  We report on detailed Monte Carlo inversion analysis of the Lyman limit
system observed at z = 2.917 in the VLT/UVES spectrum of the quasar HE
0940-1050. Metal absorption lines of carbon and silicon in three ionization
stages and numerous atomic hydrogen lines have been analyzed simultaneously. It
is found that in order to match the observations, the shape of the ultraviolet
background ionizing spectrum of Haardt & Madau (1996) should be modified: a
spectrum with a higher intensity of the emission feature at 3 Ryd is required.
It is also found that synthetic galactic spectra (or different mixtures of them
with power law spectra) cannot reproduce the observations, indicating that the
stellar contribution to the ionizing background is negligible at z ~= 3. For
the first time a very low carbon abundance of [C/H] = -2.93+/-0.13 and the
abundance ratio [Si/C] = 0.35+/-0.15 are directly measured in the Lyman limit
system with N(H I) = 3.2 10^{17} cm^{-2}. If the absorber at z = 2.917 provides
an example of a pristine gas enriched by the nucleosynthetic products of early
generations of stars, then the measured value of [Si/C] seems to indicate that
the initial mass functions for these stellar populations are constrained to
intermediate masses, M_up <= 25M_solar.
