A common column density threshold for scattering at 3.6 mum and
  water-ice in molecular clouds
  Context: Observations of scattered light in the 1-5 $\mu$m range have
revealed dust grains in molecular cores with sizes larger than commonly
inferred for the diffuse interstellar medium. It is currently unclear whether
these grains are grown within the molecular cores or are an ubiquitous
component of the interstellar medium. Aims: We investigate whether the large
grains necessary for efficient scattering at 1-5 mum are associated with the
abundance of water-ice within molecular clouds and cores. Methods: We combined
water-ice abundance measurements for sight lines through the Lupus IV molecular
cloud complex with measurements of the scattered light at 3.6 mum for the same
sight lines. Results: We find that there is a similar threshold for the cores
in emission in scattered light at 3.6 mum (tau_9.7=0.15pm0.05, A_K=0.4pm0.2 as
water-ice (tau_9.7=0.11pm0.01, A_K=0.19pm0.04) and that the scattering
efficiency increases as the relative water-ice abundance increases. The ice
layer increases the average grain size, which again strongly increases the
albedo. Conclusions: The higher scattering efficiency is partly due to layering
of ice on the dust grains. Although the layer can be relatively thin it can
enhance the scattering substantially.
