The Hubble Space Telescope has revealed chemical evidence for the idea that a rare type of hydrogen-depleted supergiant star is created through the merger of two white dwarfs.
快猫短视频s have located 21 so-called 鈥渆xtreme helium stars鈥� in the galaxy. The unusual stars are much larger and hotter than the Sun despite being less massive. Their name reflects their abundance of surface helium and peculiar dearth of hydrogen.
Since the discovery of the first extreme helium star in 1942, two theories have come to dominate the discussion of their origin. One, dubbed the final-flash (FF) model, suggests an outer helium layer of a cooling white dwarf ignites, producing an extra thermal pulse that balloons the star into a giant. Under such circumstances, an extreme helium star might result as the giant then contracts.
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The alternative theory, known as the double-degenerate (DD) model, begins with a binary star system involving a helium-rich white dwarf and a more massive carbon-oxygen-rich white dwarf orbiting one another for billions of years. If the two eventually got too close, the smaller star would rapidly be broken into a disc, and the larger star would consume it, becoming a supergiant star enriched in surface helium.
Chemical abundance
In 2002, Hideyuki Saio of Tohoku University in Japan and Simon Jeffery of Armagh Observatory in Northern Ireland showed how the DD model worked with a detailed computational model.
Now Jeffery and colleagues from the Indian Institute of Astrophysics and the University of Texas, US, have published chemical abundances for 17 extreme helium stars and found that they largely match the DD model鈥檚 predictions.
They used Hubble鈥檚 Imaging Spectrograph to make new measurements of seven stars in the ultraviolet and made additional optical measurements using McDonald Observatory鈥檚 2.7-metre Harlan J Smith Telescope in Texas and the 2.3-metre Vainu Bappu Telescope in India.
鈥淭he new compositions go a lot further than the previous ones,鈥� Jeffery says, with the Hubble data in particular giving intriguing new data on additional elements.
Star shedder
The team measured the stars鈥� surface abundance of 18 elements, including the heavy elements yttrium and zirconium, which are not normally seen in large amounts on the surface of stars. 鈥淲e see them in about the right quantities to match what we would expect from the DD model,鈥� Jeffery told 快猫短视频.
But one element does not line up with the prediction 鈥� oxygen. The team would expect the stars to possess very little surface oxygen because the smaller helium-rich white dwarf鈥檚 stores would behave been destroyed and turned into nitrogen in its helium-rich interior. Since the smaller dwarf essentially coats the outside of the more massive dwarf, it鈥檚 difficult to account for significant surface oxygen in the merged product. But in eight out of 10 cases the researchers measured oxygen at levels similar to the assumed initial abundances.
鈥淚鈥檓 afraid we don鈥檛 really understand it,鈥� Jeffery admits. But he adds that if a star is shredded in 3 or 4 minutes, 鈥渢here will be lots of things that we probably don鈥檛 know yet鈥�.
Journal Reference: The Astrophysical Journal (vol 638, p 454)
