Studies of ultracompact H ii regions - II. High-resolution radio continuum and methanol maser survey

Abstract

High spatial resolution radio continuum and 6.67‐GHz methanol spectral line data are presented for methanol masers previously detected by Walsh et al. (1997). Methanol maser and/or radio continuum emission is found in 364 cases towards IRAS‐selected regions. For those sources with methanol maser emission, relative positions have been obtained to an accuracy of typically 0.05 arcsec, with absolute positions accurate to around 1 arcsec. Maps of selected sources are provided. The intensity of the maser emission does not seem to depend on the presence of a continuum source. The coincidence of water and methanol maser positions in some regions suggests there is overlap in the requirements for methanol and water maser emission to be observable. However, there is a striking difference between the general proximity of methanol and water masers to both cometary and irregularly shaped ultracompact (UC) H ii regions, indicating that, in other cases, there must be differing environments conducive to stimulating their emission. We show that the methanol maser is most likely present before an observable UC H ii region is formed around a massive star and is quickly destroyed as the UC H ii region evolves. There are 36 out of 97 maser sites that are linearly extended. The hypothesis that the maser emission is found in a circumstellar disc is not inconsistent with these 36 maser sites, but is unlikely. It cannot, however, account for all other maser sites. An alternative model which uses shocks to create the masing spots can more readily reproduce the maser spot distributions.

Absolute positions of 6.7-GHz methanol masers

“…Hence, we assume that most of the target sources have an absolute position accuracy of 0.5 or higher, except for sources that are close to the celestial equator. This is a typical position accuracy of ATCA data (Caswell et al 1995c;Walsh et al 1998;Phillips et al 1998;Minier et al 2001;Caswell 2009). For the MERLIN data, the typical signal-to-noise ratio is at least 100, and all but one phase calibrator are from the Jodrell Bank-VLA Astrometric Survey, which has a position accuracy of higher than 5 ms.…”

“…Almost all 6.7-GHz masers are found to be associated with 1.2 mm emission (Hill et al 2005), while many have no associated 8.6-GHz continuum emission (Walsh et al 1998). Relevant cases can be found even within one starforming region, e.g., NGC6334 I, which possesses a maser cluster associated with a prominent ultracompact HII region and another one associated with the sub-mm core and a candidate hypercompact HII region with very weak radio continuum emission (Hunter et al 2006).…”

A search for water masers associated with class II methanol masers – II. Longitude range 341° to 6°

“…Ellingsen et al 2013;Breen et al 2010b;Ellingsen et al 2007) that the presence and/or absence of different maser transitions correspond to different evolutionary stages of the parent YSO. Masers can be associated with ultra-compact HII regions (Phillips et al 1998;Walsh et al 1998) and at younger stages embedded within infrared dark clouds (Ellingsen 2006). Breen et al (2010b) have shown that YSO with only 6.7-GHz methanol masers are at an earlier stage of evolution than those with 12.2-GHz methanol and OH masers.…”

The molecular environment of massive star forming cores associated with Class II methanol maser emission

“…The seemingly low emission measures derived for the 24 GHz continuum are unreliable due to the large beam size of the observations. Alternatively, the 24 GHz continuum sources may have been too extended and resolved-out by the larger array configuration used at 8 GHz by Walsh et al (1998). Further high spatial resolution observations at ν 24 GHz are required to derive reliable emission measures and spectral indexes to unambiguously differentiate between the two explanations.…”