(ORDO NEWS) — Astronomers led by the University of Warwick have identified the oldest star in our galaxy that is accreting debris from rotating planetesimals, making it one of the oldest rocky and icy planetary systems found in the Milky Way.
Their results conclude that the dim white dwarf, located 90 light-years from Earth, and the remnants of its orbiting planetary system are over 10 billion years old.
The fate of most stars, including our Sun, is to become white dwarfs. A white dwarf is a star that has burned all of its fuel, shed its outer layers, and is now undergoing a process of contraction and cooling.
During this process, any rotating planets will be destroyed, and in some cases destroyed, and their debris will be left to accumulate on the surface of the white dwarf.
For this study, a team of astronomers led by the University of Warwick modeled two unusual white dwarfs that were discovered by the European Space Agency’s GAIA space observatory.
Both stars are contaminated with planetary debris, one of which turned out to be unusually blue, and the other – the dimmest and reddest one found to date in the immediate galactic environment – the team subjected them to further analysis.
Using spectroscopic and photometric data from the GAIA, the Dark Energy Survey, and the X-Shooter instrument at the European Southern Observatory to determine the duration of the cooling, astronomers found that the “red” star WDJ2147-4035 is about 10.7 billion years old, of which 10.2 billion years was spent on cooling the white dwarf.
Spectroscopy involves analyzing a star’s light at different wavelengths to determine when elements in a star’s atmosphere are absorbing light of different colors and helping to determine which elements they are and in what amounts.
By analyzing the spectrum of WDJ2147-4035, the team found the presence of the metals sodium, lithium, potassium, and previously detected carbon accreting onto the star, making it the oldest metal-contaminated white dwarf discovered to date.
The second “blue” star WDJ1922+0233 is only slightly younger than WDJ2147-4035 and was contaminated with planetary debris similar in composition to the Earth’s continental crust.
The scientific team concluded that the blue color of WDJ1922+0233, despite its low surface temperature, is due to an unusual mixed helium-hydrogen atmosphere.
The debris, found in an atmosphere of almost pure helium and high gravity of the red star WDJ2147-4035, belongs to an old planetary system that survived the evolution of a star into a white dwarf, leading astronomers to conclude that this is the oldest planetary system around a white dwarf discovered in the Milky Way.
Lead author Abigail Elmes of the University of Warwick said: “Stars contaminated with metals show that the Earth is not unique, there are other planetary systems with planetary bodies similar to the Earth.
97% of all stars will become white dwarfs, and they are so ubiquitous in the universe that they are very important to understand. Cool white dwarfs, formed from the oldest stars in the galaxy, provide information about the formation and evolution of planetary systems around the oldest stars in the Milky Way.
We find the oldest stellar remnants in the Milky Way, which are polluted by planets that once looked like Earth. It’s amazing to think that this happened on the scale of 10 billion years, and that these planets died long before the Earth was formed.”
Astronomers can also use the spectra of a star to determine how fast these metals sink into the star’s core, allowing you to look back in time and determine how much of each of these metals was in the original planetary body.
Comparing this abundance with astronomical bodies and planetary material found in our solar system, we can guess what these planets were like before the star died and became a white dwarf, but in the case of WDJ2147-4035, this proved to be a difficult task.
Abigail explains: “The red star WDJ2147-4035 is a mystery, as the accreting planetary debris is very rich in lithium and potassium and is unlike anything known in the solar system.
This is a very interesting white dwarf, as its ultra-cold surface temperature, the metals contaminating it, old age, and the fact that it is magnetic make it extremely rare.”
Professor Pierre-Emmanuel Tremblay from the University of Warwick said: “When these old stars formed over 10 billion years ago, the universe was less rich in metals than it is now, as metals form in evolved stars and giant stellar explosions.
The two observed white dwarfs offer a spectacular window into planetary formation in a metal-poor, gas-rich environment that was different from the conditions when the solar system formed.”
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