Modeling Mid‐Ultraviolet Spectra. I. Temperatures of Metal‐poor Stars

Determining the properties of old stellar systems using evolutionarypopulation synthesis requires a library of reliable model stellar fluxes.Empirical libraries are limited to spectra of stars in the solar neighborhood,with nearly solar abundances and abundance ratios. We report here a first steptowards providing a flux library that includes nonsolar abundances, based oncalculations from first principles that are calibrated empirically. We havestarted with main-sequence stars, whose light dominates the mid-ultravioletspectrum of an old stellar system. We have calculated mid-ultraviolet spectrafor the Sun and nine nearby, near-main-sequence stars spanning metallicitiesfrom less than 1/100 solar to greater than solar, encompassing a range oflight-element abundance enhancements. We first determined temperatures of eightof the stars by analyzing optical echelle spectra together with themid-ultraviolet. Both could be matched at the same time only when models withno convective overshoot were adopted, and only when an approximate chromospherewas incorporated near the surface of relatively metal-rich models. Extensivemodifications to mid-UV line parameters were also required, notably the manualassignment of approximate identifications for mid-UV lines missing fromlaboratory linelists. Without recourse to additional missing opacity, thesemeasures suffice to reproduce in detail almost the entire mid-UV spectrum ofsolar-temperature stars up to one-tenth solar metallicity, and the region from2900A to 3100A throughout the entire metallicity range. Ramifications forabundance determinations in individual metal-poor stars and for age-metallicitydeterminations of old stellar systems are briefly discussed, with emphasis onthe predictive power of the calculations.