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

¹ European Space Research and Technology Centre (ESA/ESTEC), Keplerlaan 1, 2201 AZ Noordwijk, The Netherlands
e-mail: cschneider@hs.uni-hamburg.de
² Massachusetts Institute of Technology, Kavli Institute for Astrophysics & Space Research, 77 Massachusetts Avenue, Cambridge, MA 02139, USA
³ Hamburger Sternwarte, Gojenbergsweg 112, 21029 Hamburg, Germany
⁴ Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge, CB3 0HA, UK
⁵ Institute for Astronomy, 640 N. A’ohoku Place, Hilo, HI 96720, USA
⁶ University of Colorado, 392 UCB, Boulder, CO 80309, USA
⁷ Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02139, USA

Received: 24 August 2015
Accepted: 27 October 2015

Abstract

The binary system RW Aur consists of two classical T Tauri stars (CTTSs). The primary recently underwent its second observed major dimming event (ΔV ~ 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 × 10²²cm^-2 during the bright state to ≈2 × 10²²cm^-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 (≳1 μm) with a dust mass column density of ≳2 × 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.