Coincidence of maser emission from OH at 6.035 GHz and methanol at 6.668 GHz

Massive newly formed stars are obscured by dust, and their dearest signature is often at radio frequencies from strong molecular maser emission. In some cases, both hydroxyl and methanol masers are present. Maser position measurements of both species are then especially useful for exploring the immediate surroundings of the new-born star. The present study used the Australia Telescope Compact Array to observe 6.035-GHz OH maser emission in 35 fields; 30 of these were also observed at the 6.668-GHz methanol transition. In many of the fields the masers were found to be grouped into several distinct maser sites, allowing a total of 62 sites to be studied. Relative positions at 6.035 and 6.668 GHz could be registered to an accuracy of 0.1 arcsec. 29 sites harbour both 6.035-GHz OH and 6.668-GHz methanol, and most of these have sufficient maser features to reveal that the distributions of OH and methanol are largely co-extensive, with total extents between 10 and 30 mpc (milliparsec). Elongated morphologies are common in both the OH and methanol distributions. The new-born star associated with each group of masers is not visible, but is expected to generate an enveloping ultracompact H II region. Such H II regions have been detected (as compact radio sources, mostly by searching continuum images made from the present observations) at 23 of the present maser sites. For these, the projected distribution of the masers was mapped relative to the H II region. In most cases, the masers are confined to a zone that covers only a portion of the H rr region, and is commonly offset from the centre of the H II region, but rarely lies at its projected boundary. The masing zone can be plausibly identified as centred on the (unseen) principal exciting star of the HII region, with the maser distribution corresponding to a torus or ring, in some cases small enough to suggest that it may be embedded within the H II region. The fact that the typical extent is confined to less than 30 mpc can be used in some cases to establish distances once the angular extent has been measured. The sample of maser sites studied here is OH-selected and thus biased towards sites with strong OH emission. The high rate of strong H II region detections among them indicates a marked trend (but not a close correlation) for H II regions to be more prominent towards OH-favoured maser sites than methanol-favoured sites.