Natural mass limit for neutron-star pairs?

Many stars that begin life as heavy-weights die a spectacular death. Gravity squeezes the core of such stars so forcefully that protons and electrons fuse; the center becomes a ball of neutrons. A rebounding shock wave then moves out from the compact core, ejecting the star's outer layers in a colossal explosion called a supernova. Only the naked core remains, a blob of nuclear material with more mass than the sun packed into a sphere only 20 kilometers in diameter.

Welcome to the realm of neutron stars, in which a speck of material just big enough to cover the period at the end of this sentence weighs about 100,000 tons.

In theory, the mass of neutron stars ranges from one-tenth to about twice the mass of the sun. But astronomers have known for more than a decade that at least some of these stars -- those that exist in pairs -- have a much more restricted mass range. Now, a new analysis of neutron-star pairs supports the assertion that formation mechanisms, rather than such general considerations as the stability of superdense matter, limit the mass of such neutron stars.

In addition, says Lee Samuel Finn of Northwestern University in Evanston, Ill., knowing the masses of binary neutron stars will prove invaluable for analyzing observations with a set of gravitational wave detectors now under construction.

Finn's study, detailed in the Oct. 3 PHYSICAL REVIEW LETTERS, examines existing data on four pairs of neutron stars. One star in each pair acts as a celestial lighthouse, beaming radio waves toward Earth at intervals so precise it rivals the steadiness of the best atomic clocks. Tightly bound by gravity, the other neutron star betrays its presence by periodically delaying or advancing the arrival of the radio signals beamed by the first.

Using radio telescopes to detect the signals, astronomers have determined the mass of each member of the neutron-star duos. All of them seem to be about 1.4 times as massive as the sun.

The four pairs represent a tiny fraction of the total number of neutron stars thought to have other neutron stars as partners. But according to Finn, his analysis of this small sample reveals with high statistical certainty that all binary neutron stars range between 1.01 and 1.6 times the mass of the sun.

The finding has several implications, Finn notes. One possibility is that nature's way of making neutron stars, typically in a supernova explosion, may simply not permit them to have more mass than his analysis indicates -- whether or not they have a partner. In that case, the restricted mass range could help fine-tune theories about neutron-star formation, Finn says. Alternatively, he notes, the mass limits may not apply to neutron stars that lack a partner. However, scientists have no way of measuring the mass of isolated neutron stars.

The mass limits have additional significance, finn says. The collision of two neutron stars represents a key source of gravity waves sought by the laser interferometer gravitational wave observatory (LIGO). This pair of detectors, to be located near Livingston, La., and in Hanford, Wash., is slated for completion in 1999. The intensity of the waves depends largely on the mass of the neutron stars, and "of all the sources of gravitational radiation that we can anticipate, this is the one that LIGO is most likely to see," he says.