The HIFI spectral survey of AFGL2591 (CHESS). I. Highly excited linear
  rotor molecules in the high-mass protostellar envelope
  We aim to reveal the gas energetics in the circumstellar environment of the
prototypical high-mass protostellar object AFGL2591 using space-based
far-infrared observations of linear rotor molecules.
  Rotational spectral line signatures of CO, HCO+, CS, HCN and HNC from a
490-1240 GHz survey with Herschel/HIFI, complemented by ground-based JCMT and
IRAM 30m spectra, cover transitions with E(up)/k between 5 and ~300 K (750K for
12C16O, using selected frequency settings up to 1850 GHz). The resolved
spectral line profiles are used to separate and study various kinematic
components.
  The line profiles show two emission components, the widest and bluest of
which is attributed to an approaching outflow and the other to the envelope. We
find evidence for progressively more redshifted and wider line profiles from
the envelope gas with increasing energy level, qualitatively explained by
residual outflow contribution picked up in the systematically decreasing beam
size. Integrated line intensities for each species decrease as E(up)/k
increases from <50 to 700K. We constrain the following: n(H2)~10^5-10^6 cm^-3
and T~60-200K for the outflow gas; T=9-17K and N(H2)~3x10^21 cm^-2 for a known
foreground absorption cloud; N(H2)<10^19 cm^-2 for a second foreground
component.
  Our spherical envelope radiative transfer model systematically underproduces
observed line emission at E(up)/k > 150 K for all species. This indicates that
warm gas should be added to the model and that the model's geometry should
provide low optical depth pathways for line emission from this warm gas to
escape, for example in the form of UV heated outflow cavity walls viewed at a
favorable inclination angle. Physical and chemical conditions derived for the
outflow gas are similar to those in the protostellar envelope, possibly
indicating that the modest velocity (<10 km/s) outflow component consists of
recently swept-up gas.
