Astronomers led by the University of Warwick have identified the oldest star in our galaxy that is accumulating debris from orbiting planetesimals, making it one of the oldest rocky and icy planetary systems discovered in the Milky Way.
Their findings are published today (5 November) in the Royal Astronomical Society Monthly Notices and conclude that a faint white dwarf 90 light-years from Earth, along with the remnants of its orbiting planetary system, are over 10 billion years old.
The fate of most stars, including those like our sun, is to become a white dwarf. A white dwarf is a star that has burned all its fuel and lost its outer layers and is now undergoing a shrinking and cooling process. During this process, all orbiting planets will be disrupted and in some cases destroyed, with their debris accumulating on the surface of the white dwarf.
For this study, the team of astronomers, led by the University of Warwick, modeled two unusual white dwarfs that were detected by the European Space Agency’s GAIA space observatory. Both stars are polluted with planetary debris, with one being unusually blue, while the other is the faintest and reddest found to date in the local galactic neighborhood – the team submitted the two to further analysis.
Using spectroscopic and photometric data from GAIA, the Dark Energy Survey and the European Southern Observatory’s X-Shooter instrument to determine how long it has been cooling, astronomers have found that the “red” star WDJ2147 -4035 is about 10.7 billion years old, of which 10.2 billion years have been spent cooling as a white dwarf.
Spectroscopy involves analyzing light from the star at different wavelengths, which can detect when elements in the star’s atmosphere are absorbing light at different colors and help determine which elements it is. and their quantity. By analyzing the spectrum of WDJ2147-4035, the team discovered the presence of the metals sodium, lithium, potassium and tentatively detected carbon accumulating on the star, making it the oldest metal-polluted white dwarf discovered. nowadays.
The second “blue” star WDJ1922+0233 is only slightly younger than WDJ2147-4035 and has been polluted by planetary debris similar in composition to Earth’s continental crust. The science team concluded that the blue color of WDJ1922+0233, despite its cold surface temperature, is caused by its unusual mixed atmosphere of helium and hydrogen.
Debris found in the nearly pure helium, high-gravity atmosphere of the red star WDJ2147-4035 comes from an ancient planetary system that survived the star’s evolution into a white dwarf, which has led astronomers to conclude that it is the oldest planetary system around a white dwarf discovered in the Milky Way.
Lead Author Abbigail Elms, Ph.D. student in the physics department at the University of Warwick, said: “These metal-polluted stars show that Earth is not unique, there are other planetary systems with planetary bodies similar to Earth. 97% of all stars will become a white dwarf star and they are so ubiquitous in the universe that they are very important to understand, especially those that are extremely cold.Formed from the oldest stars in our galaxy, white dwarfs cold weather provides information about the formation and evolution of planetary systems around the oldest stars in the Milky Way.”
“We are finding the oldest stellar remnants of the Milky Way that are polluted by once-Earth-like planets. It’s amazing to think that this happened on the scale of 10 billion years, and that these planets died long before Earth was even formed.”
Astronomers can also use the star’s spectra to determine how fast these metals are sinking into the star’s core, allowing them to look back in time and determine the abundance of each of these metals. metals in the original planetary body. By comparing these abundances to astronomical bodies and planetary material found in our own solar system, we can guess what these planets would have looked like before the star died and became a white dwarf – but in the case of WDJ2147-4035, it turned out to be difficult.
Abbigail explains: “The red star WDJ2147-4035 is a mystery because the accreted planetary debris is very rich in lithium and potassium and unlike anything known in our own solar system. It is a very interesting white dwarf because its ultra-cold surface temperature, the metals that pollute it, its old age, and the fact that it is magnetic, make it extremely rare.”
Professor Pier-Emmanuel Tremblay from the Department of Physics at the University of Warwick said: “When these old stars were formed more than 10 billion years ago, the universe was less rich in metals than it seems. is now, since metals form in evolved stars and gigantic stellar explosions. The two observed white dwarfs provide an exciting window into planetary formation in a metal-poor, gas-rich environment that was different from conditions when the solar system formed.
Abbigail Elms et al, Spectral analysis of ultracool white dwarfs polluted by planetary debris, Royal Astronomical Society Monthly Notices (2022). DOI: 10.1093/mnras/stac2908
Provided by the University of Warwick
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