EXTERNALLY HEATED PROTOSTELLAR CORES IN THE OPHIUCHUS STAR-FORMING REGION*

Abstract

We present APEX 218 GHz observations of molecular emission in a complete sample of embedded protostars in the Ophiuchus star-forming region. To study the physical properties of the cores, we calculate H 2 CO and c -C 3 H 2 rotational temperatures, both of which are good tracers of the kinetic temperature of the molecular gas. We find that the H 2 CO temperatures range between 16 K and 124 K, with the highest H 2 CO temperatures toward the hot corino source IRAS 16293-2422 (69–124 K) and the sources in the ρ  Oph A cloud (23–49 K) located close to the luminous Herbig Be star S1, which externally irradiates the ρ  Oph A cores. On the other hand, the c -C 3 H 2 rotational temperature is consistently low (7–17 K) in all sources. Our results indicate that the c -C 3 H 2 emission is primarily tracing more shielded parts of the envelope whereas the H 2 CO emission (at the angular scale of the APEX beam; 3600 au in Ophiuchus) mainly traces the outer irradiated envelopes, apart from in IRAS 16293-2422, where the hot corino emission dominates. In some sources, a secondary velocity component is also seen, possibly tracing the molecular outflow.