Ro-vibrational excitation of an organic molecule (HCN) in protoplanetary disks^⋆

¹ Max-Planck-Institut für Extraterrestrische Physik, Gießenbachstrasse 1, 85748 Garching, Germany
e-mail: simonbruderer@gmail.com
² Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
³ SRON Netherlands Institute for Space Research, PO Box 800, 9700 AV Groningen, The Netherlands

Received: 17 September 2014
Accepted: 28 November 2014

Abstract

Context. Organic molecules are important constituents of protoplanetary disks. Their ro-vibrational lines observed in the near- and mid-infrared are commonly detected toward T Tauri disks. These lines are the only way to probe the chemistry in the inner few au where terrestrial planets form. To understand this chemistry, accurate molecular abundances have to be determined. This is complicated by excitation effects that include radiative pumping. Most analyses so far have made the assumption of local thermal equilibrium (LTE), which may not be fulfilled because of the high gas densities required to collisionally thermalize the vibrational levels of the molecules.

Aims. The non-LTE excitation effects of hydrogen cyanide (HCN) are studied to evaluate (i) how the abundance determination is affected by the LTE assumption; (ii) whether the ro-vibrational excitation is dominated by collisions or radiative pumping; and (iii) which regions of protoplanetary disks are traced by certain vibrational bands.

Methods. Starting from estimates for the collisional rate coefficients of HCN, non-LTE slab models of the HCN emission were calculated to study the importance of different excitation mechanisms. Using a new radiative transfer model, the HCN emission from a full two-dimensional disk was then modeled to study the effect of the non-LTE excitation, together with the line formation. We ran models tailored to the T Tauri disk AS 205 (N) where HCN lines in both the 3 μm and 14 μm bands have been observed by VLT-CRIRES and the Spitzer Space Telescope.

Results. Reproducing the observed 3 μm/14 μm flux ratios requires very high densities and kinetic temperatures (n> 10¹⁴ cm^-3 and T> 750 K), if only collisional excitation is accounted for. Radiative pumping can, however, excite the lines easily out to considerable radii ~10 au. Consequently, abundances derived from LTE and non-LTE models do not differ by more than a factor of about 3. Models with both a strongly enhanced abundance within ~1 au (jump abundance) and constant abundance can reproduce the current observations, but future observations with the MIRI instrument on JWST and METIS on the E-ELT can easily distinguish between the scenarios and test chemical models. Depending on the scenario, ALMA can detect rotational lines within vibrationally excited levels.

Conclusions. Pumping by the continuum radiation field can bring HCN close enough to the LTE so that no big deviations in derived abundances are introduced with the LTE assumption, but the line profiles are substantially altered. In non-LTE models, accounting for collisional and radiative excitation, the emitting region can be much larger than in LTE models. Because HCN can be radiatively pumped to considerable radii, deriving a small emitting region from observations can thus point to the chemical abundance structure (e.g., jump abundance). Owing to their level structure, CO₂ and C₂H₂ are expected to act in a similar way, facilitating studies of the warm inner disk chemistry.