A Differential Measurement of Circumstellar Extinction for AA Tau's 2011 Dimming Event

Abstract

AA Tau is a classical T Tauri star with a highly inclined, warped circumstellar disk. For decades, AA Tau exhibited photometric and spectroscopic variability that were successfully modelled as occultations of the primary star by circumstellar material. In 2011, AA Tau entered an extended faint state, presumably due to enhanced levels of circumstellar dust. We use two sets of contemporaneous optical-NIR spectra of AA Tau, obtained in December of 2008 and 2014, to directly measure the wavelength-dependent extinction associated with the dust enhancement driving AA Tau's 2011 optical fade. Taken alone, AA Tau's apparent optical-NIR increased extinction curve cannot be fit well with standard extinction laws. At optical wavelengths, AA Tau's dimming event is consistent with predictions of common extinction models for an increase of $A_V=2$, but no such model reproduces AA Tau's color-color excess at NIR wavelengths. We show that veiling emission accounts for the apparent anomalous NIR extinction: after including this veiling flux, AA Tau's dimming event is consistent with a standard $A_V=2$ extinction law across the full optical-NIR range. We also report an increase in AA Tau's mid-IR flux since its 2011 fade, and suggest that an increase in the height of AA Tau's inner disk is the most likely explanation for both the IR brightening and the additional extinction along the line of sight. In addition to informing our understanding of AA Tau, this analysis demonstrates that caution should be exercised when inferring extinction (and stellar parameters) from the NIR color-color excess of young stars with measurable NIR veiling fluxes.