An infrared and millimeter-wave spectroscopic study of the DR21 outflow

Abstract

In this thesis, new high-angular resolution infrared and millimeter-wave spectroscopic observations of the enigmatic outflow activity associated with the luminous DR21 star-form ing region are presented and discussed. The intent is to use these observations to undertake a detailed investigation of the physical nature of the central driving engine and the related dynamical processes involved in collimating the hypersonic outflow gas.

In the infrared, large-scale mapping and high-spectral resolution profile measurements of the vibrational H2 v = l-0 S(l) line are used to investigate the morphology and kinematic structure of the hot, dense gas that is collisionally excited behind fast shocks. The H2 emission delineates a highly-collimated pair of bipolar jets that extend over a projected distance of ~ 5 pc, centred on the DR21 molecular cloud core; this is undoubtedly the most luminous (in H2 line emission) and extended galactic outflow source yet discovered. Furthermore, the H2 line profiles at certain locations within the jets possess high-velocity wings that extend to beyond 100 km s-1 from the DR21 rest velocity. These observations pose interesting dynamical consequencies as at such high velocities H 2 should be entirely dissociated.

In an attempt to derive the mass distribution and velocity structure of the molecular gas participating in the outflow, and hence the driving force and associated mechanical luminosity, detailed observations were also undertaken at millimeter-wavelengths in the CO J= 1 -0 and CS J = l-0 , J= 2-l lines. It is found that the DR21 outflow is considerably more massive and energetic than any other outflow source studied to date. Another feature unique to the DR21 region is the discovery of extended high-velocity CS emission that is dynamically associated with the outflow lobes and extends to a distance of ~ 3 pc from the cloud core; this component presumably originates from am bient gas that has been swept up and compressed by the outflow. The high-velocity CS may be overabundant by 2 orders of magnitude, in good agreement with current numerical models of post-shock chemistry.

The CS observations further reveal the existence of an extremely massive, slowly rotating disc of high-density neutral gas that surrounds the central outflow source. It is most probable that the large momentum flux in outflow material derives from efficient mass-loss from the surface of this disc, mediated via a centrifugally propelled, magneto-hydrodynamic wind. An additional confinment mechanism is required to collimate the outflow at large distances from the flow origin. If this confinment is primarily pressure driven, then sudden changes in the ambient cloud pressure could induce a succession of oblique shocks within the outflow that may give rise to the periodic clumpy structure that characterizes the H2 emission-line jets. Other consequencies of the pressure-confinment mechanism are discussed and a broad resemblance to extragalactic radio jets is remarked upon.