A MONTHLY SURVEY OF WATER MASERS ASSOCIATED WITH LOW-MASS STARS

We have performed a multiepoch survey of water masers toward low-lumin osity young stellar objects (YSOs). These objects were selected to be in an early phase of stellar evolution based on IRAS colors and associ ation with molecular outflows or dense cores. The sample was restricte d to YSOs in nearby clouds (d less than or equal to 450 pc) with lumin osities <120 L.. Seven new sources of maser emission have been found r esulting in a total of 22 YSOs with L <120 L, known to display maser a ctivity. We present monitoring data (at least two epochs of observatio n) for 47 sources. Four objects were monitored On a semiregular basis for more than 5 years. Based on a coherent 13 month subset of this sur vey, we estimate that the maser phase in low-mass embedded sources occ upies about one-third of the duration of the embedded phase of evoluti on. Some spectrally isolated water maser features are seen to appear a nd disappear on timescales less than about 2 months. This characterist ic timescale is consistent with shock crossing times for reasonable va lues of the shock velocity and size and is thus suggestive that the ma sers originate in the shocked gas. Some embedded sources can have long periods of inactivity (similar to 5 yr) when no masers are seen to ou r detection limit. Sources with stronger masers and more complex spect ra can maintain emission over this same time period. We determined thr eshold values of the CO mass-loss rate (4 x 10(-5) M. yr(-1)), 6 cm ra dio luminosity (0.25 kpc(2) mJy), and the IRAS in-band luminosity (25 L.) above which water masers were always detected. The isotropic lumin osity of the water masers in any given source can vary over more than 2 orders of magnitude in a period of several months. Thus, a single ep och measurement of this luminosity may provide a misleading result, es pecially if compared to other physical parameters such as the bolometr ic luminosity.