Organic molecules in the Galactic center

A&A 455, 971-985 (2006)
DOI: 10.1051/0004-6361:20065190

Hot core chemistry without hot cores

M. A. Requena-Torres¹, J. Martín-Pintado¹, A. Rodríguez-Franco^{1, 2}, S. Martín³, N. J. Rodríguez-Fernández⁴ and P. de Vicente⁵

¹ Departamento de Astrofísica Molecular e Infrarroja, Instituto de Estructura de la Materia-CSIC C Serrano 121, 28006 Madrid, Spain
    e-mail: requena@damir.iem.csic.es
² Escuela Universitaria de Óptica, Departamento de Matemática Aplicada (Biomatemática), Universidad Complutense de Madrid, Avenida Arcos de Jalón s/n, 28037 Madrid, Spain
³ Instituto de Radioastronomía Milimétrica Av. Divina Pastora 7, Local 20, 18012 Granada, Spain
    e-mail: martin@iram.es
⁴ Observatoire de Bordeaux, L3AB (UMR 5804), CNRS/Université Bordeaux 1, BP 89, 2 rue de l'Observatoire, 33270 Floirac, France
    e-mail: nemesio.rodriguez@obs.u-bordeaux1.fr
⁵ Observatorio Astronómico Nacional, Centro Astronómico de Yebes Apartado 148, 19080 Guadalajara, Spain
    e-mail: p.devicente@oan.es

(Received 13 March 2006 / Accepted 11 April 2006)

Abstract
Aims.We study the origin of large abundances of complex organic molecules in the Galactic center (GC).
Methods.We carried out a systematic study of the complex organic molecules CH₃OH, C₂H₅OH, (CH₃)₂O, HCOOCH₃, HCOOH, CH₃COOH, H₂CO, and CS toward 40 GC molecular clouds. Using the LTE approximation, we derived the physical properties of GC molecular clouds and the abundances of the complex molecules. The abundances of complex organic molecules in the GC are compared with those measured in hot cores and hot corinos, in which these complex molecules are also abundant.
Results.The CH₃OH abundance between clouds varies by nearly two orders of magnitude from $2.4\times10^{-8}$ to $1.1\times10^{-6}$ . The abundance of the other complex organic molecules relative to that of CH₃OH is basically independent of the CH₃OH abundance, with variations of only a factor 4-8. We find that both the abundance and the abundance ratios of the complex molecules relative to CH₃OH in hot cores are similar to those found in the GC clouds. However, hot corinos show different abundance ratios than observed in hot cores and in GC clouds. The rather constant abundance of all the complex molecules relative to CH₃OH suggests that all complex molecules are ejected from grain mantles by shocks. Frequent (~10 $^5\,$ years) shocks with velocities >6 km s^-1 are required to explain the high abundances in gas phase of complex organic molecules in the GC molecular clouds. The rather uniform abundance ratios in the GC clouds and in Galactic hot cores indicate a similar average composition of grain mantles in both kinds of regions. The Sickle and the Thermal Radio Arches, affected by UV radiation, show different relative abundances in the complex organic molecules due to the differentially photodissociation of these molecules.

Key words: astrochemistry -- ISM: clouds -- ISM: molecules -- radio lines: ISM -- Galaxy: center