B stars seen at high resolution by XMM-Newton^⋆

Groupe d’Astrophysique des Hautes Énergies, STAR, Université de Liège, Quartier Agora (B5c, Institut d’Astrophysique et de Géophysique), Allée du 6 Août 19c, 4000 Sart Tilman, Liège, Belgium
e-mail: naze@astro.ulg.ac.be

Received: 13 July 2017
Accepted: 24 August 2017

Abstract

We report on the properties of 11 early B stars observed with gratings on board XMM-Newton and Chandra, thereby doubling the number of B stars analysed at high resolution. The spectra typically appear soft, with temperatures of 0.2−0.6 keV, and moderately bright (log [L_X/L_BOL] ~ −7) with lower values for later type stars. In line with previous studies, we also find an absence of circumstellar absorption, negligible line broadening, no line shift, and formation radii in the range 2−7 R_⋆. From the X-ray brightnesses, we derived the hot mass-loss rate for each of our targets and compared these values to predictions or values derived in the optical domain: in some cases, the hot fraction of the wind can be non-negligible. The derived X-ray abundances were compared to values obtained from the optical data, with a fair agreement found between them. Finally, half of the sample presents temporal variations, either in the long-term, short-term, or both. In particular, HD 44743 is found to be the second example of an X-ray pulsator, and we detect a flare-like activity in the binary HD 79351, which also displays a high-energy tail and one of the brightest X-ray emissions in the sample.