Galactic cold cores

Abstract

Context. The project Galactic Cold Cores has carried out Herschel photometric observations of interstellar clouds where the Planck satellite survey has located cold and compact clumps. The sources represent different stages of cloud evolution from starless clumps to protostellar cores and are located in different Galactic environments. Aims. We examine this sample of 116 Herschel fields to estimate the submillimetre dust opacity and to search for variations that might be attributed to the evolutionary stage of the sources or to environmental factors, including the location within the Galaxy. Methods. 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 investigated the systematic errors affecting these parameters and used modelling to correct for the expected biases. The ratio of 250 μm and J band opacities is correlated with the Galactic location and the star formation activity. We searched for local variations in the ratio τ(250 μm)/τ(J) using the correlation plots and opacity ratio maps. Results. We find a median ratio of τ(250 μm)/τ(J) = (1.6 ± 0.2) × 10 −3 , which is more than three times the mean value reported for the diffuse medium. Assuming an opacity spectral index β = 1.8 instead of β = 2.0, the value would be lower by ∼30%. No significant systematic variation is detected with Galactocentric distance or with Galactic height. Examination of the τ(250 μm)/τ(J) maps reveals six fields with clear indications of a local increase of submillimetre opacity of up to τ(250 μm)/τ(J) ∼ 4 × 10 −3 towards the densest clumps. These are all nearby fields with spatially resolved clumps of high column density. Conclusions. We interpret the increase in the far-infrared opacity as a sign of grain growth in the densest and coldest regions of interstellar clouds.

Structure and stability in TMC-1: Analysis of NH3molecular line andHerschelcontinuum data

“…Planck Collaboration XXV (2011) examined the Taurus region and found β > 1.8 in structures with temperatures below 14 K. Juvela et al (2012) used an average value of 2 when determining the T dust and N(H 2 ) on the fields observed during the Herschel open time key programme Galactic Cold Cores. Recently Juvela et al (2015b) examined the sub-millimeter dust opacity and gave the average value of β close to 1.9 and they indicated that it can be even higher in the coldest regions. The variations of β were described in detail by Juvela et al (2015a).…”

“…We fitted the spectral energy distribution (SED) of each pixel with B ν (T dust )ν β , where B ν (T ) is the Planck-function for a black body with colour temperature T dust at ν frequency. The value of β varies between 1.8 to 2.2 in the cold dense interstellar medium (ISM) anti-correlates with temperature and correlates with column density and galactic position (Juvela et al 2015b); see Appendix A.2 for further information. In our calculations the SED was fitted to all three SPIRE data points with a fixed β = 2 spectral index and returned T dust for each pixel.…”

Spatially associated clump populations in Rosette from CO and dust maps

“…The lower limit rather represents non-coagulated dust in diffuse clouds so that it is probably not representative for Rosette. For Planck cold cores Juvela et al (2015) estimated 1.5 × 10 −25 cm 2 /H when converted to 300 µm. Taking this range, we estimate that the dust emissivity has an accuracy of 30 − 50%.…”

Herscheland SCUBA-2 observations of dust emission in a sample ofPlanckcold clumps

“…To study the variations in the submillimetre dust opacity, Juvela et al (2015b) compared the 250 µm optical depths to the near-infrared (NIR) extinctions. The typical value of τ(250µm)/τ(J) in the fields was found to be 1.6 × 10 −3 , which is more than twice the values in diffuse medium (Planck Collaboration et al 2014a,b) but consistent with other Planck studies of molecular clouds (Planck Collaboration et al 2014c) (see also, e.g.…”