Spitzer Space Telescope Observations of Magnetic Cataclysmic Variables: Possibilities for the Presence of Dust in Polars

C. S. Brinkworth, D. W. Hoard, S. Wachter, S. B. Howell, David R. Ciardi, P. Szkody, T. E. Harrison, G. T. van Belle, and A. A. Esin

The Astrophysical Journal 659, 1541 (2007)

Abstract

We present Spitzer photometry of six short-period polars, EF Eri, V347 Pav, VV Pup, V834 Cen, GG Leo, and MR Ser. We have combined the Spitzer IRAC (3.6-8.0 μm) data with the 2MASS JHKs photometry to construct the SEDs of these systems from the near- to mid-IR (1.235-8 μm). We find that five out of the six polars have flux densities in the mid-IR that are substantially in excess of the values expected from the stellar components alone. We have modeled the observed SEDs with a combination of contributions from the white dwarf, secondary star, and either cyclotron emission or a cool, circumbinary dust disk to fill in the long-wavelength excess. We find that a circumbinary dust disk is the most likely cause of the 8 μm excess in all cases, but we have been unable to rule out the specific (but unlikely) case of completely optically thin cyclotron emission as the source of the observed 8 μm flux density. While both model components can generate enough flux at 8 μm, neither dust nor cyclotron emission alone can match the excess above the stellar components at all wavelengths. A model combining both cyclotron and dust contributions, possibly with some accretion-generated flux in the near-IR, is probably required, but our observed SEDs are not sufficiently well sampled to constrain such a complicated model. If the 8 μm flux density is caused by the presence of a circumbinary dust disk, then our estimates of the masses of these disks are many orders of magnitude below the mass required to affect CV evolution.