CO Spectral Line Energy Distributions in Galactic Sources: Empirical
  Interpretation of Extragalactic Observations
  The relative populations in rotational transitions of CO can be useful for
inferring gas conditions and excitation mechanisms at work in the interstellar
medium. We present CO emission lines from rotational transitions observed with
Herschel/HIFI in the star-forming cores Orion S, Orion KL, Sgr B2(M), and W49N.
Integrated line fluxes from these observations are combined with those from
Herschel/PACS observations of the same sources to construct CO spectral line
energy distributions (SLEDs) from $5\leq J_u\leq48$. These CO SLEDs are
compared to those reported in other galaxies, with the intention of empirically
determining which mechanisms dominate excitation in such systems. We find that
CO SLEDs in Galactic star-forming cores cannot be used to reproduce those
observed in other galaxies, although the discrepancies arise primarily as a
result of beam filling factors. The much larger regions sampled by the Herschel
beams at distances of several Mpc contain significant amounts of cooler gas
which dominate the extragalactic CO SLEDs, in contrast to observations of
Galactic star-forming regions which are focused specifically on cores
containing primarily hot molecular gas.
