Almost half of the stars in our galaxy, like the sun, are solitary. The other half is made up of stars that orbit other stars in pairs or multiples, with orbits so close that certain stellar systems might fit between Earth and the moon. Astronomers from MIT and other institutions have discovered a stellar binary, or pair of stars, with an unusually short orbit that appears to circle each other every 51 minutes. The system appears to be a cataclysmic variable, a rare class of binaries in which a star comparable to our sun circles snugly around a white dwarf, a hot, dense core of a burned-out star.
When two stars approach one other over billions of years, a cataclysmic variable develops, leading the white dwarf to begin accreting or eating material away from its partner star. This process may produce large, fluctuating bursts of light, which astronomers imagined centuries ago were the product of some unknown disaster. The newly found system, ZTF J1813+4251, is a cataclysmic variable with the shortest orbit yet discovered. Unlike previous observations of similar systems, the scientists watched this catastrophic variable as the stars obscured each other numerous times, allowing the researchers to carefully analyze the parameters of each star.
The researchers used these observations to run models of what the system is likely to be doing today and how it should evolve over the next hundreds of millions of years. They believe that the stars are in a state of flux and that the sun-like star has been orbiting and "donating" most of its hydrogen atmosphere to the hungry white dwarf. The sun-like star will eventually be reduced to a dense, helium-rich core. In another 70 million years, the stars will move even closer together, with an ultrashort orbit lasting only 18 minutes, before expanding and drifting apart. Researchers at MIT and elsewhere anticipated decades ago that such catastrophic variables will migrate to ultrashort orbits. This is the first firsthand observation of a transitional system.
According to Kevin Burdge, a Pappalardo Fellow at MIT's Department of Physics, "we caught one of these systems in the act of switching from hydrogen to helium accretion." People hypothesized that these particles would enter ultrashort orbits, and it was long contested whether they might get short enough to release observable gravitational waves. This revelation puts an end to it.
Burdge and colleagues publish their findings in Nature. The study's co-authors come from a variety of universities, including Harvard and the Smithsonian Center for Astrophysics. The new system was identified inside a massive catalog of stars studied by the Zwicky Transient Facility (ZTF), a survey that employs a camera linked to a telescope at the Palomar Observatory in California to obtain high-resolution images of large regions of the sky. The study took over 1,000 photos of each of the more than 1 billion stars in the sky, charting how their brightness changed over days, months, and years. Burdge scoured the catalog for indications of systems with ultrashort orbits, the dynamics of which can be so severe that they should generate stunning bursts of light and gravitational waves.
According to Burdge, who is looking for new gravitational-wave sources in the sky, gravitational waves are allowing us to investigate the cosmos in a whole new way.
Burdge searched the ZTF data for stars that appeared to flash regularly, with a period of less than an hour, a frequency that often indicates a system of at least two closely orbiting objects, with one crossing the other and temporarily obscuring its light.
Burdge stated, this thing appeared, where I saw an eclipse happening every 51 minutes, and I said, OK, this is definitely a binary."
He and his colleagues refined the technique further by utilizing the W.M. Keck Observatory in Hawaii and the Gran Telescopio Canarias in Spain. They discovered that the system was unusually "clean," since they could observe the light shift with each eclipse. They were able to precisely determine each object's mass, radius, and orbital period because of their clarity. They discovered that the first object was most likely a white dwarf, measuring 1/100th the size of the sun and weighing around half its mass. The second object was a sun-like star nearing the end of its existence, measuring one-tenth the size and mass of the sun (about the size of Jupiter). Every 51 minutes, the stars appeared to orbit each other.
However, things didn't add up. Burdge stated, "This one star appeared to be the sun, but the sun cannot fit into an orbit shorter than eight hours, so what's going on here?"
Nearly 30 years ago, experts including MIT emeritus professor Saul Rappaport predicted that ultrashort-orbit systems will exist as catastrophic variables. The sun-like star should burn out when the white dwarf orbits it and consumes its light hydrogen, leaving a core of helium, an element denser than hydrogen and heavy enough to hold the dead star in a tight, ultrashort orbit.
Burdge discovered that ZTF J1813+4251 was most likely a catastrophic variable in the process of changing from hydrogen- to a helium-rich body. The discovery both verifies Rappaport and others' forecasts and is the shortest orbit catastrophic variable discovered to date. This is a unique mechanism, according to Burdge. We were doubly fortunate to come upon a system that both addresses a major outstanding question and is one of the most beautifully behaved catastrophic variables known.
Journal Information: Kevin Burdge, A dense 0.1-solar-mass star in a 51-minute-orbital-period eclipsing binary, Nature (2022). DOI: 10.1038/s41586-022-05195-x. www.nature.com/articles/s41586-022-05195-x