Disentangling metallicity and age for turnoff stars

This is a progress report on calculations of near-ultraviolet spectra with Ben Dorman at NASA/Goddard, for the ultimate purpose of extracting age and metallicity from extragalactic spectra. We are calculating from first princ iples a grid of spectra covering 2200-3400 Angstrom using the Kurucz progra m SYNTHE, beginning with stars of metallicity less than one-fifth solar ([F e/H] < -0.7). For these stars, LTE calculations using known opacities and l ine lists including only transitions measured in the laboratory, coupled wi th standard line-blanketed LTE models, provide a satisfactory match to the spectra of turnoff stars of temperatures T-eff = 5750 K - 6250 K. For more metal-rich stars, two problems arise: lines without a laboratory identifica tion become increasingly influential, and the cores of all strong lines bec ome too strong. These problems must be addressed to match near-UV spectra o f turnoff stars of solar metallicity or higher.