Class II Methanol Masers in Star Formation Regions

Details Class II Methanol Masers in Star Formation Regions Class II Methanol Masers in Star Formation Regions

Jan 1996

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996phdt........15e&link_type=abstract

PhD Thesis, Physics Department, University of Tasmania, AUS, 1996.

Physics

14

Scientific paper

In 1991 maser emission from the 51 - 60A+ transition of methanol at a frequency of 6.7 GHz was discovered by Menten (1991). This transition is more common and stronger than the 12.2-GHz (20 - 3-1E) transition discovered four years previously. This thesis contains the results of a detailed study of 6.7-GHz methanol 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 methanol 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 methanol masers were detected during the course of the survey, 57 of these being new detections. These new 6.7-GHz methanol masers are generally weaker than those already known, but otherwise their spectral appearance is similar to those detected towards OH and 12.2-GHz methanol masers. The sample of 6.7-GHz methanol masers has been used to critically evaluate several IRAS-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 methanol masers. In addition, a search for 6.7-GHz methanol megamasers was carried out toward 10 Extragalactic sources, nearly all of which are known OH or water megamasers. No methanol megamasers were detected with a peak flux comparable to the OH or water megamasers in the galaxies searched. Single dish spectra of 6.7- and 12.2-GHz methanol masers are often complicated, with many spectral features spread over a velocity range of 10 km/s or 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 water masers the high resolution spatial morphology typically shows little or no simple structure. Conversely, high resolution observations of 6.7- and 12.2-GHz methanol masers (Norris et al. 1988; 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 methanol masers. It is shown that the position of the methanol masers with respect to the continuum emission is consistent with the masers originating in a circumstellar disc. Very Long Baseline Interferometry (VLBI) has been used to image strong class II methanol 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 methanol masers associated with NGC 6334F shows that five of the spots are coincident to within the positional errors of the observations (≅ 4 milli-arcseconds). VLBI observations were also used to measure the size of the 6.7- and 12.2-GHz methanol 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. This thesis is available on WWW at: http://reber.phys.utas.edu.au/~sellings/thesis.html

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