2009ApJ...694...29O

We present a model aimed to reproduce the observed spectral energy distribution (SED) as well as the ammonia line emission of the G31.41+0.31 hot core. The hot core is modeled as an infalling envelope onto a massive star that is undergoing an intense accretion phase. We assume an envelope with a density and velocity structure resulting from the dynamical collapse of a singular logatropic sphere. The stellar and envelope physical properties are determined by fitting the observed SED. From these physical conditions, the emerging ammonia line emission is calculated and compared with subarcsecond resolution VLA data of the (4,4) transition taken from the literature. The only free parameter in this line fitting is the ammonia abundance. The observed intensities of the main and satellite ammonia (4,4) lines and their spatial distribution can be well reproduced provided the steep increase of the gas-phase ammonia abundance in the hotter (>100 K), inner regions of the core produced by the sublimation of icy mantles where ammonia molecules are trapped is taken into account. The model predictions for the (2,2), (4,4), and (5,5) transitions, obtained with the same set of parameters, are also reasonably in agreement, given the observational uncertainties, with the single-dish spectra of the region available in the literature. The best fit is obtained for a model with a central star of ∼25M_☉, a mass accretion rate of ∼3x10^–3 M_☉/yr, and a total luminosity of ∼2x10⁵ L_☉. The outer radius of the envelope is 30,000 AU, where kinetic temperatures as high as ∼40 K are reached. The gas-phase ammonia abundance ranges from ∼2x10^–8 in the outer region to ∼3x10^–6 in the inner region. To our knowledge, this is the first time that the dust and molecular line data of a hot molecular core, including subarcsecond resolution data that spatially resolve the structure of the core, have been simultaneously explained by a detailed, physically self-consistent model. This modeling shows that hot, massive protostars are able to excite high excitation ammonia transitions up to the outer edge (∼ 30,000 AU) of the large scale infalling envelopes.