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