New Class-I methanol masers at 25 GHz

The methanol molecule is very rich for observable transitions and has by far the largest number of detected transitions, both in thermal and maser emission. This fact makes masers in this molecule very attractive for theoretical researches. Observationally, methanol masers were divided into two classes (Menten, 1991). Masers in Class-I transitions are usually seen apart from infrared sources and ultra-compact HII regions, while Class-II masers tend to associate with them. For bright methanol lines even a simple model can reproduce this classification (e.g., Cragg et al. 1992). The strong radiation from a nearby source quenches the Class-I maser transitions and increases the strength of the Class-II masers. However, when a large number of transitions is considered, such models are usually far from agreement. Recently, there has been a considerable progress in the theory of the Class-II methanol masers (e.g., Sobolev and Deguchi 1994). But the models of the Class-I masers still remain very crude, mostly due to poorly known collisional excitation rates. The most widespread Class-I methanol maser is that at 44 GHz. About a hundred maser sites are known. Less common maser lines were observed over a wide range of frequencies, from as low as 9.9 GHz up to 229 GHz. Among the Class-I methanol masers there is a series (J2 - J1 E) of transitions near 25 GHz. The strongest maser (about 100 Jy) at the J = 5 transition is observed towards the Orion molecular cloud (OMC-1) which is known to be the most famous Class-I source. This line was thought to be rare, although no comprehensive surveys with negative results were published. Sometimes, even theoreticians tried to explain the rareness of this line (e.g., Sobolev et al. 2003). Therefore, it would be a very interesting task to prove observationally that the line is really rare.

A search for new masers

Figure. 1: Spectra of the newly detected 25-GHz methanol masers.

The new 12-mm system at the ATCA provides a unique opportunity to search for 25-GHz masers in the southern sky. The southern sky is more attractive because of a massive Class-I methanol-maser survey of southern star-forming regions carried out at Parkes at 44 GHz (Slysh et al., 1994). This survey gives an excellent target list. About ten bright 44-GHz masers were examined for the presence of the 25-GHz maser (J = 5). Most observations were done in spring and summer, and therefore the phase stability was not very good. This resulted in a rather high flux-density limit of about 5 Jy for non-detections. In addition, there are no interferometric images for most southern Class-I methanol masers. As a result, the positions are known with single-dish accuracy only. Therefore, some weak masers may be missed. The line was detected in four sources 305.21+0.21, 305.36+0.20, 333.23-0.05 and 343.12-0.06. The phase was self-calibrated, thus removing the influence of unknown position and a poor phase-stability. After this it was possible to average together the data from all baselines, considerably reducing the noise level. The spectra of these masers after such data reduction are shown in Figure 1.

Interestingly, all new detections at 25 GHz have very simple spectra containing a single feature, although corresponding 44-GHz masers may have multiple features. In addition, there is no straight correlation between the 44-GHz and 25-GHz flux densities. The brightest known 44-GHz maser, M8E (about 520 Jy), showed no emission at 25 GHz. An experiment to further investigate the 25-GHz methanol masers will be proposed for the next term.

References
Cragg D.M., Johns K.P., Godfrey P.D., Brown R.D., 1992, MNRAS, 259, 203
Menten K.M., 1991, in Proc. of the Third Haystack Observatory Meeting, eds. Haschick A.D., Ho P. T. P., 119
Slysh V.I., Kalenskii S.V., Val'tts I.E., Otrupcek R., 1994, MNRAS, 268, 464
Sobolev A.M., Deguchi S., 1994, A&A, 291, 569
Sobolev A.M., Watson W.D., Okorokov V.A., 2003, ApJ, 590, 333

Maxim Voronkov
(Maxim.Voronkov@csiro.au)

News
Public