  Models of poststarburst (or "K+A") galaxies are constructed by combining
fully three-dimensional hydrodynamic simulations of galaxy mergers with
radiative transfer calculations of dust attenuation. Spectral line catalogs are
generated automatically from moderate-resolution optical spectra calculated as
a function of merger progress in each of a large suite of simulations. The
mass, gas fraction, orbital parameters, and mass ratio of the merging galaxies
are varied systematically, showing that the lifetime and properties of the K+A
phase are strong functions of merger scenario. K+A durations are generally less
than ~0.1-0.3 Gyr, significantly shorter than the commonly assumed 1 Gyr, which
is obtained only in rare cases, owing to a wide variation in star formation
histories resulting from different orbital and progenitor configurations.
Combined with empirical merger rates, the model lifetimes predict
rapidly-rising K+A fractions as a function of redshift that are consistent with
results of large spectroscopic surveys, resolving tension between the observed
K+A abundance and that predicted when one assumes the K+A duration is the
lifetime of A stars (~1 Gyr). The effects of dust attenuation, viewing angle,
and aperture bias on our models are analyzed. In some cases, the K+A features
are longer-lived and more pronounced when AGN feedback removes dust from the
center, uncovering the young stars formed during the burst. In this picture,
the K+A phase begins during or shortly after the bright starburst/AGN phase in
violent mergers, and thus offers a unique opportunity to study the effects of
quasar and star formation feedback on the gas reservoir and evolution of the
remnant. Analytic fitting formulae are provided for the estimates of K+A
incidence as a function of merger scenario.
