  We used the Space Telescope Imaging Spectrograph (STIS) on the Hubble Space
Telescope (HST) to observe the semi-forbidden CIII] emission in Green Pea
galaxies at 0.13 < z < 0.3. We detect CIII] emission in 7/10 galaxies with
CIII] equivalent widths that range from 2-10\AA~. The observed CIII] emission
line strengths are consistent with the predictions from photoionization models
which incorporate the effects of binary stellar evolution with young stellar
ages < 3-5 Myrs, and high ionization parameters (logU > -2). The hard ionizing
radiation from young massive stars, and high nebular temperatures at
low-metallicities can account for the observed high equivalent widths of CIII]
and [OIII] emission lines. The Green Pea galaxies do not show a significant
correlation between the Ly$\alpha$ and CIII] equivalent widths, and the
observed scatter is likely due to the variations in the optical depth of
Ly$\alpha$ to the neutral gas. Green Pea galaxies are likely to be
density-bounded, and we examined the dependence of CIII] emission on the Lyman
continuum optical depth. The potential LyC leaker galaxies in our sample have
high CIII] equivalent widths that can only be reproduced by starburst ages as
young as < 3 Myrs and harder ionizing spectra than the non-leakers. Among the
galaxies with similar metallicities and ionization parameters, the CIII]
equivalent width appears to be stronger for those with higher optical depth to
LyC, as expected from the photoionization models. Further investigation of a
larger sample of CIII]-emitters is necessary to calibrate the dependence of
CIII] emission on the escape of LyC radiation, and to enable application of the
CIII] diagnostics to galaxies in the reionization epoch.
