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Ski or jump? How neutron star kicks can break up clusters

Last Updated on Thursday, 09 July 2015 12:47
Published on Wednesday, 08 July 2015 23:01

Contenta smallVISTA infrared image of the globular star cluster Messier 55. Credit: ESO. Click for a full-size imageA supernova explosion at the end of a large star’s life can leave the collapsed core, or neutron star, hurtling away from its dust and gas envelope at hundreds of kilometres per second.  Now, astronomers have found that even a tiny number of these neutron star ‘natal kicks’ can have a dramatic effect on the lifetime of surrounding star clusters. The fast-moving neutron stars can cause star clusters to lose mass and break apart up to four times more quickly. The discovery will be presented by Filippo Contenta at the National Astronomy Meeting in Llandudno.

 

Contenta, a PhD student at the University of Surrey, said, “Astronomers have known for some time that there are two distinct scenarios for the break up of star clusters: a smooth, gradual loss and a sudden discarding of mass.  You can think of it as the difference between skiing down a gentle slope and jumping off a cliff.  Because the dividing line between the two modes is very sharp, it’s not surprising that a very small effect can make a big difference.  Our study suggests that natal kicks from neutron stars could be one of the triggers that sends star clusters into ‘jumping’ modes of rapid dissolution rather than the more gradual ‘skiing’.”

 

While neutron star natal kicks are known to occur, their frequency, distribution and cause are still uncertain. The high velocities from the kicks should send neutron stars beyond the gravitational pull of the star clusters they are born in. However, because the neutron stars are very hard to observe, their retention fraction in globular clusters is still unknown. Contenta and his colleagues ran a series of computer simulations of the evolution of different sized clusters, either with or without the presence of neutron star natal kicks. They found that even though neutron stars accounted for less than 2 per cent of the star cluster by mass, the presence of kicks could have a big effect on the cluster’s evolution.

 

In particular, the presence of kicks and the ‘jumping’ mode of rapid mass loss meant that the clusters never achieved the high degree of central concentration of stars, observed in about 20 per cent of clusters, which is often considered to be an outcome of the process of ‘core collapse’.  On the other hand, all longer-lived ‘skiing’ models showed signatures of core collapse, a phenomenon resulting from the slow dynamical evolution of a stellar system, driven by gravitational encounters between individual stars.

 

“We know that multiple factors affect the rate at which star clusters lose mass,” said Contenta. “But while our study has focused on kicks by neutron stars, perhaps the main lesson to learn is that apparently minor changes can have very large effects.”

 


Media contacts

 

Dr Robert Massey

Royal Astronomical Society

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Ms Anita Heward

Royal Astronomical Society

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Dr Sam Lindsay

Royal Astronomical Society

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Science contacts

 

Mr Filippo Contenta

University of Surrey

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Further information

 

"Neutron star natal kicks and the long-term survival of star clusters", F. Contenta, A. L. Varri and D. C. Heggie, Monthly Notices of the Royal Astronomical Society, Volume 449, Issue 1, L100-104, 2015

 


Notes for editors

 

The Royal Astronomical Society National Astronomy Meeting (NAM 2015) will take place in Llandudno, Wales, from 5-9 July. NAM 2015 will be held in conjunction with the annual meetings of the UK Solar Physics (UKSP) and Magnetosphere Ionosphere Solar-Terrestrial physics (MIST) groups. The conference is principally sponsored by the Royal Astronomical Society (RAS) and the Science and Technology Facilities Council (STFC). Follow the conference on Twitter

 

The Royal Astronomical Society (RAS), founded in 1820, encourages and promotes the study of astronomy, solar-system science, geophysics and closely related branches of science. The RAS organises scientific meetings, publishes international research and review journals, recognises outstanding achievements by the award of medals and prizes, maintains an extensive library, supports education through grants and outreach activities and represents UK astronomy nationally and internationally. Its more than 3800 members (Fellows), a third based overseas, include scientific researchers in universities, observatories and laboratories as well as historians of astronomy and others. Follow the RAS on Twitter

 

The Science and Technology Facilities Council (STFC) is keeping the UK at the forefront of international science and tackling some of the most significant challenges facing society such as meeting our future energy needs, monitoring and understanding climate change, and global security. The Council has a broad science portfolio and works with the academic and industrial communities to share its expertise in materials science, space and ground-based astronomy technologies, laser science, microelectronics, wafer scale manufacturing, particle and nuclear physics, alternative energy production, radio communications and radar. It enables UK researchers to access leading international science facilities for example in the area of astronomy, the European Southern Observatory. Follow STFC on Twitter