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 \mu m of the near edge-on disks IRS 46 and GV Tau 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 and excitation temperatures for detected lines and upper limits for non-detections .
Mid-infrared bands of HCN , C _ { 2 } H _ { 2 } and CO _ { 2 } are clearly detected toward both sources .
As found previously for IRS 46 by Lahuis et al .
( 2006 ) , the HCN and C _ { 2 } H _ { 2 } absorption arises in warm gas with excitation temperatures of 400–700 K , whereas the CO _ { 2 } absorption originates in cooler gas of \sim 250 K. Absolute column densities and their ratios are comparable for the two sources .
No other absorption features are detected at the 3 \sigma level .
Column density limits of the majority of molecules predicted to be abundant in the inner disk — C _ { 2 } H _ { 4 } , C _ { 2 } H _ { 6 } , C _ { 6 } H _ { 6 } , C _ { 3 } H _ { 4 } , C _ { 4 } H _ { 2 } , CH _ { 3 } , HNC , HC _ { 3 } N , CH _ { 3 } CN , NH _ { 3 } and SO _ { 2 } — are determined and compared with disk models .
Simulations are also performed for future higher spectral resolution instruments .
The inferred abundance ratios and limits with respect to C _ { 2 } H _ { 2 } and HCN are roughly consistent with models of the chemistry in high temperature gas .
Models of UV irradiated disk surfaces generally agree better with the data than pure X-ray models .
The limit on NH _ { 3 } /HCN implies that evaporation of NH _ { 3 } -containing ices is only a minor contributor .
The inferred abundances and their limits also compare well with those found in comets , suggesting that part of the cometary material may derive from warm inner disk gas .
The high resolution simulations show that future instruments on JWST , ELTs , SOFIA and SPICA can probe up to an order of magnitude lower abundance ratios and put important new constraints on the models , especially if pushed to high S / N ratios .