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R.C. Peterson (UCO/Lick)
The near-UV spectrum from 2200Å\ to 3400Å\ in metal-poor turnoff stars is calculated from first principles, using Kurucz atmospheres and line lists available from his website at http://cfaku5.harvard.edu. Comparisons made against archival spectra from the International Ultraviolet Explorer (IUE) and the Hubble Space Telescope (HST) show that an excellent match is achieved for stars of solar temperature more metal-poor than one-thirtieth the solar abundance. As metallicity becomes higher, however, the strength of strong lines becomes progressively too strong in the spectral synthesis calculations versus the observed spectra. This is remedied by altering the surface temperature gradient of the models, making temperature drop more slowly to the extent required to match the profiles of the very strong MgII doublet at 2800Å. Once this model change has been made, the near-UV spectra of solar-temperature stars of one-fifth solar metallicity or less show an excellent match throughout the 2200--3400Å\ region to archival spectra of individual stars.
By coadding spectra calculated for individual stars of constant metallicity at temperatures and luminosities corresponding to Milky Way globular-cluster color-magnitude diagrams, a synthetic near-UV spectrum of the integrated light of a globular cluster is obtained. Comparisons are reported which illustrate the degree to which the age and metallicity of metal-poor globular clusters in Andromeda can be deduced from archival HST near-UV spectra.