Mapping photodissociation and shocks in the vicinity of Sagittarius A^{∗⋆,⋆⋆}

¹ Centro de Astrobiología (CSIC/INTA), Ctra. de Torrejón a Ajalvir km 4, 28850 Torrejón de Ardoz, Madrid, Spain
e-mail: arancha@damir.iem.csic.es
² European Southern Observatory, Alonso de Córdova 3107, Vitacura, Casilla 19001, Santiago 19, Chile
³ Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA

Received: 26 October 2009
Accepted: 12 October 2010

Abstract

Aims. We study the chemistry in the harsh environments of galactic nuclei using the nearest one, the Galactic center (GC).

Methods. We have obtained maps of the molecular emission within the central five arcminutes (12 pc) of the GC in selected molecular tracers: SiO(2–1), HNCO(5_0,5–4_0,4), and the J =  1  →  0 transition of H¹³CO⁺, HN¹³C, and C¹⁸O at an angular resolution of 30′′ (1.2 pc). The mapped region includes the circumnuclear disk (CND) and the two surrounding giant molecular clouds (GMCs) of the Sgr A complex, known as the 20 and 50 km s^-1 molecular clouds. Additionally, we simultaneously observed the J =  2  →  1 and J =  3  →  2 transitions of SiO toward selected positions to estimate the physical conditions of the molecular gas using the large velocity gradient approximation.

Results. The SiO(2–1) emission shows all the molecular features identified in previous studies, covering the same velocity range as the H¹³CO⁺(1–0) emission, which also presents a similar distribution. In contrast, HNCO(5–4) emission appears in a narrow velocity range mostly concentrated in the 20 and 50 km s^-1 GMCs. A similar trend follows the HN¹³C(1–0) emission. The HNCO column densities and fractional abundances present the highest contrast, with difference factors of  ≥ 60 and 28, respectively. Their highest values are found toward the cores of the GMCs, whereas the lowest ones are measured at the CND. SiO abundances do not follow this trend, with high values found toward the CND, as well as the GMCs. By comparing our abundances with those of prototypical Galactic sources we conclude that HNCO, similar to SiO, is ejected from grain mantles into gas-phase by nondissociative C-shocks. This results in the high abundances measured toward the CND and the GMCs. However, the strong UV radiation from the Central cluster utterly photodissociates HNCO as we get closer to the center, whereas SiO seems to be more resistant against UV-photons or it is produced more efficiently by the strong shocks in the CND. This UV field could be also responsible for the trend found in the HN¹³C abundance.

Conclusions. We discuss the possible connections between the molecular gas at the CND and the GMCs using the HNCO/SiO, SiO/CS, and HNCO/CS intensity ratios as probes of distance to the Central cluster. In particular, the HNCO/SiO intensity ratio is proved to be an excellent tool for evaluating the distance to the center of the different gas components.