Astronomy and Astrophysics – Astronomy
Scientific paper
May 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011iaus..280p.243l&link_type=abstract
The Molecular Universe, Posters from the proceedings of the 280th Symposium of the International Astronomical Union held in Tole
Astronomy and Astrophysics
Astronomy
Scientific paper
A long-standing problem in cosmochemistry has been relating the distribution of oxygen isotope ratios in refractory grains (CAIs) in meteorites to the environment of the solar nebula. Recent work (Clayton 2002; Yurimoto & Kuramoto 2004; Lyons & Young 2005; Lee et al. 2008) has argued that self-shielding during photodissociation of CO in the solar nebula or parent cloud yields oxygen isotope ratios in water (formed from the liberated O) that form a consistent mixing line with the meteorite grains. The CO self-shielding model predicts that solar oxygen isotopes are depleted in 17O by ~ 5%, as are the refractory meteorite grains. This prediction has been verified by measurements of solar wind O collected by the Genesis spacecraft (McKeegan et al. 2008). The CO self-shielding process is well known in astrochemical environments including both molecular clouds (e.g., Bally and Langer 1982; Sheffer et al. 2003) and protoplanetary disks (e.g., Smith et al. 2009). However, these observations, which are of CO isotope abundances, are not sufficiently precise to constrain the formation process for the CAI mixing line. Model calculations thus far have used shielding functions from van Dishoeck & Black (1988), and assumed the same isotope ratios (actually, ratio of ratios) observed in the meteorites, and are therefore not an independent test. Recent synchrotron CO photolysis experiments (Chakraborty et al. 2008) measured very large wavelength-dependent isotope effects that are apparently not compatible with the CO self-shielding theory for meteorite oxygen isotopes. Here, I present results of line-by-line calculations for CO isotopologue dissociation in a vertically mixed solar nebula model. I use the same model I used in Lyons & Young (2005), but all shielding functions (CO and H2) have been replaced with line-by-line spectra. I use published molecular constants (Eidelsberg et al. 1991; Visser et al. 2009; McCandliss et al. 2002) and a Draine radiation field. Most results thus far are within 5-20% of the observed meteorite mixing line, and depend on gas temperature and the H2 column abundance. The CO molecular constants contain many uncertainties, especially for C17O, and ongoing spectral measurements at the Soleil synchrotron are being undertaken to remedy this situation (see Stark et al., this meeting).
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