Detection of pre-main-sequence binaries using spectro-astrometry

The technique of spectro-astrometry (measuring the wavelength dependence of the position of an object) has been used to observe a number of pre-main-sequence stars. Several previously known binaries with separations down to 100 milliarcseconds (mas) were easily detected in these observations by the presence of a broad feature in the position spectrum on the Hα line. The observations show that T CrA is a pre-main-sequence binary with a PA of 275°, and a separation of at least 140 mas. The binary period may be short enough for orbital motion to be detectable. The technique has the potential to find new binaries with smaller separations than have been possible by other methods. The observations can also reveal small-scale structure associated with the outflows in these systems as is clearly seen by the detection of structure corresponding to the jet in Z CMa.