Herschel observations of the Sgr B2 cores: Hydrides, warm CO, and cold
  dust
  Sagittarius B2 (Sgr B2) is one of the most massive and luminous star-forming
regions in the Galaxy and shows chemical and physical conditions similar to
those in distant extragalactic starbursts. We present large-scale far-IR/submm
photometric images and spectroscopic maps taken with the PACS and SPIRE
instruments onboard Herschel. The spectra towards the Sgr B2 star-forming
cores, B2(M) and B2(N), are characterized by strong CO line emission, emission
lines from high-density tracers (HCN, HCO+, and H2S), [N II] 205 um emission
from ionized gas, and absorption lines from hydride molecules (OH+, H2O+, H2O,
CH+, CH, SH+, HF, NH, NH2, and NH3). The rotational population diagrams of CO
suggest the presence of two gas temperature components: an extended warm
component, which is associated with the extended envelope, and a hotter
component, which is seen towards the B2(M) and B2(N) cores. As observed in
other Galactic Center clouds, the gas temperatures are significantly higher
than the dust temperatures inferred from photometric images. We determined
far-IR and total dust masses in the cores. Non-local thermodynamic equilibrium
models of the CO excitation were used to constrain the averaged gas density in
the cores. A uniform luminosity ratio is measured along the extended envelope,
suggesting that the same mechanism dominates the heating of the molecular gas
at large scales. The detection of high-density molecular tracers and of strong
[N II] 205 um line emission towards the cores suggests that their morphology
must be clumpy to allow UV radiation to escape from the inner HII regions.
Together with shocks, the strong UV radiation field is likely responsible for
the heating of the hot CO component. At larger scales, photodissociation
regions models can explain both the observed CO line ratios and the uniform
L(CO)/LFIR luminosity ratios.
