On the ionisation fraction in protoplanetary disks

III. The effect of X-ray flares on gas-phase chemistry

Astronomy Unit, Queen Mary, Mile End Road, London E1 4NS, UK e-mail: [M.Ilgner;R.P.Nelson]@qmul.ac.uk

Received: 29 March 2006
Accepted: 11 May 2006

Abstract

Context. Recent observations of the X-ray emission from T Tauri stars in the Orion nebula have shown that they undergo frequent outbursts in their X-ray luminosity. These X-ray flares are characterised by increases in luminosity by two orders of magnitude, a typical duration of less than one day, and a significant hardening of the X-ray spectrum.

Aims.It is unknown what effect these X-ray flares will have on the ionisation fraction and dead-zone structure in protoplanetary disks. We present the results of calculations designed to address this question.

Methods.We have performed calculations of the ionisation fraction in a standard α-disk model using two different chemical reaction networks. We include in our models ionisation due to X-rays from the central star, and calculate the time-dependent ionisation fraction and dead-zone structure for the inner 10 AU of a protoplanetary disk model.

Results.We find that the disk response to X-ray flares depends on whether the plasma temperature increases during flares and/or whether heavy metals (such as magnesium) are present in the gas phase. Under favourable conditions the outer disk dead-zone can disappear altogether, and the dead-zone located between  AU can disappear and reappear in phase with the X-ray luminosity.

Conclusions.X-ray flares can have a significant effect on the dead-zone structure in protoplanetary disks. Caution is required in interpreting this result as the duration of X-ray bursts is considerably shorter than the growth time of MHD turbulence due to the magnetorotational instability.