The ZZ Ceti Instability Strip for The Most Massive White Dwarf Pulsators

TL;DR

The paper investigates the ZZ Ceti instability strip at high white dwarf masses by performing time-series photometry on 31 massive DA white dwarfs in the MWDD 100 pc sample. Using multi-site, multi-band observations and a consistent Fourier-analysis pipeline, it identifies 16 pulsators (including the record-setting J0959$-$1828 with $M\approx 1.32\,M_\odot$) and 15 non-variables, uncovering clear trends: the weighted mean period increases as $T_{ m eff}$ decreases and decreases with increasing mass, while pulsation power generally peaks in the strip middle and falls toward the edges. The results also suggest the ZZ Ceti strip may not be pure for massive WDs, potentially due to weak magnetism suppressing pulsations, and highlight the potential for using crystallized-core interiors to probe WD structure via asteroseismology. Overall, the study expands the sample of massive ZZ Ceti stars and provides empirical constraints on mass- and temperature-dependent pulsation properties critical for interior and core composition inferences.

Abstract

We present time-series photometry of 31 massive DA white dwarfs with $M\gtrsim 0.9~M_\odot$ 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 non-variable 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 $1.27~M_\odot$ 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 non-variable 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.

Figures (35)

• Figure 1: Color magnitude diagram of the massive ($M > 0.9~M_\odot$) white dwarfs with $T_{\rm eff}\geq11,000$ K in the MWDD 100 pc sample and the Pan-STARRS footprint from jewett2024. The dotted lines show the evolutionary sequences for 0.9, 1.1, and $1.3~M_\odot$ white dwarfs with C/O cores bergeron19. The green stars mark both the previously known and the newly discovered pulsating white dwarfs in the sample. Similarly, the red points mark the previously known and newly discovered NOVs.
• Figure 2: Light curves (left) and their Fourier transforms (right panels) for J0039$-$0357 observed on 4 different nights. The horizontal black dashed lines indicate 4 times the average amplitude in the Fourier transforms, while the vertical dashed blue lines mark the significant frequencies. Table \ref{['tab:freqs']} provides a list of the frequencies detected for each observation.
• Figure 3: Light curves (left) and their Fourier transforms (right panels) for J0039$-$0357 observed on 2024 Nov 11 at the GTC using HiPERCAM. The corresponding filter is labeled above the Fourier Transform. Table \ref{['tab:freqs_hiper']} provides a list of the frequencies detected for each observation.
• Figure 4: Light curves and Fourier transforms for J0158$-$2503.
• Figure 5: Light curves and Fourier transforms for J0912$-$2642