WATER MASERS IN THE CIRCUMSTELLAR ENVIRONMENTS OF YOUNG STELLAR OBJECTS

We present a high-resolution radio and millimeter-wavelength study of five low-mass young stellar objects with known water maser emission: R NO 15 FIR, Orion A-W, L1157, B361, and L1251A. These objects are cold IRAS sources with far-infrared luminosities ranging from less than 6 t o 40 L-circle dot. Radio continuum observations are used to locate pre cisely the young stellar object(s) responsible for the far-infrared em ission and to investigate their relationship to the water masers and t racers of their stellar winds. Compact radio continuum emission was de tected within the IRAS error ellipse for Orion A-W, L1157, and L1251A; the spectral indices of their radio emission are consistent with ther mal ionized winds. High-resolution VLA H2O observations located pairs of masers associated with these radio sources within projected distanc es of 50, 235, and 150 AU, respectively, clearly placing these masers in the circumstellar environments of the young stellar objects. In Ori on A-W, the strongest maser feature was used to self-calibrate the lin e and continuum data, resulting in the detection of a lambda = 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 m otions expected for a stellar wind origin.