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 m and have
and
,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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