The most evolved sources in the Hi-GAL survey

Abstract

Far-infrared and submillimetre surveys as the Herschel Galactic Plane Infrared Survey (Hi-GAL) represent an irreplaceable knowledge base about early phases of star formation, permitting statistical analysis based on thousands of Galaxy-wide distributed sources. Those with a regular spectral energy distribution in the Herschel wavelength range 70-500 $\mu$m span a variety of evolutionary stages, from quiescent to star forming clumps and, within the latter class, from mid-infrared dark clumps to sources appearing very bright also at shorter wavelengths (e.g. Spitzer 24 $\mu$m). A fraction of these clumps hosts the formation of high mass stars, which are expected to reach the zero-age main sequence and to develop a HII region in their surroundings while they are still embedded in their parental large-scale dusty envelope. This paper aims at selecting and studying in detail a robust sample of Hi-GAL clumps supposed to be candidate to host a HII region in their interior. They are expected to be the most evolved sources in the Hi-GAL catalogue. The Galactic locations and the physical properties (temperature, mass, bolometric luminosity and temperature, and surface density) of these sources are discussed here. The large number (1199) of selected sources constitutes an important starting point for planning further interferometric programs, aimed at resolving possible cores hosting a young high-mass star.