Water Ice Deposition and Growth in Molecular Clouds
  In interstellar clouds the deposition of water ice onto grains only occurs at
visual extinctions above some threshold value A_th. At extinctions greater than
A_th there is a (near-linear) correlation between the inferred column density
of the water ice and A_V. For individual cloud complexes such as Taurus,
Serpens and Rho-Ophiuchi, A_th and the gradients of the correlation are very
similar along all lines of sight. We have investigated the origin of this
phenomenon, with careful consideration of the various possible mechanisms that
may be involved and have applied a full chemical model to analyse the
behaviours and sensitivities in quiescent molecular clouds. Our key results
are: (i) the ubiquity of the phenomenon points to a common cause, so that the
lines of sight probe regions with similar, advanced, chemical and dynamical
evolution, (ii) for Taurus and Serpens; A_th and the slope of the correlation
can be explained as resulting from the balance of freeze-out of oxygen atoms
and photodesorption of H2O molecules. No other mechanism can satisfactorily
explain the phenomenon, (iii) A_th depends on the local density, suggesting
that there is a correlation between local volume density and column density,
(iv) the different values of A_th for Taurus and Serpens are probably due to
variations in the local mean radiation field strength, (v) most ice is accreted
onto grains that are initially very small (<0.01 microns), and (vi) the very
high value of A_th observed in Rho-Ophiuchi cannot be explained in the same
way, unless there is complex microstructure and/or a modification to the
extinction characteristics.
