A Study of the Cometary Globules in the Gum Nebula
Creators: name: Sridharan Tirupati Kumara
Publisher: name: Zenodo
Year: 1992
Subjects: star formation, interstellar medium

description: A Study of the Cometary Globules in the Gum Nebula PhD Thesis, Oct 01, 1992, Indian Institute of Science, Bangalore, India &amp; Raman Research Institute, Bangalore, India The first part of the thesis deals with the development of a wide-band mechanically tuned local oscillator using the Gunn diode for use with the 10.4m millimeter-wave Leighton radio telescope at the Raman Research Institute. This provides sufficient power to efficiently operate two cryogenic Schottky mixers (dual polarization) and tunes over the frequency range 75-115 GHz covering most of the 3-mm atmospheric transmission window (W-band). Rotational transitions of many astrophysically important molecules including CO fall in this range. A study of the cometary globules (CGs) in the Gum Nebula forms the second part. The CGs are characterized by compact, dusty heads with long faintly luminous tails extending on one side and narrow bright rims on the other side. There exists a significant population of such CGs in the Gum Nebula, distributed over a region ~ 80 parsec in radius with their tails pointing away from an apparent common center. Some of the heads have embedded young stars. In the region bounded by the CGs there are a few massive hot stars including zeta Puppis believed to be the most luminous star in the southern sky. It has been suspected that the morphological appearance of the CGs may be due to the influence of these stars. In order to understand the kinematics and the origin of the system, a study was undertaken using the first rotational transition of the carbon monoxide molecule. The study consisted of ^12CO observations of the heads and the tails of the CGs. In addition, the Globule No.22 was mapped in both ^12CO and ^13CO. An analysis of this data has led to the following findings: 1. The system of CGs is expanding with respect to a common morphological center at ~12 kms^-1. The expansion age is ~6 Myr. 2. Some of the tails observed show systematic velocity gradients. If the tails were formed due to the elongation resulting from these velocity gradients then the estimated stretching age is ~3 Myr. In order to clarify if externally triggered star formation is going on in this region, an analysis of the locations of the embedded young stellar objects (YSOs, identified from the Infra-Red Astronomy Satellite data) in the dark clouds in the Gum-Vela region was undertaken. This study has shown that the YSOs have a statistically significant tendency to fall on the sides of dark clouds facing the morphological center rather than the far sides, supporting external triggering. From the above analysis we come to the following conclusions: 1. The rough agreement between the expansion age and the tail-stretching age suggests a common origin for the expansion and the formation of the tails. The presence of young stars of comparable ages in the heads of some of the globules suggests that the processes responsible for the expansion may have also triggered star formation in them. 2. The radiation pressure from the hot stars in the central region or the stellar winds from them cannot account for the momentum of the expanding globules. It is more likely that the <em>rocket effect </em>arising out of the heating and the consequent anisotropic ablation of the globules supplied the necessary momentum. From the space motion of the star zeta Puppis we suggest that it had a massive companion which exploded as a supernova half a million years ago. The combined effect of the ultra-violet radiation and the stellar wind from this binary as well as from other stars in the neighbourhood significantly affected the parent molecular cloud in which they formed. This resulted in much of the molecular material in the vicinity being blown away except the numerous regions of enhanced density (condensations) in the original molecular cloud. Continued effect of the radiation and stellar winds resulted in these condensations being set in motion, as well as developing cometary tails.