The nature of the 2014-2015 dim state of RW Aurigae revealed by X-ray, optical, and near-IR observations

The binary system RW Aur consists of two classical T Tauri stars (CTTSs). The primary recently underwent its second observed major dimming event (Delta V similar to 2 mag). We present new, resolved Chandra X-ray and UKIRT near-IR (NIR) data as well as unresolved optical photometry obtained in the dim state to study the gas and dust content of the absorber causing the dimming. The X-ray data show that the absorbing column density increased from N-H < 0.1 x 10(22) cm(-2) during the bright state to approximate to 2 x 10(22) cm(-2) in the dim state. The brightness ratio between dim and bright state at optical to NIR wavelengths shows only a moderate wavelength dependence and the NIR color-color diagram suggests no substantial reddening. Taken together, this indicates gray absorption by large grains (greater than or similar to 1 mu m) with a dust mass column density of greater than or similar to 2 x 10(-4) g cm(-2). Comparison with N-H shows that an absorber responsible for the optical/NIR dimming and the X-ray absorption is compatible with the ISM's gas-to-dust ratio, i.e., that grains grow in the disk surface layers without largely altering the gas-to-dust ratio. Lastly, we discuss a scenario in which a common mechanism can explain the long-lasting dimming in RW Aur and recently in AA Tau.