

Received: 22 February 2019
Accepted: 15 April 2019

Abstract

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.