  A re-evaluation of time-averaged accretion rates at DBZ-type white dwarfs
points to historical, time-averaged rates significantly higher than the
currently observed episodes at their DAZ counterparts. The difference between
the ongoing, instantaneous accretion rates witnessed at DAZ white dwarfs, which
often exceed 1e8 g/s, and those inferred over the past 1e5-1e6 yr for the DBZ
stars can be a few orders of magnitude, and therefore must result from
high-rate episodes of tens to hundreds of years so they remain undetected to
date. This paper explores the likelihood that such brief, intense accretion
episodes of gas-phase material can account for existing data. For reasonable
assumptions about the circumstellar gas, accretion rates approaching or
exceeding 1e15 g/s are possible, similar to rates observed in quiescent
cataclysmic variables, and potentially detectable with future x-ray missions or
wide-field monitoring facilities. Gaseous debris that is prone to such rapid
accretion may be abundant immediately following a tidal disruption event via
collisions and sublimation, or if additional bodies impinge upon an extant
disk. Particulate disk matter accretes at or near the Poynting-Robertson drag
rate for long periods between gas-producing events, consistent with rates
inferred for dusty DAZ white dwarfs. In this picture, warm DAZ stars without
infrared excesses have rates consistent with accretion from particulate disks
that remain undetected. This overall picture has implications for quasi-steady
state models of accretion and the derived chemical composition of asteroidal
debris in DBZ white dwarfs.
