(CN) — Astronomers have caught a glimpse of the moment debris from a crumbling planet falls into the ultra-hot surface of a white dwarf star, which had previously only been observed indirectly.
The discovery marks the first-time scientists have been able to directly measure this phenomenon, and confirms decades of indirect evidence seen in over a thousand stars to date. Using spectroscopic imaging at optical and ultraviolet wavelengths, scientists previously determined that 25–50% of all white dwarf stars have atmospheres polluted with heavy elements like iron, calcium and magnesium, which allows researchers to determine where those elements originated.
A team of researchers from the University of Warwick in Coventry, England, published their findings Wednesday in a study in the journal Nature.
“We have finally seen material actually entering the star’s atmosphere,” said Dr. Tim Cunningham, a postdoctoral research fellow in astronomy and astrophysics at the University of Warwick, in a related statement. “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.”
Cunningham explained that previous measurements of accretion rates relied heavily on spectroscopy and were dependent on sophisticated computer models, which calculate how fast a given element sinks out of the atmosphere and into a star. Those models can help researchers determine how much material is falling into a star’s atmosphere as an accretion rate. Cunningham said the trick then is to work backwards and figure out how much of an element was in the original source planet, moon or asteroid.
White dwarfs are an exceptionally dense type of star that astronomers believe to be the final form of any star not dense enough to become a neutron star or a black hole. Their volume is near that of Earth’s, while their mass is closer to that of Earth’s Sun — a star becomes a white dwarf after burning up all its fuel and shedding its outer layers. The nearest white dwarf star is Sirius B, part of a binary star system about 8.6 light years away from Earth.
As material is pulled into a white dwarf, it impacts the surface and forms a layer of shock-heated plasma with temperatures soaring between 180,000 to 1.8 million degrees Fahrenheit. As that layer of hot plasma settles onto the surface of the star and begins to cool down, it emits X-rays which can be detected and measured using advanced instruments.
“White dwarf accretion should be accompanied by intense heating of the infalling material, sufficient to promote cooling via X-ray emission,” said the authors in the study. “This has been observed directly for white dwarfs accreting from stellar companions but never for a white dwarf accreting planetary debris.”
The authors analyzed a nearby white dwarf in their study, dubbed G29-38, using the Chandra X-ray observatory, which is normally used to detect X-rays emanating from black holes and neutron stars. By using Chandra, the team successfully isolated G29-38 from other sources of X-rays for the first time to specifically view just those emissions coming from the white dwarf, confirming decades of indirect spectroscopic observations. Afterwards, the team employed a source detection algorithm to determine the sky density and ensure that what they witnessed didn’t originate from another source nearby.
“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. This detection provides the first direct evidence that white dwarfs are currently accreting the remnants of old planetary systems,” Cunningham said. “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.”
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