Chemistry in disks^*

II. Poor molecular content of the AB Aurigae disk

¹ Astrophysikalisches Institut und Universitäts-Sternwarte, Schillergässchen 2-3, 07745 Jena, Germany e-mail: martin@astro.uni-jena.de
² Université Bordeaux 1, Laboratoire d'Astrophysique de Bordeaux (LAB), France
³ CNRS/INSU – UMR 5804, BP 89, 33270 Floirac, France
⁴ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
⁵ IRAM, 300 rue de la piscine, 38406 Saint Martin d'Hères, France

Received: 21 December 2007
Accepted: 10 September 2008

Abstract

Aims. We study the molecular content and chemistry of a circumstellar disk surrounding the Herbig Ae star AB Aur at (sub-)millimeter wavelengths. Our aim is to reconstruct the chemical history and composition of the AB Aur disk and to compare it with disks around low-mass, cooler T Tauri stars.

Methods. We observe the AB Aur disk with the IRAM Plateau de Bure Interferometer in the C- and D-configurations in rotational lines of CS, HCN, C₂H, CH₃OH, HCO⁺, and CO isotopes. Using an iterative minimization technique, observed columns densities and abundances are derived. These values are further compared with results of an advanced chemical model that is based on a steady-state flared disk structure with a vertical temperature gradient, and gas-grain chemical network with surface reactions.

Results. We firmly detect HCO⁺ in the 1–0 transition, tentatively detect HCN, and do not detect CS, C₂H, and CH₃OH. The observed HCO⁺ and ¹³CO column densities as well as the upper limits to the column densities of HCN, CS, C₂H, and CH₃OH are in good agreement with modeling results and those from previous studies.


Conclusions. The AB Aur disk possesses more CO, but is less abundant in other molecular species compared to the DM Tau disk. This is primarily caused by intense UV irradiation from the central Herbig A0 star, which results in a hotter disk where CO freeze out does not occur and thus surface formation of complex CO-bearing molecules might be inhibited.