Class II methanol masers in star formation regions
Creators: name: Ellingsen, Simon Peter
Publisher: name: University Of Tasmania
Year: 2023

description: In 1991 maser emission from the 51-60 A+ transition of CH3OH at a frequency of 6.7 GHz was discovered by Menten (1991a). This transition is more common and stronger than the 12.2-GHz (2 0-3_ 1E) transition discovered four years previously. This thesis contains the results of a detailed study of 6.7-GHz CH3 OH maser emission over a wide range of angular resolutions. The University of Tasmania 26-m radio telescope has been used to perform a sensitive search for 6.7-GHz CH 3 OH masers in a 28.5 square-degree region of the Galactic Plane. The search is complete, within a well defined velocity and flux density range. One hundred and eight 6.7-GHz CH 3 OH masers were detected during the course of the survey, 57 of these being new detections. These new 6.7-GHz CH3OH masers are generally weaker than those already known, but otherwise their spectral appearance is similar to those detected towards OH and 12.2-GHz CH3 OH masers. The sample of 6.7-GHz CH3 OH masers has been used to critically evaluate several IRA S-based search techniques and we find that all these techniques fail to detect a large fraction of the masers. Two targeted searches of the Large and Small Magellanic Clouds have been performed, resulting in the detection of three 6.7-GHz CH 3 OH masers. In addition, a search for 6.7-GHz CH3OH megamasers was carried out toward 10 Extragalactic sources, nearly all of which are known OH or H 2 0 megamasers. No CH3 OH megamasers were detected with a peak flux comparable to the OH or H2 0 megamasers in the galaxies searched. Single dish spectra of 6.7- and 12.2-GHz CH 3 OH masers are often complicated, with many spectral features spread over a velocity range of 10 km s -1or more. High resolution observations of these maser sources show that each of the spectral features arise from a different region in the gas cloud. For OH and H2 0 masers the high resolution spatial morphology typically shows little or no simple structure. Conversely, high resolution observations of 6.7- and 12.2-GHz CH3 OH masers (Norris et al., 1988; Norris et al., 1993) show that many have a simple curved, or linear morphology. The Australia Telescope Compact Array has been used to observe the radio continuum emission associated with three strong 6.7-GHz CH3 OH masers. It is shown that the position of the CH 3 OH masers with respect to the continuum emission is consistent with the masers originating in a circumstellar disci Very Long Baseline Interferometry (VLBI) has been used to image strong class II CH3OH maser emission associated with two star formation regions. The milli-arcsecond resolution images detected many new maser spots, but all of these follow the general morphology revealed by lower resolution observations. Comparison of the 6.7- and 12.2-GHz images for the CH 3OH masers associated with NGC 6334F shows that five of the spots are coincident to within the positional errors of the observations (‚Äöv¢v†4 milli-arcseconds). VLBI observations were also used to measure the size of the 6.7- and 12.2-GHz CH3OH maser spots. These show that the maser spots contain structure on two different scales, one of the order of tens of astronomical units, the other between a few and ten astronomical units. These findings are supported by the imaging data and the VLBI observations of Menten et al. (1988; 1992). The sizes of the 6.7- and 12.2-GHz spots toward the same sources are similar, which suggests that they are not broadened by interstellar scattering.