11 November 2014

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

computer simulations

Scientists from the Centre for Star and Planet Formation at Copenhagen University in collaboration with colleagues at the University of Goettingen, have modelled the formation of the oldest known star in the Milky Way using high-resolution computer simulations. They performed a cosmological simulation on a supercomputer using the abundance pattern of the star. This simulation allows an improved understanding of the evolution from the first to the second generation of stars in our Universe. The results are published in the Astrophysical Journal Letters.

The first generation of stars originated from the primordial gas, and were mainly formed by hydrogen and helium, with their masses ranging from ten to five hundred times the mass of our Sun. Heavier elements like iron, silicon, carbon, and oxygen were created inside the first generation stars by nuclear reactions and spread out in supernova explosions to the surrounding medium. The gas, polluted with the ejected heavy elements, then formed the second generation of stars.

The simulation was performed by Dr. Stefano Bovino (University of Goettingen) who explains:

- Our simulations indicate that the gas efficiently cools during the formation of the star. Such conditions favour the formation of low-mass stars.

The presence of heavy elements provides additional mechanisms for the gas to cool. The studied star, called SMSS J031300.-36-670839.3, has a higher carbon abundance relative to the other heavy elements, and because of its peculiar composition it is very important for the scientists to follow and model the chemical evolution instead of using a pre-calculated evolution track with a standard composition.

This simulation became feasible through the development of KROME, a new package for modelling chemistry and microphysics of an astrophysical environment. KROME has been developed by an international collaboration led by Dr. Tommaso Grassi at the Centre for Star and Planet Formation in Copenhagen. They will use KROME to study a wide range of conditions to understand the formation of the most metal-poor stars observed in the Milky Way with an unprecedented detail, including several key physical processes that will significantly increase the quality of the models.

Using the very complete chemical model introduced by KROME, it has been shown for the first time how low mass stars can form out of gas with such a low concentration of heavier elements.

Tommaso Grassi explains:

- The first principles calculation of metal cooling included in KROME has demonstrated that the cooling in collapsing low metallicity gas has so far been underestimated. With our more complete model even light molecular cloud-cores can cool efficiently enough during the gravitational collapse for them to form a low-mass star, solving a long standing riddle of how the oldest low-mass stars in the Galaxy formed.


Tommaso Grassi
Tlf: +39 338 6831471

Troels Haugbølle
Associate Professor
Mobil: 60 66 29 80