Abstract
It is now realised that outflows from young stars are an essential part of the star-formation process and may even be necessary for disk accretion to occur. While the flows themselves can stretch for several parsecs, they are nevertheless generated within a radius of at most a few AU from their source. Such distances correspond, for the nearest star-forming regions, to angular scales of around a few tens of milliarcseconds. Therefore, until optical/near-infrared interferometry becomes widely available, our best chance of exploring how jets from young stars are produced lies with the Hubble Space Telescope (HST) . Already HST data, and in particular data obtained with the Space Telescope Imaging Spectrograph (STIS), are giving us vital clues regarding their formation and we are beginning to make direct comparison with model predictions. Here we review recent advances in the field. Not only is it now possible to measure jet diameter and velocity close to the source but also important quantities like temperature, ionization fraction, as well as electron and total density, both along and across the flow. The values obtained appear to be in agreement with popular magneto-centrifugal jet launching models. Finally, the recent discovery of possible jet rotation is discussed. Of major importance is the finding that the amount of rotation observed suggests outflows are responsible for removing angular momentum from disks, thereby allowing accretion to occur.
