Oxygen in the sun

We review elemental and isotopic abundances of oxygen in the Sun and their implications for solar physics and Solar System formation. The oxygen content of the Sun, expressed as the O/H atom ratio, is 468 ppm. This value, obtained relatively recently, is significantly lower than earlier estimates, but is supported by observations of several photospheric lines. The new, lower solar oxygen abundance (and a new, lower solar carbon abundance) are compatible with many astrophysical constraints but are in conflict with helioseismology-based constraints on the standard model of the Sun. The oxygen isotopic composition of the Sun is surprisingly poorly known, despite the fact that the Sun contains nearly all (∼98%) of the Solar System's oxygen. Spectroscopy of the solar photosphere suggests a slight enhancement in ¹⁸O/¹⁶O compared to terrestrial composition, but with a precision of 6%. One measurement of solar oxygen trapped in lunar metal grains indicates that the Sun is depleted in ¹⁶O by a few percent, whereas another indicates an enrichment of similar magnitude; some models predict that the Sun is enriched in ¹⁶O by a few percent and others predict that solar and terrestrial oxygen are of similar isotopic composition. It is hoped that forthcoming isotopic measurements on solar wind sampled by the Genesis sample return mission will bring some closure to this issue.