Chemical features in the circumnuclear disk of the Galactic center⋆
Max Planck Institute for Radio Astronomy,
Auf dem Huegel 69,
2 Academia Sinica Institute of Astronomy and Astrophysics, No.1, Sec. 4, Roosevelt Rd, 10617, Taipei R.O.C., Taiwan
3 University College London, Gower Street, London, WC1E 6BT, UK
4 European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching, Germany
5 Institut de Radio Astronomie Millimétrique, 300 rue de la Piscine, Dom. Univ., 38406 St., Martin d’Hères, France
6 European Southern Observatory, Avda. Alonso de Córdova 3107, Vitacura, 3107, Santiago, Chile
7 Department of Earth and Space Sciences, Chalmers University of Technology, Onsala Observatory, 439 92 Onsala, Sweden
8 Centro de Astrobiología (CSIC-INTA), Ctra. de Torrejón Ajalvir km 4, 28850 Torrejón de Ardoz, Madrid, Spain
Received: 18 July 2015
Accepted: 9 October 2015
Aims. The circumnuclear disk (CND) of the Galactic center is exposed to many energetic phenomena coming from the supermassive black hole Sgr A* and from stellar activities. These energetic activities can affect the chemical composition in the CND through interaction with UV photons, cosmic rays, X-rays, and shock waves. We aim to constrain the physical conditions present in the CND through chemical modeling of observed molecular species detected toward it.
Methods. We analyzed a selected set of molecular line data taken toward a position in the southwest lobe of the CND with the IRAM 30m and APEX 12-m telescopes and derived the column density of each molecule via a large velocity gradient (LVG) analysis. The determined chemical composition is compared with a time-dependent, gas-grain chemical model based on the UCL_CHEM code,which includes the effects of shock waves with varying physical parameters.
Results. We detect molecules, such as CO, HCN, HCO+, HNC, CS, SO, SiO, NO, CN, H2CO, HC3N, N2H+, and H3O+, and obtain their column densities. Total hydrogen densities obtained from LVG analysis range between 2 × 104 and 1 × 106cm-3 and most species indicate values around several × 105cm-3. These values are lower than those corresponding to the Roche limit, which shows that the CND is tidally unstable. The chemical models show good agreement with the observations in cases where the density is ~104cm-3, the cosmic-ray ionization rate is high, > 10-15s-1, or shocks with velocities > 40 km s-1 have occurred.
Conclusions. Comparison of models and observations favors a scenario where the cosmic-ray ionization rate in the CND is high, but precise effects of other factors, such as shocks, density structures, UV photons, and X-rays from the Sgr A*, must be examined with higher spatial resolution data.
Key words: ISM: molecules / Galaxy: center
© ESO, 2015