Subarcsecond resolution observations of warm water toward three deeply embedded low-mass protostars

Context. Water is present during all stages of star formation: as ice in the cold outer parts of protostellar envelopes and dense inner regions of circumstellar disks, and as gas in the envelopes close to the protostars, in the upper layers of circumstellar disks and in regions of powerful outflows and shocks. Because of its key importance in the understanding of its origin in our own solar system, following the evolution of water all the way to the planet-forming disk is a fundamental task in research in star formation and astrochemistry. Aims. In this paper we probe the mechanism regulating the warm gas-phase water abundance in the innermost hundred AU of deeply embedded (Class 0) low-mass protostars, and investigate its chemical relationship to other molecular species during these stages. Methods. Millimeter wavelength thermal emission from the para-(H2O)-O-18 3(1,3)-2(2,0) (E-u = 203.7 K) line was imaged at high angular resolution (0 ''.75; 190 AU) with the IRAM Plateau de Bure Interferometer toward the deeply embedded low-mass protostars NGC 1333-IRAS2A and NGC 1333-IRAS4A. Results. Compact (H2O)-O-18 emission is detected toward IRAS2A and one of the components in the IRAS4A binary; in addition CH3OCH3, C2H5CN, and SO2 are detected. Extended water emission is seen toward IRAS2A, possibly associated with the outflow. Conclusions. The results complement a previous detection of the same transition toward NGC 1333-IRAS4B. The detections in all systems suggests that the presence of water on less than or similar to 100 AU scales is a common phenomenon in embedded protostars and that the non-detections of hot water with Spitzer toward the two systems studied in this paper are likely due to geometry and high extinction at mid-infrared wavelengths. We present a scenario in which the origin of the emission from warm water is in a flattened disk-like structure dominated by inward motions rather than rotation. The gas-phase water abundance varies between the sources, but is generally much lower than a canonical abundance of 10(-4), suggesting that most water (>96%) is frozen out on dust grains at these scales. The derived abundances of CH3OCH3 and SO2 relative to (H2O)-O-18 are comparable for all sources pointing toward similar chemical processes at work. In contrast, the C2H5CN abundance relative to (H2O)-O-18 is significantly lower in IRAS2A, which could be due to different chemistry in the sources.