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 , which is essentially the study of unresolved features in long-slit spectra , 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 \sim 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 is 68 \pm 11 % .
This overall binary fraction 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 .
Most spectro-astrometric signatures in this sample of Herbig Ae/Be stars can be explained by the presence of a binary system .
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 .