Organic molecules in protoplanetary disks around T Tauri and Herbig Ae stars^*

¹ Leiden Observatory, PO Box 9513, 2300 RA Leiden, The Netherlands e-mail: ewine@strw.leidenuniv.nl
² Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, UK
³ Sterrenkundig Instituut Anton Pannekoek, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands
⁴ Koninklijk Nederlands Meteorologisch Instituut, PO Box 201, 3730 AE De Bilt, The Netherlands

Received: 14 January 2003
Accepted: 15 June 2004

Abstract

The results of single-dish observations of low- and high-J transitions of selected molecules from protoplanetary disks around two T Tauri stars (LkCa 15 and TW Hya) and two Herbig Ae stars (HD 163296 and MWC 480) are reported. Simple molecules such as CO, CO, HCO⁺, CN and HCN are detected. Several lines of H₂CO are found toward the T Tauri star LkCa 15 but not in other objects. No CH₃OH has been detected down to abundances of 10^-9–10^-8 with respect to H₂. SO and CS lines have been searched for without success. Line ratios indicate that the molecular emission arises from dense (10–10 cm) and moderately warm ( 20–40 K) intermediate height regions of the disk atmosphere between the midplane and the upper layer, in accordance with predictions from models of the chemistry in disks. The sizes of the disks were estimated from model fits to the ¹²CO 3–2 line profiles. The abundances of most species are lower than in the envelope around the solar-mass protostar IRAS 16293-2422. Freeze-out in the cold midplane and photodissociation by stellar and interstellar ultraviolet photons in the upper layers are likely causes of the depletion. CN is strongly detected in all disks, and the CN/HCN abundance ratio toward the Herbig Ae stars is even higher than that found in galactic photon-dominated regions, testifying to the importance of photodissociation by radiation from the central object in the upper layers. DCO⁺ is detected toward TW Hya, but not in other objects. The high inferred DCO⁺/HCO⁺ ratio of ~0.035 is consistent with models of the deuterium fractionation in disks which include strong depletion of CO. The inferred ionization fraction in the intermediate height regions as deduced from HCO⁺ is at least 10^-11–10^-10, comparable to that derived for the midplane from recent H₂D⁺ observations. Comparison with the abundances found in cometary comae is made.