My primary research field is the observation of cataclysmic variable stars (CVs) using X-ray, ultraviolet, optical, and infrared data. X-ray and ultraviolet data are obtained from astronomy satellites, such as the Chandra X-ray Observatory, the Far Ultraviolet Spectroscopic Explorer, and the Hubble Space Telescope. Optical and infrared data are obtained from ground-based observatories, such as Cerro Tololo Inter-american Observatory or the Two Micron All Sky Survey archive. Infrared data can also be obtained from astronomy satellites, such as the Spitzer Space Telescope and the Hubble Space Telescope.

CVs are binary stars in which a low mass main sequence star (the secondary star) orbits a white dwarf (the primary star). CVs typically have orbital periods ranging from a little over an hour to about a day in different systems. This means that they are very compact systems: two typical CVs placed end to end would span a distance approximately equal to the diameter of the Sun.

The secondary star in a CV is a more or less normal star like the Sun, but it is smaller (radius less than about 75% of the Sun's radius), less massive (mass less than about 70% of the Sun's mass), and cooler (temperature less than about 4500 K, compared to 5800 K for the Sun). The luminosity (i.e., a measure of the total amount of energy – or light – radiated by the star) of a CV secondary star is smaller than that of the Sun by a factor ranging from at least 10 to more than 1000. This means that the secondary stars in CVs are very dim compared to the Sun. This type of star is sometimes called a red dwarf (at temperatures cooler than the Sun, the apparent color of a star changes from yellow like the Sun, to orange, to red for the coolest stars).

The primary star in a CV is a white dwarf (WD). A WD is a stellar remnant – the core of a star like the Sun that is left after the star depletes its supply of hydrogen and helium gases required for the thermonuclear reactions that power normal stars. When a star that initially has a mass of up to about 8 times the mass of the Sun runs out of nuclear fuel, it stops generating energy (i.e., light). When this happens, the upward pressure that is generated by ebergy production in a star can no longer balance the downward force of the star's own gravity, and its core (comprising approximately the inner 10–50% of its mass) collapses to form a WD.

WDs are very small (about the size of the Earth), but they are also very massive (typical WD masses are 0.5–1.0 times the mass of the Sun). This means that they have very high densities: 1 cubic centimeter of a typical WD (about the volume of two M&M candies, or a single sugar cube) “weighs” about 2,000,000 grams (that's 2 million grams, which is equivalent to the mass of an adult elephant!). WDs can be very hot (with temperatures higher than 100,000 K – almost 20 times the temperature of the SUN) or cool (with temperatures similar to the Sun). A hot WD emits a lot of energy per square centimeter of its surface, but because it is also very small, this does not add up to a large total amount of radiated energy. Thus, WDs are typically more than 1000 times fainter than the Sun. However, because they do not contain any more usable nuclear fuel, no new energy is produced in a WD. A WD slowly radiates away the energy that was trapped inside it when it formed from a dying star, becoming cooler and dimmer. After billions of years, the WD will no longer emit any light, and become an essentially invisible black dwarf.

Its small size and large mass give a WD an intense gravitational field that squeezes the secondary star in a CV into a teardrop shape. Matter (mainly hydrogen gas) from the secondary star “leaks” out of the pointy end of the teardrop and forms a disk around the WD. This process is called mass transfer.

A particle in the disk around the WD (such as a single atom of hydrogen) undergoes viscous interactions with other particles in the disk (basically this means that the particles in the disk rub against each other). This causes the particles to lose angular momentum, and they gradually spiral inward in the disk until they impact onto the surface of the WD. This process is called accretion; the disk in a CV is often referred to as an accretion disk.

As particles spiral inward in the disk, they heat up and begin to emit light due to the friction of rubbing against their neighbors and the release of gravitational potential energy as the particles fall deeper into the gravitational field of the WD. This causes the accretion disk to heat up and emit a large amount of light, ranging from infrared at its outer edges, to ultraviolet and even X-rays near the WD. The accretion disk in a typical CV contributes the majority of the light that astronomers observe from the CV. It is difficult to observe the light output from the WD or the secondary star unless far-ultraviolet or infrared data, respectively, are available.

Artist's representation of a cataclysmic variable (by Mark Garlick). The secondary star is on the right (complete with starspots!) and the white dwarf primary star is hidden at the bright center of the accretion disk.

Disks, mass transfer, and accretion play important roles in a wide variety of astrophysically important scenarios, from the formation of stars and planets, to the power sources for active galactic nuclei and quasars. Because of their small size, rapid time scales, and apparent brightness (as viewed from the Earth), cataclysmic variables offer the best natural laboratories for observing these processes. Before we can fully understand how disk formation, mass transfer, and accretion proceed in different settings throughout the Universe, we must understand how they work in cataclysmic variables.

I also study other types of binary stars, such as

• Low mass X-ray binaries (LMXBs), in which a star like the Sun or the secondary star in a CV orbits around a neutron star (which is the possible remnant of a supernova explosion that can end the life of a very massive star).




• Non-interacting white dwarf + red dwarf binaries; these are similar to CVs except that the geometric and gravitational properties of the binary do not lead to mass transfer. These systems allow us to study the properties of a control group of CV-like binaries in which the mass transfer/accreiton faucet has been turned off. Because of the relative faintness of the WD primary star, they also offer a means of studying binaries containing extremely low mass red dwarf (or brown dwarf) secondary stars (which would be lost in the glare of a non-WD primary star).

For more information about cataclysmic variables, see The Big List of SW Sextantis Stars. Technical descriptions and results for some of my ongoing research projects can be found in the research section of my web site.