Morphology and kinematics of parsec-scale outflows from young stars

Numerous studies of young stellar objects (YSOs) have uncovered associated optical outflows, the majority of which are driven by low-mass, embedded, Class I sources Initially these outflows were thought to extend only a few thousand AU, however recent observations have shown that they can stretch for several parsecs. In this thesis I examine the more massive intermediate - mass (2M0 < M* < 10Mo) YSOs and the more evolved Class II low-mass YSOs (Classical T Tauri stars - CTTSs) for evidence of similar large - scale outflows. It is now well known that mass outflow accompanies the birth of stars. Outflows occur early in the formation process when YSOs are optically obscured by large amounts of dust although the outflow itself may be optically visible. They are an essential part of the star formation process and can remove significant amounts of excess angular momentum from a YSO as it forms. On large scales outflows may generate turbulence m their parent cloud thus helping to support the cloud against gravitational collapse. In this way outflows may even influence subsequent star formation Parsec-scale outflows, created over thousands of years, are also an important “fossil record” of the mass-ejection (and, to a lesser extent, of the accretion) history of their source. The intermediate - mass YSOs presented here were found to generate outflows of up to 8 parsecs in length. Their morphology follows trends seen in large - scale outflows from embedded low-mass sources such as decreasing spatial frequency, increasing dimensions and increasing complexity of Herbig-Haro (HH) emission with distance from the source. Extrapolating the decline in spatial frequency suggests that HH emission may exist on even greater scales than seen here 1 e tens of parsecs. Remarkably, virtually all of the optical outflows from intermediate - mass YSOs have a high degree of collimation. As very high mass YSOs are known to have poorly collimated outflows (e g the Orion Bullets), these observations therefore suggest that the transition from focused jet to poorly collimated wind must occur at higher masses than those studied here. The CTTSs observed were previously known only to drive “micro-jets” of several thousand AU m length. The wide field observations presented here however show that they also drive parsec - scale flows of the order of 0 5pc. Their degree of collimation is again very high and comparable to what is observed m the case of more embedded sources of similar mass. This implies that the collimation angle remains small even as the source evolves over 1 Myr timescales. Again one observes the same decrease in spatial frequency of HH emission with increasing distance from the source as seen for the more embedded sources. The apparent dynamical lifetimes of these large-scale outflows (around 103-4 yrs) are comparable to the inferred time interval between major accretion events that give rise to FU Orioms outbursts. This would suggest a link between major HH complexes and the FU Orioms phenomenon. Finally my research would suggest that most optical outflows (both from intermediate mass YSOs and CTTSs) have blown out of their parent cloud and are much larger than inferred from their apparent size. A proper motion study was undertaken on the CTTS-driven outflows to determine tangential velocities, particularly for the more distant HH emission, as an aid to understanding outflow dynamics. This study shows the more distant objects in the outflows to be moving with velocities of ~ 200 kins-1 while the “micro - jets” themselves are slightly faster, suggesting that the velocities remain high with distance from the source