The mass ratio and formation mechanisms of Herbig Ae/Be star binary systems

We present B- and R-band spectroastrometry of a sample of 45 Herbig Ae/Be (HAe/Be) stars in order to study their binary properties. All but one of the targets known to be binary systems with a separation of similar to 0.1-2.0 arcsec are detected by a distinctive spectroastrometric signature. Some objects in the sample exhibit spectroastrometric features that do not appear attributable to a binary system. We find that these may be due to light reflected from dusty haloes or material entrained in winds. We present eight new binary detections and four detections of an unknown component in previously discovered binary systems. The data confirm previous reports that HAe/Be stars have a high binary fraction, 74 +/- 6 per cent in the sample presented here. We use a spectroastrometric deconvolution technique to separate the spatially unresolved binary spectra into the individual constituent spectra. The separated spectra allow us to ascertain the spectral type of the individual binary components, which in turn allows the mass ratio of these systems to be determined. In addition, we appraise the method used and the effects of contaminant sources of flux. We find that the distribution of system mass ratios is inconsistent with random pairing from the initial mass function, and that this appears robust despite a detection bias. Instead, the mass ratio distribution is broadly consistent with the scenario of binary formation via disc fragmentation.