We analyze the star formation properties of a sample of 21 shell galaxies and
30 early-type galaxies members of interacting pairs, located in low density
environments (Longhetti et al 1998a, 1998b).
The study is based on new models developed to interpret the information
coming from `blue' Hδ/FeI, H+K(CaII) and \D4000 line-strength indices
proposed by Rose (1984; 1985) and Hamilton (1985).
We find that the last star forming event that occurred in the nuclear region
of shell galaxies is statistically old (from 0.1 up to several Gyr) with
respect to the corresponding one in the sub-sample of pair galaxies (<0.1 Gyr
or even ongoing star formation).
If the stellar activity is somehow related to the formation of shells, as
predicted by several dynamical models of galaxy interaction, shells have to be
considered long lasting structures.
Since pair members show evidence of very recent star formation, we suggest
that either large reservoirs of gas have to be present to maintain active star
formation, if these galaxies are on periodic orbits, or most of the pair
members in the present sample are experiencing unbound encounters.Comment: 12 pages, including 7 figures - Accepted for publication in A&
