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Description
For the project Galactic cold cores, Herschel photometric observations were carried out as a follow-up of cold regions of interstellar clouds previously identified with the Planck satellite. The aim of the project is to derive the physical properties of the population of cold sources and to study its connection to ongoing and future star formation. We build a catalogue of cold sources within the clouds in 116 fields observed with the Herschel PACS and SPIRE instruments. We wish to determine the general physical characteristics of the cold sources and to examine the correlations with their host cloud properties. From Herschel data, we compute colour temperature and column density maps of the fields. We estimate the distance to the target clouds and provide both uncertainties and reliability flags for the distances. The getsources multi-wavelength source extraction algorithm is employed to build a catalogue of several thousands of cold sources. Mid-infrared data are used along with a colour and position criteria to separate starless and protostellar sources. We also propose another classification method based on sub-millimetre temperature profiles. We analyse the statistical distributions of the physical properties of the source samples. We provide a catalogue of ~4000 cold sources within or near star forming clouds, most of which are located either in nearby molecular complexes (<1kpc) or in star forming regions of the nearby galactic arms (~2kpc). About 70% of the sources have a size compatible with an individual core, and 35% of those sources are likely gravitationally bound. Significant statistical differences in physical properties are found between starless and protostellar sources, in column density - dust temperature, mass - size, and mass - dust temperature diagrams. The core mass functions are very similar to those previously reported for other regions. On statistical grounds we find that gravitationally bound sources have higher background column densities (median N_bg_(H_2_)~5x10^21^cm-2) than unbound sources (median N_bg_(H_2_)~3x10^21^cm-2). These values of N_bg_(H_2_) are higher for larger dust temperature of the external layers of the parent cloud. However, only in few cases do we find clear N_bg_(H_2_) thresholds for the presence of cores. The dust temperature of cloud external layers shows clear variations with galactic location, as possibly do the source temperatures. Our data support a more complex view of star formation than in the simple idea of a column density threshold. They show a clear influence of the surrounding UV-visible radiation on how cores distribute in their host clouds, with possible variations on the Galactic scale.
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