NEW DETECTIONS OF 5(1)-6(0) A(+)-METHANOL MASERS TOWARDS IRAS SOURCES

We present the results of our second search for 6.7-GHz methanol masers towards colour-selected IRAS sources. Five hundred and twenty IRAS sources that meet the far-infrared colour criteria set by Wood and Churchwell for ultra-compact I-III regions were searched for 6.7-GHz methanol maser emission, to a sensitivity limit of 5 Jy. Thirty one new maser sources were detected. We also compare the FIR colours of the newly detected maser sources with those detected by Schutte et al. (1993) and the IRAS counterparts of sources that have both methanol and hydroxyl maser emission. It was found that the average flux distribution of the newly detected sources differs significantly from that of all other known 6.7-GHz methanol maser sources. It is argued that the differences may be due either to intrinsic differences between the three groups of sources or to interstellar extinction. An analysis of the relation between the 6.7-GHz maser and IRAS flux densities shows that the maser flux density is always less than the 100 mu m flux density while only three sources have a maser flux density greater than the 60 mu m flux density. Far-infrared pumping of the 6.7-GHz methanol masers is therefore in principle viable although it was found that the apparent maser efficiency will exceed 10 per cent for a significant number of sources in the case of FIR pumping by photons between 50 and 100 I Lm The overall detection rates on the IRAS [25 - 12] vs [60 - 12] two-colour diagram are also presented. Possible new search strategies for masers in colour-selected IRAS sources are discussed.