Thermal methanol emission in the DR21 complex Interferometric maps: a comparison with maser emission

We present interferometric observations with∼3.5′′ resolution of the 2k → 1k methanol lines towards the methanol masers in the DR21/DR21(OH) complex. These transitions are not masing and hence our measurements can be used to place limits on the methanol mass associated with the maser spots. We do not find in general good correlation between the peaks of “non-maser” methanol emission and the positions of Class I methanol masers. In particular, we put an upper limit on 2k → 1k emission towards the 80 → 71A maser DR 21(OH)-1 from Plambeck and Menten (1990) (hereafter PM90). We do find however evidence for coincidence between the peak of the 21 → 11E emission in our map with the Class I masers towards DR21-W. We conclude that methanol Class I masers are likely to originate in a region considerably smaller than our synthesised beam (3.5′′) and with methanol column density of order 1016 cm−2. It seems reasonable that they might form in regions of diameter 1000 AU and mass 0.01 M . Our data also show differences between the distribution of methanol and that of other species which suggest large spatial variations in the methanol abundance. In particular, we see evidence for high methanol column density towards the submillimeter continuum source DR21(OH)-MM1. In the western lobe of the DR21 outflow, the thermal methanol emission appears to be “sandwiched” between vibrationally excited H2 emission, tracing the outflow shock front, and CS emission tracing the dense ambient medium. This is consistent with the idea that methanol can be enhanced in shock regions, where the outflow impinges on the surrounding dense molecular clumps, releasing methanol from dust grain ice mantles.