An H2CO 6 cm Maser Pinpointing a Possible Circumstellar Torus in IRAS 18566+0408

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Abstract
We report observations of 6 cm, 3.6 cm, 1.3 cm, and 7 mm radio continuum, conducted with the Very Large Array, toward IRAS 18566+0408, one of the few sources known to harbor H2CO 6 cm maser emission. Our observations reveal that the emission is dominated by an ionized jet at centimeter wavelengths. Spitzer IRAC images from GLIMPSE support this interpretation, given the presence of 4.5 μm excess emission at approximately the same orientation as the centimeter continuum. The 7 mm emission is dominated by thermal dust from a flattened structure almost perpendicular to the ionized jet; thus, the 7 mm emission appears to trace a torus associated with a young massive stellar object. The H2CO 6 cm maser is coincident with the center of the torus-like structure. Our observations rule out radiative pumping via radio continuum as the excitation mechanism for the H2CO 6 cm maser in IRAS 18566+0408.
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