Dust entrainment in photoevaporative winds. The impact of X-rays

Context. X-ray- and extreme ultraviolet (XEUV) driven photoevaporative winds acting on protoplanetary disks around young T Tauri stars may crucially impact disk evolution, affecting both gas and dust distributions.Aims. We investigate the dust entrainment in XEUV-driven photoevaporative winds and compare our results to existing magnetohydrodynamic and EUV-only models.Methods. We used a 2D hydrodynamical gas model of a protoplanetary disk irradiated by both X-ray and EUV spectra from a central T Tauri star to trace the motion of passive Lagrangian dust grains of various sizes. The trajectories were modelled starting at the disk surface in order to investigate dust entrainment in the wind.Results. For an X-ray luminosity of L-X = 2 x 10(30) erg s(-1) emitted by a M-* = 0.7 M-circle dot star, corresponding to a wind mass-loss rate of M-w similar or equal to 2.6 x 10(-8)M(circle dot) yr(-1), we find dust entrainment for sizes a(0) less than or similar to 11 mu m (9 mu m) from the inner 25 AU (120 AU). This is an enhancement over dust entrainment in less vigorous EUV-driven winds with M-w similar or equal to 10(-10)M(circle dot) yr(-1). Our numerical model also shows deviations of dust grain trajectories from the gas streamlines even for mu m-sized particles. In addition, we find a correlation between the size of the entrained grains and the maximum height they reach in the outflow.Conclusions. X-ray-driven photoevaporative winds are expected to be dust-rich if small grains are present in the disk atmosphere.