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

TL;DR

This study investigates the nature of the dimming event in AA Tauri by analyzing multi-wavelength data, revealing the location and composition of the absorbing material and its gas-to-dust ratio compared to the interstellar medium.

Contribution

It provides new insights into the location and composition of the absorber during AA Tauri's dim state using X-ray, UV, optical, and NIR observations, and compares gas-to-dust ratios at different disk radii.

Findings

The absorber is located beyond 1 au from the star.

The gas-to-dust ratio of the absorber matches the ISM ratio.

Inner disk material shows an elevated gas-to-dust ratio.

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 H$_2$ 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 ($\lesssim0.1\,$au).