THE ASTRONOMICAL JOURNAL, 115 : 1599È1609, 1998 April 1998. The American Astronomical Society. All rights reserved. Printed in U.S.A. ( WATER MASERS IN THE CIRCUMSTELLAR ENVIRONMENTS OF YOUNG STELLAR OBJECTS LEBE  E S. GRISSOM AND BRUCE A. MEEHAN WILKING Department of Physics and Astronomy, University of Missouri at St. Louis, 8001 Natural Bridge Road, St. Louis, MO 63121 ; lebeem=newton.umsl.edu, brucew=newton.umsl.edu MARK J. CLAUSSEN National Radio Astronomy Observatory, Array Operations Center, P.O. Box O, Socorro, NM 87801 ; mclausse=nrao.edu LEE G. MUNDY Department of Astronomy, University of Maryland, College Park, MD 20742 ; lgm=astro.umd.edu AND ALWYN WOOTTEN National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA 22903 ; awootten=nrao.edu Received 1997 August 25 ; revised 1997 December 10 ABSTRACT We present a high-resolution radio and millimeter-wavelength study of Ðve low-mass young stellar objects with known water maser emission : RNO 15 FIR, Orion AÈW, L1157, B361, and L1251A. These objects are cold IRAS sources with far-infrared luminosities ranging from less than 6 to 40 Radio L _ . 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 AÈW, L1157, and L1251A ; the spectral indices of their radio emission are consistent with thermal ionized winds. High-resolution VLA observations located pairs of masers associated with these radio H 2 O 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 AÈW, the strongest maser feature was used to self-calibrate the line and continuum data, resulting in the detection of a j \ 1.3 cm contin- uum source o†set 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 identiÐed with gravitationally unbound motions expected for a stellar wind origin. Key words : circumstellar matter È ISM : jets and outÑows È masers È radio continuum È stars : preÈmain-sequence 1. INTRODUCTION Water maser emission is a common feature of deeply embedded young stellar objects (YSOs). These YSOs have Class 0 or Class I spectral energy distributions that rise steeply at far-infrared wavelengths with little or no emission at near-infrared wavelengths, indicative of extensive circum- stellar dust A recent survey of a (Lada 1991; Andre 1995). sample of Class 0/Class I sources identiÐed by the Infrared Astronomical Satellite (IRAS) has revealed that maser H 2 O activity is common in YSOs with L \ 120 with 24 of L _ , the studyÏs original 42 objects displaying highly variable maser emission et al. et al. H 2 O (Wilking 1994 ; Claussen The low to intermediate luminosity of these YSOs 1996). implies a low mass for the central object (\3 For M _ ). low-mass YSOs, unlike massive stars, maser activity appears to be concentrated within several hundred AU of the YSO Vogel, & Myers (Wootten 1989 ; Terebey, 1992 ; Chernin 1995). The presence of water maser emission from the 6 16 ] 5 23 transition at 22 GHz is well correlated with the presence of high-velocity molecular gas and appears to be another indi- cator of mass loss in YSOs Palagi, & Tofani (Felli, 1992 ; et al. This suggests that water masers arise in Wilking 1994). dense shock fronts created by the interaction of a strong stellar wind with circumstellar or ambient cloud material Hollenbach, & McKee This maser emission (Elitzur, 1989). may be produced by a combination of low-velocity, nondis- sociative shocks (C-type shocks, v \ 40 km s~1) or fast, dissociative shocks (J-type shocks) ; nondissociative shock models are more efficient in exciting maser emission and better at explaining higher frequency water maser emission & Neufeld Alternatively, models of infal- (Kaufman 1996). ling gas onto protostars have shown that water maser emis- sion in low- to intermediate-luminosity YSOs may be excited in accretion Hollenbach, & Tielens (Ceccarelli, 1996). Ultimately, high-resolution studies that reveal the posi- tions and space velocities of masers relative to the YSOs can distinguish between outÑow or infall models for maser emission. For example, VLA observations of the water masers associated with the cold YSO L1448C show that the masers are well aligned with the highly collimated molecu- lar outÑow and that their velocities are too high to arise in a Keplerian disk OutÑow excitation is also (Chernin 1995). suspected for water masers found toward two YSOs in the Ophiuchus cloud : YLW 16A and the southern component of the protoÈbinary system IRAS 16293[2422 et (Terebey al. While evidence for maser excita- 1992 ; Wootten 1989). tion via a disk/wind interaction or infall has not been found in low- to intermediate-luminosity YSOs, they have been suggested for higher luminosity YSOs. For example, SiO masers in Orion IRc2 have been modeled as arising in a rotating and expanding disk Wright, & Carls- (Plambeck, trom A disk origin for water masers has also been 1990). proposed for the luminous YSO IRAS 0038]6312 (Fiebig et al. 1996). 1599