Limits on the cosmic-ray ionization rate toward massive young stars
Abstract
Recent models of the envelopes of seven massive protostars are used to analyze observations of H_3+ infrared absorption and HCO+p submillimeter emission lines toward these stars, and to constrain the cosmic-ray ionization rate zeta_CR . The HCO+p gives best-fit values of zeta_CR =(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 H_3+ data. No relation of zeta_CR 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 H_2h and HCO+p results and the correlation of N(H_2h) with heliocentric distance suggest that intervening clouds contribute significantly to the H_2h absorptions in the more distant regions. The most likely absorbers are low-density (<~ 104 cm-3) clouds with most carbon in neutral form or in CO.
- Publication:
-
Astronomy and Astrophysics
- Pub Date:
- June 2000
- DOI:
- 10.48550/arXiv.astro-ph/0006246
- arXiv:
- arXiv:astro-ph/0006246
- Bibcode:
- 2000A&A...358L..79V
- Keywords:
-
- STARS: FORMATION;
- STARS: CIRCUMSTELLAR MATTER;
- ISM: COSMIC RAYS;
- ISM: MOLECULES;
- ISM: STRUCTURE;
- Astrophysics
- E-Print:
- To be published in A&