| A new survey for 6.668-GHz methanol emission has yielded 57 masers. The survey covers a narrow strip along the Galactic plane, spanning the longitude range 330.Ëි to 339.Ëි, and was made with the Australia Telescope Compact Array. The position measurements have subarcsecond accuracy, a considerable improvement for the 36 masers which were known from previous studies 21 masers are new detections. The methanol maser emission arises from the dusty envelopes of newly formed massive stars. The high accuracy of the present methanol positions facilitates detailed comparisons with corresponding emission from other maser species (such as hydroxyl and water), with the HII region or stellar wind ionized by the star (sometimes detectable in the radio continuum), and with the associated dust (commonly detected as emission in the far infrared). A comparison has been made with preliminary results from a new hydroxyl 1665-MHz survey of the same region, which contains at least 31 OH masers. 25 of the sources are in common, i.e., the methanol and OH species of masers coincide to within 1 arcsec. The larger number of methanol masers is to be expected, since the surveys are sensitivity-limited and methanol masers are typically 16 times stronger than their OH counterparts. The ratio of methanol-to-OH peak intensity covers a wide range, with some maser sites strongly methanol-favoured and others strongly OH-favoured, but there is no evidence to suggest that a distinct population of exclusively methanol masers exists. Rather, the methanol-to-OH ratio is a measure of different physical conditions and may be a useful diagnostic for the evolutionary state of the HII region or the mass of its exciting star. There are indications that many methanol-favoured sites have only weak associated HII regions, an may thus be tracers of stars that are less massive or at an earlier stage of evolution. |
