Please activate JavaScript!
Please install Adobe Flash Player, click here for download

Foeschungszentrum Jülich - Research in Jülich 1_2013

15 RESEARCH AT THE CENTRE | Merging Stars 1|2013 Research in Jülich The Orion Nebula, seen here through the Hubble Space Telescope, is a dense system of gases and stars. This multicoloured emission nebula is one of the most productive stellar nurseries in the Milky Way. stars in clusters are binaries, while in fields – such as the one through which the Sun is travelling – this applies only to 50% of the stars. Like a piece in a jigsaw puzzle, this discrepancy corresponds to another one that’s the point of departure for Christi- na Korntreff’s research. “In clusters of stars, many binary stars orbit each other very closely. This is much less frequently the case in fields.” This means that some binaries disappear when clusters dis- perse in the course of time and become fields. But why is this so? Two years ago, another astrophysicist named Steven Stahler proposed a theo- ry to explain this phenomenon. It postu- lates that the gas in the cluster restricts the orbit of stars until, after millions of years, they merge to form a larger celes- tial body. This would mean that binary stars are only an episode in the life of some stars, like the caterpillar in a but- terfly’s. However, astronomical theories that describe a development are difficult to verify. These processes take far too long to observe – if they can be seen at all with a telescope. “It would be very con- venient if it were possible to put the Ori- on Nebula into a box and look at what happens in time lapse,” says Christina Korntreff. And that’s – at least roughly speak- ing – what they do at JSC. In order to get to the bottom of the disappearing binary stars, she and her colleagues used the JUROPA supercomputer to simulate a cluster of stars with the physical dimen- sions of the Orion Nebula: 4,000 stars and a comparable distribution of mass- es, periods of orbit, and gases. Calculat- ing the gravitational forces as precisely as possible was a special challenge in this simulation. Gravitational equations including more than two bodies are tricky – like equations to determine the number pi, they can only be solved ap- proximately. Mathematicians refer to this as an n-body problem. “I was lucky to be able to fall back on an existing sim- ulation code. It describes particularly close binary stars separately, which pre- tre (JSC). She is the only astrophysicist in the Computational Science group, which comprises researchers from very different disciplines. What she finds fascinating about as- trophysics is that “we can learn anything at all about the stars when they’re so far away”. In fact, astronomical observa- tions provide nothing but snapshots – but theoretical astrophysicists like Christina Korntreff help us to get a bet- ter idea of what the universe looks like by interpreting these data. BINARY STARS DISAPPEAR For example, we know that approxi- mately 60% of all stars are binary stars. Some of them orbit each other in a few hours, others take thousands of years. But what is striking is that about 75% of vents their internal interaction from slowing down the entire simulation,” says the astrophysicist. GAS ACTS AS HANDBRAKE As part of the simulation of the clus- ter, she then calculated how, according to Stahler’s theory, gas and binary stars interact and the consequences of these interactions for the population of bina- ries in the cluster. “We were surprised how clearly our simulation reproduces the binary star populations in fields and clusters,” says Korntreff. This suggests that what happens in the star clusters is exactly what Stahler’s theory predicts: The gravitational tug of the stars attracts gas, which becomes so dense that its force of attraction slows down the move- ment of the star – like a handbrake that is on slightly. This causes the orbit to be- come smaller and smaller. Within a peri- od of a million years, binary stars that initially circled one another at a distance of a hundred times that between the Sun and the Earth approach each other and begin to exchange matter – a bit like what happens in an umbilical cord – until they finally merge. “Thanks to this simulation, we now have a better understanding of how bi- nary stars disappear and what happens when a cluster forms,” says the astro- physicist, on the significance of her find- ings for science. Her work with super- computers benefited from the exchange of know-how with colleagues at JSC. Her PhD supervisor was also at hand to offer expert advice – Prof. Susanne Pfalzner from the Max Planck Institute for Radio Astronomy (MPIfR) in Bonn. Together with her and another colleague from MPIfR, Christina Korntreff recently pub- lished initial results in the journal Astron- omy and Astrophysics. :: Christoph Mann Institute Publication

Pages