Hot and cold: A study of molecular hydrogen jets and carbon monoxide molecular outflows from young stars.

摘要

This thesis examines outflows from young low-mass accreting protostars. The outflow phenomenon appears to be ubiquitous to all newborn stars, and is often the first observational evidence for embedded young stars. Outflows may be crucial to removing angular momentum from the accretion disk, thereby allowing material to accrete onto the stellar core. In addition, since they can transport supersonic gas over parsec-scale distances, outflows probably contribute to changing the chemistry of the molecular cloud, adding to the cloud's overall turbulence, and may even affect the efficiency of star formation by physically disrupting the infall environments around other protostars. Because young stellar objects (YSOs) are usually still embedded within their parent molecular clouds, they are shrouded by the gas and dust that they are born from, and hence are invisible at optical wavelengths. Outflows are however often energetic enough to punch out of the densest parts of the molecular clouds into less extincted regions. A proper study of all the aspects of protostellar flows [including optical Herbig-Haro (HH) objects, near-infrared (NIR) jets in the upsilon = 1--0 S(1) H2 line at 2.12 mum, and molecular maps of outflows in various transitions of 12CO] thus requires observations at a variety of lines that are populated under different excitation conditions, and which are detected at, a wide range of wavelengths.; This thesis consists of new observations and interpretations of two regions: the Barnard 5 cloud containing the IRS 1 flow, and the OMC-2 and OMC-3 cloud cores, containing a rich cluster of young stars and criss-crossing H 2 jets. 12CO J = 2--1 mapping, H2 and optical emission line imaging, and high resolution optical and NIR spectroscopy have revealed the locations of the H2 emission with respect to the molecular gas; provided support for bow shock entrainment models for the acceleration of CO bearing gas; showed evidence for H2 heating by a magnetic precursor or HH-object-induced fluorescence; and showed that many H2 knots can be described as bow shocks with forward and reverse shock line profiles.; In addition, comparisons of CO, Halpha, and H2 structures within 20'' of the Barnard 5 IRS 1 source supports an outflow acceleration model where both a jet and a wide angle wind emanate from the YSO or accretion disk. CO filaments found parallel to the IRS 1 flow may trace magnetosonic perturbations excited by major mass loss episodes of IRS 1. Luminosity masses of the outflow lobes are derived using a new technique which corrects for the optical depth of the line at each velocity channel. A comparison of mass spectra of flows from this work as well as from other authors reveals possible differences in how low and high mass YSOs entrain ambient gas into outflows as they evolve. Finally the numbers of flows found in the observed clouds are consistent with the hypothesis that outflows can provide part of the turbulent support within molecular clouds. The efficiency of this pressure support however drops in the case of OMC-2/3 where the main cloud is a narrow ridge.