Solar Carbon Monoxide, Thermal Profiling, and the Abundances of C, O, and Their Isotopes

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Abstract
A solar photospheric thermal profiling analysis is presented, exploiting the infrared (2.3-4.6 μm) rovibrational bands of carbon monoxide (CO) as observed with the McMath-Pierce Fourier transform spectrometer (FTS) at Kitt Peak, and from above the Earth's atmosphere by the Shuttle-borne ATMOS experiment. Visible continuum intensities and center-limb behavior constrained the temperature profile of the deep photosphere, while CO center-limb behavior defined the thermal structure at higher altitudes. The oxygen abundance was self-consistently determined from weak CO absorptions (for C/O ≡ 0.5). Our analysis was meant to complement recent studies based on three-dimensional (3D) convection models, which, among other things, have revised the historical solar oxygen (and carbon) abundance downward by a factor of nearly 2, although in fact our conclusions do not support such a revision. Based on various considerations, an O = 700 ± 100 ppm (parts per million relative to hydrogen) is recommended; the large uncertainty reflects the model sensitivity of CO. New solar isotopic ratios also are reported: 12C/13C = 80 ± 1, 16O/17O = 1700 ± 220, and 16O/18O = 440 ± 6—all significantly lower than terrestrial. CO synthesis experiments utilizing a stripped down version of the 3D model—which has large temperature fluctuations in the middle photosphere, possibly inconsistent with CO movies from the Infrared Imaging Spectrometer (IRIS), and a steeper mean temperature gradient than matches visible continuum center-limb measurements—point to a lower oxygen abundance (~500 ppm) and isotopic ratios closer to terrestrial. A low oxygen abundance from CO—and other molecules like OH—thus hinges on the reality of the theoretically predicted midphotospheric convective properties.
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