VLBA Observations of 12 GHz Methanol Masers toward W3(OH)

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Abstract
We have conducted VLBA observations of the 12 GHz CH3OH masers toward the ultracompact H II region W3(OH). The angular resolution of ≈1 mas is sufficient to resolve most of the maser spots and determine their sizes and brightness temperatures. The FWHM sizes of the spots are found from ≤2 to 10 AU with brightness temperatures between 109 and 6 × 1011 K. The actual brightness temperatures may be higher (by a factor ≤10) than the measured values, because most of the spots have spectral profiles unresolved with our velocity resolution (0.28 km s-1). Our data also suggest the presence ofa spatially extended component of maser emission with size ≥20 mas and brightness temperature≤109 K. The comparison of our data with previous 6.7 GHz VLBI observations shows that there is a good spatial (≤10 mas) and velocity (within 0.3 km s-1) correspondence between numerous 12 and 6.7 GHz CH3OH spots. Comparing our maps with those of Menten et al. yields relative proper motions of a few km s-1 in magnitude. A "linear structure" of approximately 200 AU in size (P.A. = 141°) is found in the northern methanol maser clump, which also shows a regular trend of radial velocities with projected positions (a velocity gradient of 2400 km s-1 pc-1).

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