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

TL;DR

This study finds a consistent threshold linking water-ice presence and increased scattering at 3.6 μm in molecular clouds, suggesting ice layering on dust grains enhances their size and scattering efficiency.

Contribution

It demonstrates a common column density threshold for water-ice and scattering, indicating ice layering influences grain growth and scattering in molecular cores.

Findings

Thresholds for water-ice and scattering are similar.

Ice layering increases grain size and albedo.

Scattering efficiency correlates with water-ice abundance.

Abstract

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.