Nelemans - Ultracompact binary stars - Physics Today July...

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11/15/2006 09:37 AM Ultracompact binary stars - Physics Today July 2006 Page 1 of 8 Figure 1 URL: Published: July 2006 [Permission to reprint or copy this article/photo must be obtained from Physics Today. Call 301-209-3042 or e-mail with your request.] ARTICLES Ultracompact binary stars Double stars with tight, rapid orbits enable astronomers to study issues ranging from binary-star evolution to the internal structure of white dwarfs and neutron stars. In addition, they may emit directly observable gravitational waves. Gijs Nelemans July 2006, page 26 Stars most often don't shine alone: Many of the specks of light visible at night actually are binaries comprising two stars orbiting each other. The orbits in general are well described by Newton's law of gravity, just like the planetary orbits in our solar system; the exceptional cases can be explained by Einstein's theory of general relativity. Thus, the closer the stars are to each other, the shorter their orbital period as described by Kepler's third law. At the end of the 1960s, binary stars with periods of less than one hour were discovered, and astrophysicists recognized that the two stars are so close together that an ordinary star like the Sun could not fit in their orbits. 1,2 The close proximity of the objects suggests that both components are stellar remnants—white dwarfs, helium stars, neutron stars, or even black holes—formed after stars exhaust their nuclear fuel. Box 1 offers a brief tutorial on stellar evolution and describes those remnants. As the binary stars mutually orbit, they emit gravitational waves that remove energy from the binary system. As a result, the two stars get closer to each other. In time, they can get so close that gas from the outer layers of one of the stars falls onto its companion in a process called accretion. For the remainder of this article, I will mostly discuss those so- called interacting ultracompact binaries. By now about 30 such objects are known. They naturally divide in two classes, based on their observed characteristics. In ultracompact x- ray binaries, gas falls onto a neutron star whose potential well is so deep that the gas heats up to millions of kelvin and produces abundant x rays. The second class comprises the AM CVn stars, so named because the first of them was the variable star AM CVn discovered in the constellation Canes Venatici (hunting dogs). In AM CVn systems gas falls onto a white dwarf, which has a much shallower potential well than a neutron star. The gas is heated to only about 100 000 K, and most radiation is emitted at optical and UV wavelengths in the range of 100–600 nm. Most studies of AM CVn systems have been done in that wavelength range, in particular by Joseph Patterson (Columbia University), Jan-Erik Solheim (University of Tromsø, Norway), Brian Warner (University of Cape Town, South Africa), Thomas Marsh (University of Warwick, UK), and
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Nelemans - Ultracompact binary stars - Physics Today July...

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