Galactic cold cores V. Dust opacity

TL;DR

This study measures dust opacity in interstellar clouds using Herschel and near-infrared data, revealing increased opacity in dense regions likely due to grain growth, with implications for understanding star formation environments.

Contribution

It provides the first detailed analysis of submillimetre dust opacity variations across multiple Galactic cold cloud fields, highlighting grain growth in dense regions.

Findings

Median tau(250um)/tau(J) ratio is 1.6e-3, over three times the diffuse medium value.

Six nearby fields show increased submillimetre opacity up to 4e-3.

No significant variation with Galactocentric distance or Galactic height.

Abstract

The project Galactic Cold Cores has made Herschel observations of interstellar clouds where the Planck satellite survey has located cold and compact clumps. The sources range from starless clumps to protostellar cores. We examine 116 Herschel fields to estimate the submillimetre dust opacity and its variations. The submillimetre dust opacity was derived from Herschel data, and near-infrared observations of the reddening of background stars are converted into near-infrared optical depth. We studied the systematic errors affecting these parameters and used modelling to correct for the expected biases. The ratio of 250um and J band opacities is correlated with the cloud location and star formation activity. We find a median ratio of tau(250um)/tau(J)= (1.6+-0.2)*10^-3, which is more than three times the mean value in diffuse medium. Assuming a spectral index beta=1.8 instead of beta=2.0, the value would be lower by ~30%. No significant systematic variation is detected with Galactocentric distance or with Galactic height. The tau(250um)/tau(J) maps reveal six fields with clear increase of submillimetre opacity of up to tau(250um)/tau(J) ~ 4*10^-3. These are all nearby fields with spatially resolved clumps of high column density. We interpret the increase in the far-infrared opacity as a sign of grain growth in the densest and coldest regions of interstellar clouds.