Water Maser Detections toward 70 μ m Infrared Dark Clumps at Early Star Formation Stage

Water masers provided powerful probes of shocked gas and protostellar activity during the early stages of star formation. Using Very Large Array K -band observations of the 22 GHz water line, we report the first detections of water masers toward dense cores in five 70 μ m infrared dark clumps, providing evidence for active star formation at an early evolutionary stage. We detected 10 unresolved maser spots comprising 13 velocity components, with peak flux densities of 13–499 mJy and deconvolved FWHMs of 0.51–3.54 km s −1 . Eleven components had FWHMs below 1 km s −1 , while their velocity offsets from the systemic velocities traced by the NH 3 (1, 1) main line ranged from 0.83 to 50.73 km s −1 . Four water maser spots were associated with 1.3 mm continuum emission, three of which were also associated with molecular outflows traced by CO 2–1 emission. No CO outflow signature was detected toward the remaining spots within the observed field of view, suggesting that water masers could trace deeply embedded protostellar activity whose outflows were not yet detectable in CO emission. For AGAL031, the nondetection of Stokes V emission toward the strongest maser component yields an upper limit on the line-of-sight magnetic field strength on the order of a few hundred mG. Our results showed that star formation was already active in 70 μ m infrared dark clumps and that 22 GHz water masers provided insights into early protostellar environments that remained difficult to probe with common star formation tracers.