Methanol masers — all class!

Methanol masers are well established as signposts of the early stages of star formation, many being associated with sources that have not developed an Hii region. There are more than 20 different methanol transitions that have been observed to show maser emission and these are divided into two classes. The empirical division defined by Menten (1991) is that the class-II transitions (of which the 6.7 GHz is the best known) are closely associated with infrared sources, OH masers and HII regions. The class-I transitions are also found towards star-forming regions, but offset (up to a parsec) from infrared sources, other maser transitions etc. Theoretical models of methanol masers suggest that the class-I masers arise in an environment where collisional processes dominate and class-II masers form where radiative processes dominate. Class-I methanol masers are not known to be closely associated with any other type of astrophysical object and are relatively poorly studied. However, they are hypothesised to be associated with molecular outflows and so may be a signpost for an even earlier stage in high-mass star formation than are the class-II masers.

A search with the Mopra telescope for 95-GHz class-I methanol masers towards a statistically complete sample of 6.7-GHz class-II methanol masers achieved a detection rate of 38% (Ellingsen, 2004). Contrary to expectations in more than half the cases the velocity ranges of the two classes of masers partially overlap. However, with a single-dish it is not possible to determine the nature of the relationship between the two classes.

In late September 2004 we used the new 5-antenna millimetre system at the Compact Array in the H75 configuration to image the 95-GHz class-I masers detected at Mopra. The weather was kind and the system performed very well with a system temperature between 200 – 300 K on all antennas. We observed 5 × 5 arcminute cuts on each source at a range of hour angles and were able to determine the absolute position of the 95-GHz masers to arcsecond accuracy. The 6.7-GHz class-II masers in each of these sources have previously been imaged with the Compact Array and the new 95-GHz observations have revealed a number of surprises. In particular, they show that the emission from the class-I transition that overlaps the velocity range of the 6.7-GHz masers is often from the same region (to within an arcsecond) as the class-II maser site. Also, the class-I methanol masers are much more widely distributed throughout the star forming region than are the class-II masers.

The figure shows our observations of 328.809+0.633, a well studied southern high-mass star forming region. The greyscale is the integrated class-I methanol maser emission, the thick contours are the 3.5-mm continuum emission (which is the site of the class-II maser emission in this source) and the thin contours are a renzogram of the class-I emission. The class-I maser emission is located in at least eight different clusters spread over a region of the order of 30 arcseconds. There is some emission from the class-I transition at the class-II maser site, but the strongest class-I masers are significantly offset from the class-II masers. The completion of the millimetre upgrade has made the Compact Array a powerful tool for studying the nature of class-I methanol masers and the high-mass star formation process.

Simon Ellingsen (University of Tasmania), Jim Caswell (ATNF), Maxim Voronkov (ATNF) and Giuseppe Cimò (University of Tasmania)
(Simon.Ellingsen@utas.edu.au)

References

Ellingsen, S.P., 2004, submitted to MNRAS
Menten, K.M., 1991, in Haschick, A.D., Ho P.T.P. eds, ASP Conf. Ser. Vol. 16, p. 119