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Where Do Dead Stars Come From?

Vicky Kalogera, associate professor, WCAS Physics & Astronomy

Where do white dwarfs, neutron stars, and black holes originate? These astral entities, known as compact objects, are the endpoints of ordinary stars when they run out of nuclear fuel in their cores. Once a stellar core is dead, gravity can either be balanced by pressure forces of high-density matter (and thus form a white dwarf or a neutron star), or win altogether and lead to complete gravitational collapse and the formation of a black hole.

These extreme physical states are believed to lie at the heart of some of the most energetic phenomena in the universe such as gamma-ray bursts and X-ray binary systems. Compact objects are also expected to be detectable sources of gravitational waves.

Vicky Kalogera, physics and astronomy, and her research group thoroughly examine this question of origin. Over years a "standard" picture has crystallized: neutron stars and black holes are thought to form from stars more massive than about 10 times the mass of the sun in supernova explosions powered by the collapse of the inner stellar core. As part of the same standard picture, it has been widely accepted that immediate progenitors must satisfy a lower mass limit of two times the solar mass and that explosions forming neutron stars are asymmetric leading to recoil kicks of 200-300 kilometers per second (km/s), whereas black hole formation is less dramatic with either no or very small (of about 20km/s) kicks.

Kalogera, in close collaboration with postdoctoral associate Bart Willems and students, has led recent studies that challenge the standard picture. Specifically, they have developed a methodology for following backwards in time the evolution of binary systems with neutron stars or black holes; this allows them to derive quantitative constraints on their progenitors.

The results have been surprising: they have shown that in a specific galactic system known as the double pulsar, the second neutron star has formed from an unusually low-mass progenitor of about 1.5 times the solar mass with a recoil kick lower than about 100km/s. A study similar in concept of an X-ray binary harboring a black hole has provided the first evidence that some black holes form with significant recoil kicks, in excess of 100km/s.

These results (published in The Astrophysical Journal and Physical Review D) clearly challenge our standard picture concerning the formation of neutron stars and black holes, and they provide evidence for the actual diversity of compact object progenitors and formation processes.

This research is supported by the David and Lucile Packard Foundation and the National Science Foundation Astronomy Division through a CAREER grant.

- Adapted from Office for Research Annual Report 2007