X-ray news from RW Auriga: Optical dimming associated with iron rich corona and exceptionally high absorbing column density

RW Aur is a young, low-mass star which is variable on a wide range of time scales, as rotation, magnetic fields, outflows, and circumstellar disk interact. RW Aur A has undergone a visual dimming event in 2011 (ΔV∼2 mag) lasting about half a year and further dimming 2014-2016 (ΔV up to 3 mag). Visual and IR observations indicate that the cause for this dimming is a gray absorber that moved into the line-of-sight. This dimming is also associated with changes in the outflow. We report on new Chandra X-ray data taken in 2017, when the optical flux was almost 2 mag below the long-term average. The observed X-ray spectrum is more absorbed by a column density of a few 1023cm−2

and shows significantly more hot plasma than in X-ray observations taken before the dimming. An emission feature at 6.62 keV is present indicating an Fe abundance an order of magnitude above Solar, in contrast with previous sub-Solar Fe abundance measurements.

Comparing X-ray absorbing column density NH and optical reddening AV, we find that either the gas-to-dust ratio in the absorber is orders of magnitude higher than in the ISM or the absorber has undergone significant dust evolution. Given the high column density coupled with changes in the X-ray spectral shape, this absorber is probably located in the inner disk. We speculate that a break-up of planetesimals or a terrestrial planet could supply large grains causing gray absorption; some of these grains would be accreted and enrich the stellar corona with iron which could explain the inferred high abundance.