A Herschel study of YSO evolutionary stages and formation timelines in two fields of the Hi-GAL survey

We present a first study of the star-forming compact dust condensations revealed by Herschel in the two 2 degrees x2 degrees Galactic Plane fields centered at [l, b] = [30 degrees, 0 degrees] and [l, b] = [59 degrees, 0 degrees], respectively, and observed during the science demonstration phase for the Herschel Infrared GALactic plane survey (Hi-GAL) key-project. Compact source catalogs extracted for the two fields in the five Hi-GAL bands (70, 160, 250, 350 and 500 mu m) were merged based on simple criteria of positional association and spectral energy distribution (SED) consistency into a final catalog which contains only coherent SEDs with counterparts in at least three adjacent Herschel bands. These final source lists contain 528 entries for the l = 30 degrees field, and 444 entries for the l = 59 degrees field. The SED coverage has been augmented with ancillary data at 24 mu m and 1.1 mm. SED modeling for the subset of 318 and 101 sources (in the two fields, respectively) for which the distance is known was carried out using both a structured star/disk/envelope radiative transfer model and a simple isothermal grey-body. Global parameters like mass, luminosity, temperature and dust properties have been estimated. The L(bo)l/M-env ratio spans four orders of magnitudes from values compatible with the pre-protostellar phase to embedded massive zero-age main sequence stars. Sources in the l = 59 degrees field have on average lower L/M, possibly outlining an overall earlier evolutionary stage with respect to the sources in the l = 30 degrees field. Many of these cores are actively forming high-mass stars, although the estimated core surface densities appear to be an order of magnitude below the 1 g cm(-2) critical threshold for high-mass star formation.