Astronomy research at Fresno State is carried out by Prof. Ringwald and his students. It is mainly about cataclysmic variable binary stars (CVs) and related objects such as novae and black holes, their evolution, and the physics of their accretion disks and outflows. For an easy primer on "What is a nova?," see here. For a more detailed primer on CVs, see here.
My Ph. D. thesis was the first optically selected population study of CVs. It was selected by color, and not by the behavior of these stars' outbursts, which had biased previous surveys. The space density found, 6 × 10-6 pc-3, suggested a larger-than-expected fraction of the lowest-luminosity objects. I have since been publishing detailed studies of individual systems. (Painting by Dana Berry, Space Telescope Science Institute)
Along the way, John Thorstensen and I identified a new class of faint nova-likes, with mysterious, but consistent behavior. John named them the SW Sex stars, and much has been made of them since. They probably aren't physically different from other CVs, "just" being deeply eclipsing. Still, it is just this high inclination that reveals their properties, since explained as from disk outflow and the stream-disk interaction by Coel Hellier, although questions remain about magnetic propellers and the role of mass loss.
Another by-product of the survey was finding interesting objects previously misclassified as CVs. One was PG 1002+506, a high-latitude Be star I found with Robert Rolleston, Rex Saffer, and John Thorstensen. If on the main sequence, as befits a Be star, it's over 10 kpc above the Galactic plane! How did such a young star get there? Was it flung out of the Galaxy? Or did it somehow form in the halo? (The simulation of a rapidly rotating Be star and the disk it extrudes is from Owocki, Cranmer, & Blondin 1994, ApJ, 424, 887.)
My second CV population study, with Tim Naylor and Koji Mukai, was a spectroscopic atlas of classical novae with outbursts between 1783 and 1986. The spectra were taken to see evolution over decades or centuries. They show little evidence of nova hibernation, or of any changes at all, aside from fading of the nebular lines. Old novae look remarkably similar to each other, except for effects wholly attributable to their orbital inclinations. (Hubble Space Telescope image by F. Paresce)
BZ Cam, long called just 0623+71, or "the one in the bow-shock nebula" (image by Hollis et al.), was the first CV found with an optical spectrum revealing a wind from its accretion disk, shown by intermittent P Cygni profiles in its H alpha and He I 5876 Å lines. I used William Herschel Telescope to obtain time-resolved spectra of these lines with 0.04 nm/pixel dispersion and 30-second time resolution. They revealed the acceleration law in a CV wind for the first time, a linear acceleration to 1700 km/s in 6 to 8 minutes. They also showed a subsequent linear deceleration in 30 to 40 minutes, perhaps an effect of dilution of the wind as it expands and cools. No periodicity from rotational outflow is obvious. Raman Prinja, Christian Knigge, and I extended this study into the ultraviolet, with Hubble Space Telescope's Goddard High Resolution Spectrograph.
In collaboration with Koji Mukai, we used Chandra X-ray Observatory to observe the origin of the soft X-rays in the old nova DQ Herculis. We found that the X-rays are from a point-like source and show a shallow partial eclipse. We interpret this as due to scattering of the unseen central X-ray source, probably in an accretion disk wind. DQ Her is a prototype of a class of stars, the Intermediate Polars. Click here for a painting of the disrupted disk of one of these systems by Mark Garlick.
In collaboration with Knox Long, Cyndi Froning, Raman Prinja, and Christian Knigge, I was involved in a project with NASA's Far Ultraviolet Spectroscopic Explorer (FUSE) spacecraft. This project is similar to the one mentioned above with Hubble Space Telescope, although we had a brighter target, the nova-like variable RW Sextantis. Observing in the far ultraviolet allowed us to detect the Lyman lines and other indicators of variability in the accretion disk's wind.
In collaboration with Steve Saar, Gaitee Hussain, Chris Johns-Krull, I had a run on Keck I, at the time the largest telescope on Earth, on Mauna Kea, Hawaii. The objective was to see if the low states in cataclysmic variables such as VY Scl are caused by spots on the mass-losing secondary stars.
Steve Saar, Vinay Kashyap, and I extended these observations with Hubble Space Telescope, to detect ultraviolet emission lines excited by the magnetism of the mass-losing secondary star of the polar system, AM Herculis. We did indeed find evidence for strong magnetism near L1.
Also with Hubble Space Telescope, a snapshot survey of nova shells, which are the gaseous, resolved nebular remnants expelled by classical novae. The snapshots were brief exposures, taken a few at a time between larger projects, to make the best use of telescope time. It proved "rather successful" for discovering nova shells, with ten previously unknown shells found among 32 novae observed. Shown here are the nova shells of QU Vul (Nova Vul 1984 #2) (left) and V351 Pup (Nova Pup 1991) (right).
Searches for Thorne-Zytkow objects, red supergiants with neutron stars in their centers. For all we know, Betelgeuse might be one (Hubble Space Telescope image by A. Dupree).
Studies of detached post-common envelope binaries: the image is by Mark Somers, and shows the cool companion's heated face coming and going, as it orbits the hot white dwarf. Here you can get my star catalog of stars from this program, ``Composite-Spectrum and Related Stars That Are Candidate Detached Post-Common-Envelope Binaries, v. 2.3.''
Time-resolved photometry of detached post-common envelope binaries, to search for irradiation variations, ellipsoidal variations, and eclipses (with Fresno State graduate student Simon Gonzalez)
Searches for variability in hot subdwarf stars, such as from pulsations, or from eclipses, irradiation variations, or ellipsoidal variations in binary systems, or possibly from unrecognized and misclassified cataclysmic variables (with Fresno State graduate student Melissa Blacketer)
Fresno State has a Campus Observatory, on the grounds of the Downing Planetarium. It is well instrumented, and is used by Fresno State students for the Observational Astronomy course (PHYS 151), and for a variety of science programs
For more on opportunities with these instruments, click here.
Last updated 2015 November 7. Web page by Prof. Ringwald
Department of Physics, California State University, Fresno. Please read this disclaimer.