Chandra spectroscopy of the hot star β Crucis and the discovery of a pre-main-sequence companion

In order to test the O star wind-shock scenario for X-ray production in less lu-minous stars with weaker winds, we made a pointed 74 ks observation of the nearby early B giant, β Cru (B0.5 III), with the Chandra High Energy Transmission Grating Spectrometer. We find that the X-ray spectrum is quite soft, with a dominant ther-mal component near 3 million K, and that the emission lines are resolved but quite narrow, with half-widths of 150 km s−1. The forbidden-to-intercombination line ratios of Ne ix and Mg xi indicate that the hot plasma is distributed in the wind, rather than confined near the photosphere. It is difficult to understand the X-ray data in the context of the standard wind-shock paradigm for OB stars, primarily because of the narrow lines, but also because of the high X-ray production efficiency. A scenario in which the bulk of the outer wind is shock heated is broadly consistent with the data, but not very well motivated theoretically. It is possible that magnetic channeling could explain the X-ray properties, although no field has been detected on β Cru. We de-tected periodic variability in the hard (hν> 1 keV) X-rays, modulated on the know