  White dwarf stars frequently experience external pollution by heavy elements,
and yet the intrinsically carbon-enriched DQ spectral class members fail to
exhibit this phenomenon, representing a decades-old conundrum. This study
reports a high-resolution spectroscopic search for Ca II in classical DQ white
dwarfs, finding that these stars are stunted both in pollution frequency and
heavy element mass fractions, relative to the wider population. Compared to
other white dwarf spectral classes, the average external accretion rate is
found to be at least three orders of magnitude lower in the DQ stars. Several
hypotheses are considered which need to simultaneously account for i) an
apparent lack of accreted metals, ii) a dearth of circumstellar planetary
material, iii) an observed deficit of unevolved companions in post-common
envelope binaries, iv) relatively low helium mass fractions, and remnant masses
that appear smaller than for other spectral classes, v) a high incidence of
strong magnetism, and vi) modestly older disk kinematics. Only one hypothesis
is consistent with all these constraints, suggesting DQ white dwarfs are the
progeny of binary evolution that altered both their stellar structures and
their circumstellar environments. A binary origin is already suspected for the
warmer and more massive DQ stars, and is proposed here as an inclusive
mechanism to expose core carbon material, in a potential evolutionary
unification for the entire DQ spectral class. In this picture, DQ stars are not
descended from DA or DB white dwarfs that commonly host dynamically-active
planetary systems.
