Observing Carbon and Oxygen Carriers in Protoplanetary Disks at Mid-infrared Wavelengths

Dana E Anderson,Colette Salyk,Arthur D Bosman,Ke Zhang,Edwin A Bergin,L Ilsedore Cleeves,Kamber R Schwarz,Geoffrey A Blake

doi:10.3847/1538-4357/abd9c1

Abstract

Abstract Infrared observations probe the warm gas in the inner regions of planet-forming disks around young Sun-like T Tauri stars. In these systems, H2O, OH, CO, CO2, C2H2, and HCN have been widely observed. However, the potentially abundant carbon carrier CH4 remains largely unconstrained. The James Webb Space Telescope (JWST) will be able to characterize mid-infrared fluxes of CH4 along with several other carriers of carbon and oxygen. In anticipation of the JWST mission, we model the physical and chemical structure of a T Tauri disk to predict the abundances and mid-infrared fluxes of observable molecules. A range of compositional scenarios are explored involving the destruction of refractory carbon materials and alterations to the total elemental (volatile and refractory) C/O ratio. Photon-driven chemistry in the inner disk surface layers largely destroys the initial carbon and oxygen carriers. This causes models with the same physical structure and C/O ratio to have similar steady-state surface compositions, regardless of the initial chemical abundances. Initial disk compositions are better preserved in the shielded inner disk midplane. The degree of similarity between the surface and midplane compositions in the inner disk will depend on the characteristics of vertical mixing at these radii. Our modeled fluxes of observable molecules respond sensitively to changes in the disk gas temperature, inner radius, and total elemental C/O ratio. As a result, mid-infrared observations of disks will be useful probes of these fundamental disk parameters, including the C/O ratio, which can be compared to values determined for planetary atmospheres.

Highlights

(Exo)planetary studies are revealing the chemical composition of planets in our galaxy
The high excitation temperatures derived for these molecules suggest that they are present in the warm inner regions of disks, consistent with the radii associated with terrestrial planet formation
The aim of this work is to understand if we can ascertain both the chemical history and bulk composition of the inner disk via nearto-mid-infrared observations, such as those anticipated by the upcoming James Webb Space Telescope (JWST)

Summary

Introduction

(Exo)planetary studies are revealing the chemical composition of planets in our galaxy. Investigation of the elemental composition of gas in the inner regions of protoplanetary disks, up to a few astronomical units from the central star, is of particular interest because this region is thought to be representative of the terrestrial planet-forming environment in our own solar system. We present theoretical models investigating how well emission of observable molecules including, H2O, OH, CO2, C2H2, HCN, and CH4, responds to changes in the major carbon and oxygen carriers in the inner regions of protoplanetary disks. The aim of this work is to understand if we can ascertain both the chemical history and bulk composition of the inner disk via nearto-mid-infrared observations, such as those anticipated by the upcoming James Webb Space Telescope (JWST). We explore several different chemical scenarios to determine how the fluxes of observable species are affected

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Conclusion