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

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 infrared absorption and H¹³CO⁺ submillimeter emission lines toward these stars, and to constrain the cosmic-ray ionization rate . The H¹³CO⁺ gives best-fit values of  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 data. No relation of 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 and H¹³CO⁺ results and the correlation of N() with heliocentric distance suggest that intervening clouds contribute significantly to the absorptions in the more distant regions. The most likely absorbers are low-density ( 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)

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Contents

• 1. Introduction
• 2. Models
• 3. Results
• 4. Comparison with HCO⁺
• 5. Contributions by foreground layers
• Acknowledgements
• References

© European Southern Observatory (ESO) 2000

Online publication: June 20, 2000
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