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تسجيل مستخدم جديدA Herschel study of YSO evolutionary stages and formation timelines in two fields of the Hi-GAL survey
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We present a first study of the star-forming compact dust condensations revealed by Herschel in the two 2 times 2 degr Galactic Plane fields centered at [l;b] = [30degr; 0 degr] and [l;b] = [59degr; 0 degr], 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 = 30degr field, and 444 entries for the l = 59degr 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 Lbol/Menv 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 = 59degr field have on average lower L/M, possibly outlining an overall earlier evolutionary stage with respect to the sources in the l = 30degr 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.
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