Galactic cold cores II. Herschel study of the extended dust emission around the first Planck detections

TL;DR

This study uses Herschel and Planck data to analyze dust emission in cold interstellar clouds, revealing variations in dust properties and temperature, and assessing the impact of observational uncertainties.

Contribution

It provides the first detailed analysis of dust emission properties in Planck-detected cold clouds, including opacity, spectral index, and temperature variations, using combined Herschel and Planck data.

Findings

Dust opacity varies from 0.05 to 0.2 cm^2/g, higher in dense regions.

Average dust spectral index is around 1.9-2.2, increasing in colder areas.

Line-of-sight temperature variations can explain spectral index changes without dust property alterations.

Abstract

Within the project Galactic Cold Cores we are carrying out Herschel photometric observations of cold interstellar clouds detected with the Planck satellite. The three fields observed as part of the Herschel science demonstration phase (SDP) provided the first glimpse into the nature of these sources. We examine the properties of the dust emission within the fields. We determine the dust sub-millimetre opacity, look for signs of spatial variations in the dust spectral index, and estimate how the apparent variations of the parameters could be affected by different sources of uncertainty. We use the Herschel observations where the zero point of the surface brightness scale is set with the help of the Planck satellite data. We derive the colour temperature and column density maps of the regions and determine the dust opacity by a comparison with extinction measurements. By simultaneously fitting the colour temperature and the dust spectral index values we look for spatial variations in the apparent dust properties. With a simple radiative transfer model we estimate to what extent these can be explained by line-of-sight temperature variations, without changes in the dust grain properties. The analysis of the dust emission reveals cold and dense clouds that coincide with the Planck sources and confirm those detections. The derived dust opacity varies in the range kappa(250um) ~ 0.05-0.2 cm^2/g, higher values being observed preferentially in regions of high column density. The average dust spectral index beta is ~ 1.9-2.2. There are indications that beta increases towards the coldest regions. The spectral index decreases strongly near internal heating sources but, according to radiative transfer models, this can be explained by the line-of-sight temperature variations without a change in the dust properties.