Water Masers in the Circumstellar Environments of Young Stellar Objects

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Abstract
We present a high-resolution radio and millimeter-wavelength study of five low-mass young stellar objects with known water maser emission: RNO 15 FIR, Orion A–W, L1157, B361, and L1251A. These objects are cold IRAS sources with far-infrared luminosities ranging from less than 6 to 40 L⊙. Radio continuum observations are used to locate precisely the young stellar object(s) responsible for the far-infrared emission and to investigate their relationship to the water masers and tracers of their stellar winds. Compact radio continuum emission was detected within the IRAS error ellipse for Orion A–W, L1157, and L1251A; the spectral indices of their radio emission are consistent with thermal ionized winds. High-resolution VLA H2O observations located pairs of masers associated with these radio sources within projected distances of 50, 235, and 150 AU, respectively, clearly placing these masers in the circumstellar environments of the young stellar objects. In Orion A–W, the strongest maser feature was used to self-calibrate the line and continuum data, resulting in the detection of a λ = 1.3 cm continuum source offset 50 ± 17 AU from the strongest maser. In part because of the small separations of the masers and continuum sources, none of the masers could be identified with gravitationally unbound motions expected for a stellar wind origin.

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