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1 Introduction


The early phases of stellar evolution are characterized by interaction between a newly born star and its environment, resulting in the formation of molecular outflows, highly collimated jets, Herbig-Haro objects, emission or reflection nebulae and molecular masers. Methanol maser emission arises from several transitions, the strongest being the 50-61A+ line at 6.7 GHz, first detected by Menten (1991a) and recognized as typical of the Class II masers (Menten 1991b). Their fluxes are often larger than the associated OH masers fluxes.

Several surveys of the 6.7 GHz masers have been done in the Southern hemisphere (MacLeod et al. 1992; MacLeod & Gaylard 1993; Schutte et al. 1993; Caswell et al. 1995a), whilst in the Northern hemisphere no surveys in this line have been done since the discovery survey by Menten (1991a) who searched for the 6.7 GHz masers mostly toward OH masers.

Schutte et al. (1993) detected 35 masers at 6.7 GHz toward IRAS sources with colours typical for HII-regions, without known OH or other Class II methanol maser counterparts and showed that colour-selected IRAS objects are good candidate sources for the 6.7 GHz maser search. Schutte et al. (1993) also report different detection rates of methanol masers in different directions, supporting the idea that large-scale variations of overall methanol abundance in the Galaxy may exist. It is of interest to test with a larger sample of sources whether these variations really exist and try to understand their nature. Therefore we observed at 6.7 GHz a sample of 231 IRAS sources in the Northern hemisphere which satisfy the colour criteria by Wood & Churchwell (1989) for UC HII regions (WC objects), i.e. sources, which are not identified as stars and extragalactic objects, have good detections at 25 and 60 $\mu$m and have $\lg (F_{25}/F_{12})\;\gt\;0.57$and $\lg (F_{60}/F_{12})\;\gt\;1.3$,independently of their relation to known OH or CH3OH masers. Combining our results with data taken from the literature we can study the overall methanol abundance distribution in star-forming regions in a large part of the Galaxy.

In addition to colour-selected IRAS sources, we observed also objects that were identified as ultracompact HII-regions by Helfand et al. (1992) on the basis of the comparison of their IRAS and radio properties (52 objects). We extended our observations to other sources connected to the star formation process. Our source list contains 140 bipolar outflows. In addition, we observed a group of 6 objects, which consists of methanol maser and thermal sources from the surveys of Friberg et al. (1988); Slysh et al. (1994a); Peng & Whiteoak (1992). Many of these sources are associated with IRAS WC objects. The total number of the observed IRAS WC sources is 326. The others are supposed to be related to stars with low and intermediate mass. The goal of this work was to better understand the connection of methanol masers with other phenomena typical of star-forming regions. At present only the connection between the Class II methanol masers and ultracompact HII-regions seems to be very likely. Slysh et al. (1995) suggested a connection between high-velocity motions and the Class II J0-J-1E methanol masers. However, the situation is unclear, and we expected that our survey of 6.7 GHz sources toward a large sample of bipolar outflows will clarify it.



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