Massive stars play a key role in shaping the local Universe but their formation process remains enigmatic. Emission from the 6.67 GHz, class II masing transition of methanol has proven a powerful tracer of where massive star formation regions are at their earliest evolutionary stages. Presented here are multi-wavelength observations to investigate the molecularlionised gas and (proto) stellar populations in cores traced by methanol maser emission, with the aim of investigating the earliest evolutionary stages of massive star formation and the role of methanol masers as a diagnostic of the state that star formation has reached within a core. Observations of para-ammonia [NH3 (1,1) --- (5,5)] and 24 GHz continuum emission were taken with the Australia Telescope Compact Array towards 21 southern Galactic hot molecular cores traced by 6.7 GHz methanol maser emission. NH3 was detected toward every region. For each core we extract and fit characteristic NH3 spectra and calculate the continuum properties. We split the cores into four groups based on their observed association with NH3 , 24 GHz continuum and methanol maser emission. We find evidence based on the kinetic temperatures and gas kinematics that the cores in the groups are at different evolutionary stages. In addition, we find 24 GHz continuum emission towards some methanol masers which had no 8 GHz continuum counterparts. We posit these could be hyper-compact HII regions: more dense and younger ionised gas than the ultra-compact HII regions. The NH3 (4, 4) and (5, 5) emission is always unresolved and at the methanol maser location, suggesting the methanol masers are located at the warmest part of the core. The fact that several cold cores also contained methanol masers suggests they trace regions at stages shortly after a suitable powering source has formed, right through to relatively evolved UCHII regions. While remaining a good general tracer of young MSF regions, the presence of a methanol maser does not single out any particular evolutionary stage. High resolution (~0.36"), multiple-filter, mid-IR images using Michelle on Gemini North were taken toward three hot molecular cores signposted by methanol maser emission. We find each region contains multiple, very red, luminous (50-40,000 Leo) sources, some of which lie at extremely small angular separations corresponding to linear separations of ?1700AU. Assuming the MlR point sources are embedded stellar objects, and the observed gas mass provides the bulk of the reservoir from which the stars formed, it is difficult to generate the observed distributions for the most massive cluster members from the gas in the cores using standard forms of the IMF. Deep, wide-field J, Hand Ks images were taken with IRIS2 on the Anglo Australian Telescope, towards the massive star formation region G305.2+0.2 and combined with 3.6, 4.5, 5.8 and 8.0??m data from the GLIMPSE survey on the Spitzer Space Telescope. After removing contamination from foreground stars, sources were separated, based on their IR colour, to investigate the properties of the embedded stellar populations. While strong extended emission in the GLIMPSE images hampered investigation towards the known sites of massive star formation, a sizable population of IR excess sources are found in the surrounding region free from these completeness effects. Investigation reveals the recent star formation activity in the region is more widespread than previously known. The embedded cluster in the region, G305.24+0.204 may have played a role in triggering further sites of star formation. Finally, we investigate the distribution of IR excess sources towards the cluster, in particular their apparent lack towards the centre compared with its immediate environs.
| Identifer | oai:union.ndltd.org:ADTP/258313 |
| Date | January 2007 |
| Creators | Longmore, Steven Neil, Physics, Faculty of Science, UNSW |
| Publisher | Publisher:University of New South Wales. Physics |
| Source Sets | Australiasian Digital Theses Program |
| Language | English |
| Detected Language | English |
| Rights | http://unsworks.unsw.edu.au/copyright, http://unsworks.unsw.edu.au/copyright |
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