The relationship between class I and class II methanol masers

The Australia Telescope National Facility Mopra millimetre telescope has been used to search for 95.1-GHz class I methanol masers towards 62 6.6-GHz class II methanol masers. A total of 26 95.1-GHz masers were detected, 18 of these being new discoveries. Combining the results of this search with observations reported in the literature, a near complete sample of 66 6.6-GHz class II methanol masers has been searched in the 95.1-GHz transition, with detections towards 38 per cent (25 detections; not all of the sources studied in this paper qualify for the complete sample, and some of the sources in the sample were not observed in the present observations).
There is no evidence of an anticorrelation between either the velocity range, or peak flux density of the class I and II transitions, contrary to suggestions from previous studies. The majority of class I methanol maser sources have a velocity range that partly overlaps with the class II maser transitions. The presence of a class I methanol maser associated with a class II maser source is not correlated with the presence (or absence) of main-line OH or water masers. Investigations of the properties of the infrared emission associated with the maser sources shows no significant difference between those class II methanol masers with an associated class I maser and those without. This may be consistent with the hypothesis that the objects responsible for driving class I methanol masers are generally not those that produce main-line OH, water or class II methanol masers.     

Class I methanol masers: masers with extended green objects

We have compared the results of a number of published class I methanol maser surveys with the catalogue of high-mass outflow candidates identified from the Galactic Legacy Infrared Mid-Plane Survey Extraordinaire survey (known as extended green objects or EGOs). We find class I methanol masers associated with approximately two-thirds of EGOs. Although the association between outflows and class I methanol masers has long been postulated on the basis of detailed studies of a small number of sources, this result demonstrates the relationship for the first time on a statistical basis. Despite the publication of a number of searches for class I methanol masers, a close physical association with another astrophysical object which could be targeted for the search is still lacking. The close association between class I methanol masers and EGOs therefore provides a large catalogue of candidate sources, most of which have not previously been searched for class I methanol masers. Interstellar masers and outflows have both been proposed to trace an evolutionary sequence for high-mass star formation, therefore a better understanding of the relationship between class I methanol masers and outflow offers the potential for comparison and amalgamation of these two evolutionary sequences.     

Multitransition study and new detections of class II methanol masers

We have used the ATNF Mopra antenna and the SEST antenna to search in the directions of several class II methanol maser sources for emission from six methanol transitions in the frequency range 85–115 GHz. The transitions were selected from excitation studies as potential maser candidates. Methanol emission at one or more frequencies was detected from five of the maser sources, as well as from Orion KL. Although the lines are weak, we find evidence of maser origin for three new lines in G345.01+1.79, and possibly one new line in G9.62+0.20.
The observations, together with published maser observations at other frequencies, are compared with methanol maser modelling for G345.01+1.79 and NGC 6334F. We find that the majority of observations in both sources are consistent with a warm dust (175 K) pumping model at hydrogen density ∼10⁶ cm⁻³ and methanol column density ∼ 5×10¹⁷ cm⁻². The substantial differences between the maser spectra in the two sources can be attributed to the geometry of the maser region.     

A GLIMPSE-based search for 6.7-GHz methanol masers and the lifetime of their spectral features

The University of Tasmania Mt Pleasant 26-m and Ceduna 30-m radio telescopes have been used to search for 6.7-GHz class II methanol masers towards 200 GLIMPSE ( The Galactic Legacy Infrared Mid-Plane Survey Extraordinaire ) sources. The target regions were selected on the basis of their mid-infrared colours as being likely to be young high-mass star formation regions and are either bright at 8.0 μm, or have extreme [3.6]–[4.5] colour. Methanol masers were detected towards 38 sites, nine of these being new detections. The prediction was that approximately 20 new 6.7-GHz methanol masers would be detected within 3.5 arcmin of the target GLIMPSE sources, but this is the case for only six of the new detections. A number of possible reasons for the discrepancy between the predicted and actual number of new detections have been investigated. It was not possible to draw any firm conclusions as to the cause, but it may be because many of the target sources are at an evolutionary phase prior to that associated with 6.7-GHz methanol masers. Through comparison of the spectra collected as part of this search with those in the literature, the average lifetime of individual 6.7-GHz methanol maser spectral features is estimated to be around 150 yr, much longer than is observed for 22-GHz water masers.     

The Torun catalogue of 6.7 GHz methanol masers

We report the observations of 289 methanol maser sources at 6.7 GHz obtained over a two month period with the Torun 32 m telescope. The data form a catalogue of all objects north of δ = –22° brighter than 7.5 Jy in the peak emission. The positions of sub‐arcsecond accuracy are updated for 76 % of the objects. We find that about one third of the sources show changes in the peak fluxes by a factor of two or more on time scales of 8.5–9.5 years (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

A search for 22-GHz water masers within the giant molecular cloud associated with RCW 106

We report the results of a blind search for 22-GHz water masers in two regions, covering approximately half a square degree, within the giant molecular cloud associated with RCW 106. The complete search of the two regions was carried out with the 26-m Mount Pleasant radio telescope and resulted in the detection of nine water masers, five of which are new detections. Australia Telescope Compact Array (ATCA) observations of these detections have allowed us to obtain positions with arcsecond accuracy, allowing meaningful comparison with infrared and molecular data for the region. We find that for the regions surveyed there are more water masers than either 6.7-GHz methanol, or main-line OH masers. The water masers are concentrated towards the central axis of the star formation region, in contrast to the 6.7-GHz methanol masers which tend to be located near the periphery. The colours of the GLIMPSE point sources associated with the water masers are similar to those of 6.7-GHz methanol masers, but slightly less red. We have made a statistical investigation of the properties of the ¹³CO and 1.2-mm dust clumps with and without associated water masers. We find that the water masers are associated with the more massive, denser and brighter ¹³CO and 1.2-mm dust clumps. We present statistical models that are able to predict those ¹³CO and 1.2-mm dust clumps that are likely to have associated water masers, with a low misclassification rate.     

A search for 4750- and 4765-MHz OH masers in southern star-forming regions

We have used the Australia Telescope Compact Array (ATCA) to make a sensitive  (5 σ ≃100 mJy)  search for maser emission from the 4765-MHz ²Π_1/2   F =1→0  transition of OH. 55 star formation regions were searched and maser emission with a peak flux density in excess of 100 mJy was detected toward 14 sites, with 10 of these being new discoveries. In addition we observed the 4750-MHz ²Π_1/2   F =1→1  transition towards a sample of star formation regions known to contain 1720-MHz OH masers, detecting marginal maser emission from G348.550−0.979. If confirmed this would be only the second maser discovered from this transition.
The occurrence of 4765-MHz OH maser emission accompanying 1720-MHz OH masers in a small number of well-studied star formation regions has led to a general perception in the literature that the two transitions favour similar physical conditions. Our search has found that the presence of the excited-state 6035-MHz OH transition is a much better predictor of 4765-MHz OH maser emission from the same region than 1720-MHz OH maser emission is. Combining our results with those of previous high-resolution observations of other OH transitions we have examined the published theoretical models of OH masers and find that none of them predicts any conditions in which the 1665-, 6035- and 4765-MHz transitions are inverted simultaneously.     

A comprehensive search for extragalactic 6.7-GHz methanol masers

We have used the Australia Telescope Compact Array (ATCA) to search for 6.7-GHz methanol maser emission towards 87 galaxies. We chose the target sources using several criteria, including far-IR luminosities and the presence of known OH megamasers. In addition, we searched for methanol masers in the nearby starburst galaxy NGC 253, making a full spectral-line synthesis image. No emission was detected in any of the galaxies, with detection limits ranging from 25 to 75 mJy. This is surprising, given the close association of OH and methanol masers in Galactic star formation regions, and significantly constrains models of OH megamaser emission. This absence of maser emission may be a result of low methanol abundances in molecular clouds in starburst galaxies.     

Modelling methanol and hydroxyl masers in star-forming regions

Class II methanol masers are found in close association with OH main-line masers in many star-forming regions, where both are believed to flag the early stages in the evolution of a massive star. We have studied the formation of masers in methanol and OH under identical model conditions for the first time. Infrared pumping by radiation from warm dust at temperatures >100 K can account for the known maser lines in both molecules, many of which develop simultaneously under a range of conditions. The masers form most readily in cooler gas (<100 K) of moderately high density  (10⁵–10⁸ cm^-3)  , although higher gas temperatures and/or lower densities are also compatible with maser action. The agreement between the current model (developed for methanol) and the established OH maser trends is very encouraging, and we anticipate that further tuning of the model will further improve such agreement.
We find the gas-phase molecular abundance to be the key determinant of observable maser activity for both molecules. Sources exhibiting both 6668-MHz methanol and 1665-MHz OH masers have a typical flux density ratio of 16; our model suggests that this may be a consequence of maser saturation. We find that the 1665-MHz maser approaches the saturated limit for OH abundances >10^−7.3, while the 6668-MHz maser requires a greater methanol abundance >10⁻⁶. OH-favoured sources are likely to be less abundant in methanol, while methanol-favoured sources may be less abundant in OH or experiencing warm (>125 K), dense (∼10⁷ cm⁻³) conditions. These abundance requirements offer the possibility of tying the appearance of masers to the age of the new-born star via models of gas-phase chemical evolution following the evaporation of icy grain mantles.     

Spectra of OH masers at 6035 and 6030 MHz

The Parkes radio telescope has been used to study circular polarization in the spectra of masers at the 6035- and 6030-MHz transitions of excited OH. The targets were 91 previously catalogued sites of 6035-MHz maser emission. A few were not detected, primarily because of variability. However, the 6035-MHz intensity variations seldom exceed a factor of 2 over several years, with a handful of dramatic exceptions.
Towards many targets, the present observations have provided the first high-sensitivity search for the 6030-MHz transition and yielded 33 detections. All of the 6030-MHz maser features have 6035-MHz counterparts closely matching in velocity. For matching features, the 6030-MHz emission is most commonly weaker than the 6035-MHz emission by an order of magnitude but, in a few cases, is several times stronger. The detection statistics are well accounted for by very recent developments in maser modelling. However, the occasional occurrence of 6030-MHz maser emission stronger than at 6035-MHz poses a new challenge for the theory.
Spectra with good frequency resolution at 6030 and 6035 MHz yield many valuable measurements of magnetic fields. At each transition, the field can be inferred from a small frequency separation between the right-hand and left-hand circularly polarized features, attributed to the Zeeman effect in a magnetic field of a few mG. In the many instances where a 'Zeeman pair' on the 6035-MHz spectrum has features matched by the 6030-MHz spectrum, this provides convincing corroboration of the magnetic field, in both direction and magnitude.
Several prominent absorption features occur at 6035 MHz, and usually have matching absorption at 6030 MHz of similar, or slightly smaller, depth.     

Positions of hydroxyl masers at 1665 and 1667 MHz

The Australia Telescope Compact Array has been used to observe more than 200 1665-MHz hydroxyl masers south of declination −16° and derive their positions with typical rms uncertainties of 0.4 arcsec. Many of the 1665-MHz maser sites are found to have 1667-MHz OH maser counterparts which are coincident, within the errors.
The resulting position list presented here includes all well-documented, previously reported 1665-MHz masers close to the Galactic plane in the galactic longitude range 230° (through 360°) to 13°. Nearly 50 newly discovered masers are also listed, chiefly in the longitude range 312° to 356°, where the observations were conducted as an intensive survey of a continuous zone close to the Galactic plane.
Many of the maser sites are discussed briefly so as to draw attention to those possessing properties that are unusual among this large sample. Most of the masers are of the variety found in star-forming regions – at the sites of newly formed massive stars and their associated ultracompact H ii regions. The new, accurate, positions reveal coincidences of the OH masers with the continuum radio emission, with the infrared emission from dust that accompanies such regions, and with emission from other maser species such as methanol at 6668 MHz and water at 22 GHz.
By-products of the survey, also presented here, include measurements of at least 11 objects that are not associated with massive star-forming regions. They comprise several OH/IR stars (detected at the 1667- or 1665-MHz transition of OH, though commonly found to be most prominent at the 1612-MHz transition) and several unusual masers that may pinpoint other varieties of late-type stars or protoplanetary nebulae.     

Methanol masers at 107.0 and 156.6 GHz

From a search of more than 80 southern class II methanol maser sites, we report measurements of 22 masers at 107.0 GHz and four at 156.6 GHz, mostly new discoveries. Class II sites, recognized by their strong emission at the 6.6-GHz methanol transition, are indirect indicators of new-born massive stars, and several hundred have been documented; only a handful of these had previously been found to exhibit maser emission at the 107.0- or 156.6-GHz transition. The present survey increases the number of known 107.0-GHz masers to 25, providing a sufficiently large sample to assess their general properties. For the stronger ones, our position measurements confirm that, to an accuracy of 5 arcsec, they coincide with the dominant maser emission at 6.6 GHz. Intensity variations exceeding 50 per cent have occurred in some 107.0-GHz maser features that we observed in both 1996 October and 1998 June.
We find that masers are rare at the 156.6-GHz transition. Two new detections increase the total now known to four. Each 156.6-GHz maser is substantially weaker than its corresponding 107.0-GHz maser. Despite the scarcity of masers, our 156.6-GHz spectra at most observed sites show emission, but apparently of a quasi-thermal variety; it is usually accompanied by somewhat weaker thermal emission at 107.0 GHz, and the intensity ratio of the transitions allows us to begin exploration of the physical characteristics of the small molecular clouds (diameter less than 60 mpc) at these sites. The thermal emission thus provides estimates of the environmental conditions that are needed to support strong masing from spots that are apparently embedded within these clouds.