Water maser survey toward low-mass young stellar objects in the northern sky: Observational constraints on maser excitation conditions

We present the results from a multiepoch H2O maser survey toward low-mass young stellar objects using the Nobeyama 45 m telescope and the Very Large Array. Our Nobeyama survey is the first complete H2O maser survey toward known Class 0 sources in the northern sky (delta > -35 degrees). During the series of the monitoring observations, we detected the maser emission toward none of the 31 pre-protostellar cores, 15 of 30 Class 0, two of 32 Class I, and zero of nine Class II sources. From this, we conclude that Class 0 sources are favorable sites to harbor the masers: the detection rates are derived to be 39.7% for Class 0, 4.0% for Class I, and 0.0% for Class II sources taking time variation into account. In addition, we found that the H2O maser luminosities in low-mass stars are more closely related to the luminosities of 100 AU scale radio jets rather than the mechanical luminosities of large-scale CO outflows. This fact suggests that the masers are associated with the shocked regions that are impacted by neutral protostellar jets emanating from the central stars. The drastic decrease of the maser detection rate in Class I sources is likely to be caused by the dissipation of dense gas around the central objects. We base this on the fact that the radio jets are found to have similar luminosities in Class 0 and Class I. It seems difficult even for active protostellar jets to excite masers in the remaining tenuous gas around Class I sources.