Disk evolution -- Oral Presentation
X-ray diagnostics of grain depletion in matter accreting onto T
Tauri stars
Paola Testa, MIT Kavli Institute for Astrophysics and Space Research
Jeremy Drake (CfA), Lee Hartmann (CfA)
Recent analysis of high resolution Chandra X-ray spectra has
shown that the Ne/O abundance ratio is remarkably constant in stellar
coronae. Based on this result, we point out the utility of the Ne/O
ratio as a discriminant for accretion-related X-rays from T Tauri stars,
and for probing the measure of grain-depletion of the accreting material
in the inner disk. We apply the Ne/O diagnostic to the classical T Tauri
stars BP Tau and TW Hya---the two stars found to date whose X-ray
emission appears to originate, at least in part, from accretion activity.
We show that TW Hya appears to be accreting material which is significantly
depleted in O relative to Ne. In constrast, BP Tau has an Ne/O abundance
ratio consistent with that observed for post-T Tauri stars. We interpret
this result in terms of the different ages and evolutionary states of
the circumstellar disks of these stars. In the young BP Tau disk (age
$\sim 0.6$ Myr) dust is still present near the disk corotation radius
and can be ionized and accreted, re-releasing elements depleted onto
grains. In the more evolved TW Hya disk (age $\sim 10$ Myr), evidence
points to ongoing coagulation of grains into much larger bodies that can
resist the drag of inward-migrating gas, and accreting gas is consequently
depleted of grain-forming elements.