Interstellar CO A-X bands in the spectrum of ζ Oph were recorded at high S/N with grating G160M of the Goddard High Resolution Spectrograph on the Hubble Space Telescope. Isotopic fractionation of CO is severe: 12CO/13CO = 167, C16O/C18O ≃ 1550 and C16O/C17O > 5900 are found where 12C/13C = 70, 16O/ 18O = 500, and 16O/17O = 2600 are observed or expected. Standard models of the æ Oph cloud predict 12CO/13CO ≲ 70. The higher observed ratio suggests that photodissociation of CO, not the isotopic charge exchange reaction (13C+ + CO ↔ 12C+ + 13CO), is the dominant influence on the 12CO/13CO ratio.
All Science Journal Classification (ASJC) codes
- Astronomy and Astrophysics
- Space and Planetary Science