The report of a new determination of the orbit of the massive white dwarf Sirius B is being presented by Dr. Jay B. Holberg of the Lunar and Planetary Laboratory, of the University of Arizona in Tucson, Arizona to the American Astronomical Society meeting in Nashville, Tennessee. The new orbit for Sirius B is important because it directly yields an improved estimate of the mass of this very famous and unique white dwarf. Sirius B is one of just a few white dwarfs whose masses are known accurately enough to be used to test a fundamental theory of the interiors of white dwarf stars.
Sirius, in the constellation Canis Majoris, is the brightest and also one of the nearest stars in the sky. It consists of two stars; the familiar bright star known as Sirius A and a tiny faint companion known as Sirius B, which orbits Sirius A every fifty years. In 1844 the German astronomer Friedrich Bessel theoretically predicted that an unseen companion orbited Sirius. The companion was accidently observed by the American telescope maker Alvin G. Clark in 1862. By the 1920's it had become clear that the small faint companion was fundamentally different from other stars. Sirius B is by far the most famous example of a white dwarf star. White dwarfs are the remnant cores of stars that have exhausted their nuclear fuel, shrunk to objects the size of the earth and are continuing to slowly cool. Unlike stars such as the sun, the interiors of white dwarfs are composed of ultra-dense degenerate matter, consisting of a fluid of electrons and nuclei, produced by intense gravitational forces that have crushed individual atoms out of existence.
Sirius B is the nearest known white dwarf, at a distance of only 8.6 light years from the sun, yet because of the closeness at which it orbits the brightest star in the sky, it is one of the most difficult white dwarfs to observe. The primary importance of Sirius B is that it is the most massive of only four white dwarfs for which it has been possible to obtain an accurate mass estimate based on a study of their orbits. In the 1930's the Nobel prize winner Subrahmanyan Chandrasekhar, using the then new ideas of quantum mechanics and relativity, developed a very specific theoretical relationship between the masses and radii of white dwarf stars. By carefully measuring the mass of Sirius B and comparing its predicted radius with its measured radius, it is possible to make a detailed test of Chandrasekhar's theoretical mass-radius relationship. To obtain an accurate estimate of the mass of Sirius B, however, it is first necessary to accurately determine its orbital period and the distances at which the two stars orbit their mutual center of gravity.
Over the past one hundred years astronomers have made numerous determinations of the orbit of Sirius B, the last major orbit solution was completed in 1960. The work presented here involves a new determination of the orbit of Sirius B from over 2000 separate visual observations of the relative positions of the two stars made by over one hundred astronomers during the 19th and 20th centuries, as well as a nearly one hundred more precise photographic observations obtained during the 20th century. A histogram showing the distribution with time of these observations, which now cover nearly three complete orbits is shown in Figure 1. This is the first time all of these diverse sources have been combined to determine the orbit. Figure 2 illustrates how these same observations compare to the best fitting orbit. A key parameter describing the new orbit is an improved orbital period of 50.178 ± 0.024 years. The new masses for Sirius B and Sirius A, respectively are 1.00 ± 0.01 and 2.02 ± 0.03 solar masses. For Sirius B, this represents a slight reduction from previous estimates of the mass and places the white dwarf in better agreement with the theoretical mass-radius relationship.
The present work, which involves the existing ground-based observations of Sirius B, is just a first step in a program to significantly improve the estimates of the mass and radius of Sirius B. Other elements of this program include observations from space with the Hubble Space Telescope and the Far Ultraviolet Spectroscopic Explorer satellite to make several independent estimates of the mass and radius and surface temperature of Sirius B. When these observations are completed in the next several years astronomers should have the most detailed test yet of the relation between the mass and radius of a white dwarf as well as a firm estimate of the consistency of several commonly used methods of measuring the masses and radii of all white dwarf stars.
This work was supported in part by the Space Telescope Science Institute.
For More Information Contact:
Dr. Jay Holberg, 520 621-4571
holberg@argus.lpl.arizona.edu