The vanishing of the long photometric cycle in AU Monocerotis

Abstract

Double periodic variables (DPVs) are a group of semi-detached interacting binaries that exhibit a long photometric cycle with an average length of $\sim33$ times the orbital period of the system. It has been proposed that this long photometric cycle originates from a modulated mass transfer rate from the donor star, which itself is driven by an internal magnetic dynamo. One of the most well-studied DPVs in the Milky Way is AU Monocerotis (AU Mon). We aim to enhance our understanding of the long photometric cycle in AU Mon by characterising its behaviour through the analysis of available photometric data from several databases and surveys. We summarise previous findings on the system and analyse its published multi-wavelength photometry from different sources, covering 46.3 years, to study the variability of its light curve. We find that the orbital period has remained constant over recent decades, but the long cycle of approximately 417 days vanished around 2010. From an O-C analysis, we conclude that the system is experiencing a change in its orbital period of no greater than $0.038\pm0.040$ s yr$^{-1}$, and thus, imposing a value of $2\times10^{-8}$ M$_\odot$ yr$^{-1}$ for $\dot{M}$ in a fully conservative mass transfer regime. A time-series analysis of the disentangling light curve in the Ic filter shows a transient periodicity of approximately 1910 days lasting at least 2000 days before it also disappears around the year 2020. An analysis of the available APASS light curves around the year 2013 shows a strong periodicity at approximately 280 days, which appears to be stronger in the Z filter. We report what is the second observation of the sudden disappearance of the long cycle in a DPV, after the Galactic DPV TYC 5353-1137-1. The disappearance of the long cycle in AU Mon is a strong constraint for current models that aim to explain the long cycle in DPVs.