Exploring organic chemistry in planet-forming zones
  Over the last few years, the chemistry of molecules other than CO in the
planet-forming zones of disks is starting to be explored with Spitzer and
high-resolution ground-based data. However, these studies have focused only on
a few simple molecules. The aim of this study is to put observational
constraints on the presence of more complex organic and sulfur-bearing
molecules predicted to be abundant in chemical models of disks and to simulate
high resolution spectra in view of future missions. High S/N Spitzer spectra at
10-30 micron of the near edge-on disks IRS46 and GVTau are used to search for
mid-infrared absorption bands of various molecules. These disks are good
laboratories because absorption studies do not suffer from low line/continuum
ratios that plague emission data. Simple LTE slab models are used to infer
column densities (or upper limits) and excitation temperatures. Bands of HCN,
C2H2 and CO2 are clearly detected toward both sources. The HCN and C2H2
absorption arises in warm gas with Tex of 400-700 K, whereas the CO2 absorption
originates in cooler gas of app. 250 K (as in Lahuis 2006). No other absorption
features are detected. Limits of those molecules are determined and compared
with disk models. The inferred ratios wrt. C2H2 and HCN are roughly consistent
with models of the chemistry in high-T gas. Models of UV irradiated disk
surfaces generally agree better than pure X-ray models. The limit on NH3/HCN
implies that evaporation of NH3 containing ices is only a minor contributor.
The inferred ratios also compare well with those found in comets, suggesting
that part of the cometary material may derive from warm inner disk gas. High
resolution simulations show that future instruments on JWST, ELTs, SOFIA and
SPICA can probe up to an order of magnitude lower ratios and put important new
constraints on the models, especially if pushed to high S/N ratios.
