Astronomy and Astrophysics – Astrophysics
Scientific paper
Feb 1987
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1987apj...313..400h&link_type=abstract
Astrophysical Journal, Part 1 (ISSN 0004-637X), vol. 313, Feb. 1, 1987, p. 400-407.
Astronomy and Astrophysics
Astrophysics
32
Abundance, Carbon Monoxide, M Stars, Stellar Envelopes, Stellar Spectrophotometry, Supergiant Stars, Absorption Spectra, Emission Spectra, Line Spectra, Spectral Resolution, Stellar Models
Scientific paper
New observations are reported of the 1.3 mm emission lines of C-12O and C-13O in the circumstellar envelope of Alpha Ori. The CO source size is small and is probably limited in extent by photodissociation. Model calculations of the excitation and self-shielding of CO are used to interpret the data, and for a standard distance to Alpha Ori an unshielded CO photodissociation rate 8/100 billionths per sec is required for consistency with the observations. This rate may be representative of the value in the general interstellar medium, although a chromospheric contribution to the radiation which dissociates CO cannot be ruled out. CO is significantly underassociated so that ionized carbon is probably an important species for the overall ionization structure of the envelope. The C-12O/C-13O ratio determined from the emission-line observations is consistent with previous low values determined from absorption-line measurements of both the envelope and the photosphere. Exchange reactions of carbon ions with CO are probably important in maintaining the molecular isotopic ratio close to that of the carbon ions in the presence of dissociating radiation.
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