  The impact of the cluster environment on the evolution of dwarf galaxies is
investigated by comparing the properties of a sample of dwarf irregulars (dIs)
in the Virgo Cluster with a control sample of nearby ("field") dIs having
oxygen abundances derived from [O III]4363 measurements and measured distances
from resolved stellar constituents. Spectroscopic data are obtained for H II
regions in 11 Virgo dIs distributed in the central and outer regions of the
cluster. To ensure that oxygen abundances are derived in a homogeneous manner,
oxygen abundances for field and Virgo dIs are computed using the bright-line
method and compared with abundances directly obtained from [O III]4363, where
available. They are found to agree to within about 0.2 dex with no systematic
offset. At a given optical luminosity, there is no systematic difference in
oxygen abundance between the sample of Virgo dIs and the sample of nearby dIs.
However, five of the eleven Virgo dIs exhibit much lower baryonic gas fractions
than field dIs at comparable oxygen abundances. Using field dIs as a reference,
a gas-deficiency index for dIs is constructed, making it possible
quantitatively to identify which galaxies have lost gas. For the Virgo sample,
some of the dwarfs are gas-deficient by a factor of 30. The gas-deficiency
correlates roughly with the X-ray surface brightness of the intracluster gas.
Ram-pressure stripping can best explain the observed gas-poor dIs in the
cluster sample. Together with the lack of significant fading and reddening of
the gas-poor dIs compared to gas-normal dIs, these observations suggest that
the gas-poor dIs in Virgo have recently encountered the intracluster medium for
the first time. Faded remnants of gas-poor dIs in Virgo will resemble bright
dwarf ellipticals presently seen in the cluster core.
