The fate of most stars, including those like our Sun, is to Our Sun is just one of many stars that will eventually become a white dwarf. In our galaxy, more than 300,000 white dwarf stars have been spotted, with many of them thought to be accreting debris from planets and other things that once orbited them.

Astronomers at the University of Warwick have detected for the first time the moment when debris from destroyed planets collides with the surface of a white dwarf star.

They used X-rays to find the rocky and gaseous stuff that a planetary system leaves behind after colliding with and being eaten by its host star.

The findings, which were published in the journal Nature today, are the first direct measurement of rocky material accretion onto a white dwarf and confirm decades of indirect evidence of accretion in over a thousand stars. The reported event took place billions of years after the planetary system was formed.

Most stars, including our Sun, will eventually turn into white dwarfs. Our galaxy has about 300,000 white dwarf stars, many of which accrete garbage from planets and other things that earlier orbited them.

For decades, astronomers have employed spectroscopy at optical and ultraviolet wavelengths to detect the abundances of elements on the star’s surface and establish the composition of the object from which it emerged. Spectroscopic measurements suggest that 25–50 percent of white dwarfs contain heavy elements such as iron, calcium, and magnesium polluting their atmospheres, giving astronomers indirect evidence that these objects are constantly accreting.

Until recently, however, astronomers had not witnessed the material being drawn into the star.

“We have finally seen material actually entering the star’s atmosphere,” says Dr Tim Cunningham of the University of Warwick Department of Physics, adding, “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.”

A white dwarf is a star that has consumed all its fuel and lost its outer layers, potentially killing or disturbing any orbiting bodies. As material from such bodies is drawn into the star at a fast enough pace, it collides with the star’s surface, generating shock-heated plasma. This plasma, which has a temperature of 100,000 to a million degrees kelvin, settles on the surface and releases detectable X-rays as it cools.

X-rays are like visible light, but far more powerful. They’re made up of extremely fast-moving electrons (the outer shells of atoms, which make up all the matter around us). In astrophysics, X-rays are the primary fingerprint of material pouring down on exotic phenomena such as black holes and neutron stars. They are well recognized for their usage in medicine.

The limited amount of X-rays that reach Earth can be lost among other strong X-ray sources in the sky, making detection difficult. The astronomers using the Chandra X-ray Observatory, which is generally used to detect X-rays from accreting black holes and neutron stars, studies study the neighboring white dwarf G29–38.

They were able to isolate the target star from other X-ray sources thanks to Chandra’s enhanced angular resolution, and they saw X-rays from an isolated white dwarf for the first time. It backs up decades of observations of material accreting into white dwarfs based on spectroscopic data.

Dr. Cunningham adds: “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. 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.”

Source: 10.1038/s41586-021-04300-w

Image Credit: University of Warwick/Mark Garlick

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