  We present in this paper the deepest Arecibo HI observations of Virgo cluster
dwarf ellipticals (dEs) taken to date. Based on this data we argue that a
significant fraction of Virgo cluster dEs recently underwent evolution. Our new
observations consist of HI 21-cm line observations for 22 classified dE
galaxies with optical radial velocities consistent with membership in the Virgo
cluster. Cluster members VCC 390 and VCC 1713 are detected with HI masses M(HI)
= 6*10^7 M_solar and 8*10^7 M_solar, respectively, while M(HI) in the remaining
20 dE galaxies have upper limits as low as 5*10^5 M_solar. We combine our
results with those for 27 other Virgo cluster dEs with HI observations in the
literature, 7 of which have HI detection claims. New optical images from the
WIYN telescope of 5 of these HI-detected dEs, along with archival data, suggest
that seven of the claimed detections are real, yielding a ~ 15% detection rate.
These HI-detected classified dEs are preferentially located near the periphery
of the Virgo cluster. Three Virgo dEs have observed HI velocity widths > 200
km/s, possibly indicating the presence of a large dark matter content, or
transient extended HI. We discuss the possible origins of these objects and
argue that they originate from field galaxies accreted onto high angular
momentum orbits by Virgo in the last few Gyr. As a result these galaxies are
slowly transformed within the cluster by gradual gas stripping processes,
associated truncation of star formation, and passive fading of stellar
populations. Low-mass early-type cluster galaxies are therefore currently being
produced as the product of cluster environmental effects. We utilize our
results to estimate the recent (past 1-3 Gyr) average mass accretion rate into
the Virgo cluster, finding dM/dt ~ 50 M_solar/year.

Astrophysics