Infrared Properties of Cataclysmic Variables in the 2 Micron All‐Sky Survey Second Incremental Data Release

Abstract

Cataclysmic variables (CVs) have "traditionally" been observed primarily at short wavelengths because accretion-generated luminosity, which peaks in the optical-ultraviolet, dominates the radiated energy of most systems. Hence, relatively little is known about their infrared (IR) properties. Investigating CVs in the IR will contribute to the understanding of key system components that are expected to radiate at these wavelengths, such as the cool outer disk, accretion stream, and secondary star. We have compiled the near-IR J, H, and Ks band photometry of all cataclysmic variables contained in the sky coverage of the Second Incremental Data Release of the 2 Micron All Sky Survey (2MASS). This data comprises 251 cataclysmic variables with reliably identified near-IR counterparts and S/N > 10 photometry in one or more of the three near-IR bands. In addition to tables containing the 2MASS data, we present a set of near-IR finding charts for selected systems. A comparison between the 2MASS photometry and various literature sources of near-IR photometry of cataclysmic variables shows good agreement after allowing for differences in photometric systems and the intrinsic variability of cataclysmic variables. The bulk of our analysis consists of an exploration of near-IR color-color diagrams of the main cataclysmic variable classes. Results from this analysis include: (1) dwarf novae in outburst and quiescence occupy distinct regions of their color-color diagram; (2) nova-like CVs (and dwarf novae in outburst) have colors similar to F-K main-sequence stars, although this does not imply that they have F-K type secondary stars; (3) polars and intermediate polars also occupy distinct regions in their color-color diagram, with most polars having colors essentially indistinguishable from late (M0+) main-sequence stars; (4) there is no strong correlation between age and color for novae, except that many old novae (>75 yr since outburst) have colors similar to F-K main-sequence stars; and (5) there are unusual and unexplained loci of data points in all the color-color diagrams that warrant further investigation in the IR. Except in the case of the polars, near-IR photometry of cataclysmic variables does not isolate the luminosity contribution of their secondary stars. In general, the near-IR colors of cataclysmic variables are significantly and systematically offset blueward of the spectral type of secondary star expected at their orbital periods. This blue contamination of the near-IR light almost certainly originates from the accretion process. For a few systems, their near-IR color is redder than the secondary star expected at their orbital period. One effect that can explain some, but not all, of the red-excess cataclysmic variables is the presence of an evolved secondary star. We suggest that this can also be caused by the luminosity contribution of the cool outer regions of prominent accretion disks. There is at least a weak trend of redder color in higher inclination systems (in which the disk rim would be most visible and most obscure the hot inner region) that supports this hypothesis.