Early magnetic B-type stars: X-ray emission and wind properties

Abstract

We present a comprehensive study of X-ray emission and wind properties of massive magnetic early B-type stars. Dedicated XMM-Newton observations were obtained for three stars xi1 CMa, V2052 Oph, and zeta Cas. We report the first detection of X-ray emission from V2052 Oph and zeta Cas. The observations show that the X-ray spectra of our program stars are quite soft. We compile the complete sample of early B-type stars with detected magnetic fields to date and existing X-ray measurements, in order to study whether the X-ray emission can be used as a general proxy for stellar magnetism. We find that hard and strong X-ray emission does not necessarily correlate with the presence of a magnetic field. We analyze the UV spectra of five non-supergiant B stars with magnetic fields by means of non-LTE iron-blanketed model atmospheres. The latter are calculated with the PoWR code, which treats the photosphere as well as the the wind, and also accounts for X-rays. Our models accurately fit the stellar photospheric spectra in the optical and the UV. The parameters of X-ray emission, temperature and flux are included in the model in accordance with observations. We confirm the earlier findings that the filling factors of X-ray emitting material are very high. Our analysis reveals that the magnetic early type B stars studied here have weak winds. The mass-loss rates are significantly lower than predicted by hydrodynamically consistent models. We find that, although the X-rays strongly affect the ionization structure of the wind, this effect is not sufficient in reducing the total radiative acceleration. When the X-rays are accounted for at the intensity and temperatures observed, there is still sufficient radiative acceleration to drive stronger mass-loss than we empirically infer from the UV spectral lines. (abridged)Comment: 20 pages, accepted by MNRA

Detection of magnetic field in the B2 star ρ Ophiuchi A with ESO FORS2

“…Following the formalism of Petit et al (2013), with the stellar parameters listed in Table 2, a rotation period of 1.2 days, and a lower limit on the dipolar magnetic field of 1.5 kG, we derived a Keplerian co-rotation radius of 2.1 stellar radii and a lower limit on the Alfvén radius of 17.6 stellar radii. For these calculations, we further adopted a mass-loss rate of 1.4×10 −10 M yr −1 (Krtička 2014) and a terminal velocity of 700km s −1 (Prinja 1989;Oskinova et al 2011). Following the results of Petit et al (2013), ρ Oph A should host a centrifugal magnetosphere, and the interplay between the stellar magnetic field and rotation rate may be able to sustain a circumstellar disk.…”

B fields in OB stars (BOB): FORS 2 spectropolarimetric follow-up of the two rare rigidly rotating magnetosphere stars HD 23478 and HD 345439

“…Our analysis of the X-ray spectra of HR 5907 and HR 7355 shows that these stars are even harder X-ray sources than σ Ori E; the emission-measure-weighted temperature of the hot plasma in HR 5907 is 80 MK and that in HR 7355 is 60 MK, for instance. Given that the winds in magnetic B-type dwarf stars are weak, with velocities not exceeding a few hundred km s −1 (Oskinova et al 2011b), it is very difficult to explain such high temperatures in the framework of the magnetically confined wind shock model (Babel & Montmerle 1997). On the other hand, non-thermal emission can be expected from stars with strong magnetic fields (Leto et al 2006).…”

High-mass X-ray binaries in the Milky Way

“…Initial 1D hydrodynamic simulations showed that clumps merge and grow in size while moving away from the stellar surface, leading to large variations in the local density (up to 4 orders of magnitude) and velocity (a factor of few). In these simulations, collisions between clumps were also shown to be able to produce a remarkable amount of X-rays (Feldmeier, 1995;Feldmeier et al, 1997;Cohen et al, 2010;Oskinova et al, 2011Oskinova et al, , 2012Leutenegger et al, 2013). 2D hydrodynamical simulations later questioned the formation of large clumps, as in the multi-dimensional approach these structures are disrupted by the thin-shell and Rayleigh-Taylor instabilities (Dessart and Owocki, 2002.…”