Star HD 222925 is a cool and unassuming star in the southern constellation of Tucana. It might appear just another of the 100 billion stars in the Milky Way, but new analysis has revealed that it is quite unique. It has the widest range of elements found in a star beyond the solar system.
Researchers have found 65 different elements present in the star and 42 of them are heavy metals such as selenium, silver, tellurium, platinum, gold, and thorium. These elements are rarely found in stars and astronomers still do have not a precise model for how they form. But, as reported in The Astrophysical Journal Supplement Series, the existence of HD 222925 might just change that.
“To the best of my knowledge, that’s a record for any object beyond our solar system. And what makes this star so unique is that it has a very high relative proportion of the elements listed along the bottom two-thirds of the periodic table. We even detected gold,” University of Michigan astronomer Ian Roederer, who led the study, said in a statement. “These elements were made by the rapid neutron capture process. That’s really the thing we’re trying to study: the physics in understanding how, where and when those elements were made.”
Heavy elements are created by the so-called "r-process", or rapid neutron-capture process. This is a set of nuclear reactions responsible for creating about half of the natural elements in the periodic table that are heavier than iron. Iron is the heaviest element produced by standard nuclear fusion.
The r-process requires a lot of energy and it has been proposed that some types of supernovae can provide that type of energy but models have always lacked something. In 2017, the first direct observation of a neutron star collision suggested that these cataclysmic events can also create r-process elements.
And that’s where HD 222925 is key. Its abundance of heavy elements can be seen as a gold standard. The correct model, favoring one scenario (a massive supernovae), the other (a merger of neutron stars), or a combination of the two, will need to be able to explain what astronomers have observed in this particular star.
“We now know the detailed element-by-element output of some r-process event that happened early in the universe,” added co-author Professor Anna Frebel, from the Massachusetts Institute of Technology. “Any model that tries to understand what’s going on with the r-process has to be able to reproduce that.”
This work is a solid step in finally understanding how and where the r-process happens.