Paula Szkody is a professor of astronomy at the University of Washington. She is well known for her work with dwarf novae and magnetic CVs. Paula has participated in projects with the EUVE, HST, IUE, ROSAT, Ginga, ASCA, RXTE, Chandra, XMM, FUSE and Galex satellites and is an active user of the UW observatories Apache Point Observatory (APO) and Manastash Ridge Observatory (MRO), as well as National Observatories KPNO, CTIO and the MMT and Keck.
We're going to talk with Paula about her work with the Sloan Digital Sky Survey (SDSS), the CVs discovered to date, and the hundreds of new CVs they are currently finding.
CVnet: How does the SDSS differ from previous surveys finding CVs, like the Palomar Green Survey?
Paula: The SDSS reaches much fainter magnitudes than previous surveys, finding CVs down to 21st magnitude whereas the PG survey limit was near 16th and the Hamburg Survey was near 18th. This means we can reach larger distances and also find systems with lower accretion rates that are fainter than those found in previous surveys. The end result is that we have less of the selection effect that is pre-disposed to finding higher accretion rate systems and can get a better idea of the true distribution of CVs. However, it should be kept in mind that SDSS has its own selection biases in that spectra are not taken of all objects. Instead, color selection is used and so some objects may be missed.
CVnet: How will these survey results aid in our understanding of the evolution and distribution of CVs in the Galaxy?
Paula: If we can determine the periods for the large number of CVs coming out of this survey (near 200), we can compare the distribution of orbital period and type of CV with population models in existence to understand the angular momentum losses that drive our picture of evolution in close binaries. Since SDSS covers the sky out of the galactic plane, the survey results can be compared with previous surveys in the galactic plane to obtain a scale height for CVs.
CVnet: How are the CVs sorted out from all the stars, galaxies and quasars SDSS is observing?
Paula: The SDSS CVs are found from their spectra as emission line sources or those showing cyclotron harmonics. We run a program on point sources (thus eliminating galaxies) that selects all systems showing emission or absorption lines of hydrogen near their rest wavelengths (thus eliminating quasars). Some hand searches of plates are also done. To have a spectrum taken, objects are color selected from the SDSS imaging. Quasars and galaxies have the most spectral fibers allocated. Stars obtain a small number of fibers and CVs have one allocated per plate. Since CVs have a large range in color (blue if the accretion disk dominates, red if cyclotron or a secondary star dominates and both blue and red if the accretion disk does not contribute much to the light and the underlying stars are evident), we rely on overlaps with QSO colors, hot white dwarfs and serendipity (very blue or red objects) to pick up most CVs. Our one fiber is used to pick out the objects which have both blue and red colors as these are almost all WD plus M dwarf binaries or CVs.
CVnet: Even though the primary aim is to find faint CVs, you have discovered some systems well within the reach of amateur equipment. Are there any particular systems for which follow-up observations or continued monitoring by amateurs might prove useful or rewarding?
Paula: The bright saturation limit is near 15th magnitude so there are several systems that are bright enough for amateurs. In addition, systems that are faint during the SDSS observations may have brighter outbursts or higher states of accretion. For followup work, we have been concentrating on the systems near 17-19th mag (the lowest accretors). This means that the bright, mostly novalike systems could use photometric light curves. In addition, no outburst records exist for most of the systems. If a dwarf nova outburst can be observed, it would determine the classification of these systems.
CVnet: Is it possible, or even likely, that there may be some UGWZs (or TOADs) lurking amongst these new CVs?
Paula: It is extremely likely there are UGWZs (TOADs) among these systems. Many show the characteristics of very low accretion that result in the extreme outbursts of TOADs. The most likely ones are the systems which show broad absorption surrounding the emission lines - this means that the white dwarf is showing up (the absorption lines are from the white dwarf) and the accretion disk (creating the emission) is very tenuous.
CVnet: What is the most surprising result of your experiment thus far?
Paula: I think the most surprising thing is the number of magnetic systems of extremely low accretion that we have been finding. They are evident from very large, sawtooth appearing cyclotron humps in the SDSS spectra and very strange colors in the photometry. Models that have been fit to the continua by Gary Schmidt and Lilia Ferrario show that the accretion rates are about 1000 times less than most AM Her stars or Polars. There apparently is no mass transfer stream and the accretion likely takes place from a wind. The temperatures of the white dwarfs in these systems are very cold (~5000K), implying they are very old and have never been heated by accretion. Thus, these objects may be pre-Polars, on the way to becoming Polars. The numbers being found may imply a large population of these objects and a different angular momentum loss than for typical CVs as their periods are all above the gap.
CVnet: Will you come back and talk to us again about future results?
Paula: The main part of the survey will end in June 2005, with the final data release soon after. Hopefully, we will have some summary of all the CVs known by 2006 and can make more generalities about what we have found. I am always happy to answer questions about the SDSS CVs at any time.
Sloan Digital Sky Survey Website
Cataclysmic Variables from SDSS I, First Results
Cataclysmic Variables from SDSS II
Cataclysmic Variables from SDSS III
Apache Point Observatory
Manastash Ridge Observatory
University of Washington Astronomy