WATER MASERS ASSOCIATED WITH LOW-MASS STARS - A 13 MONTH MONITORING SURVEY

We report the results of a 13 month survey of embedded young stellar o bjects (YSOs) for H2O masers using the Haystack 37 m telescope. YSOs w ere selected to be in an early phase of evolution based on IRAS colors and association with molecular outflows or dense cores. The sample wa s restricted to YSOs in nearby clouds (d < 450 pc) with luminosities < 120 L.. Six new sources of water maser emission have been found, rais ing the total number of maser sources in this luminosity range to 20. Three sources (L1448C, L1534B, and VLA 1623) have luminosities less th an 3 L.. Of the YSOs monitored on a regular basis, we estimate that ma ser activity occupies at least one-third of the duration of the embedd ed state. All sources with L > 25 L. displayed some maser activity. Al though the maser strength of a given source can vary by two orders of magnitude, there is a general correlation between maser strength and Y SO luminosity. For YSOs with L < 25 L., no single property, such as th e CO mass-loss rate or luminosity at lambda = 6 cm, successfully predi cts maser activity. We propose that other factors, such as the inclina tion of the outflow axis, may be important in enhancing the detectabil ity of the masers in lower luminosity sources.