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

TL;DR

This study investigates the 2014-2015 dimming of RW Aur, revealing gray absorption by large grains and suggesting grain growth in disk surface layers, with implications for understanding similar dimming events in young stellar objects.

Contribution

It provides the first detailed multi-wavelength analysis of RW Aur's dimming, showing large grain growth and consistent gas-to-dust ratios, and proposes a common mechanism for long-lasting dimming events.

Findings

Absorbing column density increased during dimming.

Gray absorption indicates large dust grains.

Gas-to-dust ratio remains consistent with ISM values.

Abstract

The binary system RW Aur consists of two classical T~Tauri stars (CTTSs). The primary recently underwent its second observed major dimming event ($\Delta V\,\sim2\,$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 $N_H<0.1\times10^{22}\,$cm$^{-2}$ during the bright state to $\approx2\times10^{22}\,$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 ($\gtrsim1\,\mu$m) with a dust mass column density of $\gtrsim2\times10^{-4}\,$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.