Using NASA’s Chandra X-ray Observatory and ESA’s XMM-Newton satellite, astronomers studied unusual X-ray activity in three white dwarfs: KPD 0005 + 5106, PG 1159- 035 and WD 0121-756.
Most stars, including the Sun, will turn white dwarfs after they start to run out of fuel, expand and cool into a red giant, and then lose their outer layers. This evolution leaves behind a stellar core that slowly fades over billions of years.
Typically, white dwarfs emit low energy x-rays. However, KPD 0005 + 5106, PG 1159-035 and WD 0121-756 also had surprisingly bright X-ray emission at higher energies.
KPD 0005 + 5106 stood out among this group. It emitted high-energy x-rays that steadily increased and decreased in brightness every 4.7 hours.
This recurrent ebb and flow of x-rays indicates that KPD 0005 + 5106 has an object orbiting it – either a very low-mass star or a Jupiter-like planet.
Material from the low-mass star or planet could strike the north and south poles of the white dwarf, creating a bright spot emitting high-energy x-rays.
As the white dwarf and her mate orbit each other, this hot spot would come in and go, causing a steady increase and decrease in high-energy x-rays.
“We didn’t know this white dwarf had a mate until we saw the x-ray data,” said Dr. You-Hua Chu, an astronomer at the Institute of Astronomy and Astrophysics at Academia Sinica.
“We looked for the mate with optical telescopes, but we didn’t see anything, which means it is a very faint star, a brown dwarf or a planet.”
KPD 0005 + 5106 is located approximately 1,300 light years away in the constellation Cassiopeia.
It is one of the hottest white dwarf stars known, with a surface temperature of around 200,000 K.
“The companion object is nearly 805,000 km (500,000 miles) from the white dwarf, only one-thirtieth the distance from Mercury to the Sun. Whatever object is, it is blown up by heat,” said Dr. Jesús Toala, astronomer at the National Autonomous University of Mexico.
Astronomers examined what would happen if this object were a planet of Jupiter’s mass – a possibility that fits the data more easily than a dark star or a brown dwarf.
In their models, the white dwarf would pull matter from the planet onto the white dwarf, a process in which the planet could only survive a few hundred million years before being ultimately destroyed.
This stolen material swirls around the white dwarf, which glows in the x-rays that Chandra can detect.
“This is a slow disappearance for this object which is essentially torn apart by constant gravitational forces. It would be a very unpleasant place, ”said Dr Martín A. Guerrero, astronomer at the Institute of Astrophysics of Andalusia.
The other two white dwarfs – PG 1159-035 and WD 0121-756 – were also considered solitary objects, but they show similar energetic X-ray emission to KPD 0005 + 5106. By analogy, this suggests that they may also have weak companions, perhaps planets.
“The harsh X-ray emission from seemingly unique white dwarfs is fueled by accretion from sub-stellar companions or giant planets, and is modulated by orbital motion with a period of 4.7 hours,” concluded the authors.
The results appear in the Astrophysics Journal.
You-Hua Chu et al. 2021. Hard X-ray emission associated with white dwarfs. IV. Signs of accretion of substellar companions. ApJ 910, 119; doi: 10.3847 / 1538-4357 / abe5a5