A White Dwarf in Waiting: A Precursor Star with a Mysterious Massive Companion
In their new study, Gautham Adamane Pallathadka and collaborators report the discovery of a rare and fascinating binary star system: SDSS J022932.28+713002.7, or SDSS J0229+7130 for short. At its heart is a bloated, still-evolving star called a proto-white dwarf, specifically an extremely low-mass (ELM) one. This type of object forms when a star loses much of its mass to a nearby companion before completing its natural life cycle. What makes SDSS J0229+7130 especially intriguing is that its companion is invisible and surprisingly massive, possibly a very heavy white dwarf or even a neutron star.
Observations and Data Collection
The discovery emerged from data collected during the Sloan Digital Sky Survey (SDSS-V) and other surveys including Gaia, ZTF (Zwicky Transient Facility), and TESS. Initial observations from SDSS showed the telltale signs of a white dwarf: strong hydrogen lines in the spectrum and large changes in radial velocity, suggesting it's orbiting another object. Follow-up spectroscopy and photometry were key to understanding the system’s characteristics. Spectra helped identify the visible star’s motion and properties, while precise brightness measurements from lightcurves revealed regular fluctuations caused by the star’s shape being distorted due to the gravitational pull of its unseen partner.
Analyzing the Stellar Properties
To analyze these data, the team used a combination of models and simulations. They fit both the star’s light and its spectrum to determine surface gravity, temperature, and metallicity. They also corrected for extinction, or the dimming caused by interstellar dust, which can significantly affect these measurements. The final fits indicated that the primary star is about 0.18 times the mass of the Sun and is still contracting into its final white dwarf state. Meanwhile, the unseen companion is at least 1.19 solar masses, above the average for white dwarfs and potentially in the range of neutron stars.
Lightcurves and Orbital Period
By studying the system's lightcurve, the periodic brightening and dimming due to the distorted star’s orbit, and combining that with the measured velocities, the authors could determine the orbital period: just under 36 hours. This short orbit means the two stars are close together. Despite their proximity, there are no eclipses observed, implying the system is tilted rather than edge-on. Simulations with PHOEBE, a software tool used to model eclipsing binary stars, helped refine these estimates further and ruled out a main sequence star companion, which would be too bright to remain unseen.
Formation Scenario and Evolutionary History
The authors suggest that this system likely formed through a process called stable Roche lobe overflow, where the now-visible star once donated its outer layers to the more massive companion. This is common in close binary evolution and consistent with the observed properties of SDSS J0229+7130. They also discuss two possible models for the system, one with higher extinction (implying a hotter, heavier primary) and one with lower extinction. The lower extinction solution matched theoretical models more closely and was chosen as the fiducial interpretation.
Significance of the Discovery
This discovery is important not only for understanding how ELM white dwarfs form, but also because such binary systems are potential sources of gravitational waves and may even be the precursors to more dramatic cosmic events like supernovae or pulsars. SDSS J0229+7130 appears to be a stable system that will eventually settle into a long-lived compact binary, with its white dwarf primary orbiting a massive, and still mysterious, compact companion.
Source: Pallathadka
