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Heavy stars injected by supernovae jets
Posted: 16 November 2011

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Unusually enriched ancient stars in the early Universe could have gained their heavy elements from nearby supernovae explosions supplementing the gas clouds from which they were born.

Stars with the heaviest elements, such as gold, platinum and uranium, are something of a conundrum in the early Universe, as they are usually only seen in later generations of stars that have had time to fuse these heavier elements in their cores. But in the outer parts of the Milky Way, which hosts ancient stars from our own Galaxy's youth, between one and two percent of the stellar inhabitants contain unusual quantities of these elements compared with 'normal' heavy elements such as iron, oxygen and carbon.

"These stars are very old; one has an age estimate of 13.2 billion years which is comparable to the age of the Universe," describes Terese Hansen of the University of Copenhagen, who adds that these stars have masses of 0.8 solar masses. Such stars are small but live long, swelling up into red giants at the ends of their lives before cooling into white dwarfs.

Captured by the Very Large Telescope, this image shows the halo of ancient stars surrounding the Sombrero galaxy. These halo stars should possess primitive materials poor in heavy elements like gold, platinum and uranium. The presence of surprisingly heavy stars in the Milky Way's halo could be explained by the injection of heavy material via jets blasting out in supernovae explosion. Image: ESO/P. Barthel.

Hansen's colleagues had already studied abnormal stars like these over several years using the European Southern Observatory's large telescopes in Chile, and spent an additional four years following 17 stars in the northern sky with the Nordic Optical Telescope on La Palma. Hansen noted that three of the stars had clear orbital motions, and were in binary systems.

One theory of enriched stars in binary systems results from one of the stellar pair exploding, drenching its partner in layers of the heavy elements. But in the examples studied by Hansen's team, both stars are still present, so the enrichment points to a different mechanism.

"The binary system that we have found in the sample does not show sign of a supernova explosion – the secondary stars are most probably main sequence stars with masses around 0.6 solar masses – so the enrichment in heavy elements happened before the stars formed," Hansen tells Astronomy Now. "The stars have then formed from an enriched gas cloud, some in binary systems and some as single stars."

The observation therefore supports another proposed theory, that jets from early supernovae explosions of massive stars preferentially enriched some portions of nearby gas clouds, providing the seeds from which these heavy stars formed.

"First the star creates the elements up to iron by fusion in the core and then when it explodes elements heavier than iron are created in the explosion," says Hansen.