HUBBLE-SPACE-TELESCOPE OBSERVATIONS OF JETS FROM YOUNG STARS

We report on Hubble Space Telescope (HST) emission line ([S II]lambda lambda 6716,6731, H alpha, and [O I]lambda 6300) and nearby continuum imaging of the HL Tau and HH 30 jets from our own HST program as well as a study of HST Archive [S II]lambda lambda 6716,6731 images of the HH land HH 34 jets. It is found in all cases that these jets are well resolved in the lateral direction (with FWHM diameters greater than or similar to 0.2'') as far as we can follow them to their source which, in the case of HH 30, is as close as 0.25'' (35 AU). Assuming the jet has essentially zero angular width at its origin, one can deduce a lo wer bound for the initial opening angle, and the values obtained are v ery large indeed (e.g., greater than or similar to 60 degrees fur the HH 30 jet and counterjet), Our data are shown to support models in whi ch the jet is initially poorly focused before being asymptotically col limated into a ''column'' with a diameter of order a few tens of AU. A s regards the origin of the knots seen in these jets, it is found that many of the knots in the HH 1 and HH 34 jets resemble internal bow sh ocks, at least far away from their driving source (greater than or sim ilar to 5'' and 10'' in the case of HH 1 and HH 34 jets, respectively) . This is consistent with models in which the knots are attributed to ''internal working surfaces'' caused by temporal variations in the out flow from the source. It is found in the case of the HH 30 jet, howeve r, that its knots, at least close to the source, might have another or igin.