FIRST DETECTION OF WATER VAPOR IN A PRE-STELLAR CORE

Abstract

Water is a crucial molecule in molecular astrophysics as it controls much of\nthe gas/grain chemistry, including the formation and evolution of more complex\norganic molecules in ices. Pre-stellar cores provide the original reservoir of\nmaterial from which future planetary systems are built, but few observational\nconstraints exist on the formation of water and its partitioning between gas\nand ice in the densest cores. Thanks to the high sensitivity of the Herschel\nSpace Observatory, we report on the first detection of water vapor at high\nspectral resolution toward a dense cloud on the verge of star formation, the\npre-stellar core L1544. The line shows an inverse P-Cygni profile,\ncharacteristic of gravitational contraction. To reproduce the observations,\nwater vapor has to be present in the cold and dense central few thousand AU of\nL1544, where species heavier than Helium are expected to freeze-out onto dust\ngrains, and the ortho:para H2 ratio has to be around 1:1 or larger. The\nobserved amount of water vapor within the core (about 1.5x10^{-6} Msun) can be\nmaintained by Far-UV photons locally produced by the impact of galactic cosmic\nrays with H2 molecules. Such FUV photons irradiate the icy mantles, liberating\nwater wapor in the core center. Our Herschel data, combined with radiative\ntransfer and chemical/dynamical models, shed light on the interplay between gas\nand solids in dense interstellar clouds and provide the first measurement of\nthe water vapor abundance profile across the parent cloud of a future\nsolar-type star and its potential planetary system.\n