Pair of stars in “impossible” close dance – Sun-like star and white dwarf have orbital periods of only 51 minutes

Stellar record: Astronomers have seen a pair of stars orbiting each other closer and faster than meets the eye at first glance. The pair of white dwarf and sun-like star takes just 51 minutes to complete one orbit — less than ever measured for such a pair. This “tight dance” is possible because the white dwarf has sucked almost all the hydrogen out of its partner, leaving almost only the denser helium core, as the astronomers report in “Nature.”

Most stars in the cosmos are binary stars – and some orbit very closely. For example, the two young giant stars in the binary star MY Camelopardalis take only 1.2 days to complete one orbit and touch each other with their gas shells. Many white dwarfs in binary stars transfer material from their stellar partner and get very close to it. Such “catastrophic variables” can have an orbital time of as little as 75 minutes.

Turnaround time of only 51 minutes

Now astronomers have discovered a pair of stars that orbit more closely than any other catastrophic pair ever observed. The ZTF J1813+4251 system, discovered by Kevin Burdge of the Massachusetts Institute of Technology (MIT) and his team, is located about 3,000 light-years away in the constellation Hercules and has been detected in data from the Zwicky Transient Facility (ZTF) at the Palomar Observatory in California.

“I came across this object that seems to go through an eclipse every 51 minutes,” Burdge reports. This is the shortest orbit ever observed in a catastrophic variable. To learn more about the system, astronomers observed ZTF J1813+4251 using high-resolution spectroscopes from the Keck Observatory in Hawaii and Gran Telescopio Canarias in Spain. This allowed them to determine the mass, radius and orbital periods of the two stars.

White Dwarf and Sun-Like Star

The surprising result: The pair, orbiting so closely, was a white dwarf and a Sun-like main sequence star with a mass of about one solar mass. “So one star in this pair looks like the sun, but the sun doesn’t fit into an orbit with an orbit of less than eight hours,” Burdge explains. “So what’s going on up there?” A Sun-like star is actually far too “thick” to allow for an orbital period of just 51 minutes.

Nevertheless, according to the measurements, both stars are less than half a ray of sunshine from each other. The spectral analyzes provided an explanation for this “impossible” proximity. Because they revealed that the supposedly sun-like star showed fewer hydrogen signatures, but an unusually large number of helium spectral lines. This suggests the white dwarf has already siphoned off much of its hydrogen shroud from its partner, the astronomers explain.

Transitioning to the helium star

But that means: This pair’s Sun-like star is almost “stripped down” to its compact core. “As a result, this star has a temperature comparable to that of the Sun, but its density is 100 times higher because of its helium-rich composition,” the team reports. As a result, the star is only as big as Jupiter, but very massive in proportion, which explains its unusually short orbit around the white dwarf.

“So this is a very special system,” Burdge explains. “Because this is the rare case of a catastrophic star pair in the process of transitioning from hydrogen to helium accretion.” This means that the white dwarf has already sucked so much hydrogen out of its partner that soon only helium will be left. Then the stellar “cannibal” will also pull this gas from its partner, reducing the radius even further. The binary number ZTF J1813+4251 is now the first thing astronomers have been able to ‘capture’ during this transition.

Further shortening is coming

Burdge and his team expect the binary system’s already ultra-short orbital time to shorten even further in the future. In about 70 million years, they estimate, the ZTF J1813+4251 will have completed its full transition to a helium system. Then both stars could have come so close that they only need 18 minutes to orbit each other, as the astronomers report.

However, it doesn’t stop there: because the aspirated star cools and expands due to its material loss, the two will drift apart again after the minimum of their orbit. “The system will take 300 million years and then again it will take about 30 minutes for one orbit as a catastrophic helium variable,” the team explains.

The exotic pair of stars thus gives astronomers valuable insights into the behavior of such disparate partners — confirming some of the previously only theoretical assumptions about their evolution. (Nature, 2022; doi: 10.1038/s41586-022-05195-x)

Source: Massachusetts Institute of Technology

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