The MgHB′2Σ+–X2Σ+Transition: A New Tool for Studying Magnesium Isotope Abundances

We have identified lines from the 0-3, 0-4, 0-5, 0-6, 0-7, 1-3, 1-4, 1-7, and 1-8 bands of the 24 MgH B ' 2 Σ + - X 2 Σ + transition in sunspot umbral spectra. Lines of the 0-7 and 1-8 bands in the uncluttered 7500 Å region are the most obvious, but B ' 2 Σ + - X 2 Σ + lines have been tracked as far to the blue as 5300 Å. In combination with weak lines of the 0-7 bands of the 25 MgH and 26 MgH isotopes, the solar isotope ratio 24 Mg: 25 Mg: 26 Mg has been measured as 76:12:12, in agreement with the much better determined terrestrial ratio 79:10:11. The intensity distribution of bands with v '' from 4 to 8 has been measured and found to show no anomalies; the excitation temperature of 3100 K agrees well with a value of 3200 K determined from SiO in a sunspot spectrum. The lines of the MgH B ' 2 Σ + - X 2 Σ + transition are much more cleanly separated and much less blended than lines from the stronger A 2 Π- X 2 Σ + transition. The B ' 2 Σ + - X 2 Σ + lines should prove useful in isotopic abundance analyses for stars where the A 2 Π- X 2 Σ + transition is too strong to yield useful results.