A common column density threshold for scattering at 3.6 μ m and water-ice in molecular clouds

Context. Observations of scattered light in the 1-5 μm 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 μm 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 μm 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 μm (τ9.7 = 0.15 ± 0.05, AK = 0.4 ± 0.2) as water-ice (τ9.7 = 0.11 ± 0.01, AK = 0.19 ± 0.04) 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.