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