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(Abridged) Through spectrally unresolved observations of high-J COtransitions, Herschel-PACS has revealed large reservoirs of warm (300 K) andhot (700 K) molecular gas around low-mass protostars. We aim to shed light onthe excitation and origin of the CO ladder observed toward protostars, and onthe water abundance in different physical components using spectrally resolvedHerschel-HIFI data. Observations are presented of the highly excited CO lineJ=16-15 with Herschel-HIFI toward 24 low-mass protostellar objects. Thespectrally resolved profiles show two distinct velocity components: a broadcomponent with an average FWHM of 20 km/s, and a narrower component with a FWHMof 5 km/s that is often offset from the source velocity. The average rotationaltemperature over the entire profile, as measured from comparison between COJ=16-15 and 10-9 emission, is ~300 K. A radiative-transfer analysis shows thatthe average H2O/CO column-density ratio is ~0.02, suggesting a total H2Oabundance of ~2x10^-6. Two distinct velocity profiles observed in the HIFI lineprofiles suggest that the CO ladder observed with PACS consists of twoexcitation components. The warm component (300 K) is associated with the broadHIFI component, and the hot component (700 K) is associated with the offsetHIFI component. The former originates in either outflow cavity shocks or thedisk wind, and the latter in irradiated shocks. The ubiquity of the warm andhot CO components suggests that fundamental mechanisms govern the excitation ofthese components; we hypothesize that the warm component arises when H2 stopsbeing the dominant coolant. In this scenario, the hot component arises incooling molecular H2-poor gas just prior to the onset of H2 formation. Highspectral resolution observations of highly excited CO transitions uniquely shedlight on the origin of warm and hot gas in low-mass protostellar objects.

astronomy and astrophysics

Origin of warm and hot gas emission from low-mass protostars:  Herschel-HIFI observations of CO J=16-15. I. Line profiles, physical  conditions, and H2O abundance