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 $\rho$ Oph A cloud (23-49 K) located close to the luminous Herbig Be star S 1, which externally irradiates the $\rho$ 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.

arXiv comment19 pages, 9 figures, accepted for publication in ApJ

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