- 3706 - ORBITING STARS - smallest orbits yet discovered? Astronomers have discovered a stellar binary, or pair of stars, with an extremely short orbit, appearing to circle each other every 51 minutes. The system seems to be one of a rare class of binaries known as a "cataclysmic variable," in which a star similar to our sun orbits tightly around a white dwarf, which is a hot, dense core of a burned-out star.
-------------- 3706 - ORBITING STARS - smallest orbits yet discovered?
- Nearly half the stars in our galaxy are solitary like the sun. The other half comprises stars that circle other stars, in pairs and multiples, with orbits so tight that some stellar systems could fit between Earth and the Moon.
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- A “cataclysmic variable” occurs when the two stars draw close, over billions of years, causing the white dwarf to start accreting, or eating material away from its partner star. This process can give off enormous, variable flashes of light.
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- The newly discovered system, “ZTF J1813+4251“, is a cataclysmic variable with the shortest orbit detected to date. Unlike other such systems observed in the past, the astronomers caught this cataclysmic variable as the stars eclipsed each other multiple times, allowing the team to precisely measure properties of each star.
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- With these measurements, the researchers ran simulations of what the system is likely doing today and how it should evolve over the next hundreds of millions of years. They conclude that the stars are currently in transition, and that the sun-like star has been circling and "donating" much of its hydrogen atmosphere to the voracious white dwarf.
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- The sun-like star will eventually be stripped down to a mostly dense, helium-rich core. In another 70 million years, the stars will migrate even closer together, with an ultrashort orbit reaching just 18 minutes, before they begin to expand and drift apart.
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- This is a rare case where astronomers caught one of these systems in the act of switching from hydrogen to helium accretion. Astronomers predicted these objects should transition to ultra-short orbits, and it was debated for a long time whether they could get short enough to emit detectable gravitational waves.
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- The astronomers discovered the new system within a vast catalog of stars, observed by the “Zwicky Transient Facility” (ZTF), a survey that uses a camera attached to a telescope at the Palomar Observatory in California to take high-resolution pictures of wide swaths of the sky.
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- The survey has taken more than 1,000 images of each of the more than 1 billion stars in the sky, recording each star's changing brightness over days, months, and years.
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- Looking through the ZTF data for stars that appeared to flash repeatedly, with a period of less than an hour, a frequency that typically signals a system of at least two closely orbiting objects, with one crossing the other and briefly blocking its light.
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- The algorithm sifted out about 1 million stars that appeared to flash every hour or so. This search zeroed in on ZTF, a system that resides about 3,000 light years from Earth, in the Hercules constellation.
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- Astronomers found that the first object was likely a white dwarf, at 1/100th the size of the sun and about half its mass. The second object was a sun-like star near the end of its life, at a tenth the size and mass of the sun, about the size of Jupiter. The stars also appeared to orbit each other every 51 minutes.
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- As the white dwarf orbits the sun-like star and eats away its light hydrogen, the sun-like star should burn out, leaving a core of helium, an element that is more dense than hydrogen, and heavy enough to keep the dead star in a tight, ultra-short orbit.
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- The system was a “cataclysmic variable“, in the act of transitioning from a hydrogen- to helium-rich body. This is a special system that answers a big open question, and is one of the most beautifully behaved cataclysmic variables known.
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October 12, 2022 ORBITING STARS - smallest orbits yet discovered? 3706
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