Observation of a Binary White Dwarf System with Mass Transfer

A team of astrophysicists has made significant strides in understanding mass transfer processes in binary star systems by observing a unique pair of white dwarfs. This study, published in The Astrophysical Journal, provides one of the clearest images to date of how ultracompact white dwarfs exchange mass when they are in close proximity to each other.

Binary star systems are often the site of dramatic and violent interactions, where one star distorts and siphons material from its companion. However, the exact nature of mass transfer in systems where the stars orbit extremely close to one another remains poorly understood. White dwarfs are the remnants of stars similar to the Sun, having exhausted their nuclear fuel and shed their outer layers. Emma Chickles and her team from MIT meticulously examined millions of images of binary stars, particularly white dwarfs, captured over the past decade through various stellar observation programs.

The researchers observed the brightness variations of ATLAS J1013−4516, a binary system consisting of two white dwarfs. This variation corresponds to the mutual eclipse of the two stars, occurring over an exceptionally brief period of just 8.56 minutes. Such a short orbital period indicates a very small distance between the stars, allowing one of the components to actively draw material from its companion and transfer it onto a compact and overheated accretion disk (25900 K).

ATLAS J1013−4516 is categorized as part of the AM Canum Venaticorum (AM CVn) family, a rare class of ultracompact binary systems where one white dwarf accretes helium-rich material from a degenerate or semi-degenerate donor star. These systems typically have orbital periods ranging from about 5 to 70 minutes, and they are distinguished from cataclysmic variables by their continuous blue spectrum, strong He I/He II emission, and the absence of Balmer lines.

Using the LMAS (Large Lenslet Array Magellan Spectrograph), the team confirmed the presence of a helium-dominated accretion disk, while high-speed photometry from ULTRACAM highlighted pronounced primary and secondary eclipses. Chickles and her colleagues established a decade-long timing baseline by utilizing light curves from the ATLAS and Gaia surveys, as well as high-speed images from ULTRACAM on the New Energy Telescope and proto-Lightspeed on the Magellan telescope. From this temporal baseline, they measured the orbital period derivative at -1.60 × 10^-12 s.s^-1. This measurement allows for a direct examination of the balance between angular momentum loss due to gravitational wave emission and the orbital expansion induced by mass transfer. Such measurements have only been performed for a few ultracompact binaries with significant, non-zero orbital period derivatives.

This investigation sheds light on the structural response of the donor star to mass loss. The researchers constrained the masses of both the accreting and donor stars based on stable mass transfer arguments, assuming that angular momentum loss is primarily due to gravitational wave emission. The donor star has a mass of 0.1 solar masses, while the accreting star weighs 0.87 solar masses.

Chickles and her team predict the characteristic gravitational wave signature of the binary system, which could be detected by future space-based gravitational wave observatories such as LISA (Laser Interferometer Space Antenna). They anticipate a signal-to-noise ratio for LISA greater than 10 over four years, making ATLAS J1013−4516 a promising candidate for studying long-term orbital evolution in the mass transfer regime.

Thanks to nearly a decade of observations from ATLAS, Gaia, ULTRACAM, and proto-Lightspeed, Emma Chickles and her team have measured a negative orbital period derivative for ATLAS J1013−4516, whose amplitude is lower than predictions based on gravitational wave emission processes. This suggests that the ongoing mass transfer partially compensates for angular momentum losses due to gravitational waves, thus favoring a semi-degenerate and thermally inflated helium donor star. Coupled with geometric constraints from the eclipses, the observation of ATLAS J1013−4516 allows for precise testing of the influence of the donor's structure on secular orbital evolution, particularly at the shortest periods observed in white dwarf binaries undergoing accretion.

Recent findings have revealed a limited yet diverse population of ultracompact AM CVn systems with orbital periods under approximately 10 minutes, showcasing a wide variety of light curve morphologies and both positive and negative orbital period derivatives. ATLAS J1013−4516 exemplifies this diversity while being one of the few systems in this regime to exhibit deep eclipses and accurately measured orbital periods, making it a particularly valuable model for testing mass transfer processes through the disk and the thermodynamics of helium donors.