Herschel observations of the Sagittarius B2 cores: Hydrides, warm CO, and cold dust ⋆
Departamento de Astrofísica. Centro de Astrobiología.
CSIC-INTA, Torrejón de
2 RAL Space, Rutherford Appleton Laboratory, Oxfordshire, OX11 0QX, UK
3 Institute for Space Imaging Science, Department of Physics & Astronomy, University of Lethbridge, AB T1K3M4 Lethbridge, Canada
4 Spitzer Science Center, 91125 Pasadena, USA
5 INAF-IFSI, 00133 Roma, Italy
6 Dept. of Physics & Astronomy, University College London,Gower Street, WC1E 6 BT London, UK
7 Commissariat À l’Énergie Atomique, Service d’Astrophysique, Saclay, 91191 Gif-sur-Yvette, France
8 CASA, University of Colorado, Boulder, 80309, USA
Received: 7 February 2013
Accepted: 16 June 2013
Context. Sagittarius B2 is one of the most massive and luminous star-forming regions in the Galaxy and shows a very rich chemistry and physical conditions similar to those in much more distant extragalactic starbursts.
Aims. We present large-scale far-infrared/submillimeter photometric images and broadband spectroscopic maps taken with the PACS and SPIRE instruments onboard Herschel.
Methods. High angular resolution dust images (complemented with Spitzer MIPS 24 μm images) as well as atomic and molecular spectral maps were made and analyzed in order to constrain the dust properties, the gas physical conditions, and the chemical content of this unique region.
Results. The spectra towards the Sagittarius B2 star-forming cores, B2(M) and B2(N), are characterized by strong CO line emission (from J = 4 to 16), emission lines from high-density tracers (HCN, HCO+, and H2S), [N ii] 205μm emission from ionized gas, and a large number of absorption lines from light hydride molecules (OH+, H2O+, H2O, CH+, CH, SH+, HF, NH, NH2, and NH3). The rotational population diagrams of CO suggest the presence of two different gas temperature components: an extended warm component with Trot ~ 50–100 K, which is associated with the extended envelope, and a hotter component at Trot ~ 200 K and Trot ~ 300 K, which is only seen towards the B2(M) and B2(N) cores, respectively. As observed in other Galactic center clouds, such gas temperatures are significantly higher than the dust temperatures inferred from photometric images (Td ≃ 20–30 K). We determined far-IR luminosities (LFIR(M) ~ 5 × 106 L⊙ and LFIR(N) ~ 1.1 × 106 L⊙) and total dust masses (Md(M) ~ 2300 M⊙ and Md(N) ~ 2500 M⊙) in the cores. Non-local thermodynamic equilibrium models of the CO excitation were used to constrain the averaged gas density (n(H2) ~ 106 cm-3) in the cores (i.e., similar or lower than the critical densities for collisional thermalization of mid- and high-J CO levels). A uniform luminosity ratio, L(CO)/LFIR ~ (1−3) × 10-4, is measured along the extended envelope, suggesting that the same mechanism dominates the heating of the molecular gas at large scales.
Conclusions. Sgr B2 shows extended emission from warm CO gas and cold dust, whereas only the cores show a hotter CO component. The detection of high-density molecular tracers and of strong [N ii] 205 μm line emission towards the cores suggests that their morphology must be clumpy to allow UV radiation to escape from the inner H ii 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.
Key words: dust, extinction / Galaxy: center / infrared: ISM / ISM: individual objects: Sagittarius B2 / ISM: lines and bands / ISM: molecules
© ESO, 2013