5 Remarks on associations and variability

32 out of 70 new methanol masers have now been associated with at least one of the following emitters: radio continuum, CO, CS, NH₃ lines, H₂O and OH masers. 12 of them have an associated ultracompact HII region (Backer et al. 1994) and 15 objects exhibit the CS(2-1) emission (Bronfman et al. 1996). We identified 5 new sources associated with both 5 GHz radio continuum and CS line. Those associations strongly confirm that the methanol emission is an indicator of dense gas in regions of recent formation of massive stars (Menten 1991). To our knowledge nearly half of our new detections were not searched for radio continuum or lines.

Although the present survey differs significantly in spectral resolution and sensitivity from previous studies usually performed in various conditions, the existing data sets appear to be valuable to assess the variability of methanol masers on the time-scale of the 4 and 7-8 year intervals between observing runs. We found 96 sources with the 6.7 GHz maser data taken at least at two epochs and their line parameters being published. In previous section we compared the peak flux densities for all of them, but in several cases when spectra were published we also estimated the relative intensities of individual features. Four groups of objects were distinguished. The first one contains 23 sources without recognizable variations of maser intensity within the measurement accuracy of about 15%. In the second group of 11 sources most of them show weak variations of about 20% or poorly documented variations. We classified 46 sources as variable assuming that their peak flux densities changed by more than 20% but usually less than 50%. Into this group we also included sources with weak variations of peak flux densities but with considerable (more than 20%) changes of relative intensities of maser features. The fourth group is composed of 16 sources with strong variations of peak flux densities or intensity ratios of individual features commonly higher than 50%. Into this group we included two objects 19220+1432 and 19388+2357 undetected in our survey but previously observed as quite strong masers. This crude classification indicates that more than 65% of objects exhibit moderate or strong variations of maser emission and about 15% of sources do not vary at all.

Acknowledgements

We are grateful to Andrzej Kepa, Eugeniusz Pazderski, Roman Feiler and Sawomir Jakubowicz, who have made a major contribution to the successful construction of the autocorrelation spectrometer and the receiver. This work was supported by KBN grant 2P03D01415.