Astrophysicists model the formation of the oldest star in 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 Göttingen

A team of researchers led by Dr. Stefano Bovino at the Institute for Astrophysics Göttingen (IAG) has conducted high-resolution simulations investigating the formation of the oldest-known star in our galaxy, SMSS J031300.36-670839.3, on a Cray supercomputer of the North-German Supercomputing Alliance.

Using the star's abundance patterns, the scientists have performed cosmological simulations which include 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.

The stars of the first generation have formed out of a primordial gas consisting only of hydrogen and helium.

Their mass was ranging from ten to five hundred times the mass of our Sun.

Nuclear processes in the interior of these stars have created heavy elements like iron, silicon, carbon and oxygen.

When these stars died during the first supernova explosions, the heavy elements have been ejected, and stars of the second generation could form.

"Even for the oldest-known star in the Milky Way galaxy, our simulations indicate that the gas efficiently cools due to the presence of heavy elements," says Dr. Bovino. Such conditions favour the formation of low-mass stars.

The results therefore strongly suggest that the transition to the second generation already occurred after the first supernova explosion.

"The heavy elements provide additional mechanisms for the gas to cool, and it is very important to follow their chemical evolution," explains Dr. Tommaso Grassi from the Center for Star and Planet Formation at the University of Copenhagen.

The scientists have considered SMSS J031300.36-670839.3 for their study, as 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 forming out of the metal-enriched gas, providing the chemical conditions right after the first supernova explosion," says Prof. Dominik Schleicher at the IAG.

While this star has 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 mass of the stars mostly depends on the temperature of the gas, as gravity needs to overcome the thermal pressure during star formation," says Dr. Muhammad Latif, a scientist in the Göttingen Collaborative Research Center 963 on Astrophysical Flow Instabilities and Turbulence.


The new simulations became feasible through the development of the chemistry package KROME, an effort led by Dr. Grassi in 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 our Milky Way galaxy.

More information: "Formation of carbon-enhanced metal-poor stars in the presence of far ultraviolet radiation," Stefano Bovino et al., 2014, Astrophysical Journal Letters, Volume 790, L35: dx.doi.org/10.1088/2041-8205/790/2/L35 , On Arxiv: arxiv.org/abs/1406.4450

Heavy Metal in the Early Cosmos - Simulation

This simulation shows heavy-element-bearing sheets of an exploding star's debris streaming into the center of a cosmic dark matter halo. 

Upon arriving in the center, the streams will enable the formation of the first low-mass stars, when the universe was still only about 200 million years old. 

Image courtesy Jeremy Ritter, Milos Milosavljevic, and Volker Bromm, The University of Texas at Austin.

Ab initio: "From the beginning." It's a term used in science to describe calculations that rely on established mathematical laws of nature, or "first principles," without additional assumptions or special models.

Milos Milosavljevic

But when it comes to the phenomena that Milos Milosavljevic is interested in calculating, we're talking really ab initio, as in from the beginning of time onward.

Things were different in the early eons of the universe.

The cosmos experienced rapid inflation; electrons and protons floated free from each other; the universe transitioned from complete darkness to light; and enormous stars formed and exploded to start a cascade of events leading to our present-day universe.

Working with Chalence Safranek-Shrader and Volker Bromm at the University of Texas at Austin, Milosavljevic recently reported the results of several massive numerical simulations charting the forces of the universe in its first hundreds of millions of years using some of the world's most powerful supercomputers, including the National Science Foundation (NSF) -supported Stampede, Lonestar and Ranger (now retired) systems at the Texas Advanced Computing Center.

The results, described in the Monthly Notices of the Royal Astronomical Society in January 2014, refine how the first galaxies formed, and in particular, how metals in the stellar nurseries influenced the characteristics of the stars in the first galaxies.

"The universe formed at first with just hydrogen and helium," Milosavljevic said. "But then the very first stars cooked metals and after those stars exploded, the metals were dispersed into ambient space."

This simulation shows hydrodynamic instability triggered by rapid cooling in a heavy-element-enriched cosmic dark matter halo when the universe was only 300 million years old.

The instability drives turbulence which breaks the flow into fragments. 

Some fragments undergo gravitational collapse and set to fragment into progressively smaller units. 

From left to right and top to bottom, the six panels show projections of gas density, and the horizontal bar has length 1 pc = 3.26 light years.

Credit: Chalence Safranek-Shrader, Milos Milosavljevic, and Volker Bromm, the University of Texas at Austin

More Information: 'Heavy metal in the early cosmos' Monthly Notices of the Royal Astronomical Society in January 2014 - Milos Milosavljevic, Chalence Safranek-Shrader and Volker Bromm