The accretion process (whereby material falls onto an object) is fundamental to many astrophysical situations. The process of star and planetary formation must involve accretion, and it is thought that accretion of material onto a central, massive black hole may be the power source for quasars, the most luminous objects in the Universe! In most of these situations, however, our ability to view the accretion process is shrouded by cooler, obscuring material. Hence, the physical processes involved in accretion are not well understood.
Cataclysmic binary stars are ideal laboratories for studying the physics of the accretion process. They are close double star systems with orbital periods of a few hours to a few days. Studies of the optical light variations in these systems are often sufficient to determine the masses and separations of the stars and the viewing angle. These studies tell us that the partners in this "stellar waltz" are usually a typical low mass star (not too different from the sun) and a white dwarf star (a star which has burned all its nuclear fuel and contracted until its diameter is only about 1% that of the sun). Cataclysmic variables are relatively common and different systems offer enough variety to provide information on accretion in a range of different physical situations. Most important, however, is that they are relatively nearby and unobscured by surrounding material so that the accretion process can be observed directly. Look here for an artist's conceptual image of a cataclysmic variable.
Because the separation of the stars is quite small by astronomical standards, the gravitational attraction of the white dwarf is strong enough to capture material shed by the normal star and pull this material onto itself. However, the stellar waltz prevents this material from being accreted directly. Instead, this material tends to spiral in onto the white dwarf star, forming a disk of accreting material. The gas temperatures can get very high, and these systems radiate most of their energy in the ultraviolet and far ultraviolet part of the spectrum, which is inaccessible from the surface of the earth. Hence, far ultraviolet observations are crucial for learning about the hottest portions of these systems.
HUT scientists obtained the first high quality spectra of cataclysmic variable stars in the 900 - 1200 angstrom range during Astro-1. This spectral range is important because the inner part of the accretion disk (that extends down to the surface of the white dwarf) radiates predominantly below 1200 angstroms; as a result HUT spectra provide unique insights into the details of the accretion process. Additionally, there are many emission lines in the HUT wavelength range that are "diagnostics" of the physical conditions in various parts of these systems (i.e. the inner or outer parts of the accretion disk or accretion column, the heated face of the companion star, a "corona" of material extending above the disk, etc.). HUT observations at various binary orbital phases also made it possible to isolate the locations and characteristics of various emission components. By directly observing the white dwarf stars, for instance, HUT scientists can characterize their properties much more accurately than has ever been done before.
Cataclysmic variables have been studied for many years by amateur and professional astronomers because they have periodic light variations (due to their changing orbit), and also undergo semi-regular outbursts in which their light output changes by factors of 100 or more. The reason for these outbursts is not well understood, and could be due either to changes in the rate of mass transfer or to some instability that occurs in the accretion disk itself. One of the goals for HUT on Astro-2 is to follow at least one such system throughout its outburst period to discern what happens during one of these outbursts. This will require some luck, and a good bit of real-time replanning of observations to catch one of these systems in outburst. Information on the outburst status of key cataclysmic variables will be provided to HUT astronomers in near real-time by the American Association of Variable Star Observers (AAVSO), a world-wide network of amateur astronomers.
One of the most important factors influencing accretion in some cataclysmic variables is magnetic fields. If the magnetic field is strong enough, it can disrupt the accretion disk and channel material onto the magnetic poles of the white dwarf star. Many processes occur in the column of material as it plunges onto the white dwarf, and on the heated surface of the white dwarf itself. No magnetic cataclysmic variables were observed during Astro-1, so this is a new area of investigation for Astro-2.
HUT scientists will also take advantage of HUT's special detector system to study short time scale (milliseconds to minutes) variability in accretion disk and magnetic systems. They will attempt to observe short term pulsations in the UV, produced by resonances in of the accretion disk or perhaps even by a single blob of matter orbiting the inner edge of the disk. In eclipsing (i.e. edge-on) systems, the normal star (which does not produce UV light) can be used to shadow portions of the system as it orbits the white dwarf. By studying changes in the spectrum as a function of the position of the normal star, the temperature structure of the accretion disk can be studied in detail. In magnetic systems, the light variations as a function of the rotation period will be used to measure the size of the hot spot which is produced where the material funneled on to the magnetic poles slams into the surface of the white dwarf. Thus, HUT will permit us to view a variety of waltzing stars in unprecedented detail.
William P. Blair, and the HUT team