Radio continuum emission associated with Class II methanol maser sources |
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| Authors: | D J van der Walt E Churchwell M J Gaylard S Goedhart |
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| Institution: | Space Research Unit, Potchefstroom University for CHE, Private Bag X6001, Potchefstroom, 2520 South Africa;Astronomy Department, University of Wisconsin, 475 N Charter Street, Madison, WI 53706;Hartebeesthoek Radio Astronomy Observatory, PO Box 433, Krugersdorp, 1740, South Africa |
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| Abstract: | Class II methanol masers are believed to be associated with high-mass star formation. Recent observations by Walsh et al. and Phillips et al. reported a very low detection rate of radio continuum emission toward a large sample of 6.7-GHz methanol masers. These results raise questions about the evolutionary phase and/or the mass range of the exciting stars of the masers. Here we report the results of a VLA search for 8.4-GHz continuum emission from the area around five Class II methanol masers, four of which were not detected by Walsh et al. at 8.6 GHz. Radio continuum emission was detected in all five fields although only two of the nine maser spot groups in the five fields were found to be superimposed on radio continuum sources that appear to be ultra-compact H ii (UCH ii ) regions. This suggests that continuum counterparts for some masers might be found in further surveys for which the sensitivity level is lower than 1 mJy beam?1 . Considering our results as well as observations from other studies of methanol masers we conclude that masers without radio continuum counterparts are most likely associated with high-mass stars in a very early evolutionary stage, either prior to the formation of a UCH ii region or when the H ii region is still optically thick at centimetre wavelengths. With one exception all maser spot groups in the five fields were found to be associated with mid-infrared objects detected in the Midcourse Space Experiment survey. |
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| Keywords: | masers circumstellar matter stars: formation stars: pre-main-sequence H ii regions radio continuum: ISM |
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