A monthly survey of water masers associated with low-mass stars

We have performed a multiepoch survey of water masers toward low-luminosity young stellar objects (YSOs). These objects were selected to be in an early phase of stellar evolution based on IRAS colors and association with molecular outflows or dense cores. The sample was restricted to YSOs in nearby clouds (d less than or equal to 450 pc) with luminosities <120 L.. Seven new sources of maser emission have been found resulting in a total of 22 YSOs with L <120 L, known to display maser activity. We present monitoring data (at least two epochs of observation) 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 survey, we estimate that the maser phase in low-mass embedded sources occupies about one-third of the duration of the embedded phase of evolution. Some spectrally isolated water maser features are seen to appear and disappear on timescales less than about 2 months. This characteristic timescale is consistent with shock crossing times for reasonable values of the shock velocity and size and is thus suggestive that the masers originate in the shocked gas. Some embedded sources can have long periods of inactivity (similar to 5 yr) when no masers are seen to our detection limit. Sources with stronger masers and more complex spectra can maintain emission over this same time period. We determined threshold values of the CO mass-loss rate (4 x 10(-5) M. yr(-1)), 6 cm radio luminosity (0.25 kpc(2) mJy), and the IRAS in-band luminosity (25 L.) above which water masers were always detected. The isotropic luminosity 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 epoch measurement of this luminosity may provide a misleading result, especially if compared to other physical parameters such as the bolometric luminosity.