Water masers associated with infrared dark cloud cores

We present a survey of the 6(16)-5(23) H2O maser transition toward a sample of 140 compact cores in infrared dark clouds using the Very Large Array. Strong (> 1 Jy) H2O maser emission was found associated with 17 cores, indicative of star formation in these cores. We infer that the cores with H2O masers have embedded protostars. Cores associated with maser emission have masses of 12 to M-circle dot, similar to the mass range in the entire sample. The H2O maser detection rate (12%) toward the compact, cold cores is much lower than that toward high-mass protostellar objects and ultracompact H (II) regions. The detection rate of H2O masers is significantly higher for higher mass cores than for lower mass cores. We suggest that some of the most massive infrared dark cloud cores without H2O maser emission are at an evolutionary stage earlier than the protostellar phases. They are prime candidates for high-mass starless cores.