Letter to the Editor

APEX mapping of H₃O in the Sgr B2 region

¹ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
² National Institute for Space Research (SRON), Postbus 800, 9700 AV Groningen, The Netherlands e-mail: vdtak@sron.rug.nl
³ European Southern Observatory, Casilla 19001, Santiago, Chile

Received: 27 March 2006
Accepted: 22 May 2006

Abstract

Context.The cosmic-ray ionization rate  of dense molecular clouds is a key parameter for their dynamics and chemistry.

Aims.Variations of  are well established, but it is unclear if these are related to source column density or to Galactic location.

Methods.Using the APEX telescope, we have mapped the 364 GHz line of H₃O⁺ in the Sgr B2 region and observed the 307 GHz line at selected positions. With the IRAM 30-m telescope we have observed the HO 203 GHz line at the same positions.

Results.Strong H₃O⁺ emission is detected over a ~3  2 pc region, indicating an H₃O⁺ column density of 10¹⁵–10¹⁶ cm^-2 in an 18'' beam. The H₃O⁺ abundance of ~ 3  10^-9 and H₃O⁺/H₂O ratio of ~1/50 in the Sgr B2 envelope are consistent with models with  ~ 4  10^-16  s^-1, 3 lower than derived from H observations toward Sgr A, but 10 that of local dense clouds.

Conclusions.The ionization rates of interstellar clouds thus seem to be to first order determined by the ambient cosmic-ray flux, while propagation effects cause a factor of ~3 decrease from diffuse to dense clouds.