Stellar record: Astronomers have spotted a pair of stars orbiting each other more closely and faster than first appears possible. The pair of white dwarf and sun-like star needs only 51 minutes for one orbit – less than ever measured for such a pair. This “tight dance” is possible because the white dwarf has sucked out almost all of the hydrogen from its partner and almost only the denser helium core is left, as the astronomers report in “Nature”.
Most of the stars in the cosmos are binary stars – and some of them orbit each other extremely closely. The two young giant stars in the double star MY Camelopardalis For example, they only need 1.2 days for one orbit and touch each other with their gas shells. Many white dwarves in binary systems suck off material from their star partner and also come very close to it. Such “cataclysmic variables” can have orbital times as short as 75 minutes.
Orbital period of only 51 minutes
Now astronomers have discovered a pair of stars that orbit each other more closely than any other cataclysmic pair ever observed. Discovered by Kevin Burdge of the Massachusetts Institute of Technology (MIT) and his team, the ZTF J1813+4251 system lies around 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 appears to be going through an eclipse every 51 minutes,” reports Burdge. This is the shortest orbital period ever observed in a cataclysmic variable. To find out more about the system, astronomers observed ZTF J1813+4251 using the high-resolution spectroscopes at Keck Observatory in Hawaii and Gran Telescopio Canarias in Spain. This enabled 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 each other 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 orbital period of less than eight hours,” explains Burdge. “So what’s going on up there?” A sun-like star is actually far too “fat” to allow for an orbital period of just 51 minutes.
Nevertheless, according to the measurement data, both stars are less than half a solar radius apart. 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 that the white dwarf has already siphoned off much of its hydrogen envelope from its partner, the astronomers explain.
In transition to the helium star
But that means: The sun-like star of this pair is almost “stripped” down to its compact core. “As a result, this star has a temperature comparable to the sun, but its density is 100 times higher due to its helium-rich composition,” reports the team. As a result, the star is only as large as Jupiter, but very heavy in proportion, which explains its unusually close orbit around the white dwarf.
“So this is a very special system,” explains Burdge. “Because this represents the rare case of a cataclysmic star pair that is in the process of switching from hydrogen to helium accretion.” What is meant by this is that the white dwarf has already sucked so much hydrogen from its partner that soon only helium is left is left. Then the stellar “cannibal” will also pull this gas from its partner, reducing its radius even further. The ZTF J1813+4251 binary is now the first that astronomers have been able to “catch” during this transition.
Further shortening is imminent
Burdge and his team expect that the already ultra-short orbital period of the binary system will shorten even further in the future. In around 70 million years, they estimate, ZTF J1813+4251 will have completed the complete switch to a helium system. Then both stars could have come so close to each other that they only need 18 minutes for one orbit around each other, as the astronomers report.
However, it will not stop there: Because the sucked star cools down as a result of its loss of material and expands again, the two will drift apart again after the minimum of their orbital period. “The system will need 300 million years to then need around 30 minutes for one orbit again as a cataclysmic helium variable,” explains the team.
The exotic pair of stars thus provides astronomers with valuable insights into the behavior of such dissimilar partners – and confirms some of the previously only theoretical assumptions about their development. (Nature, 2022; doi: 10.1038/s41586-022-05195-x)
Quelle: Massachusetts Institute of Technology