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Thermal methanol emission in the DR 21 complex
Interferometric maps: a comparison with maser emission
S. Liechti 1 and
C.M. Walmsley 2, 3
Received 22 July 1996 / Accepted 22 October 1996
We present interferometric observations with resolution of the methanol lines towards the methanol masers in the DR 21/DR 21(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 emission towards the A maser DR 21(OH)-1 from Plambeck and Menten (1990) (hereafter PM90). We do find however evidence for coincidence between the peak of the E emission in our map with the Class I masers towards DR 21-W. We conclude that methanol Class I masers are likely to originate in a region considerably smaller than our synthesised beam () and with methanol column density of order . It seems reasonable that they might form in regions of diameter 1000 AU and mass 0.01 . 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 DR 21(OH)-MM1.
In the western lobe of the DR 21 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.
Key words: ISM: molecules ISM: individual objects: DR 21; DR 21(OH) Radio lines: ISM stars: formation
Send offprint requests to: C.M. Walmsley
Online publication: June 30, 1998