Title: CO2 infrared emission as a diagnostic of planet-forming regions of disks,
Abstract: [Abridged] The infrared ro-vibrational emission lines from organic molecules
in the inner regions of protoplanetary disks are unique probes of the physical
and chemical structure of planet forming regions and the processes that shape
them. The non-LTE excitation effects of carbon dioxide (CO2) are studied in a
full disk model to evaluate: (i) what the emitting regions of the different CO2
ro-vibrational bands are; (ii) how the CO2 abundance can be best traced using
CO2 ro-vibrational lines using future JWST data and; (iii) what the excitation
and abundances tell us about the inner disk physics and chemistry. CO2 is a
major ice component and its abundance can potentially test models with
migrating icy pebbles across the iceline. A full non-LTE CO2 excitation model
has been built. The characteristics of the model are tested using non-LTE slab
models. Subsequently the CO2 line formation has been modelled using a
two-dimensional disk model representative of T-Tauri disks. The CO2 gas that
emits in the 15 $\mu$m and 4.5 $\mu$m regions of the spectrum is not in LTE and
arises in the upper layers of disks, pumped by infrared radiation. The v$_2$ 15
$\mu$m feature is dominated by optically thick emission for most of the models
that fit the observations and increases linearly with source luminosity. Its
narrowness compared with that of other molecules stems from a combination of
the low rotational excitation temperature (~250 K) and the inherently narrower
feature for CO2. The inferred CO2 abundances derived for observed disks are
more than two orders of magnitude lower than those in interstellar ices
(~10$^5$), similar to earlier LTE disk estimates. Line-to-continuum ratios are
low, of order a few %, thus high signal-to-noise (S/N > 300) observations are
needed for individual line detections. Prospects of accurate abundance
retreival with JWST-MIRI and JWST-NIRSpec are discussed.