On the ionisation fraction in protoplanetary disks III. The effect of X–ray flares on gas-phase chemistry

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 0.5 < R < 2 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.