  We test the theory that lenticular (S0) galaxies form from spirals whose star
formation has been shut down. We use the globular cluster specific frequency
S_N, defined as the number of globular clusters normalised by the galaxy
luminosity as a diagnostic. NTT/EMMI long-slit spectroscopic observations of 11
S0 galaxies at z < 0.006 are used to measure the absorption-line indices,
Hdelta, Hgamma, Mgb, Fe5270 and Fe5335 within the central r_e/8. By inverting
single-stellar population models, luminosity-weighted mean ages, metallicities
and alpha-element abundance ratios are derived. We estimate the amount of
fading a galaxy has undergone by comparing each galaxy's S_N with its deviation
from the mean spiral S_N. Galaxies with higher S_N have older stellar
populations. Moreover, we find that the zero-point and amount of fading is
consistent with a scenario where lenticulars are formed by the quenching of
star formation in spiral galaxies. Our data also rule out any formation method
for S0s which creates a large number of new globular clusters. We confirm that
previous results showing a relationship between S_N and color are driven by the
S_N - Age relation. Five galaxies show detectable Hbeta, [OIII], Halpha or
[NII] emission lines. However, only in the two youngest galaxies is this
emission unambiguously from star formation. Our results are consistent with the
theory that S0 galaxies are formed when gas in normal spirals is removed,
possibly as a result of a change in environment. The on-going star formation in
the youngest galaxies hints that the timescale of quenching is ~< 1 Gyr. We
speculate, therefore, that the truncation of star formation is a rather gentle
process unlikely to involve a rapid burst of star formation.
