Hitting a New Low: The Unique 28 h Cessation of Accretion in the TESS Light Curve of YY Dra (DO Dra)

TL;DR

This paper reports the first detailed observation of a complete 28-hour cessation of accretion in the intermediate polar YY Dra, revealing a rare low state with no flickering and implications for white dwarf magnetic field measurements.

Contribution

It documents a unique, well-characterized low accretion state in YY Dra, providing insights into accretion cessation and its frequency in intermediate polars.

Findings

First observation of a complete 28-hour accretion cessation in YY Dra.

Identification of multiple low states reaching the same faint magnitude.

Potential for future magnetic field measurements via Zeeman splitting.

Abstract

We present the Transiting Exoplanet Surveying Satellite (TESS) light curve of the intermediate polar YY Draconis (YY Dra, also known as DO Dra). The power spectrum indicates that while there is stream-fed accretion for most of the observational period, there is a day-long, flat-bottomed low state at the beginning of 2020 during which the only periodic signal is ellipsoidal variation and there is no appreciable flickering. We interpret this low state to be a complete cessation of accretion, a phenomenon that has been observed only once before in an intermediate polar. Simultaneous ground-based observations of this faint state establish that when accretion is negligible, YY Dra fades to $g=17.37\pm0.12$, which we infer to be the magnitude of the combined photospheric contributions of the white dwarf and its red dwarf companion. Using survey photometry, we identify additional low states in 2018-2019 during which YY Dra repeatedly fades to -- but never below -- this threshold. This implies relatively frequent cessations in accretion. Spectroscopic observations during future episodes of negligible accretion can be used to directly measure the field strength of the white dwarf by Zeeman splitting. Separately, we search newly available catalogs of variable stars in an attempt to resolve the long-standing dispute over the proper identifier of this system.