Late B-type stars and their candidate companions resolved with Chandra

We present the first results from a series of Chandra observations carried out with the aim to examine the origin of X-ray emission in main-sequence late B-type stars. X-ray detections of late-B and early A-type stars have remained a mystery as none of the two major theories for stellar X-ray emission applies in this spectral range: while O- and early B-type stars drive strong winds that are subject to instabilities, late-type stars produce X-rays as a result of magnetic dynamo action. Since any dynamo works only in the presence of a convective zone, early-type stars are not magnetically active. We use high spatial resolution X-ray observations to enlighten the prevalent speculation that previously unknown late-type or low-mass companion stars are the sites of the X-ray emission, instead of the B-type primaries. Here we present the results for HD1685, HD113703, HD123445, HD133880, and HD169978. Adaptive optics observations have recently revealed at least one faint object near each of these B-type stars (at separation of 1-6). Four of the new infrared objects show infrared colors and magnitudes typical for low-mass pre-main sequence stars, and are likely true companions to the similar to10-50 Myr old B-type stars. These multiple systems are now resolved for the first time in X-ray light. We uncover that four of the new companions are X-ray emitters, and the fifth one is likely to be a weak X-ray source below the detection limit. Three of the B-type primaries are X-ray dark down to the detection limit of L(x)similar to10(28) erg/s. But we do detect X-ray emission from the position of HD1685 A and HD169978 A. The latter one indeed is a spectroscopic binary. The characteristics of all X-ray sources are compatible with those of typical young late-type stars: hard X-ray spectrum (kT>0.5 keV) and high X-ray luminosity (log L(x)similar to29...30 erg/s). Spectroscopic observations in the infrared will solve the question whether the one remaining X-ray detected B-star in our sample, HD1685 A, also has an even closer companion or whether this is an intrinsic X-ray emitter.