A radio and near-infrared study of 6.7 GHz methanol maser sources
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The 6.7 GHz 51 - 60 A+ methanol maser transition was discovered relatively recently, in 1991. The exact nature of these masers is not known to date, but it seems likely that they are closely associated with high mass stars (M ≥ 10 Mo) in their earliest stages of evolution. Since the molecular cloud is optically thin to emission at radio wavelengths, the methanol masers can provide information about conditions deep in the star formation region. Twelve southern 6.7 GHz methanol maser sources were imaged in the near infrared (NIR) at I, J, H and K bands using the 1.5-m telescope at the Cerro Tololo Interamerican Observatory (Chile). Astrometry accurate to 0.5 arcsec and photometry down to a limiting magnitude of 14 was obtained. The positions of known H 11 regions, water masers, hydroxyl masers and mid- and far-infrared objects in the region are examined in order to try to determine the nature of the methanol maser sources. Seven out of 14 methanol maser sites were found to be within 8 kAU of a NIR source with colours characteristic of a deeply embedded source. In three cases, no NIR source, H 11 region, water maser or hydroxyl maser could be found in likely association with the methanol masers, leaving the methanol maser as the only indication that star formation is taking place at these locations. An intensive programme was started in January 1999 to monitor a sample of 56 sources at 6.7 GHz using the Hartebeesthoek 26-m telescope. The observations were taken at 1-2 week intervals, with daily observations when possible if a maser was seen to be varying rapidly. It was found that the majority of the sources have a significant level of variability. In addition, nine sources were found to have periodic or quasi-periodic variations. The source G9.62+0.20E was the first such source detected in the dataset, and is the first reported instance of a periodic maser associated with a star formation region. High-resolution images were obtained of the source G9.62+0.20E during a flare in 2001 using the Very Long Baseline Array (VLBA). The maser spots increased in intensity, with no changes to their morphology or relative positions during the flare. This indicates that the flare originated in an increase in radiation beyond the maser regions.