The,molecular structure of Herbig-Haro objects, outflows and jets from young stellar objects
Davis, Christopher John
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The observations discussed in this thesis are of regions of star-formation where outflow is believed to be occurring. Specifically, radio, m illim eter/sub-m illim eter and infrared observations are made to illustrate the molecular structure of outflows and related phenomena.CO J= 2-1 observations of the molecular outflow from the infrared reflection nebulae GGD27 IRS complement earlier CO J = l-0 observations and further allow us to revise previous estimates of the outflow mass and energetics. The mass derived here is comparable to mass estimates for other outflows from similarly luminous sources, though it is a factor of ~ 1 0 less than that derived from the earlier CO 1-0 observations. This difference is due to the choice of excitation tem perature used in deriving the outflow mass; here combining CO 2-1 observations with the 1-0 studies allows us to calculate Tex more precisely. I also show that the conical outflow overlaps the plane of the sky, and find that the observed molecularflow is not powerful enough to excite the associated HH nebulae HH80/81.Studies of the molecular environment around the HH objects HH1 and 2, and towards the optical jet/H H bow-shock system HH34 reveal the presence of dense, quiescent molecular material associated with all three HH objects. The presence of such stationary, molecular clumps, particularly evident in HH2 and HH34, does not support current popular HH models.Observations of these regions in a number of molecular species also show how contrasting and sometimes misleading observations in different molecular gas tracers can be: In HH2 peaks seen in NH3 (1,1) do not coincide with the well resolved clump observed in H CO + J=3-2, whilst the C180 J = 2 -l emission shows no clear peak at all. These contrasting morphologies are thought to be due to the differing excitation requirements of the observed lines. However, in HH34 chemical abundance variations are also significant: Towards the exciting source of the optical stellar jet and HH bow-shock, the NH3 is underabundant. Indeed, in many other outflow sources, a similar NH3 depletion is observed towards the central source. Instead, the NH3 delineates a toroidal structure that encircles the central gas-density peak and the outflow source.I also show how near-IR imaging of the shocked H2 emission from molecular outflows (particularly those embedded within molecular clouds) may be used to search for infrared counterparts to optical stellar jets. The H2 structure in L1448 is thought to illustrate the interface between the molecular outflow and an associated stellar wind. These observations thus support the idea that CO (molecular) outflows result from an entrainment process as an atom ic stellar wind shocks ambient cloud gas. M oreover, they represent the first steps towards observing collimated stellar jets in most, if not all, CO outflows.Future m illimeter/sub-millimeter and infrared observations and pertinent questions concerning the outflow process are also discussed.