Galactic cold cores

II. Herschel study of the extended dust emission around the first Planck detections^{⋆,⋆⋆,⋆⋆⋆}

¹ Department of Physics, PO Box 64, 00014 University of Helsinki, Finland
e-mail: mika.juvela@helsinki.fi
² CNRS, IRAP, 9 Av. colonel Roche, BP 44346, 31028 Toulouse Cedex 04, France
³ IPAC, Caltech, Pasadena, USA
⁴ IAS, Université Paris-Sud, 91405 Orsay Cedex, France
⁵ Laboratoire AIM, CEA/DSM-CNRS-Université Paris Diderot, IRFU/Service d’Astrophysique, CEA Saclay, 91191 Gif-sur-Yvette, France
⁶ Jodrell Bank Centre for Astrophysics, University of Manchester, Oxford Road, Manchester, M13 9PL, UK
⁷ European Space Agency, European Space Astronomy Centre, Villanueva de la Cañada, Madrid, Spain
⁸ Canadian Institute for Theoretical Astrophysics (CITA), 60 St. George St., Toronto ON M5S 3H8, Canada
⁹ LERMA & UMR 8112 du CNRS, Observatoire de Paris, 61 Av. de l’Observatoire, 75014 Paris, France
¹⁰ Laboratoire d’Astrophysique de Marseille, 38 rue F. Joliot-Curie, 13388 Marseille Cedex 13, France

Received: 12 October 2010
Accepted: 29 December 2010

Abstract

Context. Within the project Galacticcoldcores 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. The aim of the project is to derive the physical properties of the full cold core population revealed by Planck.

Aims. We examine the properties of the dust emission within the three fields observed during the SDP. 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.

Methods. 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.

Results. 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 κ(250   μm) ~ 0.05−0.2 cm² g^-1, higher values being observed preferentially in regions of high column density. The average dust spectral index β is  ~1.9−2.2. There are indications that β 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.