Warm molecular layers in protoplanetary disks

¹ Department of Earth and Planetary Sciences, Kobe University, Kobe 657-8501, Japan
² Leiden Observatory, PO Box 9513, 2300 RA Leiden, The Netherlands
³ Departments of Physics and Astronomy, The Ohio State University, Columbus, OH 43210, USA

Corresponding author: Y. Aikawa, aikawa@kobe-u.ac.jp

Received: 15 October 2001
Accepted: 8 January 2002

Abstract

We have investigated molecular distributions in protoplanetary disks, adopting a disk model with a temperature gradient in the vertical direction. The model produces sufficiently high abundances of gaseous CO and HCO⁺ to account for line observations of T Tauri stars using a sticking probability of unity and without assuming any non-thermal desorption. In regions of radius AU, with which we are concerned, the temperature increases with increasing height from the midplane. In a warm intermediate layer, there are significant amounts of gaseous molecules owing to thermal desorption and efficient shielding of ultraviolet radiation by the flared disk. The column densities of HCN, CN, CS, H₂CO, HNC and HCO⁺ obtained from our model are in good agreement with the observations of DM Tau, but are smaller than those of LkCa15. Molecular line profiles from our disk models are calculated using a 2-dimensional non-local-thermal-equilibrium (NLTE) molecular-line radiative transfer code for a direct comparison with observations. Deuterated species are included in our chemical model. The molecular D/H ratios in the model are in reasonable agreement with those observed in protoplanetary disks.