  Evaporation of water ice above 100 K in the inner few 100 AU of low-mass
embedded protostars (the so-called hot core) should produce quiescent water
vapor abundances of ~10^-4 relative to H2. Observational evidence so far points
at abundances of only a few 10^-6. However, these values are based on spherical
models, which are known from interferometric studies to be inaccurate on the
relevant spatial scales. Are hot cores really that much drier than expected, or
are the low abundances an artifact of the inaccurate physical models? We
present deep velocity-resolved Herschel-HIFI spectra of the 3(12)-3(03) lines
of H2-16O and H2-18O (1097 GHz, Eup/k = 249 K) in the low-mass Class 0
protostar NGC1333 IRAS2A. A spherical radiative transfer model with a power-law
density profile is unable to reproduce both the HIFI data and existing
interferometric data on the H2-18O 3(13)-2(20) line (203 GHz, Eup/k = 204 K).
Instead, the HIFI spectra likely show optically thick emission from a hot core
with a radius of about 100 AU. The mass of the hot core is estimated from the
C18O J=9-8 and 10-9 lines. We derive a lower limit to the hot water abundance
of 2x10^-5, consistent with the theoretical predictions of ~10^-4. The revised
HDO/H2O abundance ratio is 1x10^-3, an order of magnitude lower than previously
estimated.
