EVIDENCE FOR A DENSE, COLD SCREEN TOWARDS IRAS-18507+0121

Self-absorbed profiles has been seen towards the core containing IRAS 18507 + 0121, a probable high-mass protostar, in the millimeter wave l ines of HCO+, (HCO+)-C-13, CS, and (CS)-S-34. Especially, the J = 1 -- > 0 line of (HCO+)-C-13 is found to be stronger than that of HCO+. We have modeled this core in order to determine the physical conditions i n it. To generate synthetic spectra from the model core, are used a mu lti-level, non-LTE radiative transfer code that employs a large-veloci ty-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 (similar to 4 K) and dense (3 x 10(4) cm(-3)) intervening, but comoving, screen or envelope. The mo del parameters adjusted to reproduce the observed profiles are (a) the temperature, density, and velocity fields in the model core and (b) t he temperature, density, and linewidth in the screen. Comparing the ob served profiles with the model spectra at both the on-center and one o ff-center positions, we have been able to reliably determine the physi cal conditions in the core.