First pairs of white dwarf–main sequence binaries discovered in clusters shine new light on stellar evolution

Astronomers at the University of Toronto (U of T) have discovered the first pairs of white dwarf and main sequence stars—”dead” remnants and “living” stars—in young star clusters. Described in a new study published in The Astrophysical Journal, this breakthrough offers new insights into an extreme phase of stellar evolution, and one of the biggest mysteries in astrophysics.

Scientists can now begin to bridge the gap between the earliest and final stages of binary star systems—two stars that orbit a shared center of gravity—to further our understanding of how stars form, how galaxies evolve, and how most elements on the periodic table were created. This discovery could also help explain cosmic events like supernova explosions and gravitational waves, since binaries containing one or more of these compact dead stars are thought to be the origin of such phenomena.

Most stars exist in binary systems. In fact, nearly half of all stars similar to our sun have at least one companion star. These paired stars usually differ in size, with one star often being more massive than the other. Though one might be tempted to assume that these stars evolve at the same rate, more massive stars tend to live shorter lives and go through the stages of stellar evolution much faster than their lower mass companions.

In the stage where a star approaches the end of its life, it will expand to hundreds or thousands of times its original size during what we call the red giant or asymptotic giant branch phases. In close binary systems, this expansion is so dramatic that the dying star’s outer layers can sometimes completely engulf its companion. Astronomers refer to this as the common envelope phase, as both stars become wrapped in the same material.

The common envelope phase remains one of the biggest mysteries in astrophysics. Scientists have struggled to understand how stars spiraling together during this critical period affects the stars’ subsequent evolution. This new research may solve this enigma.

Remnants left behind after stars die are compact objects called white dwarfs. Finding these post-common envelope systems that contain both a “dead” stellar remnant and a “living” star—otherwise known as white dwarf-main sequence binaries—provides a unique way to investigate this extreme phase of stellar evolution.

“Binary stars play a huge role in our universe,” says lead author Steffani Grondin, a graduate student in the David A. Dunlap Department for Astronomy & Astrophysics at U of T. “This observational sample marks a key first step in allowing us to trace the full life cycles of binaries and will hopefully allow us to constrain the most mysterious phase of stellar evolution.”

The researchers used machine learning to analyze data from three major sources: the European Space Agency’s Gaia mission—a space telescope that has studied over a billion stars in our galaxy—along with observations from the 2MASS and Pan-STARRS1 surveys. This combined data set enabled the team to search for new binaries in clusters with characteristics resembling those of known white dwarf-main sequence pairs.

Even though these types of binary systems should be very common, they have been tricky to find, with only two candidates confirmed within clusters prior to this research. This research has the potential to increase that number to 52 binaries across 38 star clusters.

Since the stars in these clusters are thought to have all formed at the same time, finding these binaries in open star clusters allows astronomers to constrain the age of the systems and to trace their full evolution from before the common envelope conditions to the observed binaries in their post-common envelope phase.

“The use of machine learning helped us to identify clear signatures for these unique systems that we weren’t able to easily identify with just a few datapoints alone,” says co-author Joshua Speagle, a professor in the David A. Dunlap Department for Astronomy & Astrophysics and Department of Statistical Sciences at U of T. “It also allowed us to automate our search across hundreds of clusters, a task that would have been impossible if we were trying to identify these systems manually.”

“It really points out how much in our universe is hiding in plain sight—still waiting to be found,” says co-author Maria Drout, also a professor in the David A. Dunlap Department for Astronomy & Astrophysics at U of T. “While there are many examples of this type of binary system, very few have the age constraints necessary to fully map their evolutionary history. While there is plenty of work left to confirm and fully characterize these systems, these results will have implications across multiple areas of astrophysics.”

Binaries containing compact objects are also the progenitors of an extreme stellar explosion called a Type Ia supernova and the sort of merger that causes gravitational waves, which are ripples in spacetime that can be detected by instruments such as the Laser Interferometer Gravitational-Wave Observatory (LIGO). As the team uses data from the Gemini, Keck and Magellan Telescopes to confirm and measure the properties of these binaries, this catalog will ultimately shed light on the many elusive transient phenomena in our universe.