Title: Warm CO gas as a tracer of the inner 50 AU of proto-planetary disks.

Author list: G. van der Plas

Abstract: The CO molecule is a very versatile tracer that can trace many
different physical regimes of Proto Planetary (PP) disks via its different
rotational and ro-vibrational transitions. In this talk we present
observations of fundamental ro-vibrational CO emission, which is a great
tool to study the disk surface of the inner few AU of PP disks via
thermally excited CO molecules, and the inner 50 AU via CO molecules that
are fluoresced by UV radiation.

Observations of ro-vibrational CO emission from PP disks surrounding Herbig
Ae/Be stars suggest a connection between disk geometry and CO excitation
conditions. The disk shape (flared or self-shadowed) determines the radial
location (and hence the temperature) of the CO emission: Self shadowed
disks show (as expected) CO emission from the hot inner disk (? 2000K, R_CO
= 0.5 - 5 AU) which likely is thermally excited, whereas the CO from
flaring disks is cooler and originates from larger distances (? 1000K, R_CO
= 10 - 50 AU).

We use the thermo-chemical disk code ProDiMo to study the influence of PAH
abundance and dust-to-gas ratio on CO emission from our PP disks, and find
a possible mechanism for the suppression of the CO emission in the inner
disks of the flared PP disks around HAEBE stars.