Magnesium Hydride in the Sun

Low noise photoelectric scans of the MgH |A²Π -X²∑ (0,0)| band in the spectrum of the solar disk are examined and accurate equivalent widths obtained. Analysis of these widths with a recent photospheric model atmosphere provides an estimate for the band oscillator strength: |$f\,0,0\,=\,0.055$| . A discussion of these weak MgH and strong Mg I lines in the disk spectrum and weak lines of MgH in the penumbral spectrum shows that limits can be set on the isotopic abundance ratios 25Mg/24Mg and 26Mg/24Mg. These limits are consistent with the terrestrial abundance ratios. Recent analyses of sunspot spectra which claim a marked overabundance of 25Mg and 25Mg relative to 24Mg are critically examined. It is proposed that these results are in error owing to the neglect of a contribution to the spectrum from bright umbral dots. A new method of analysis is outlined.