Methanol Masers: Tracer of Star Formation
The Onsala VLBI group has been conducting an extensive study of the Northern hemisphere methanol masers using the European VLBI Network (EVN) and the VLBA since early 1997. In parallel, a blind survey of the Galactic plane has been undertaken in order to discover new 6.7 GHz methanol masers using the Onsala-25m antenna, as well as a search for new class II methanol maser lines at higher frequencies (85-112 GHz). This thesis reports the results in these three observational projects and shows that methanol masers are excellent tracers of early stages in the process of massive star formation.
VLBI observations of 6.7 and 12.2 GHz methanol masers toward fifteen star-forming regions (namely IRAS20126+4104, NGC7538, S252, W75N, W48, G31.28+0.06, S231, S255, S269, MonR2, G9.62+0.20, CepA, W51, G59.78+0.06 and G29.95-0.02) have given four main results. First, the majority of the observed methanol maser sites do not coincide spatially with UC HII regions. Instead, the methanol maser sites may trace a stage of massiveion prior to the development of observable ionised regions. Secondly, in many cases the methanol maser components form a line with a linear velocity gradient along it, which is consistent with a rotating disk seen edge-on. The masers partially delineate the edge-on disk and probably lie only in front of the star. There is also some evidence for methanol masers tracing outflows in a few sources. Preliminary proper motion studies of one source, G9.62+0.20, show that the maser components lying in the lines are moving apart. Finally, the intrinsic structures of the masing regions consist of a core and a halo.
The first results of the unbiased survey after 35 square degrees observed toward the Galactic plane, show a poor detection rate of new methanol masers in the regions far away from the galactic centre. Instead, most of the detected methanol maser sites are located in the inner part of the Galactic plane. This confirms that the distribution of methanol maser sources is correlated to the distribution of massive star-forming regions. Some methanol maser sites do not exhibit any traditional emission of young massive stars, indicating that the methanol masers could trace deeply embedded massive protostars.
The search for new methanol maser lines in the frequency range 85-112 GHz shows that strong thermal emissions are present at the frequencies of maser line candidates. Intense masers are only observed at 107 GHz. Likely detections of new methanol masers at 85.5, 94.5, 108.9 and 111.3 GHz are reported.
In summary, the collected evidence resulting from multi-wavelength observations at high and low resolutions, demonstrates that class II methanol masers are directly involved in the earliest process of massive star formation and are potentially powerful tracers of stellar evolution.