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Astron. Astrophys. 358, L79-L82 (2000)


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Letter to the Editor

Limits on the cosmic-ray ionization rate toward massive young stars

F.F.S. van der Tak and E.F. van Dishoeck

Sterrewacht, Postbus 9513, 2300 RA Leiden, The Netherlands

Received 5 May 2000 / Accepted 26 May 2000

Abstract

Recent models of the envelopes of seven massive protostars are used to analyze observations of [FORMULA] infrared absorption and H13CO+ submillimeter emission lines toward these stars, and to constrain the cosmic-ray ionization rate [FORMULA]. The H13CO+ gives best-fit values of [FORMULA][FORMULA] s-1, in good agreement with diffuse cloud models and with recent Voyager/Pioneer data but factors of up to 7 lower than found from the [FORMULA] data. No relation of [FORMULA] with luminosity or total column density is found, so that local (X-ray) ionization and shielding against cosmic rays appear unimportant for these sources. The difference between the [FORMULA] and H13CO+ results and the correlation of N([FORMULA]) with heliocentric distance suggest that intervening clouds contribute significantly to the [FORMULA] absorptions in the more distant regions. The most likely absorbers are low-density ([FORMULA] cm-3) clouds with most carbon in neutral form or in CO.

Key words: stars: formation – stars: circumstellar matter – ISM: cosmic rays – ISM: molecules – ISM: structure

Send offprint requests to: Floris van der Tak (vdtak@strw.leidenuniv.nl)

© European Southern Observatory (ESO) 2000

Online publication: June 20, 2000

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