Researchers at the University of Toronto (U of T) have identified the first combinations of white dwarf stars and main sequence stars—known respectively as “dead” remnants and “living” stars—in young star clusters. This significant discovery, detailed in a recent publication in The Astrophysical Journal, provides new perspectives on an extreme phase of stellar evolution, which is one of the great unresolved questions in astrophysics.
This advancement allows scientists to connect the initial and final phases of binary star systems—where two stars orbit a shared gravitational center—thus enhancing our comprehension of how stars form, how galaxies develop, and how the majority of elements in the periodic table were formed. It could also shed light on cosmic occurrences such as supernovae and gravitational waves, which are believed to originate from binaries that contain one or more of these compact dead stars.
Binary systems are quite common among stars; in fact, nearly half of stars like our sun have at least one neighboring star. These paired stars typically vary in size, with one commonly being more massive than the other. Despite assumptions about their similar evolution rates, heavier stars generally have shorter lifespans and progress through evolutionary stages much quicker than their less massive companions.
As a star nears its end, it expands to hundreds or even thousands of times its original volume, entering what is known as the red giant or asymptotic giant branch phases. In close binary systems, this dramatic expansion can lead to the dying star’s outer layers completely engulfing its companion. Astronomers describe this phenomenon as the common envelope phase, indicating that both stars are surrounded by the same material.
The common envelope phase remains one of the most perplexing phenomena in astrophysics. Researchers have faced challenges understanding how the gravitational interaction of stars spiraling together during this crucial period influences their later evolution. This new research may help resolve this mystery.
After a star’s death, it leaves behind a compact remnant known as a white dwarf. Discovering these post-common envelope systems that include both a “dead” star and a “living” star—termed white dwarf-main sequence binaries—offers a unique opportunity to explore this extreme evolution phase.
“Binary stars are fundamentally important in our universe,” remarks lead author Steffani Grondin, a graduate student in the David A. Dunlap Department for Astronomy & Astrophysics at U of T. “This observational sample represents an essential first step in tracking the life cycles of binary systems, and we hope it will help clarify one of the most enigmatic phases of stellar evolution.”
The research team utilized machine learning algorithms to analyze data sourced from three significant missions: the European Space Agency’s Gaia project—a space observatory that has examined over a billion stars in our galaxy—as well as observations from the 2MASS and Pan-STARRS1 surveys. This comprehensive dataset allowed the team to search for new binaries in clusters that mirror the characteristics of known white dwarf-main sequence pairs.
Although these binary systems are expected to be prevalent, they have proven challenging to locate, with only two candidates confirmed in clusters before this study. The research could potentially raise that total to 52 binaries across 38 star clusters. Given that stars in these clusters are believed to have formed concurrently, identifying these binaries in open clusters could enable astronomers to determine the age of these systems and trace their entire evolutionary path from before the common envelope phase to the observed binaries in their post-common envelope state.
“By employing machine learning, we were able to identify distinct characteristics of these unique systems that we couldn’t easily pinpoint with limited data alone,” explains co-author Joshua Speagle, a professor in the David A. Dunlap Department for Astronomy & Astrophysics and the Department of Statistical Sciences at U of T. “It also facilitated the automation of our search across numerous clusters, which would have been impractical with manual identification.”
“This highlights how much in our universe remains unnoticed—still waiting to be discovered,” adds co-author Maria Drout, another professor in the David A. Dunlap Department for Astronomy & Astrophysics at U of T. “While numerous examples of this type of binary system exist, very few have the necessary age constraints to completely map their evolutionary journey. Although much work remains to confirm and thoroughly characterize these systems, these findings will impact various fields of astrophysics.”
Binaries that include compact objects are also considered the origins of extreme stellar explosions known as Type Ia supernovae and the kinds of mergers that produce gravitational waves, which are ripples in spacetime detectable by instruments like the Laser Interferometer Gravitational-Wave Observatory (LIGO). As the research team employs data from the Gemini, Keck, and Magellan telescopes to validate and analyze the properties of these binaries, this catalogue will ultimately enhance our understanding of many elusive transient phenomena in the universe.
Organizations contributing to this research include the David A. Dunlap Department of Astronomy & Astrophysics, Dunlap Institute for Astronomy & Astrophysics, the Department of Statistical Sciences, and the Data Sciences Institute at the University of Toronto, along with the National Technical Institute for the Deaf and Center for Computational Relativity and Gravitation at the Rochester Institute of Technology, the Department of Astronomy & The Institute for Astrophysical Research at Boston University, and the Department of Astronomy at the University of California, Berkeley.
