Cornell University

JUNE 8, 2000

How neutron stars get their kicks: Cornell researcher poses rocket theory

ROCHESTER, N.Y. -- When a massive star runs out of nuclear fuel something extraordinary happens in the space of a few seconds: The star's core collapses from a radius of 1,000 miles into a tight, dense ball -- a neutron star -- with a radius of only 10 miles, and then with fearsome energy the massive stellar "envelope," equal to about 10 solar masses, is ejected into outer space.

With the explosion, the newly formed neutron star is sent whizzing along at several hundred miles a second, sometimes more than 600 miles a second, compared with the original star, which typically was dawdling along at a few tens of miles a second. Apparently, the neutron star is "kicked" at birth. But what is the origin of this kick that causes the furious acceleration of the newly born neutron star?

In an attempt to answer this question, Dong Lai, an assistant professor of astronomy at Cornell University, Ithaca, N.Y., proposes two possible ways of firing up the kick. He calls them the "mass rocket" and the "neutrino rocket." Lai presented his theories today at the 196th meeting of the American Astronomical Society at the Convention Center in Rochester, N.Y. His presentation was part of a symposium, "Supernovae Here and There."

"Despite decades of intensive investigations, our understanding of core-collapse supernovae remains significantly incomplete," says Lai. "A major unsolved problem of supernova research is the origin of pulsar kicks and the fact that supernovae are not symmetrical in shape."

Neutron stars, one group of which are called pulsars, are the product of supernovae, the vast explosions in space of dying stars larger than eight solar masses. During the course of the massive star's evolution, nuclear burning turns the star's central region into iron. As the iron core grows bigger than 1.4 solar masses (the so-called Chandrasekhar limit), it collapses under its own gravity.

When the density of the collapsing core reaches that of nuclear matter, the core rebounds and sends off a shock wave that blows the overlying stellar envelope to bits.

Theoretical astrophysicists have expended much effort over the past two decades in trying to understand how the core rebounds and how the shock wave propagates out to make an explosion.

The remnant at the center of the explosion, the neutron star, is a highly dense object that has been compressed under huge gravitational forces. It has long been known that the velocity of neutron stars is much greater than that of the stars from which they emerged. And in the last few years, observational evidence has left little doubt that neutron star kicks are the cause.

Besides explaining the great speed of pulsars, kicks are required in order to explain other phenomena, such as the peculiar properties of binary pulsar systems. Moreover, many recent observations have revealed that supernovae generally are not spherical in shape. "Imagine standing in a boat and throwing a large rock into the water. The boat will travel in the opposite direction to the force of the rock. Something like that must be happening with neutron stars," says Lai.

Working with Peter Goldreich at the California Institute of Technology, Lai has proposed a theory he calls the "mass rocket." Shortly before its collapse, the iron core, which eventually will become the neutron star, is surrounded by a nuclear burning shell. The iron core is beset by disturbance near its boundary in the form of gravity waves -- like waves on the surface of the ocean. The burning shell can pump energy into the waves in the same way that the wind makes the ocean waves get bigger. In some cases, a small disturbance grows so that one side of the core becomes denser than the other side.

Then, as the core collapses, the disturbance is further amplified by gravity. After the core bounces, the outgoing shock wave encounters different densities in different directions, resulting in asymmetrical mass ejection -- the supernova.

Another theory, which Lai has studied with Phil Arras, a former Cornell graduate student now at the University of Toronto, and Yong-Zhong Qian at the University of Minnesota, is the "neutrino rocket." This theory relies on the intense magnetic field that surrounds the newly formed neutron star. After the core falls in on itself, it releases its stored heat in the form of ghostlike particles called neutrinos. As the neutrinos zig-zag out of the hot (hundreds of billions of degrees) neutron star, they interact with the neutron star matter via the so-called weak interaction, the force responsible for radioactivity.

Because the intense magnetic field can polarize the matter, more neutrinos will be emitted along the direction of the magnetic field than opposite to it. "Neutrinos can tell the difference between up and down in the magnetic field," says Lai.

The problem with this theory is that the magnetic field required is much larger than that observed in most neutron stars. However, in the past two years a new class of neutron stars, called magnetars, have been observed with very strong magnetic fields.

However, Lai says, "the mass rocket is, I think, perhaps the more promising theory because it is more generic and does not rely on a very strong magnetic field."

Lai's research is supported by NASA and the Alfred P. Sloan Foundation.

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