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

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 3-D convection models which, among other things, have revised the historical solar oxygen (and carbon) abundance downward by a factor of nearly two; although in fact our conclusions do not support such a revision. Based on various considerations, an ǫ 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: 12 C/ 13 C=80±1, 16 O/ 17 O=1700±220, and 16 O/ 18 O=440±6; all significantly lower than terrestrial. CO synthesis experiments utilizing a stripped down version of the 3-D model-which has large temperature fluctuations in the middle photosphere, possibly inconsistent with CO "movies" from the Infrared

Solar System Abundances of the Elements

“…In the studies by Allende Pietro et al and Asplund et al, lower equivalent widths were used, and effects of collisions of H atoms in the calculations of the statistical equilibrium of O were not considered. Ayres et al (2006) derived the O abundance from weak CO absorptions but did not find support for a lower O abundance. They recommend A(O) = 8.85, much closer to the value in AG89 (A(O) = 8.93).…”

Does the Sun have a subsolar metallicity?

“…A problem with the 3D model of Asplund et al (2000a) it that it appears to predict a slightly too steep temperature gradient as inferred from continuum center-to-limb variation (Ayres et al 2006;Koesterke et al 2008). As seen in Fig.…”

“…Ayres (2008) has carried out an analysis of the [O i] 630 nm line using one snapshot of a 3D co5bold solar atmosphere model. He followed the same procedure as Allende Prieto et al (2001) and Asplund et al (2004) for this line, namely to allow the Ni blend to vary freely in obtaining the best overall line profile fit.…”

“…This difference exceeds the quoted uncertainties in these two values. In terms of abundance, a 0.15 dex lower Ni abundance corresponds to an increase of the derived O abundance by ≈ 0.09 dex for disk-center intensity (Allende Prieto et al 2001;Caffau et al 2008), which would reduce the suggested value of Ayres (2008) to ≈ 8.72 but at the expense of a deteriorated profile fit. It remains to be determined if this is an acceptable trade-off.…”

The solar oxygen abundance from an empirical three-dimensional model

“…The debate on whether the proposed revision should be adopted has been very intense, and several papers have been published with new abundance determinations, including those of Ayres et al (2006), Socas-Navarro & Norton (2007), Ayres (2008), Centeno & Socas-Navarro (2008), Caffau et al (2008, 2011), Scott et al (2009), Pereira et al (2009. The frequency of publications on the so-called solar oxygen crisis seems to have declined in recent years, not because the problem has been satisfactorily resolved, but probably because there are no new arguments or data in favor of one view or the other.…”