Herschel Observations of Cataclysmic Variables

TL;DR

This study used Herschel observations combined with other data to analyze infrared emissions from cataclysmic variables, revealing cyclotron emission in polars and the absence of dusty circumbinary disks.

Contribution

First comprehensive infrared analysis of multiple cataclysmic variables combining Herschel, Spitzer, and WISE data, clarifying emission sources and challenging previous dust disk assumptions.

Findings

Infrared luminosities of dwarf novae are dominated by secondary stars.

Mid-infrared excesses in polars are due to cyclotron emission, not dust.

No evidence of dusty circumbinary disks in surveyed targets.

Abstract

We have used the PACS instrument on the Herschel Space Observatory to observe eight cataclysmic variables at 70 and 160 microns. Of these eight objects, only AM Her was detected. We have combined the Herschel results with ground-based, Spitzer, and WISE observations to construct spectral energy distributions for all of the targets. For the two dwarf novae in the sample, SS Cyg and U Gem, we find that their infrared luminosities are completely dominated by their secondary stars. For the two highly magnetic "polars" in our survey, AM Her and EF Eri, we find that their mid-infrared excesses, previously attributed to circumbinary dust emission, can be fully explained by cyclotron emission. The WISE light curves for both sources show large, orbitally modulated variations that are identically phased to their near-IR light curves. We propose that significant emission from the lowest cyclotron harmonics (n </= 3) is present in EF Eri and AM Her. Previously, such emission would have been presumed to be optically thick, and not provide significant orbitally modulated flux. This suggests that the accretion onto polars is more complicated than assumed in the simple models developed for these two sources. We develop a model for the near-/mid-IR light curves for WZ Sge with an L2 donor star that shows that the ellipsoidal variations from its secondary star are detected. We conclude that none of the targets surveyed have dusty circumbinary disks.