The nature of the 2014-2015 dim state of RW Aur revealed by X-ray, optical, and NIR observations

Abstract

The binary system RW Aur consists of two classical T~Tauri stars (CTTSs). The primary recently underwent its second observed major dimming event ($\hat{I}”V\sim 2$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 $N_H<0.1\times 10^{22}$cm${}^{-2}$ during the bright state to $\approx 2\times 10^{22}$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 ($\gtrsim 1\hat{I}¼$m) with a dust mass column density of $\gtrsim 2\times 10^{-4}$gcm${}^{-2}$. 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.

5 pages, 4 figures, accepted by A&A letters