Properties of Hi-GAL clumps in the inner Galaxy]{The Hi-GAL compact
  source catalogue. I. The physical properties of the clumps in the inner
  Galaxy ($-71.0^{\circ}< \ell < 67.0^{\circ}$)
  Hi-GAL is a large-scale survey of the Galactic plane, performed with Herschel
in five infrared continuum bands between 70 and 500 $\mu$m. We present a
band-merged catalogue of spatially matched sources and their properties derived
from fits to the spectral energy distributions (SEDs) and heliocentric
distances, based on the photometric catalogs presented in Molinari et al.
(2016a), covering the portion of Galactic plane $-71.0^{\circ}< \ell <
67.0^{\circ}$. The band-merged catalogue contains 100922 sources with a regular
SED, 24584 of which show a 70 $\mu$m counterpart and are thus considered
proto-stellar, while the remainder are considered starless. Thanks to this huge
number of sources, we are able to carry out a preliminary analysis of early
stages of star formation, identifying the conditions that characterise
different evolutionary phases on a statistically significant basis. We
calculate surface densities to investigate the gravitational stability of
clumps and their potential to form massive stars. We also explore evolutionary
status metrics such as the dust temperature, luminosity and bolometric
temperature, finding that these are higher in proto-stellar sources compared to
pre-stellar ones. The surface density of sources follows an increasing trend as
they evolve from pre-stellar to proto-stellar, but then it is found to decrease
again in the majority of the most evolved clumps. Finally, we study the
physical parameters of sources with respect to Galactic longitude and the
association with spiral arms, finding only minor or no differences between the
average evolutionary status of sources in the fourth and first Galactic
quadrants, or between "on-arm" and "inter-arm" positions.
