We present spectro-astrometric observations for 28 southern pre-main sequence ( PMS ) stars and investigate their circumstellar environment down to AU scales .
The structures detected in the “ position spectra ” include : ( 1 ) almost all the known binary companions in our sample ( Sz 68 , Sz 41 , HO Lup , VW Cha , S CrA , AS 205 ) , ( 2 ) companion candidates which have not been detected by infrared speckle techniques ( T CrA , MWC 300 ) , ( 3 ) monopolar and bipolar jets ( AS 353A , CS Cha ) , ( 4 ) a combination of jets and a bow shock ( VV CrA ) , and ( 5 ) a combination of a jet and stellar companion ( R CrA ) .
Results in known binaries show that this technique is capable of detecting binaries with separations down to \sim 10 milliarcsec .
Both components in each binary appear to have strikingly similar profiles in H \alpha emission , indicating a similarity of circumstellar activity ( mass accretion and/or a wind ) , and supporting the scenario of core fragmentation for the mechanism of binary formation .
The bipolar H \alpha jet in CS Cha has a spatial scale of \sim 1.5 AU , similar to that previously observed in RU Lup , and likely be heated by a mechanism other than shocks .
From the spatial scale , velocity , and H \alpha luminosity , we estimate the mean hydrogen density in the AU-scale bipolar flows to be \ga 10 ^ { 7 } cm ^ { -3 } .
The bipolar geometry in these jets can be explained by the presence of a disc gap/hole at AU scales , which could be induced by a gas-giant planet at the ice condensation radius .