Context :

Aims : The paper is devoted to the study of the underlying stellar population of a sample of 65 nearby early-type galaxies , predominantly located in low density environments , a large fraction of which show emission lines .

Methods : Ages , metallicities , and [ \alpha /Fe ] ratios have been derived through the comparison of Lick indices measured at different galacto-centric distances ( 7 apertures and 4 gradients ) with new simple stellar population ( SSP ) models that account for the presence of \alpha /Fe–enhancement .
The SSPs cover a wide range of ages ( 10 ^ { 9 } -16 \times 10 ^ { 9 } yr ) , metallicities ( 0.0004 \leq Z \leq 0.05 ) , and [ \alpha /Fe ] ratios ( 0-0.8 ) .
To derive the stellar population parameters , we use an algorithm that provides , together with the most likely solution in the ( age , Z , [ \alpha /Fe ] ) space , the probability density function along the age-metallicity degeneracy .

Results : We derive a large spread in age , with SSP-equivalent ages ranging from a few to 15 Gyrs .
Age does not show any significant trend with central velocity dispersion \sigma _ { c } , but E galaxies appear on average older than lenticulars .
On the contrary , a significant 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 .
These latter two relations do not depend on galaxy morphological type .

We have also sought possible correlations with the local galaxy density \rho _ { xyz } , but neither metallicity nor \alpha –enhancement show clear trends .
However , we find that while low density environments ( LDE ) ( \rho _ { xyz } \leq 0.4 ) contain very young objects ( from 1 Gyr to 4 Gyr ) , none of the galaxies in the higher density environments ( HDE ) ( 40 % of galaxies with a measured density ) 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 .

The good quality of the data also allow us to analyze the gradients of stellar populations .
An average negative metallicity gradient \Delta \log Z / \Delta \log ( r / r _ { e } ) \sim - 0.21 is firmly detected , while the age and \alpha –enhancement spatial distributions within r _ { e } /2 appear quite flat .
These observations suggest that , in a given galaxy , the star formation proceeded on similar timescales all across the central r _ { e } /2 region , but with an increasing efficiency toward the center .

Conclusions :