The destiny of yellow dwarf stars, such as our Sun, is nearly entirely influenced by their mass. The heftiest stars, around eight to twelve times heavier than the Sun, can explode as supernovae, resulting in the most extreme entities in the cosmos—neutron stars and black holes. Conversely, low-mass stars, which include our Sun, adopt a more subdued path. As they exhaust their hydrogen and their cores grow denser, their outer envelopes expand and disperse, evolving into red giant stars. Once their cores reach peak density, they lose their outer envelopes, leaving behind remnants known as white dwarfs. A white dwarf resembles an ember from a fire that is dim and gradually cooling.
This entire process unfolds over an extensive period relative to human life: tens to hundreds of billions of years. (Our universe is 13.8 billion years old, meaning not a single white dwarf has experienced the complete aging cycle yet.) To observers, the Sun’s behavior is so stable and foreseeable that it seems timeless. However, the Sun and similar stars undergo very dynamic lifespans, following a trajectory of significant transformations before they settle into the white dwarf phase. In particular, when a Sun-like star orbits alongside another star, that companion can profoundly influence the evolutionary pathway of the Sun-like star.
A research group from Caltech, co-directed by Kareem El-Badry, assistant professor of astronomy; Shri Kulkarni, the George Ellery Hale Professor of Astronomy and Planetary Science; and Tom Prince, the Ira S. Bowen Professor of Physics, Emeritus, has been monitoring the late-stage behavior of stars by analyzing data collected through Caltech’s Zwicky Transient Facility (ZTF) at the Palomar Observatory. Every two days, ZTF’s wide-field camera thoroughly scans the northern hemisphere’s night sky; by comparing historical and current views of the sky, researchers can identify variable objects, namely, those undergoing rapid, continuous change.
The ZTF Stellar Group, as the research team is known, detected such a transient object in 2022. The object, designated Gaia22ayj, was remarkable for its swiftly pulsing signal. Initially, it was categorized as a detached double white dwarf binary—two white dwarf stars orbiting one another. However, subsequent data gathered from the W. M. Keck Observatory on Maunakea in Hawai’i prompted astronomers to reconsider that early classification. Tony Rodriguez, a graduate student in the ZTF Stellar Group, recalls examining the data with his colleagues and concluding that the object’s light curve—a measurement of its light intensity fluctuations over time—was puzzling. “Why would the light curve appear that way?” Rodriguez remembers pondering.
Driven by preliminary data acquired by El-Badry, Rodriguez aimed to gather even more information from other telescopes to deepen his understanding of Gaia22ayj. Drawing from his experiences studying similar systems, he recognized that they were viewing a white dwarf star orbited by a low-mass star (rather than another white dwarf as initially suggested), and that the Gaia22ayj system was likely to possess a strong magnetic field. The fast rotation of the white dwarf in this system also reminded him of white dwarf pulsars, which emit regular bursts of electromagnetic radiation when their poles are oriented toward our perspective on Earth. Curiously, this star pulsed every nine minutes, a slower rate than known white dwarf pulsars. Additionally, it seemed to be transferring mass to its companion white dwarf, in contrast to the known white dwarf pulsars.
Ultimately, Rodriguez concluded that Gaia22ayj represented the missing connection in the previously established life cycle of a white dwarf pulsar. “We’ve previously observed two young systems, white dwarf stars in binary formats whose rapid rotation cultivates a substantial magnetic field. And we’ve documented many mature star systems where the white dwarf spins very slowly,” Rodriguez explains. “Yet, this is the first star we’ve identified that is caught in the midst of its ‘teenage’ phase, having already established a robust magnetic field and beginning to draw matter from the companion star onto itself. We had never previously caught a system in the act of spinning so rapidly while also significantly decelerating, all while gaining mass from its partner.”
Identifying a “teenage” white dwarf pulsar is especially thrilling because this stage of a star’s development is fleeting, Rodriguez notes. How fleeting? “Approximately 40 million years,” he states. However, he emphasizes that these systems endure for billions of years, making their teenage phase less than 1 percent of their life span. In human terms, that would equate to possibly just a few months of our complete lives—if only human teenage angst could be so brief!
“Data collected at the W. M. Keck Observatory confirmed that this system exhibited a powerful magnetic field and was funneling matter onto the white dwarf,” Rodriguez remarks. “Further findings from the distinctive instruments at Palomar Observatory revealed that this system is, astonishingly, decelerating.”
The paper, authored by Rodriguez and including researchers from four different continents, is entitled “A Link Between White Dwarf Pulsars and Polars: Multiwavelength Observations of the 9.36-Minute Period Variable Gaia22ayj” and has been published in Publications of the Astronomical Society of the Pacific.
Rodriguez is a National Science Foundation (NSF) Graduate Research Fellow and a Neugebauer Scholar supported by the France A. Córdova Fund. Caltech’s ZTF is financed by the NSF and an international consortium of collaborators. Additional assistance comes from the Heising-Simons Foundation and Caltech. ZTF data are processed and archived by Caltech’s IPAC.