The Temperature versus Orbital Period relation of AM CVns: Insights from their Donors

TL;DR

This study analyzes the spectral energy distribution of 22 AM CVn systems to understand how their donor star's temperature varies with orbital period, revealing an infrared excess and deviations from models at longer periods.

Contribution

It provides the first comprehensive IR analysis of AM CVn systems with orbital periods over 35 minutes, highlighting donor contributions and temperature deviations from evolutionary predictions.

Findings

Infrared excess observed in all systems, indicating donor contribution.

Donor temperature decreases with orbital period, aligning with models for 35-45 min.

Systems with longer periods (>45 min) show higher than expected temperatures.

Abstract

We studied the spectral energy distribution (SED) of 22 known AM~CVns with orbital periods ($P_{orb}$) larger than 35~min 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 $35<P_{orb}<45~\text{min}$ are consistent with models. Systems with $P_{orb}\gtrsim45~\text{min}$ have higher temperatures than expected. The second blackbody temperature does not correlate with $P_{orb}$.