Failed Star Orbits a Dead Star Every 71 Minutes
An international team of astronomers, including Dr. Lorne Nelson of the Physics & Astronomy Department at Bishop’s University, have discovered a rare gem: A binary system consisting of a failed star, also known as a brown dwarf, and the remnant of a dead star known as a white dwarf.
What makes this binary so remarkable is that the orbital period of the two objects is only 71.2 minutes. This means that the speeds of the stars as they orbit each other are about 100 km/sec (a speed that would allow you to travel across the Atlantic in less than a minute).
It was Dr. Nelson, a member of the research team at the Centre de recherche en astrophysique du Québec, who made the announcement during a press conference at the semi-annual meeting of the American Astronomical Society in Austin, Texas this month. Matthew Lundy, a Bishop’s student, and Jonathan St-Antoine, a recent alumnus, are two of the co-authors of the paper that has been submitted to the Monthly Notices of the Royal Astronomical Society. The team made extensive use of the Observatoire astronomique du Mont-Mégantic to obtain the data.
“Using five different ground-based telescopes across three continents, we were able to deduce that this binary system consists of a failed star with a mass of about 6.7% that of the Sun (equivalent to 67 Jupiter masses) and a white dwarf that has a mass of about 40% of the sun’s mass,” explained Dr. Nelson. “We have also determined that the white dwarf will begin cannibalizing the brown dwarf in less than 250 million years making this binary the shortest-period pre-cataclysmic variable ever to have been discovered.”
The hot white dwarf star had originally been identified as WD1202-024 and was thought to be an isolated star. Dr. Saul Rappaport at the Massachusetts Institute of Technology and Andrew Vanderburg at the Harvard Smithsonian Center for Astrophysics were analyzing the brightness of more than 28,000 targets from the K2 (Kepler) space telescope when one observation caught their attention. Unlike the transits of exoplanets that pass in front of their host stars and cause a small decrease in the brightness of the star, this light curve showed reasonably deep and broad eclipses.
The team quickly devised a model for the binary showing that it was consistent with a hot white dwarf being eclipsed by a much cooler and lower-mass brown dwarf companion that is seen nearly edge-on. A 3-D animation of the orbit and its lightcurve can be found online. Particular attention should be given to how the hemisphere facing the hot white dwarf is highly irradiated while the other is quite ‘dark’.
Even with this success, some big questions remained. As Dr. Nelson said, “We had constructed a robust model but we still had to address the ‘big-picture’ issues such as how this system formed and what would be its ultimate fate.” In order to answer these questions the team used sophisticated computer models to simulate the formation and evolution of WD1202. According to their calculations, the primordial binary formed from giant clouds of hydrogen-rich gas about 3 billion years ago, and the star that ultimately produced the hot white dwarf in its core was initially not much different from our sun,.
So what will happen in the future? The team believes that the emission of gravitational waves will deplete the orbital energy of the binary so that in less than 250 million years, the separation between the white dwarf and brown dwarf will be so small that the brown dwarf will start to be cannibalized by its white dwarf neighbor. When this happens, the binary will exhibit all of the characteristics of a cataclysmic variable (CV). For this reason, the team believes that the WD1202 system can rightly be referred to as the shortest-period pre-CV that has ever been discovered.
More information on this discovery can be found at http://physics.ubishops.ca/wd1202/
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