Gas emission from debris disks around A and F stars

TL;DR

This paper introduces a new code, ontario, to model gas emission in debris disks around A and F stars, revealing that CII 157.7 micron line emission is a key observable for understanding disk gas properties.

Contribution

The paper presents a novel self-consistent code for simulating gas emission in dust-poor debris disks, incorporating detailed heating, cooling, and ionization processes.

Findings

Gas is mainly heated by photoelectric emission from dust.

CII 157.7 micron line is the dominant cooling line.

CII emission should be detectable by Herschel in these disks.

Abstract

Gas has been detected in a number of debris disk systems. This gas may have arisen from grain sublimation or grain photodesorption. It interacts with the surrounding dust grains through a number of charge and heat exchanges. Studying the chemical composition and physical state of this gas can therefore reveal much about the dust component in these debris disks. We have produced a new code, ontario, to address gas emission from dusty gas-poor disks around A--F stars. This code computes the gas ionization and thermal balance self-consistently, with particular care taken of heating/cooling mechanisms. Line emission spectra are then produced for each species (up to zinc) by statistical equilibrium calculations of the atomic/ionic energy levels. For parameters that resemble the observed beta Pictoris gas disk, we find that the gas is primarily heated by photoelectric emission from dust grains, and primarily cooled through the CII 157.7 micron line emission. The gas can be heated to a temperature that is warmer than that of the dust and may in some cases reach temperature for thermal escape. The dominant cooling line, CII 157.7 micron, should be detectable by Herschel in these disks, while the OI 63.2 micron line will be too faint. We also study the dependence of the cooling line fluxes on a variety of disk parameters, in light of the much improved sensitivity to thermal line emission in the mid/far infrared and at sub-millimeter wavelengths provided by, in particular, Herschel, SOFIA and ALMA. These new instruments will yield much new information about dusty debris disks.