  While most debris disks consist of dust with little or no gas, a fraction has
significant amounts of gas detected via emission lines of CO, ionized carbon,
and/or atomic oxygen. Almost all such gaseous debris disks known are around
A-type stars with ages up to 50 Myr. We show, using semi-analytic disk
evolution modeling, that this can be understood if the gaseous debris disks are
remnant protoplanetary disks that have become depleted of small grains compared
to the interstellar medium. Photoelectric heating by the A-stars' FUV radiation
is then inefficient, while the stars' EUV and X-ray emissions are weak owing to
a lack of surface convective zones capable of driving magnetic activity. In
this picture, stars outside the range of spectral types from A through early B
are relatively hard to have such long-lived gas disks. Less-massive stars have
stronger magnetic activity in the chromosphere, transition region, and corona
with resulting EUV and X-ray emission, while more-massive stars have
photospheres hot enough to produce strong EUV radiation. In both cases,
primordial disk gas is likely to photoevaporate well before 50 Myr. These
results come from 0D disk evolution models where we incorporate internal
accretion stresses, MHD winds, and photoevaporation by EUV and X-ray photons
with luminosities that are functions of the stellar mass and age. A key issue
this work leaves open is how some disks become depleted in small dust so that
FUV photoevaporation slows. Candidates include grains' growth, settling, radial
drift, radiation force, and incorporation into planetary systems.
