Physical Properties of Galactic Planck Cold Cores revealed by the Hi-GAL survey

Previous studies of the initial conditions of massive star formation have mainly targeted Infrared-Dark Clouds (IRDCs) toward the inner Galaxy. This is due to the fact that IRDCs were first detected in absorption against the bright mid-IR background, requiring a favourable location to be observed. By selection, IRDCs represent only a fraction of the Galactic clouds capable of forming massive stars and star clusters. Due to their low dust temperatures, IRDCs are bright in the far-IR and millimeter and thus, observations at these wavelengths have the potential to provide a complete sample of star-forming massive clouds across the Galaxy. Our aim is to identify the clouds at the initial conditions of massive star formation across the Galaxy and compare their physical properties as a function of their Galactic location. We have examined the physical properties of a homogeneous galactic cold core sample obtained with the Planck satellite across the Galactic Plane. With the use of Herschel Hi-GAL observations, we have characterized the internal structure of them. By using background-subtracted Herschel images, we have derived the H2 column density and dust temperature maps for 48 Planck clumps. Their basic physical parameters have been calculated and analyzed as a function of location within the Galaxy. These properties have also been compared with the empirical relation for massive star formation derived by Kauffmann & Pillai (2010). Most of the Planck clumps contain signs of star formation. About 25% of them are massive enough to form high mass stars. Planck clumps toward the Galactic center region show higher peak column densities and higher average dust temperatures than those of the clumps in the outer Galaxy. Although we only have seven clumps without associated YSOs, the Hi-GAL data show no apparent differences in the properties of Planck cold clumps with and without star formation.