Astronomy research at Fresno State is carried out by Professor 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.

A cataclysmic variable 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)

Coel's plot of the gas stream flowing over the disk of an SW Sex 
star 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.

A Be star, 
featuring its disk 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.)

Hubble image of the gas shell expelled from Nova Cygni 1992, in the 
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 and its bow-shock nebula 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.

Chandra X-ray 
Observatory 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.

NASA's Far Ultraviolet Spectroscopic 
Explorer spacecraftIn 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.

The Keck Telescope's primary mirror
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.

Hubble Space Telescope
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.

The gaseous shells of QU Vul (Nova 
Vul 1984 #2) (left) and V351 Pup (Nova Pup 1991) (right)
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).

The red supergiant Betelgeuse, 
compared to Earth's orbit around the Sun
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).

A white dwarf heating a cool companion 
orbiting it
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.''

Irradiation variation
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)

Hot subdwarf
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)

Soft X-ray Transient
Time-resolved photometry of X-ray binaries, including the outbursts of soft X-ray transients/black hole candidates (with Fresno State graduate students Dillon Trelawny)

Negative superhumps
Time-resolved photometry of cataclysmic variables, including the discovery of apsidal superhumps and nodal superhumps in several systems (with Fresno State students Kenia Velasco, Jonathan Roveto, Michelle Meyers, Jerry Rude, and Dillon Trelawny)

Long-term light curves from the new generation of automated telescopes (with Fresno State graduate students Kylee Ford, Kurt Shults, Eric Goeken, E'lisa Lee, and Chance Spencer)

A solar-like star having a 
superflare, enhanced by its planet's magnetic field
Searches for superflares and related chromospheric activity in solar-like stars, suspected to be enhanced by magnetized planets such as hot Jupiters (with Fresno State graduate students Randy Clark, Jerry Rude, Dillon Trelawny, and Armando Guerrero)

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Last updated 2021 May 21. Web page by Professor Ringwald (ringwald[at],
Department of Physics, California State University, Fresno. Please read this disclaimer.