The methanol line at 6 GHz is the brightest of the strong Class II methanol masers. Detection of widespread emission in this line was first reported by Menten (1991). At present extensive surveys have yielded about three hundred 6 GHz maser sites (see Caswell 1996 for references and new detections). Class II methanol masers are always found in regions of recent massive star formation and many of them are associated with known ultracompact H II regions. The widespread occurrence and high intensity of the line makes it one of the best tracers of star-forming regions at present.
Interferometric studies indicate that the line is extremely bright. For example, the brightness temperature in a number of maser spots in the prototypic Class II maser source, W3(OH), is and higher (Menten et al. 1992). Such a high value provides a strong constraint on the excitation mechanism. The other major constraint comes from the observed ratio of brightnesses of spatially coincident (Menten et al. 1992; Norris et al. 1993) maser sources in the 6 GHz and 12 GHz lines. For a sample of 131 maser sources, Caswell et al. (1995b) found that the 6 GHz to 12 GHz intensity ratio ranges from 0.39 to 85, with a median value of 3.2. For their sample of stronger 6 GHz masers, a median value of 26 was found for this ratio. So, 12 GHz features are typically weaker than their 6 GHz counterparts, though there are some sites where they are stronger (Caswell et al. 1995b, c), including NGC 6334F (Ellingsen et al. 1996a). In W3(OH) the peak 12 GHz brightness is approximately (Menten et al. 1988), so that the 6 GHz line is approximately 150 times brighter.
In the first papers dedicated to quantitative explanation of Class II methanol maser characteristics, it was shown that the phenomenon appears only when the brightness temperature of the external radiation is greater than the kinetic temperature in the source itself (Cragg et al. 1992, Zeng 1992, Peng & Whiteoak 1993a). However, these pioneering models failed to produce bright enough masers.
In the paper of Sobolev & Deguchi (1994a, Paper I) it was shown that the brightness temperatures of the 12 GHz line could be explained in a more elaborate model taking into account maser beaming and involving energy levels of torsionally excited states. Brightness was strongly influenced by background radiation of the ultracompact H II region. In Paper I the model was applied only to the E symmetry species of methanol. In the current work, the same model is applied to A -species methanol in order to investigate the 6 GHz line. Some details of the model are briefly described in the following section. It is noteworthy that the model naturally explains the recently found variability of the (Caswell et al. 1993) and (Caswell et al. 1995a) brightness by changes of geometry of the beamed maser source.
The current study is devoted to the main problems of Class II methanol maser modelling: explanation of the line brightness and the 6 GHz to 12 GHz brightness ratio. Special attention is paid to the brightness ratio and what one can deduce from its value, because these two lines are the only widespread manifestations of Class II maser activity.
© European Southern Observatory (ESO) 1997
Online publication: May 26, 1998