The Temperature versus Orbital Period Relation of AM CVns: Insights from Their Donors

Abstract

We studied the spectral energy distribution of 22 known AM CVns with orbital periods ( P orb ) larger than 35 minutes using multiwavelength public photometric data to estimate the effective temperature of the accreting white dwarf. We find an infrared (IR) excess in all systems when compared to a single blackbody, both when the disk should be extended and when it should be truncated by the accretor’s magnetic field. This suggests a dominant contribution from the donor to the IR flux. When fitting two blackbodies, the temperature of the hot component decreases with P orb , as expected by evolutionary models. Temperatures for systems with 35 <  P orb  < 45 minutes are consistent with models. Systems with P orb  ≳ 45 minutes have higher temperatures than expected. The second blackbody temperature does not correlate with P orb .