Astronomy research at Fresno State is carried out by Dr. 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: hibernating novae? 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. Indeed, 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. 1992, ApJ, 393, 217), 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.

Ongoing research projects include:

Searches for Thorne-Zytkow objects, red supergiants with neutron stars in their centers: for all we know, Betelgeuse might be one (image by A. Dupree with Hubble Space Telescope).

Here you can get my star catalog of targets for this program, ``Composite-Spectrum and Related Stars That Are Candidate Detached Post-Common-Envelope Binaries, v. 2.3.''

---

In collaboration with Steve Saar and Gaitee Hussain (Harvard) and Chris Johns-Krull ( Rice), I had a run on Keck I, the largest telescope on Earth, at 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. We have extended this work with Hubble Space Telescope, to detect ultraviolet emission lines excited by the magnetism.