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

Etxaluze, M. and Goicoechea, J. R. and Cernicharo, J. and Polehampton, E. T. and Noriega-Crespo, A. and Molinari, S. and Swinyard, B. M. and Wu, R. and Bally, J. (2013) Herschel observations of the Sagittarius B2 cores: Hydrides, warm CO, and cold dust. Astronomy and Astrophysics, 556 . Art. No. A137. ISSN 0004-6361. doi:10.1051/0004-6361/201321258.

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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 H_2S), [N ii] 205μm emission from ionized gas, and a large number of absorption lines from light hydride molecules (OH^+, H_2O^+, H_2O, CH^+, CH, SH^+, HF, NH, NH^2, and NH_3). 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 × 10^6 L_⊙ and L_(FIR)(N) ~ 1.1 × 10^6 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(H_2) ~ 10^6 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.

Item Type:Article
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URLURL TypeDescription Paper
Noriega-Crespo, A.0000-0002-6296-8960
Bally, J.0000-0001-8135-6612
Additional Information:© 2013 ESO. Article published by EDP Sciences. Received 7 February 2013. Accepted 16 June 2013. Published online 08 August 2013. Published online 08 August 2013. We are very grateful to the referee, Dr. Paul Goldsmith, for the very valuable comments and suggestions that helped to improve the clarity and the quality of this paper. We thank the HiGAL consortium for releasing the Sgr B2 data. We thank ASTROMADRID for funding support through the grant S2009ESP-1496, the consolider programme ASTROMOL: CSD2009-00038 and the Spanish MINECO (grants AYA2009-07304 and AYA2012-32032). HIPE is a joint development by the Herschel Science Ground Segment Consortium consisting of ESA, the NASA Herschel Science Center, and the HIFI, PACS, and SPIRE consortia. SPIRE has been developed by a consortium of institutes led by Cardiff University (UK) and including University of Lethbridge (Canada); NAOC (China); CEA, LAM (France); IFSI, Univ. Padua (Italy); IAC (Spain); Stockholm Observatory (Sweden); Imperial College London, RAL, UCL-MSSL, UKATC, Univ. Sussex (UK); and Caltech, JPL, NHSC, Univ. Colorado (USA). This development has been supported by national funding agencies: CSA (Canada); NAOC (China); CEA, CNES, CNRS (France); ASI (Italy); MCINN (Spain); SNSB (Sweden); STFC (UK); and NASA (USA). Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.
Funding AgencyGrant Number
Spanish MINECOAYA2009-07304
Spanish MINECOAYA2012-32032
Subject Keywords: dust, extinction; Galaxy: center; infrared: ISM; ISM: individual objects: Sagittarius B2; ISM: lines and bands; ISM: molecules
Record Number:CaltechAUTHORS:20131007-100036727
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Official Citation:Herschel observations of the Sagittarius B2 cores: Hydrides, warm CO, and cold dust M. Etxaluze, J. R. Goicoechea, J. Cernicharo, E. T. Polehampton, A. Noriega-Crespo, S. Molinari, B. M. Swinyard, R. Wu and J. Bally A&A 556 A137 (2013) DOI:
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:41703
Deposited By: Ruth Sustaita
Deposited On:07 Oct 2013 18:00
Last Modified:10 Nov 2021 04:33

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