Multiperiodic pulsations of the unique DAQ white dwarf J0551+4135: insights into a merger remnant

TL;DR

This work investigates the pulsation content of the unique ultra-massive DAQ white dwarf J0551+4135, a likely merger remnant with a carbon-enriched hydrogen atmosphere. Through 19 nights of multi-site time-series photometry from APO, Gemini, and GTC, it identifies at least ten recurrent non-radial g-mode frequencies in the 987–1180 μHz range, along with a secondary group near 1595–1700 μHz, while addressing substantial aliasing from gaps. Mass estimates derived from an extended DAQ evolutionary grid yield $M = 1.13 \pm 0.01\,M_\odot$ (CO core) and $1.12 \pm 0.01\,M_\odot$ (ONe core) with cooling ages of $1.7 \pm 0.1$ Gyr and $1.6 \pm 0.1$ Gyr, respectively; however, true asteroseismology awaits fully consistent DAQ models. The results highlight the potential of DAQ WDs as merger remnants and emphasize the need for tailored evolutionary models and extended, continuous monitoring to resolve modes and probe internal structure.

Abstract

2MASS J05513444+4135297 (herafter J0551+4135) is the only pulsating DAQ white dwarf known with a carbon and hydrogen atmosphere. Its unusual atmospheric composition and kinematics indicate a white dwarf merger origin. We present time-series photometry of J0551+4135 obtained using the Apache Point Observatory 3.5m, Gemini North 8m, and Gran Telescopio Canarias 10m telescopes. J0551+4135 exhibits variations in pulsation amplitude and frequency over time. We detect ten significant recurring peaks across different subsets of observations, with frequencies ranging from 987 to 1180~$μ$Hz, consistent with non-radial gravity ($g$)-mode oscillations. We present new evolutionary models suitable for spectroscopic characterization of DAQ white dwarfs, and derive a mass of $1.13 \pm 0.01\,M_\odot$ and a cooling age of $1.7 \pm 0.1$ Gyr for a CO core, and $1.12 \pm 0.01\,M_\odot$ and $1.6 \pm 0.1$\,Gyr for an ONe-core white dwarf, respectively. However, detailed asteroseismology of this unique pulsator has to wait until fully-consistent DAQ evolutionary models are available. Further observations, including multi-site campaigns to reduce daily aliasing and to improve the signal-to-noise ratio would be helpful for identification of additional modes and constraining the internal structure of this unique pulsator.

Figures (8)

• Figure 1: Combined normalized light curve of J0551 obtained with ARCTIC (green), GMOS (red), and HiPERCAM (blue) between November 2023 and April 2025. Each point represents normalized flux as a function of Barycentric Julian Date (BJD – 2460000). The data from different instruments were normalized by subtracting either a mean or unity flux level to ensure consistency across observing runs. The dashed horizontal line marks the zero-flux reference. The inset shows a zoomed-in view of the ARCTIC observations from 2024 January 19, covering a 4.8-hour sequence that reveals short-term variability on timescales of minutes. See section \ref{['analysis']} for more details.
• Figure 2: Lomb–Scargle periodograms (left panel) and window functions (right panel) for J0551 from different datasets: HiPERCAM (blue), ARCTIC (green), GMOS (red), and the combined dataset (black). The horizontal dashed red line indicates the 4$\sigma$ noise threshold computed from the mean noise level. The orange curves show the residual (prewhitened) periodograms after removing the identified frequencies. Vertical grey lines mark the prewhitened frequencies in each dataset.
• Figure 3: Detected pulsation frequencies from HiPERCAM (blue), ARCTIC (green), GMOS (red), and the combined dataset (black), plotted as a function of their signal-to-noise ratios. Each symbol marks a prewhitened frequency, with the symbol size reflecting its associated uncertainty. Grey vertical bands indicate frequencies detected in at least two independent datasets, merged within a tolerance of $\pm11~\mu$Hz, consistent with the alias width expected from the temporal sampling of our ground-based observations.
• Figure 4: Evolutionary tracks in the Radius-effective temperature plane for our DAQ models with different stellar masses and two core compositions. Solid (dashed) lines refer to ONe (CO) core composition. The diagonal solid and dashed lines mark the onset of crystallization.
• Figure 5: Light curves obtained with APO and Gemini.