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Description
We present a survey of the 6_16_-5_23_ H_2_O maser transition toward a sample of 140 compact cores in infrared dark clouds using the Very Large Array. Strong (>1Jy) H_2_O 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 2x10^3^M_{sun}_, similar to the mass range in the entire sample. The H_2_O 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 H_2_O 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 H_2_O maser emission are at an evolutionary stage earlier than the protostellar phases. They are prime candidates for high-mass starless cores.
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