Massive stars form in the dense optically thick cores of molecular clouds throughout the Galaxy. They actively participate in the heating of these molecular clouds and also in enriching the interstellar medium with heavy elements. Stellar winds may contribute to change the original cloud structure by in uencing nearby low mass star formation, or even by triggering the formation of new stars. Despite the fact that massive star formation is manifest throughout the Galaxy, the mechanisms involved in their formation are poorly understood. The coincidence of methanol masers and ultracompact (UC) HII regions with young massive stars, indicates their eectiveness in tracing the earliest stages of high-mass stellar evolution. This thesis concentrates on a multiwavelength study of massive star formation regions. The cornerstone of this thesis is a (sub)millimetre study of methanol maser and UC HII sources associated with massive star formation regions. The 15m Swedish ESO Submillimetre Telescope (SEST) and the 15m James Clerk Maxwell Telescope (JCMT) were used to survey continuum emission at 1.2mm and 450 & 850 m, respectively. The 1.2-mm continuum emission observations were undertaken with the SEST IMaging Bolometer Array (SIMBA) toward star-forming complexes associated with methanol maser and/or radio continuum sources. This survey detected emission toward all of the methanol maser and UC HII regions targeted, and revealed evidence of a handful of these tracers lying within the elds mapped without millimetre continuum emission. Interestingly, the SIMBA survey also revealed evidence of sources devoid of traditional star formation tracers (methanol/OH maser, UC HII regions, IRAS sources), detected solely from their millimetre continuum emission. These `mm-only' sources prompted a follow-up study with the JCMT. The 450 and 850m observations of mm-only cores discovered in the SIMBA survey, utilised the Submillimetre Common User Bolometer Array (SCUBA) on the JCMT, which detected submillimetre continuum emission toward 97 per cent of sources targeted. The SCUBA instrument also resolved a number of SIMBA sources into multiple components, and found evidence of submillimetre sources devoid of SIMBA emission. The (sub)millimetre data from the SIMBA and SCUBA surveys enabled study of the dust properties of the cores, and in particular the dust grain emissivity index , which is of order 2 for the sample. Spectral energy distribution (SED) diagrams were drawn for the SIMBA sources, employing MSX and IRAS data where possible. Eight parameters are reported for each of the sources in the sample: luminosity, temperature, mass, H2 number density (nH2 ), surface density (), radius, distance and luminosity-tomass ratio (L/M). These data show that the mm-only cores are of comparable mass to sources harbouring a methanol maser and/or radio continuum source, but are smaller, cooler, less luminous, with smaller luminosity-to-mass ratios, than these sources. This prompts the suggestion that the mm-only core is an example of the earliest stages of massive star formation prior to the onset of methanol maser emission. The mm-only sample was identied as comprising two populations, which are distinguished by temperature. The cool-mm sources are distinct from the warm-mm sources as well as from those sources with a methanol maser and/or a radio continuum source for all parameters tested but for the mass and in some instances the radius. The warm-mm on the other hand are similar to those sources with a methanol maser site and/or an UC HII region for all parameters tested. The cool-mm sources are less luminous, with lower luminosity-to-mass ratios, as well as higher H2 number and surface densities. This prompts hypotheses to be drawn regarding the nature of these two mm-only populations. The warm-mm sources are possible precursors to the methanol maser and are thus indicative of the earliest stages of massive star formation, whilst the cool-mm sources are possible examples of `failed' cores that will remain starless. It is, however, not yet clear which of these mm-only populations are forming massive stars and further work to test these hypotheses is required.
| Identifer | oai:union.ndltd.org:ADTP/273069 |
| Date | January 2006 |
| Creators | Hill, Tracey Lee, Physics, Faculty of Science, UNSW |
| Publisher | Awarded by:University of New South Wales. School of Physics |
| Source Sets | Australiasian Digital Theses Program |
| Language | English |
| Detected Language | English |
| Rights | Copyright Tracey Lee Hill, http://unsworks.unsw.edu.au/copyright |
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