Evidence for a dense, cold screen towards IRAS 18507+0121

Abstract

Self-absorbed profiles has been seen towards the core containing IRAS 18507+0121, a probable high-mass protostar, in the millimeter wave lines of HCO(+) , H(13) CO(+) , CS, and C(34) S. Especially, the J=1->0 line of H(13) CO(+) is found to be stronger than that of HCO(+) . We have modeled this core in order to determine the physical conditions in it. To generate synthetic spectra from the model core, we used a multi-level, non-LTE radiative transfer code that employs a large-velocity-gradient approximation in order to estimate the level populations. It has been demonstrated that the line profiles can be explained by a collapsing hot core hidden behind a cold ( ~ 4 K) and dense (3 times 10(4) cm(-3) ) intervening, but comoving, screen or envelope. The model parameters adjusted to reproduce the observed profiles are (a) the temperature, density, and velocity fields in the model core and (b) the temperature, density, and linewidth in the screen. Comparing the observed profiles with the model spectra at both the on-center and one off-center positions, we have been able to reliably determine the physical conditions in the core.