Nature of highly-extended CS(7-6) emission from protostellar envelopes

Abstract

Molecular lines excited at high densities and temperatures are expected to trace the inner regions of protostellar envelopes. Single-dish observations in CS(7-6) and HCN(4-3), however, reveal emission extending out to a few thousand AU from the central protostar(s). At such radial distances, simple models for collapsing condensations around low-mass protostars predict that the envelope density is an order of magnitude or more below the critical densities of CS(7-6) and HCN(4-3). In all cases resolved, the CS(7-6) emission is found to elongate along and exhibit a velocity gradient in the opposite sense to a bipolar molecular outflow. Furthermore, the CS(7-6) emission is almost always brighter on the blueshifted side of the outflow. Models proposed to explain the extended CS(7-6) and HCN(4-3) emissions invoke gas lifted and driven outwards, by perhaps a stellar wind, from either the surface of a highly-flattened envelope (pseudodisk) or the surface defining the outflow cavities of an envelope carved by a bipolar outflow. In either case, irradiation from the central protostar is invoked to heat the dispersing gas to a sufficiently high temperature to emit in CS(7-6) and HCN(4-3).

In this thesis, I present a detailed study of the CS(7-6) emission from the protostar L483 using archival data from the Atacama Submillimetre Telescope Experiment (ASTE) and the SubMillimetre Array (SMA). By combining the data from the two telescopes, I investigate in detail both the structure and kinematics of the CS(7-6) emission on scales from hundreds to thousands of AU. In addition, CO(2-1) data from the SMA provides an important comparison with the CS(7-6) data in elucidating the nature of the CS(7-6) emission. I find that the CS(7-6) emission of L483 comprises of an extended component as found by ASTE and a compact central component. Both the asymmetric brightness distribution and velocity gradient of the extended component arise from emission only within 1000 from the centre. The compact central component has a velocity gradient in the same sense as, but steeper than, the extended component along the outflow axis. Furthermore, both components exhibit the same velocity gradients across the outflow axis. I demonstrate that the spatial-kinematic structure of the CS(7-6) emission around L483 cannot be explained by previous models invoking dispersing gas. Instead, all the features observed can be explained by infalling gas at the envelope surfaces defining the outflow cavity walls. The model requires the envelope surfaces to be significantly enhanced in both density and temperature, but otherwise qualitatively resembles simple models of rotating and collapsing condensations. I successfully apply this model also to all other low-mass protostars so far found to exhibit extended CS(7-6) emission. My work shows that apparently compact, rotating, and infalling structures around protostars should not be automatically assigned to a central pseudodisk, as has been the common practice in the past.