The destruction and survival of polycyclic aromatic hydrocarbons in the disks of T Tauri stars

In Spitzer observations of Tauri stars and their disks, features of polycyclic aromatic hydrocarbons (PAHs) 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 far ultraviolet (FUV), an extreme ultraviolet (EUV) and an X-ray component with a fractional luminosity of 1%, 0.1% and 0.025%, respectively. We consider as a PAH destruction process unimolecular dissociation and present a simplified scheme to estimate the location from the star at which the molecules become photo-stable. We find that soft photons with energies below similar to 20 eV dissociate PAHs only up to short distances from the star (r < 1 AU); 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 or remove PAHs from the reach of hard photons. For standard disk models, where the surface density changes like r(-1) and the mid plane temperature like r(-0.5), the critical vertical velocity for PAH survival is proportional to r(-3/4) and equals similar to 5 m/s at 10 AU, 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 is ionized at the top of the disk and neutral at lower levels.