The theoretical instability strip of V777 Her white dwarfs

We present a new theoretical investigation of the instability strip of V777
Her (DBV) white dwarfs. We apply a time-dependent convection (TDC) treatment
to cooling models of DB and DBA white dwarfs. Using the spectroscopic
calibration for the convective efficiency, ML2/alpha=1.25, we find a wide
strip covering the range of effective temperature from 30,000 K down to
about 22,000 K at log g = 8.0. This accounts very well for the empirical
instability strip derived from a new accurate and homogenous spectroscopic
analysis of known pulsators. Our approach leads to an exact description of
the blue edge and to a correct understanding of the onset and development of
pulsational instabilities, similarly to our results of TDC applied to ZZ
Ceti white dwarfs in the recent past. We propose that, contrarily to what is
generally believed, there is practically no fuzziness on the boundaries of
the V777 Her instability strip due to traces of hydrogen in the atmospheres
of some of these helium-dominated-atmosphere stars.

Contrary to the blue edge, the red edge provided by TDC computations is far
too cool compared to the empirical one. A similar situation was observed for
the ZZ Ceti stars as well. We hence test the energy leakage argument (i.e.,
the red edge occurs when the thermal timescale in the driving region becomes
equal to the critical period beyond which gravity modes cease to exist),
which was successful to correctly reproduce the red edge of ZZ Ceti white
dwarfs. Based on this argument, the red edge is qualitatively well
reproduced as indicated above. However, upon close inspection, it may be
about 1000 K too cool compared to the empirical one, although the latter
relies on a few objects only. We also test the hypothesis of including
turbulent pressure in our TDC computations in order to provide an alternate
physical mechanism to account for the red edge. First promising results are
presented.