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

¹ INAF - Osservatorio Astronomico di Palermo, Piazza del Parlamento 1, 90134 Palermo, Italy
² European Southern Observatory, Casilla 19001, Santiago 19, Chile
³ Astrophysikalisches Institut Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany

Corresponding author: B. Stelzer, stelzer@astropa.unipa.it

Received: 10 February 2003
Accepted: 13 June 2003

Abstract

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 HD 1685, HD 113703, HD 123445, HD 133880, and HD 169978. Adaptive optics observations have recently revealed at least one faint object near each of these B-type stars (at separation of ). 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 ∼10–50 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  erg/s. But we do detect X-ray emission from the position of HD 1685 A and HD 169978 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 ( keV) and high X-ray luminosity ( erg/s). Spectroscopic observations in the infrared will solve the question whether the one remaining X-ray detected B-star in our sample, HD 1685 A, also has an even closer companion or whether this is an intrinsic X-ray emitter.