August 11-15, 2014


The eclipsing double white dwarf binary CSS 41177.

Madelon Bours (University of Warwick)

Tom Marsh (University of Warwick)

CSS 41177 is one of only five known double white dwarf binaries that show eclipses. In addition, it is the only double-lined spectroscopic binary of these five, making it uniquely suited for detailed parameter studies. We have obtained simultaneous high-speed photometry in u', g' and r' with ULTRACAM, as well as phase-resolved X-shooter spectroscopy. These data allow us to determine precise masses and radii for both white dwarfs without relying on theoretical mass-radius relations. CSS 41177 therefore provides a good test of such relations, especially because the masses, 0.32 and 0.38 Msolar, put the white dwarfs into a region of the mass-radius diagram that was previously unexplored. Our analysis of the optical light curves suggests a temperature around 24000 K for the primary white dwarf, but this produces too much FUV flux compared to measurements from GALEX. At the same time, because of parameter correlations in the light curve model, forcing a lower temperature for the primary white dwarf would imply that the cooler secondary white dwarf was located in the ZZ Ceti instability strip, although we do not observe any pulsations to an amplitude limit of 0.5%. I will describe the observations and analysis that lead to these conclusions and I will also present recently-acquired (May 2014) HST/COS UV-spectra taken with the aim of settling the temperature problem.

Mode of presentation: oral