APEX mapping of H3O+ in the Sgr B2 region

Abstract

The cosmic-ray ionization rate (zeta) of dense molecular clouds is a key parameter for their dynamics and chemistry. Variations of zeta are well established, but it is unclear if these are related to source column density or to Galactic location. 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 203 GHz line of H2O-18 at the same positions. Strong H3O+ emission is detected over a ~3x2 pc region, indicating H3O+ column densities of 10^15 - 10^16 cm^-2 in an 18" beam. The H3O+ abundance of ~3 x 10^-9 and H3O+/H2O ratio of ~1/50 in the Sgr B2 envelope are consistent with models with zeta ~4 x 10^-16 s^-1, 3x lower than derived from H3+ observations toward Sgr A, but 10x that of local dense clouds. 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.

Accepted by A&A Letters (APEX special issue); four A4 pages, two figures

References in corpus (8)

• An atomic and molecular database for analysis of submillimetre line observations
• Molecular ions in L1544. II. The ionization degree
• Enhanced cosmic-ray flux toward zeta Persei inferred from laboratory study of H3+ - e- recombination rate
• Hot and Diffuse Clouds near the Galactic Center Probed by Metastable H3+
• The far-IR spectrum of Sagittarius B2 region: Extended molecular absorption, photodissociation and photoionization
• Water in the envelopes and disks around young high-mass stars
• Confinement-Driven Spatial Variations in the Cosmic Ray Flux
• The line-of-sight distribution of water in the SgrB2 complex

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