X-ray to NIR emission from AA Tauri during the dim state

Occultation of the inner disk and gas-to-dust ratio of the absorber^⋆

¹ Hamburger Sternwarte, Gojenbergsweg 112, 21029 Hamburg, Germany
e-mail: christian.schneider@esa.int
² European Space Research and Technology Centre (ESA/ESTEC), Keplerlaan 1, 2201 AZ Noordwijk, The Netherlands
³ Center for Astrophysics and Space Astronomy, University of Colorado, 389 UCB, Boulder, CO 80309, USA
⁴ Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02139, USA
⁵ The Kavli Institute for Astronomy and Astrophysics, Peking University, Yi He Yuan Lu 5, Hai Dian Qu, Beijing 100871, PR China
⁶ Univ. Grenoble Alpes, IPAG, 38000 Grenoble, France
⁷ CNRS, IPAG, 38000 Grenoble, France
⁸ Laboratory for Atmospheric and Space Physics, University of Colorado, 392 UCB, Boulder, CO 80309, USA

Received: 24 December 2014
Accepted: 20 August 2015

Abstract

AA Tau is a well-studied, nearby classical T Tauri star, which is viewed almost edge-on. A warp in its inner disk periodically eclipses the central star, causing a clear modulation of its optical light curve. The system underwent a major dimming event beginning in 2011 caused by an extra absorber, which is most likely associated with additional disk material in the line of sight toward the central source. We present new XMM-Newton X-ray, Hubble Space Telescope FUV, and ground-based optical and near-infrared data of the system obtained in 2013 during the long-lasting dim phase. The line width decrease of the fluorescent H₂ disk emission shows that the extra absorber is located at r > 1 au. Comparison of X-ray absorption (N_H) with dust extinction (A_V), as derived from measurements obtained one inner disk orbit (eight days) after the X-ray measurement, indicates that the gas-to-dust ratio as probed by the N_H to A_V ratio of the extra absorber is compatible with the ISM ratio. Combining both results suggests that the extra absorber, i.e., material at r > 1 au, has no significant gas excess in contrast to the elevated gas-to-dust ratio previously derived for material in the inner region (≲0.1 au).