Scientists model the formation of the oldest known star in the Milky Way

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Scientists from the Universities of Gottingen and Copenhagen have modelled the formation of the oldest known star in the Milky Way using high-resolution computer simulations. Using the star’s abundance patterns, the scientists performed cosmological simulations on a supercomputer of the North-German Supercomputing Alliance which included the dynamics of gas and dark matter as well as the chemical evolution. From this simulation, the scientists expect to obtain an improved understanding of the transition from the first to the second generation of stars in the Universe. The results of their study were published in the Astrophysical Journal Letters.

Scientists model the formation of the oldest known star in the Milky Way
The illustration shows projections of the gas density, temperature and the fraction 
of ionized carbon in the central region where the star forms, in simulations with
 different abundances of the heavy elements, from 0.01 to 0.0001 times the solar value. 
The results show that a strong transition occurs for a carbon abundance of 0.01 times 
the solar value, providing a pathway for the formation of low-mass stars 
[Credit: Institute for Astrophysics Gottingen]

The stars of the first generation were formed out of a primordial gas which consisted only of hydrogen and helium. Their mass ranged from ten to five hundred times the mass of our Sun. Nuclear processes in the interior of these stars created heavy elements like iron, silicon, carbon, and oxygen. When the stars died during the first supernova explosions, the heavy elements were ejected and formed the stars of the second generation.

“Our simulations indicate that the gas efficiently cools during the process,” explains the leader of the study, Dr. Stefano Bovino from Gottingen University’s Institute for Astrophysics. “Such conditions favor the formation of low-mass stars.” The presence of heavy elements provides additional mechanisms for the gas to cool. It is therefore very important for the scientists to follow and model their chemical evolution.

The scientists chose the oldest known star of the Milky Way, called SMSS J031300.-36-670839.3 and estimated to be roughly 13.6 billion years old, because its abundance patterns were previously shown to be consistent with one single low-energy supernova. “It seems very likely that this star is indeed one of the very first stars that formed out of the metal-enriched gas,” says Gottingen University’s Prof. Dr. Dominik Schleicher. “The chemical conditions reflect those right after the first supernova explosion.”

While SMSS J031300.-36-670839.3 has only a tiny amount of heavy elements, it has a relatively higher carbon abundance. It in fact represents an entire class with similar properties, and the scientists expect a very similar formation pathway for the entire class. The new simulations became feasible through the development of the chemistry package KROME, a joint effort led by the University of Copenhagen. In the future, the scientists plan to explore a wide range of possible conditions to understand the formation of the most metal-poor stars observed in the Milky Way.
Source: Institute for Astrophysics Gottingen [July 22, 2014]

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