The molecular composition of the planet-forming regions of protoplanetary disks across the luminosity regime^⋆

¹ Leiden Observatory, Leiden University, PO Box 9513 2300 RA Leiden The Netherlands
e-mail: cwalsh@strw.leidenuniv.nl
² Department of Earth and Planetary Sciences, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, 152-8551 Tokyo, Japan
³ Max-Planck-Institut für extraterretrische Physik, Giessenbachstrasse 1, 85748 Garching, Germany

Received: 15 June 2015
Accepted: 30 July 2015

Abstract

Context. Near- to mid-infrared observations of molecular emission from protoplanetary disks show that the inner regions are rich in small organic volatiles (e.g., C₂H₂ and HCN). Trends in the data suggest that disks around cooler stars (T_eff ≈ 3000 K) are potentially (i) more carbon-rich; and (ii) more molecule-rich than their hotter counterparts (T_eff ≳ 4000 K).

Aims. We explore the chemical composition of the planet-forming region (<10 AU) of protoplanetary disks around stars over a range of spectral types (from M dwarf to Herbig Ae) and compare with the observed trends.

Methods. Self-consistent models of the physical structure of a protoplanetary disk around stars of different spectral types are coupled with a comprehensive gas-grain chemical network to map the molecular abundances in the planet-forming zone. The effects of (i) N₂ self shielding; (ii) X-ray-induced chemistry; and (iii) initial abundances, are investigated. The chemical composition in the “observable” atmosphere is compared with that in the disk midplane where the bulk of the planet-building reservoir resides.

Results. M dwarf disk atmospheres are relatively more molecule rich than those for T Tauri or Herbig Ae disks. The weak far-UV flux helps retain this complexity which is enhanced by X-ray-induced ion-molecule chemistry. N₂ self shielding has only a small effect in the disk molecular layer and does not explain the higher C₂H₂/HCN ratios observed towards cooler stars. The models underproduce the OH/H₂O column density ratios constrained in Herbig Ae disks, despite reproducing (within an order of magnitude) the absolute value for OH: the inclusion of self shielding for H₂O photodissociation only increases this discrepancy. One possible explanation is the adopted disk structure. Alternatively, the “hot” H₂O (T ≳ 300 K) chemistry may be more complex than assumed. The results for the atmosphere are independent of the assumed initial abundances; however, the composition of the disk midplane is sensitive to the initial main elemental reservoirs. The models show that the gas in the inner disk is generally more carbon rich than the midplane ices. This effect is most significant for disks around cooler stars. Furthermore, the atmospheric C/O ratio appears larger than it actually is when calculated using observable tracers only. This is because gas-phase O₂ is predicted to be a significant reservoir of atmospheric oxygen.

Conclusions. The models suggest that the gas in the inner regions of disks around cooler stars is more carbon rich; however, calculations of the molecular emission are necessary to definitively confirm whether the chemical trends reproduce the observed trends.