Complex organic molecules in protoplanetary disks^⋆

¹ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
e-mail: cwalsh@strw.leidenuniv.nl
² Astrophysics Research Centre, School of Mathematics and Physics, Queen’s University Belfast, University Road, Belfast BT7 1NN, UK
³ Department of Astronomy, Graduate School of Science, Kyoto University, 606-8502 Kyoto, Japan
⁴ National Astronomical Observatory of Japan, Osawa, Mitaka, Tokyo 181-8588, Japan
⁵ Department of Earth and Planetary Sciences, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, 152-8551 Tokyo, Japan
⁶ Departments of Physics, Chemistry and Astronomy, The Ohio State University, Columbus OH 43210, USA
⁷ Departments of Chemistry, Astronomy, and Physics, University of Virginia, Charlottesville VA 22904, USA
⁸ Department of Chemistry, Emory University, Atlanta GA 30322, USA
⁹ Department of Earth and Planetary Sciences, Kobe University, 1-1 Rokkodai-cho, Nada, 657-8501 Kobe, Japan
¹⁰ Department of Chemistry, University of Virginia, Charlottesville VA 22904, USA
¹¹ Visiting Scientist, Ural Federal University, 620075 Ekaterinburg, Russia

Received: 5 August 2013
Accepted: 7 December 2013

Abstract

Context. Protoplanetary disks are vital objects in star and planet formation, possessing all the material, gas and dust, which may form a planetary system orbiting the new star. Small, simple molecules have traditionally been detected in protoplanetary disks; however, in the ALMA era, we expect the molecular inventory of protoplanetary disks to significantly increase.

Aims. We investigate the synthesis of complex organic molecules (COMs) in protoplanetary disks to put constraints on the achievable chemical complexity and to predict species and transitions which may be observable with ALMA.

Methods. We have coupled a 2D steady-state physical model of a protoplanetary disk around a typical T Tauri star with a large gas-grain chemical network including COMs. We compare the resulting column densities with those derived from observations and perform ray-tracing calculations to predict line spectra. We compare the synthesised line intensities with current observations and determine those COMs which may be observable in nearby objects. We also compare the predicted grain-surface abundances with those derived from cometary comae observations.

Results. We find COMs are efficiently formed in the disk midplane via grain-surface chemical reactions, reaching peak grain-surface fractional abundances ~10^-6–10^-4 that of the H nuclei number density. COMs formed on grain surfaces are returned to the gas phase via non-thermal desorption; however, gas-phase species reach lower fractional abundances than their grain-surface equivalents, ~10^-12–10^-7. Including the irradiation of grain mantle material helps build further complexity in the ice through the replenishment of grain-surface radicals which take part in further grain-surface reactions. There is reasonable agreement with several line transitions of H₂CO observed towards T Tauri star-disk systems. There is poor agreement with HC₃N lines observed towards LkCa 15 and GO Tau and we discuss possible explanations for these discrepancies. The synthesised line intensities for CH₃OH are consistent with upper limits determined towards all sources. Our models suggest CH₃OH should be readily observable in nearby protoplanetary disks with ALMA; however, detection of more complex species may prove challenging, even with ALMA “Full Science” capabilities. Our grain-surface abundances are consistent with those derived from cometary comae observations providing additional evidence for the hypothesis that comets (and other planetesimals) formed via the coagulation of icy grains in the Sun’s natal disk.