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T. Lanz (U Maryland/GSFC), S.R. Heap (NASA/GSFC), T.M. Brown, I. Hubeny (NOAO/GSFC), S. Yi (Caltech)
The rest-frame ultraviolet spectrum of intermediate-age stellar populations (1 to few Gyr) is dominated by the UV flux of F-type stars at the main sequence turn-off. We have obtained high-resolution spectra of four well-studied F stars with STIS aboard HST to calibrate ultraviolet spectral features in terms of age and metallicity. The UV spectrum is mostly degenerate in terms of effective temperature and metallicity, but we found that two spectral indices, [2850/2770] and [2310/3040], allow a discrimination between these two parameters. The first index measures mainly the strength of Mg I 2852, while the second index is the mid-UV color. We find that the Mg I index is sensitive to the effective temperature, but insensitive to metallicity; the UV color is sensitive to both metallicity and T\rm eff. We discuss the importance of departures from LTE and chromospheric activity on these two spectral indices. We apply our results to estimate the time since the last major star formation episode in the early-type galaxy LBDS 53W091 at redshift z=1.55.