Unveiling the Hidden Beats: The Richest Pulsating Ultra-Massive White Dwarf
Ultra-massive white dwarfs are some of the most intriguing remnants of stellar evolution, providing unique opportunities to study the final stages of stars. In this study, Francisco De Gerónimo and collaborators report the discovery of the richest pulsating ultra-massive white dwarf to date, WD J0135+5722, which exhibits 19 distinct pulsation modes—far surpassing the previous record of 8 modes in BPM 37093.
The Importance of Pulsating White Dwarfs
White dwarfs are dense stellar remnants left behind when stars like our Sun exhaust their nuclear fuel. A special subclass, known as ZZ Ceti stars, exhibit pulsations caused by gravity waves within their interiors. These pulsations are key to understanding their internal structure and chemical composition using a technique called asteroseismology. Most white dwarfs have masses below 1.05 times the mass of the Sun and contain carbon-oxygen cores. However, ultra-massive white dwarfs, those exceeding 1.1 solar masses, might have cores composed of heavier elements like oxygen and neon, or a mix of carbon, oxygen, and neon. WD J0135+5722 offers a unique opportunity to investigate these possibilities.
Methods: Selecting a Stellar Candidate
To identify promising candidates, the team used data from prior surveys and selected objects within the ZZ Ceti instability strip—a temperature range where pulsations are observed. WD J0135+5722, located 51 parsecs away, was observed using advanced instruments such as the Gran Telescopio Canarias and the Apache Point Observatory. High-speed photometry was used to capture its subtle brightness variations over several hours.
Analyzing the Light: Pulsation Periods and Frequencies
The brightness of WD J0135+5722 fluctuates with periods ranging from 137 to 1345 seconds. These variations are linked to internal pulsation modes, which were identified by analyzing the light curves using Fourier transforms. Remarkably, 19 pulsation modes were detected, making this the richest pulsation spectrum observed in an ultra-massive white dwarf. The frequencies of these modes suggest a complex interior, though additional observations are needed to refine the analysis.
Estimating the Mass and Core Composition
The team used multiple methods to estimate the star's mass, finding values between 1.12 and 1.15 solar masses. This mass range is consistent with a significant fraction of the core being crystallized—a phenomenon where dense material solidifies under immense pressure. Depending on the core composition, the fraction of the core that is crystallized could range from 56% (carbon-oxygen core) to 86% (oxygen-neon core). Determining the exact core composition will require further analysis of the pulsation modes.
Implications and Future Directions
The discovery of WD J0135+5722 adds a crucial data point to our understanding of ultra-massive white dwarfs and their potential origins as the remnants of high-mass stars or stellar mergers. The high number of pulsation modes opens the door for detailed asteroseismological studies, which could reveal whether the core is primarily composed of carbon-oxygen, oxygen-neon, or a hybrid mixture. Such insights have broader implications for understanding the evolution of stars and the processes that govern the late stages of stellar life.
Source: De Gerónimo
