Astronomy and Astrophysics – Astronomy
Scientific paper
Apr 2000
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2000mnras.313..461r&link_type=abstract
Monthly Notices of the Royal Astronomical Society, Volume 313, Issue 3, pp. 461-468.
Astronomy and Astrophysics
Astronomy
24
Stars: Formation, Ism: Clouds, Ism: Jets And Outflows, Ism: Molecules
Scientific paper
Anomalous molecular line profile shapes are the strongest indicators of the presence of the infall of gas that is associated with star formation. Such profiles are seen for well-known tracers, such as HCO+, CS and H2CO. In certain cases, optically thick emission lines with appropriate excitation criteria may possess the asymmetric double-peaked profiles that are characteristic of infall. However, recent interpretations of the HCO+ infall profile observed towards the protostellar infall candidate B335 have revealed a significant discrepancy between the inferred overall column density of the molecule and that which is predicted by standard dark cloud chemical modelling. This paper presents a model for the source of the HCO+ emission excess. Observations have shown that, in low-mass star-forming regions, the collapse process is invariably accompanied by the presence of collimated outflows; we therefore propose the presence of an interface region around the outflow in which the chemistry is enriched by the action of jets. This hypothesis suggests that the line profiles of HCO+, as well as other molecular species, may require a more complex interpretation than can be provided by simple, chemically quiescent, spherically symmetric infall models. The enhancement of HCO+ depends primarily on the presence of a shock-generated radiation field in the interface. Plausible estimates of the radiation intensity imply molecular abundances that are consistent with those observed. Further, high-resolution observations of an infall-outflow source show HCO+ emission morphology that is consistent with that predicted by this model.
Rawlings M. C. J.
Taylor S. D.
Williams David. A.
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