Water vapor toward starless cores: The Herschel view

Abstract

Aims. Previous studies by the satellites SWAS and Odin provided stringent upper limits on the gas phase water abundance of dark clouds (x(H2O) 7000 AU and similar or equal to 2 x 10(-10) toward the center. The radiative transfer analysis shows that this is consistent with a x(o-H2O) profile peaking at similar or equal to 10(-8), 0.1 pc away from the core center, where both freeze-out and photodissociation are negligible. Conclusions. Herschel has provided the first measurement of water vapor in dark regions. Column densities of o-H2O are low, but prestellar cores such as L1544 (with their high central densities, strong continuum, and large envelopes) appear to be very promising tools to finally shed light on the solid/vapor balance of water in molecular clouds and oxygen chemistry in the earliest stages of star formation