The Apparent Sizes of the 62–61E‐Type Methanol Masers in OMC‐1

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Abstract
The 62-61 E transition of CH3OH has been observed toward the Orion KL region with an angular resolution of 007 and frequency resolution of 12 kHz (0.15 km s-1) at two epochs, 1 year apart. The observations have a sensitivity of 1 Jy beam-1 (corresponding to a main beam brightness temperature of 4 × 105 K). Only the brightest masers previously measured with 3'' resolution are detected; these are resolved into several bright features. The brightness temperatures of the detected masers range from 2 × 106 to 3 × 107 K, with apparent sizes in the range 22-56 AU. From the trends of radial velocity versus position, the observed line widths of the masers may be explained as blends of individual narrower features, each with line widths less than the frequency resolution of these measurements. The intensities of the masers increase with decreasing apparent size. This is interpreted as evidence that the excitation process plays a larger role than path length in determining maser intensity; our measurement of narrow line widths supports this interpretation. The CH3OH masers are probably collisionally pumped and formed in very turbulent regions. The kinetic temperatures are ≈ 100 K, corresponding to sound speeds of order 2 km s-1. From this, intensity variations are expected on timescales of years, or longer. At both epochs the majority of the maser features are located in the same relative positions with approximately the same flux densities. However, some of the masers appear to vary on timescales of a year. Variations in intensity of the strongest maser feature may have been detected over a year's time. Further, two weak maser features that appeared in 1990 were not detected in 1991.