Herschel-HIFI observations of high-J CO and isotopologues in
  star-forming regions: from low- to high-mass
  Context; Our understanding of the star formation process has traditionally
been confined to certain mass or luminosity boundaries because most studies
focus only on low-, intermediate- or high-mass star-forming regions. As part of
the "Water In Star-forming regions with Herschel" (WISH) key program, water and
other important molecules, such as CO and OH, have been observed in 51 embedded
young stellar objects (YSOs). The studied sample covers a range of luminosities
from <1 to >10^5 L_sol. Aims; We analyse the CO line emission towards a large
sample of protostars in terms of both line intensities and profiles. Methods;
Herschel-HIFI spectra of the 12CO 10-9, 13CO 10-9 and C18O 5-4, 9-8 and 10-9
lines are analysed for a sample of 51 YSOs. In addition, JCMT spectra of 12CO
3-2 and C18O 3-2 extend this analysis to cooler gas components. Results; All
observed CO and isotopologue spectra show a strong linear correlation between
the logarithms of the line and bolometric luminosities across six orders of
magnitude on both axes. This suggests that the high-J CO lines primarily trace
the amount of dense gas associated with YSOs. This relation can be extended to
larger (extragalactic) scales. The majority of the detected 12CO line profiles
can be decomposed into a broad and a narrow Gaussian component, while the C18O
spectra are mainly fitted with a single Gaussian. A broadening of the line
profile is also observed from pre-stellar cores to embedded protostars, which
is due mostly to non-thermal motions (turbulence/infall). The widths of the
broad 12CO 3-2 and 10-9 velocity components correlate with those of the narrow
C18O 9-8 profiles, suggesting that the entrained outflowing gas and envelope
motions are related independent of the mass of the protostar. These results
indicate that physical processes in protostellar envelopes have similar
characteristics across the studied luminosity range.
