Accretion Disks and Cataclysmic Variables Using spectroscopy and long-term photometry, Honeycutt works on the properties of cataclysmic variable stars and the accretion disks they contain. Cataclysmic variables (CVs) consist of a cool red dwarf star that is transferring gas to a companion white dwarf star, forming a spinning disk of gas around the white dwarf. This accretion disk is subject to several kinds of instabilities due to the partial ionization of hydrogen and to tidal effects induced by the red dwarf star. These instabilities sometimes result in dwarf nova outbursts, in which the disk brightens by 2 to 6 magnitudes for a typical interval of a week. CVs with sufficiently large mass transfer rates, called nova-like variables, are expected to be stable against these instabilities. The hydrogen-rich material that settles onto the white dwarf from the accretion disk eventually detonates in a thermonuclear runaway, resulting in a nova explosion. 

The time scales for photometric variability in CVs are often too long to be effectively monitored via conventional telescope scheduling. Therefore we have been using RoboScope, a fully-automated 16-inch telescope, to accumulate long-term light curves of over 100 CVs since 1990. Recent (listed in the Departmental Annual Reports) have studied the systematics of dwarf nova outbursts, the high-state/low-state behavior of nova-like CVs as the mass transfer rate varies, and the discovery of unexpected "stunted" outbursts in nova-like and old-nova CVs (systems whose accretion disks are predicted to be stable). A new companion 50-inch telescope, called SpectraBot, joined RoboScope in 1998. SpectraBot is being used for photometry on fainter CVs and will be used to obtain the kinds of automated, unattended, CV spectroscopy that will be needed to understand the unusual photometric changes being found by RoboScope. The WIYN 3.5-m telescope is used by Honeycutt and students to obtain CV spectroscopy on shorter time scales, needed to obtain the orbital periods as well as accretion disk shapes, sizes, and motions. The DensePak instrument on WIYN is used to obtain the spectra of selected nova shells at many spatial locations simultaneously, which can yield CV distances, nova shell kinematics, and ejecta abundances. 

Planetary Rings and Protostellar Disks

One of the first astrophysical disks to be discovered and recognized (by Cassini in the 17th Century) is the Saturn ring system. The rings are a massive, thin sheet of icy particles orbiting Saturn. The ring particles range in size from hail stones to large boulders and are rather sensely packed in some regions. The Voyager spacecraft in the 1980's revealed remarkably detailed structure in these rings, but only a small fraction of the features are understood even today. Durisen and NASA-Ames Research Center scientists Drs. Jeff Cuzzi, Luke Dones, and Mark Showalter have conducted several theoretical and observational research projects on the rings. In collaboration with Honeycutt and various IU graduate students, they used the WIYN Telescope to observe the 1995 passage of the Sun through the ring plane. This occurs only once every fifteen years and is a time that is especially well-suited for observations of small moons and faint ring features. Durisen and Cuzzi have also studied the effects of meteoroid bombardment on the radial structure of planetary rings, in particular the process called ballistic transport where debris ejected by meteoroid impacts sprays around the ring system. Their computer simulations show that some unusual structures which seem to be unique to the inner edges of the A and B rings are probably due to ballistic transport. The current Cassini mission to Saturn will hopefully provide critical new information on the erosion of ring particles by meteoroids. Durisen also does research on gravitational instabilities in gaseous disks around protostars and protoplanets.