2008A&A...487.1119M

The formation of massive stars goes through phases that remain heavily obscured until the object is well along on the main sequence. The identification of massive YSOs in different evolutionary phases is therefore particularly difficult, and requires a statistical approach with large samples of candidate objects to determine the observational signatures of these different phases. A mandatory activity in this context is the identification and characterisation of all the phases that a massive forming YSO undergoes. It is of particular interest to verify the observability of the phase in which the object is rapidly accreting while not yet igniting the fusion of hydrogen that marks the arrival on the ZAMS. One of the candidate prototypical objects for this phase is Mol160/IRAS23385+6053, which has been the subject of detailed studies that confirmed and strengthened the possibility that this massive YSO may be in a pre-hot core stage. We further investigate this issue by means of Spitzer imaging and spectroscopy in the 5-70 µm range. The dense core of Mol160/IRAS23385+6053, which up to now had only been detected at submillimeter and millimeter wavelengths, with only upper limits below 20 µm, has been revealed for the first time at 24 and 70 µm by Spitzer. These observations confirm the earlier assumptions that this object is dominant at far-IR wavelengths. The complete 24 µm-3.4 mm continuum cannot be fitted with a standard model of a zero-age main-sequence (ZAMS) star embedded in an envelope. A simple greybody fit yields a mass of 220 M_☉. The luminosity is slightly in excess of 3000 L_☉, which is a factor of 5 less than previous estimates when only IRAS fluxes were available between 20 and 100 µm. The source is under-luminous by the same factor with respect to UCHii regions or hot-cores of similar circumstellar mass, and simple models show that this is compatible with an earlier evolutionary stage. Spectroscopy between 5-40 µm shows that the physical conditions are typical of a photo-dissociated or photo-ionized region. The required UV illumination can be provided by some of the other sources revealed at λ≤24 µm in the same star-forming region, that can be plausibly modeled as moderately embedded intermediate-mass ZAMS stars. Our results strengthen the suggestion that the central core in Mol160/IRAS23385+6053 is a massive YSO actively accreting from its circumstellar envelope and that it has not yet begun hydrogen fusion.