# Four New Massive Pulsating White Dwarfs Including an Ultramassive DAV

## TL;DR

This paper reports the discovery of four massive ZZ Ceti white dwarfs, including an ultramassive one, with detailed pulsation analysis providing insights into their internal structure and core composition.

## Contribution

The study presents four newly discovered massive pulsating white dwarfs, including the first ultramassive ZZ Ceti, and provides preliminary asteroseismic analysis to probe their core composition.

## Key findings

- Detection of pulsations in three white dwarfs and identification of multiple periods in one.
- Preliminary asteroseismic modeling suggests high core crystallization in the ultramassive white dwarf.
- Significant increase in known massive ZZ Ceti stars, enabling tests of crystallization theories.

## Abstract

We report the discovery of four massive ($M > 0.8\,M_\odot$) ZZ Ceti white dwarfs, including an ultramassive $1.16\,M_\odot$ star. We obtained ground based, time-series photometry for thirteen white dwarfs from the Sloan Digital Sky Survey Data Release 7 and Data Release 10 whose atmospheric parameters place them within the ZZ Ceti instability strip. We detect mono-periodic pulsations in three of our targets (J1053, J1554, and J2038) and identify three periods of pulsation in J0840 (173, 327, and 797 s). Fourier analysis of the remaining nine objects do not indicate variability above the $4\langle{A}\rangle$ detection threshold. Our preliminary asteroseismic analysis of J0840 yields a stellar mass $M=1.14\pm 0.01\,M_{\odot}$, hydrogen and helium envelope masses of $M_H = 5.8 \times 10^{-7}\,M_{\odot}$ and $M_{He}=4.5 \times 10^{-4}\,M_{\odot}$, and an expected core crystallized mass ratio of 50-70\%. J1053, J1554, and J2038 have masses in the range $0.84-0.91 M_\odot$ and are expected to have a CO core; however, the core of J0840 could consist of highly crystallized CO or ONeMg given its high mass. These newly discovered massive pulsators represent a significant increase in the number of known ZZ Ceti white dwarfs with mass $M > 0.85\,M_\odot$, and detailed asteroseismic modeling of J0840 will allow for significant tests of crystallization theory in CO and ONeMg core white dwarfs.