Herschel observations of the Sagittarius B2 cores: Hydrides, warm CO, and cold dust ^⋆

¹ Departamento de Astrofísica. Centro de Astrobiología. CSIC-INTA, Torrejón de Ardoz, 28850 Madrid, Spain
e-mail: etxaluzeam@cab.inta-csic.es
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

Abstract

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 T_rot ~ 50–100 K, which is associated with the extended envelope, and a hotter component at T_rot ~ 200 K and T_rot ~ 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 (T_d ≃ 20–30 K). We determined far-IR luminosities (L_FIR(M) ~ 5 × 10⁶ L_⊙ and L_FIR(N) ~ 1.1 × 10⁶ L_⊙) and total dust masses (M_d(M) ~ 2300 M_⊙ and M_d(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)/L_FIR ~ (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)/L_FIR luminosity ratios.