  We present an analysis of the stellar populations of 102 visually-selected
early-type galaxies (ETGs) with spectroscopic redshifts (0.3<z<1.5) from
observations in the Early Release Science program with the Wide Field Camera 3
(WFC3) on {\it Hubble Space Telescope} (HST). We fit one- and two-component
synthetic stellar models to the ETGs UV-optical-near-IR spectral energy
distributions and find a large fraction (~40%) are likely to have experienced a
minor (f$\lesssim$10% of stellar mass) burst of recent (t$\lesssim$1 Gyr)
star-formation. The measured ages and mass fraction of the young stellar
populations do not strongly trend with measurements of galaxy morphology. We
note that massive (log(M[$M_{\odot}$])>10.5) recently star-forming ETGs appear
to have larger sizes. Furthermore, high-mass, quiescent ETGs identified with
likely companions populate a distinct region in the size-mass parameter space,
in comparison with the distribution of massive ETGs with evidence of RSF. We
conclude that both mechanisms of the quenching of star-formation in disk-like
ETGs and (gas-rich, minor) merger activity contribute to the formation of young
stars and the size-mass evolution of intermediate redshift ETGs. The number of
ETGs for which we have both HST WFC3 panchromatic (especially UV) imaging and
spectroscopically-confirmed redshifts is relatively small, therefore a
conclusion on the relative roles of both of these mechanisms remains an open
question.
