whole_EllingsenSimonPeter1996_thesis.pdf (11.19 MB)
Class II methanol masers in star formation regions
thesisposted on 2023-05-26, 23:54 authored by Ellingsen, Simon Peter
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.
Rights statementCopyright 1996 the Author - The University is continuing to endeavour to trace the copyright owner(s) and in the meantime this item has been reproduced here in good faith. We would be pleased to hear from the copyright owner(s). Thesis (Ph.D.)--University of Tasmania, 1996. Includes bibliographical references (leaves 155-161)