The ZZ Ceti instability strip for the most massive white dwarf pulsators

We present time-series photometry of 31 massive DA white dwarfs with M ≳ 0.9 M⊙ within the ZZ Ceti instability strip from the Montreal White Dwarf Database 100 pc sample. The majority of the targets had no previous time-series photometry available, though several were classified as nonvariable or potential pulsators in the literature. Out of the 31 candidates, we confirm 16 as pulsating. Our observations at three observatories have led us to discover the most massive pulsating white dwarf currently known, J0959−1828 (M = 1.32 or 1.27 M⊙ for a CO versus ONe core), which is slightly more massive than the previous record holder J0049−2525. We study the sample properties of massive ZZ Ceti white dwarfs and find several trends with their weighted mean periods. As predicted by theory, we see an increase in the weighted mean periods with decreasing effective temperature and a decrease in pulsation amplitudes at the red edge of the instability strip. Furthermore, the weighted mean periods decrease with increasing stellar mass. Our observations show that the ZZ Ceti instability strip may not be pure at high masses. This is likely because the nonvariable white dwarfs in the middle of the strip may be weakly magnetic, which could escape detection in the available low-resolution spectroscopy data, but may be sufficient to suppress pulsations. Extensive follow-up observations of the most massive white dwarfs in our sample have the potential to probe the interior structures and core compositions of these white dwarfs with significantly crystallized cores.