Main Sequence and White Dwarf Binaries are Hiding in Plain Sight

Some binary stars are unusual. They contain a main sequence star like our Sun, while the other is a “dead” white dwarf star that left fusion behind and emanates only residual heat. When the main sequence star ages into a red giant, the two stars share a common envelope.

This common envelope phase is a big mystery in astrophysics, and to understand what’s happening, astronomers are building a catalogue of main sequence-white dwarf binaries.

Common envelope (CE) binaries are important because they’re the progenitors for Type 1a supernovae. When the main sequence star swells into a red giant, the compact and gravitationally powerful white dwarf draws matter away from it. This matter gathers on the surface of the white dwarf until it reaches a critical point and then detonates as a supernova.

CE binaries are also important because they can merge and emit gravitational waves, another astrophysical phenomenon that needs better understanding.

In new research, astronomers from the University of Toronto identified 52 candidates with high probabilities of being CE binaries. The research is “The First Catalog of Candidate White Dwarf–Main-sequence Binaries in Open Star Clusters: A New Window into Common Envelope Evolution.” It’s published in the Astrophysical Journal and the lead author is Steffani Grondin, a graduate student in the David A. Dunlap Department for Astronomy & Astrophysics at U of T.

“Despite its importance, CE evolution may be one of the largest uncertainties in binary evolution,” the authors write in their research.

“Binary stars play a huge role in our universe,” said lead author Grondin. “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 research used massive data sets from three sources: the ESA’s Gaia spacecraft, The Pan-STARRS1 survey, and the 2MASS survey. The team used machine learning techniques to comb the dataset for candidate main sequence-white dwarf (MSWD) binaries in 299 open star clusters in the Milky Way. Open clusters were chosen because they can provide an independent age constraint for the system, allowing the researchers to trace the evolution of the binaries from before the CE phase to after the CE phase. The researchers found 52 high-probability candidates in 38 open clusters.

This number is a huge increase in the number of known MSWD binaries. Only two were known previously. Machine learning is a powerful tool that allows astronomers to work with huge data sets to uncover difficult-to-distinguish results, and this study is no exception.

“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.”

Study co-author Maria Drout is also a professor in the David A. Dunlap Department for Astronomy & Astrophysics at U of T. Drout says that the team’s results illustrate how many things in our Universe are “hiding in plain sight” if we only had the tools to see them. As our telescope and survey tools become more discerning and gather larger data sets, our machine-learning tools are making these data sets less opaque.

Drout points out that finding the MSWD binaries in open clusters is the key.

“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,” Drout explains.

The evolution of CE systems is poorly understood. Astrophysicists don’t know how energy is dissipated during the CE phase, how stellar metallicity affects the development of the CE, or how initial binary parameters predict post-CE orbital configurations. Those are just a few of their unanswered questions.

This study can’t answer all of those questions, but by producing the largest catalogue of MSWD binaries, the team is setting the stage for researchers to make progress.

Grondin and her co-researchers did follow-up spectroscopy on a subset of three systems with the Gemini and Lick observatories. They confirmed two of them to be MSWD binaries.

They also retrieved archival light curves from TESS, Kepler, and the Zwicky Transient Facility. All three candidates showed clear variability in their light curves. That could indicate rapid M-dwarf rotation or ellipsoidal modulations in a short-period binary. The researchers explain that the catalogue could be contaminated, though not very significantly, by single WDs or MS+MS binaries.

Natal kicks likely influence the results. Many of the MSWD candidates show offsets from their host clusters, suggesting that natal kicks were imparted when the WD formed or during common envelope ejection. Since 78% of the open clusters they observed lacked candidates, the authors think that some MSWD binaries were ejected from their clusters by natal kicks.

“Ultimately, this catalog is a first step to obtaining a set of observational benchmarks to better link post-CE systems to their pre-CE progenitors,” the authors write in their research.

More spectroscopic observations of the candidates will help confirm more of them as MSWD binaries. An expanded search could also help identify MSWD candidates that have been ejected from their clusters by natal kicks.

As is often the case in astronomy and astrophysics, a larger dataset is needed before researchers can reach any conclusions.

“Ultimately, this catalogue is a necessary first step in a larger effort to provide observational constraints on the CE phase,” the authors write, noting that a detailed characterization of some of the candidates in this sample is already underway. The larger sample will allow researchers to link the masses of post-CE binaries with pre-CE progenitors.

“With these observational benchmarks, this sample will aid in efforts to unlock important new insights into one of the most uncertain phases of binary evolution,” the authors conclude.