PAH destruction and survival in the disks of T Tauri stars

Siebenmorgen, Ralf
Krügel, Endrik
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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.
Comment: 17 pages, 8 figures, to be acc. by A&A
Astrophysics - Astrophysics of Galaxies, Astrophysics - Earth and Planetary Astrophysics