Astronomers using NASA’s Chandra X-ray Observatory have detected five X-ray events from G29-38, which is among the 100 closest white dwarfs. Their results are the first direct measurement of the accretion of rocky material onto a white dwarf.
An artist’s impression of the G29-38 white dwarf, accreting planetary material from a circumstellar debris disk; when the planetary material hits the white dwarf surface, a plasma is formed and cools via detectable X-ray emission. Image credit: Mark Garlick / University of Warwick.
A white dwarf is a star that has burnt up all its fuel and shed its outer layers.
Over 300,000 such stars have been discovered in our Milky Way Galaxy, and many are believed to be accreting the debris from planets and other objects that once orbited them.
As planetary material is pulled into the white dwarf at a high enough rate it slams into the stellar surface, forming a shock-heated plasma.
This plasma, with a temperature between 100,000 K to one million K, then settles on the surface, and as it cools it emits X-rays that can be detected.
“We have finally seen material actually entering the star’s atmosphere,” said Dr. Tim Cunningham, an astronomer in the Department of Physics at the University of Warwick.
“It is the first time we’ve been able to derive an accretion rate that doesn’t depend on detailed models of the white dwarf atmosphere.”
“What’s quite remarkable is that it agrees extremely well with what’s been done before.”
“Previously, measurements of accretion rates have used spectroscopy and have been dependent on white dwarf models,” he said.
“These are numerical models that calculate how quickly an element sinks out of the atmosphere into the star, and that tells you how much is falling into the atmosphere as an accretion rate.”
“You can then work backwards and work out how much of an element was in the parent body, whether a planet, moon or asteroid.”
Using NASA’s Chandra X-ray Observatory, Dr. Cunningham and colleagues observed the nearby white dwarf G29-38 (also known as ZZ Piscium).
With Chandra’s improved angular resolution over other telescopes, they could isolate the target star from other X-ray sources and viewed, for the first time, X-rays from an isolated white dwarf.
It confirms decades of observations of material accreting into white dwarfs that have relied upon evidence from spectroscopy.
“What’s really exciting about this result is that we’re working at a different wavelength, X-rays, and that allows us to probe a completely different type of physics,” Dr. Cunningham said.
“This detection provides the first direct evidence that white dwarfs are currently accreting the remnants of old planetary systems.”
“Probing accretion in this way provides a new technique by which we can study these systems, offering a glimpse into the likely fate of the thousands of known exoplanetary systems, including our own Solar System.”
A paper on the findings was published in the journal Nature.
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T. Cunningham et al. 2022. A white dwarf accreting planetary material determined from X-ray observations. Nature 602, 219-222; doi: 10.1038/s41586-021-04300-w
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