4 research outputs found
PAH chemistry and IR emission from circumstellar disks
Aims. The chemistry of, and infrared (IR) emission from, polycyclic aromatic
hydrocarbons (PAHs) in disks around Herbig Ae/Be and T Tauri stars are
investigated. The equilibrium distribution of the PAHs over all accessible
charge/hydrogenation states depends on the size and shape of the PAHs and on
the physical properties of the star and surrounding disk.
Methods. A chemistry model is created to calculate this equilibrium
distribution. Destruction of PAHs by ultraviolet (UV) photons, possibly in
multi-photon absorption events, is taken into account. The chemistry model is
coupled to a radiative transfer code to provide the physical parameters and to
combine the PAH emission with the spectral energy distribution (SED) from the
star+disk system.
Results. Normally hydrogenated PAHs in Herbig Ae/Be disks account for most of
the observed PAH emission, with neutral and positively ionized species
contributing in roughly equal amounts. Close to the midplane, the PAHs are more
strongly hydrogenated and negatively ionized, but these species do not
contribute to the overall emission because of the low UV/optical flux deep
inside the disk. PAHs of 50 carbon atoms are destroyed out to 100 AU in the
disk's surface layer, and the resulting spatial extent of the emission does not
agree well with observations. Rather, PAHs of about 100 carbon atoms or more
are predicted to cause most of the observed emission. The emission is extended
on a scale similar to that of the size of the disk. Furthermore, the emission
from T Tauri disks is much weaker and concentrated more towards the central
star than that from Herbig Ae/Be disks. Positively ionized PAHs are predicted
to be largely absent in T Tauri disks because of the weaker radiation field.Comment: 13 pages, 8 figures, accepted for publication in A&