Chemistry and line emission from evolving Herbig Ae disks

¹ Sterrewacht Leiden, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands e-mail: jonkheid@strw.leidenuniv.nl
² Max Planck Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany

Received: 23 May 2006
Accepted: 28 October 2006

Abstract

Aims. To calculate chemistry and gas temperature of evolving protoplanetary disks with decreasing mass or dust settling, and to explore the sensitivity of gas-phase tracers.

Methods. The density and dust temperature profiles for a range of models of flaring and self-shadowed disks around a typical Herbig Ae star are used together with 2-dimensional ultraviolet (UV) radiative transfer to calculate the chemistry and gas temperature. In each model the line profiles and intensities for the fine structure lines of [], [] and [] and the pure rotational lines of CO, CN, HCN and are determined.

Results. The chemistry shows a strong correlation with disk mass. Molecules that are easily dissociated, like HCN, require high densities and large extinctions before they can become abundant. The products of photodissociation, like CN and , become abundant in models with lower masses. Dust settling mainly affects the gas temperature, and thus high temperature tracers like the O and fine structure lines. The carbon chemistry is found to be very sensitive to the adopted PAH abundance. The line ratios CO/, CO/ and [] / can be used to distinguish between disks where dust growth and settling takes place, and disks that undergo overall mass loss.