Chandra spectroscopy of the hot star βCrucis 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 luminous stars with weaker winds, we made a pointed 74-ks observation of the nearby early B giant, βCrucis (βCru B0.5III), with the Chandra High Energy Transmission Grating Spectrometer. We find that the X-ray spectrum is quite soft, with a dominant thermal component near 3millionK, and that the emission lines are resolved but quite narrow, with half widths of 150kms^-1. The forbidden-to-intercombination line ratios of NeIX and MgXI 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 channelling could explain the X-ray properties, although no field has been detected on βCru. We detected periodic variability in the hard (hν > 1keV) X-rays, modulated on the known optical period of 4.58h, which is the period of the primary βCephei pulsation mode for this star. We also have detected, for the first time, an apparent companion to βCru at a projected separation of 4arcsec. This companion was likely never seen in optical images because of the presumed very high contrast between it and βCru in the optical. However, the brightness contrast in the X-ray is only 3:1, which is consistent with the companion being an X-ray active low-mass pre-main-sequence star. The companion's X-ray spectrum is relatively hard and variable, as would be expected from a post-TTauri star. The age of the βCru system (between 8 and 10Myr) is consistent with this interpretation which, if correct, would add βCru to the roster of Lindroos binaries - B stars with low-mass pre-main-sequence companions.