  (Abridged). Stellar absorption lines in the optical have been extensively
studied and constitute an important stellar population diagnostic for galaxies
in the local universe and up to moderate redshifts. Proceeding towards higher
look-back times, galaxies are younger and the ultraviolet becomes the relevant
spectral region where the dominant stellar populations shine. A comprehensive
study of ultraviolet absorption lines of stellar population models is however
still lacking. We study absorption line indices in the far and mid-ultraviolet
in order to determine age and metallicity indicators for UV-bright stellar
populations in the local universe as well as at high redshift. We explore
empirical and theoretical spectral libraries and use evolutionary population
synthesis to compute synthetic line indices of stellar population models. From
the empirical side, we exploit the IUE-low resolution library of stellar
spectra and system of absorption lines. From the theoretical side we use a
high-resolution library of Kurucz synthetic spectra. In order to select indices
that can be used as age and/or metallicity indicators for distant galaxies and
globular clusters, we compare the models to data of template globular clusters
from the Magellanic Clouds. We provide synthetic line indices in the wavelength
range 1200 Angstrom to 3000 Angstrom for stellar populations of various ages
and metallicities. This adds several new indices to the well-studied CIV and
SiIV absorptions. Based on the comparison with globular cluster data, we select
a set of 11 indices blueward of the 2000 Angstrom rest-frame that allows to
recover well the ages and the metallicities of the clusters. These indices are
ideal to study young galaxies at high redshift. We also provide the synthetic
high-resolution stellar population SEDs.
