IRAS 23385+6053: A candidate protostellar massive object

We present the results of a multi-line and continuum study towards the source IRAS 23385+6053 performed with the IRAM-30 m telescope, the Plateau de Bure Interferometer, the Very Large Array Interferometer and the James Clerk Maxwell Telescope. We have obtained single-dish maps in the C18O (1-0), C17O (1-0) and (2-1) rotational lines, interferometric maps in the CH3C2H (13-12) line, NH3(1,1) and (2,2) inversion transitions, and single-pointing observations of the CH3C2H (6-5), (8-7) and (13-12) rotational lines. The new results confirm our earlier findings, namely that IRAS 23385+6053 is a good candidate high-mass protostellar object, precursor of an ultracompact HII region. The source is roughly composed of two regions: a molecular core ∼0.03 / 0.04 pc in size, with a temperature of ∼40 K and an H2 volume density of the order of 107 cm-3, and an extended halo of diameter ≤0.4 pc, with an average kinetic temperature of ∼15 K and H2 volume density of the order of 105 cm-3. The core temperature is much smaller than what is typically found in molecular cores of the same diameter surrounding massive ZAMS stars. From the continuum spectrum we deduce that the core luminosity is between 150 and 1.6 × 104 L⊙, and we believe that the upper limit is near the ``true'' source luminosity. Moreover, by comparing the H2 volume density obtained at different radii from the IRAS source, we find that the halo has a density profile of the type nH2∝ r-2.3. This suggests that the source is gravitationally unstable. The latter hypothesis is also supported by a low virial-to-gas mass ratio (MVIR/Mgas ≤ 0.3). Finally, we demonstrate that the temperature at the core surface is consistent with a core luminosity of 103 L⊙ and conclude that we might be observing a protostar still accreting material from its parental cloud, the mass of which is at present ∼6 M⊙.