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

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

2000-06-19

arxiv.org/abs/astro-ph/0006246v1

Astronomy and Astrophysics

Astrophysics

To be published in A&A 358 (Letters); 4 pages including 3 figures

Scientific paper

Recent models of the envelopes of seven massive protostars are used to analyze observations of H3+ infrared absorption and H13CO+ submillimeter emission lines toward these stars, and to constrain the cosmic-ray ionization rate zeta. The H13CO+ gives best-fit values of zeta=(2.6+/- 1.8) x 10^-17 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 H3+ data. No relation of zeta 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 H3+ and H13CO+ results and the correlation of N(H3+) with heliocentric distance suggest that intervening clouds contribute significantly to the H3+ absorptions in the more distant regions. The most likely absorbers are low-density (<~10^4 cm^-3) clouds with most carbon in neutral form or in CO.

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