  SWAS and Odin provided stringent upper limits on the gas phase water
abundance of dark clouds (x(H2O) < 7x10^-9). We investigate the chemistry of
water vapor in starless cores beyond the previous upper limits using the highly
improved angular resolution and sensitivity of Herschel and measure the
abundance of water vapor during evolutionary stages just preceding star
formation. High spectral resolution observations of the fundamental ortho water
(o-H2O) transition (557 GHz) were carried out with Herschel HIFI toward two
starless cores: B68, a Bok globule, and L1544, a prestellar core embedded in
the Taurus molecular cloud complex. The rms in the brightness temperature
measured for the B68 and L1544 spectra is 2.0 and 2.2 mK, respectively, in a
velocity bin of 0.59 km s^-1. The continuum level is 3.5+/-0.2 mK in B68 and
11.4+/-0.4 mK in L1544. No significant feature is detected in B68 and the 3
sigma upper limit is consistent with a column density of o-H2O N(o-H2O) <
2.5x10^13 cm^-2, or a fractional abundance x(o-H2O) < 1.3x10^-9, more than an
order of magnitude lower than the SWAS upper limit on this source. The L1544
spectrum shows an absorption feature at a 5 sigma level from which we obtain
the first value of the o-H2O column density ever measured in dark clouds:
N(o-H2O) = (8+/-4)x10^12 cm^-2. The corresponding fractional abundance is
x(o-H2O) ~ 5x10^-9 at radii > 7000 AU and ~2x10^-10 toward the center. The
radiative transfer analysis shows that this is consistent with a x(o-H2O)
profile peaking at ~10^-8, 0.1 pc away from the core center, where both
freeze-out and photodissociation are negligible. Herschel has provided the
first measurement of water vapor in dark regions. Prestellar cores such as
L1544 (with their high central densities, strong continuum, and large
envelopes) are very promising tools to finally shed light on the solid/vapor
balance of water in molecular clouds.
