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In Spitzer observations of Tauri stars and their disks, PAH features are detected in less than 10% of the objects, although the stellar photosphere is sufficiently hot to excite PAHs. To explain the deficiency, we discuss PAH destruction by photons assuming that the star has beside its photospheric emission also a FUV, an EUV and an X-ray component with fractional luminosity of 1%, 0.1% and 0.025%, respectively. As PAH destruction process we consider unimolecular dissociation and present a simplified scheme to estimate the location from the star where the molecules become photo-stable. We find that soft photons with energies below ~20eV dissociate PAHs only up to short distances from the star (r < 1AU); whereas dissociation by hard photons (EUV and X-ray) is so efficient that it would destroy all PAHs (from regions in the disk where they could be excited). As a possible path for PAH survival we suggest turbulent motions in the disk. They can replenish PAHs or remove them from the reach of hard photons. For standard disk models, where the surface density changes like 1/r and the mid plane temperature like 1/r^{0.5}, the critical vertical velocity for PAH survival is proportional to r^{-3/4} and equals ~5m/s at 10AU which is in the range of expected velocities in the surface layer. The uncertainty in the parameters is large enough to explain both detection and non-detection of PAHs. Our approximate treatment also takes into account the presence of gas which, at the top of the disk, is ionized and at lower levels neutral.
Context: T Tauri stars have X-ray luminosities ranging from L_X = 10^28-10^32 erg/s. These luminosities are similar to UV luminosities (L_UV 10^30-10^31 erg/s) and therefore X-rays are expected to affect the physics and chemistry of the upper layers
Infrared and (sub-)mm observations of disks around T Tauri and Herbig Ae/Be stars point to a chemical differentiation between both types of disks, with a lower detection rate of molecules in disks around hotter stars. To investigate the potential und
Aims: We search for PAH features towards T Tauri stars and compare them with surveys of Herbig Ae/Be stars. The presence and strength of the PAH features are interpreted with disk radiative transfer models exploring the PAH feature dependence on the
We present 3.6 to 70 {mu}m Spitzer photometry of 154 weak-line T Tauri stars (WTTS) in the Chamaeleon, Lupus, Ophiuchus and Taurus star formation regions, all of which are within 200 pc of the Sun. For a comparative study, we also include 33 classica
Residual gas in disks around young stars can spin down stars, circularize the orbits of terrestrial planets, and whisk away the dusty debris that is expected to serve as a signpost of terrestrial planet formation. We have carried out a sensitive sear