  In this paper we study the underlying stellar population of a sample of 65
nearby early-type galaxies predominantly located in low density environments.
Ages, metallicities and [alpha/Fe] ratios have been derived through the
comparison of Lick indices measured at different galacto-centric distances with
new SSP models which account for the presence of alpha/Fe enhancement. The SSPs
cover a wide range of ages, metallicities and [alpha/Fe] ratios. To derive the
stellar population parameters we have devised an algorithm based on the
probability density function. We derive a large spread in age ((1-15) Gyrs).
Age does not show any significant trend with central velocity dispersion
sigma_c but E galaxies appear on average older than S0. On the contrary, an
increasing trend of metallicity and [alpha/Fe] with sigma_c is observed,
testifying that the chemical enrichment was more efficient and the duration of
the star formation shorter in more massive galaxies. We have also sought for
possible correlations with the local galaxy density but neither metallicity nor
alpha-enhancement show clear trends. However we find that while low density
environments (LDE) contain very young objects (from 1 to 4 Gyr), none of the
galaxies in the higher density environments (HDE) is younger than 5 Gyrs.
Considering the lack of environmental effect on the [alpha/Fe] ratio and the
high value of [alpha/Fe] in some young massive objects, we argue that young
galaxies in LDE are more likely due to recent rejuvenation episodes. By
comparing the number of rejuvenated objects with the total number of galaxies
in our sample, and by means of simple two-SSP component models, we estimate
that, on average, the rejuvenation episodes do not involve more than 25 % of
the total galaxy mass.
