The ASAS-SN Catalog of Variable Stars VIII: "Dipper" Stars in the Lupus Star-Forming Region

Abstract

Some young stellar objects such as T Tauri-like "dipper" stars vary due to transient partial occultation by circumstellar dust, and observations of this phenomenon inform us of conditions in the planet-forming zones close to these stars. Although many dipper stars have been identified with space missions such as $Kepler$/$K2$, ground-based telescopes offer longer term and multi-wavelength perspectives. We identified 11 dipper stars in the Lupus star forming region in data from the All-Sky Automated Survey for SuperNovae (ASAS-SN), and further characterized these using observations by the Las Cumbres Global Observatory Telescope (LCOGT) and the Transiting Exoplanet Survey Satellite $TESS$, as well as archival data from other missions. Dipper stars were identified from a catalog of nearby young stars and selected based on the statistical significance, asymmetry, and quasi-periodicity or aperiodicity of variability in their ASAS-SN light curves. All 11 stars lie above or red-ward of the zero-age main sequence and have infrared excesses indicating the presence of full circumstellar disks. We obtain reddening-extinction relations for the variability of 7 stars using our combined ASAS-SN-$TESS$ and LCOGT photometry. In all cases the slopes are below the ISM value, suggesting larger grains, and we find a tentative relation between the slope (grain size) and the $\text{K}_\text{s}-[22\:\mu\text{m}]$ infrared color regarded as a proxy for disk evolutionary state.