We present high resolution spectro-astrometry of a sample of 28 Herbig Ae/Be
and 3 F-type pre-main sequence stars. The spectro-astrometry is shown from both
empirical and simulated data to be capable of detecting binary companions that
are fainter by up to 6 magnitudes at separations larger than 0.1 arcsec. The
nine targets that were previously known to be a binary are all detected. In
addition, we report the discovery of 6 new binaries and present 5 further
possible binaries. The resulting binary fraction of 68+/-11 per cent is the
largest reported for any observed sample of Herbig Ae/Be stars, presumably
because of the exquisite sensitivity of spectro-astrometry for detecting binary
systems. The data hint that the binary frequency of the Herbig Be stars is
larger than for the Herbig Ae stars. The appendix presents model simulations to
assess the capabilities of spectro-astrometry and reinforces the empirical
findings. Two objects, HD 87643 and Z CMa, display evidence for asymmetric
outflows. Finally, the position angles of the binary systems have been compared
with available orientations of the circumprimary disc and these appear to be
co-planar. The alignment between the circumprimary discs and the binary systems
strongly suggests that the formation of binaries with intermediate mass
primaries is due to fragmentation as the alternative, stellar capture, does not
naturally predict aligned discs. The aligment extends to the most massive
B-type stars in our sample. This leads us to conclude that formation mechanisms
that do result in massive stars, but predict random angles beween the binaries
and the circumprimary disks, such as stellar collisions, are also ruled out for
the same reason.Comment: MNRAS accepted, 18 page