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We present far-IR (57-196 mu) spectra of 21 protostars in the Orion molecularclouds, obtained with the Photodetector Array Camera and Spectrometer (PACS)onboard the Herschel Space observatory, as part of the Herschel Orion ProtostarSurvey (HOPS) program. We analyzed the CO emission lines (J_up = 14-46) in thePACS spectra, extracted within a projected distance of <= 2000 AU centered onthe protostar. The total luminosity of the CO lines observed with PACS (L(CO))is found to increase with increasing L_bol. The CO rotational temperatureimplied by the line ratios increases with J, and at least 3-4 rotationaltemperature components are required to fit the observed rotational diagram. Therotational temperature components are remarkably invariant between protostarsand show no dependence on L_bol, T_bol or envelope density, implying that ifthe emitting gas is in LTE, the CO emission must arise in multiple temperaturecomponents that remain independent of L_bol over two orders of magnitudes. Theobserved CO emission can also be modeled as arising from a single temperaturegas component or from a medium with a power-law temperature distribution; bothof these require sub-thermally excited molecular gas at low densities (n(H_2)<= 10^6 cm^-3) and high temperatures (T >= 2000 K). Our results suggest thatthe contribution from PDRs along the envelope cavity walls is unlikely to bethe dominant component of the CO emission observed with PACS. Instead, the"universality" of the rotational temperatures and the observed correlationbetween L(CO) and L_bol can most easily be explained if the observed COemission originates in shock-heated, hot (T >= 2000 K), sub-thermally excited(n(H_2) <= 10^6 cm^-3) molecular gas. Post-shock gas at these densities is morelikely to be found within the outflow cavities along the molecular outflow oralong the cavity walls at radii >= several 100-1000 AU.

Herschel/PACS Spectroscopic Survey of Protostars in Orion: The Origin of Far-Infrared CO Emission