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AA Tau is a classical T Tauri star with a highly inclined, warpedcircumstellar disk. For decades, AA Tau exhibited photometric and spectroscopicvariability that were successfully modelled as occultations of the primary starby circumstellar material. In 2011, AA Tau entered an extended faint state,presumably due to enhanced levels of circumstellar dust. We use two sets ofcontemporaneous optical-NIR spectra of AA Tau, obtained in December of 2008 and2014, to directly measure the wavelength-dependent extinction associated withthe dust enhancement driving AA Tau's 2011 optical fade. Taken alone, AA Tau'sapparent optical-NIR increased extinction curve cannot be fit well withstandard extinction laws. At optical wavelengths, AA Tau's dimming event isconsistent 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 NIRwavelengths. We show that veiling emission accounts for the apparent anomalousNIR extinction: after including this veiling flux, AA Tau's dimming event isconsistent with a standard $A_V=2$ extinction law across the full optical-NIRrange. 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 mostlikely explanation for both the IR brightening and the additional extinctionalong the line of sight. In addition to informing our understanding of AA Tau,this analysis demonstrates that caution should be exercised when inferringextinction (and stellar parameters) from the NIR color-color excess of youngstars with measurable NIR veiling fluxes.

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