This image from the ALMA telescope shows star system HD101584 and the complex gas clouds surrounding the binary. It is the result of a pair of stars sharing a common outer layer during their last moments. Credit: ALMA (ESO/NAOJ/NRAO), Olofsson et al. Acknowledgement: Robert Cumming.
By Ilana MacDonald, Dunlap Institute for Astronomy & Astrophysics
A team of astronomers, led by David A. Dunlap Department for Astronomy & Astrophysics (DADDAA) graduate student Steffani Grondin, has discovered the first population of white dwarf-main sequence stellar binary candidates in open star clusters. The team also included DADDAA professors Maria Drout and Joshua Speagle, who is jointly appointed with the Department of Statistical Sciences.
This discovery will help link the initial and final states of binary star systems, which will help inform our models for the formation of stars, the chemical evolution of our galaxy, and even how most of the elements on the periodic table were created. It was made possible by using 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.
Most stars exist in binary systems – pairs of stars that orbit around a shared center of gravity. 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.
The main stage of a star’s evolution is called the “main sequence” phase. This is when hydrogen is being fused into helium in the star’s core. Our own sun is currently a main sequence star, as are about 90 percent of stars in the universe.
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.
This common envelope phase, and how stars spiral together during this critical period remains one of the biggest mysteries in astrophysics. Scientists still struggle to fully understand how this interaction affects the stars’ subsequent evolution.
Fortunately, this new research may provide a potential solution to this enigma. The remnants left behind by the stars in this study are compact objects called white dwarfs. Finding these “post-common envelope” systems that contain both a “dead” stellar remnant and “living” star therefore provide a unique way to investigate this extreme phase of stellar evolution.
Steffani Grondin pictured in front of one of the Magellan Telescopes while observing white dwarf-main sequence binaries at the Las Campanas Observatory in Chile. Credit: Steffani Grondin
“Binary stars play a huge role in our universe,” says 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.”
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,” states Speagle. “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,” added Drout. “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 a kind of extreme stellar explosion called a Type Ia supernova and the sort of merger that causes gravitational waves, that is, 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 catalogue will ultimately shed light on the many elusive transient phenomena in our universe.
Contributing institutions include the David A. Dunlap Department of Astronomy & Astrophysics, the Dunlap Institute for Astronomy & Astrophysics, the Department for Statistical Sciences, and the Data Sciences Institute at the University of Toronto, as well as 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.
About the Dunlap Institute for Astronomy & Astrophysics
The Dunlap Institute for Astronomy & Astrophysics in the Faculty of Arts & Science at the University of Toronto is an endowed research institute with over 80 faculty, postdocs, students, and staff, dedicated to innovative technology, groundbreaking research, world-class training, and public engagement.
The research themes of its faculty and Dunlap Fellows span the Universe and include: optical, infrared and radio instrumentation, Dark Energy, large-scale structure, the Cosmic Microwave Background, the interstellar medium, galaxy evolution, cosmic magnetism and time-domain science.
The Dunlap Institute, the David A. Dunlap Department of Astronomy & Astrophysics, and other researchers across the University of Toronto’s three campuses together comprise the leading concentration of astronomers in Canada, at the leading research university in the country.
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For more information, contact:
Ilana MacDonald
Public Outreach, Communications, and Events Strategist
Dunlap Institute, University of Toronto
media@dunlap.utoronto.ca
