PAH destruction and survival in the disks of T Tauri stars
  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.
