Determining the mid-plane conditions of circumstellar discs using gas and dust modelling: a study of HD 163296

Abstract

The mass of gas in protoplanetary discs is a quantity of great interest for assessing their planet formation potential. Disc gas masses are, however, traditionally inferred from measured dust masses by applying an assumed standard gas-to-dust ratio of g/d = 100. Furthermore, measuring gas masses based on CO observations has been hindered by the effects of CO freeze-out. Here we present a novel approach to study the mid-plane gas by combining C 18 O line modelling, CO snowline observations and the spectral energy distribution (SED) and selectively study the inner tens of au where freeze-out is not relevant. We apply the modelling technique to the disc around the Herbig Ae star HD 163296 with particular focus on the regions within the CO snowline radius, measured to be at 90 au in this disc. Our models yield the mass of C M . We find that most of our models yield a notably low g/d < 20, especially in the disc mid-plane (g/d < 1). Our only models with a more interstellar medium (ISM)-like g/d require C 18O to be underabundant with respect to the ISM abundances and a significant depletion of submicron grains, which is not supported by scattered light observations. Our technique can be applied to a range of discs and opens up a possibility of measuring gas and dust masses in discs within the CO snowline location without making assumptions about the gas-to-dust ratio.

Mass constraints for 15 protoplanetary discs from HD 1–0

“…If so, the implication is that ∆ g/d ≈ 20-50. This relative depletion of gas over dust is supported by the hydrostatic MCMax modelling of the SED and low-J 12 CO, 13 CO, and C 18 O lines by Boneberg et al (2016), whose best models had 9.2 < ∆ g/d < 18. It is also consistent with the inner disk value ∆ g/d ≈ 55, measured using accre- 3.…”

“…Most previous estimates of the HD 163296 gas mass relied on low-J emission lines of CO isotopologs, and used a range of modelling approaches from generic model grids to tailored modelling with physical-chemical codes. Those M gas estimates range from 8×10 −3 to 5.8×10 −1 M (Isella et al 2007;Williams & Best 2014;Boneberg et al 2016;Miotello et al 2016;Williams & McPartland 2016;Powell et al 2019;Woitke et al 2019;Booth et al 2019). The mass obtained from the most optically thin isotopolog among these, 13 C 17 O, was 2.1 × 10 −1 M (Booth et al 2019).…”

Planet-forming material in a protoplanetary disc: the interplay between chemical evolution and pebble drift

“…In the absence of direct measurements of the radial flow of gas through discs, the accretion rate onto the star combined with the disc mass provide the most reasonable estimates. For example, Boneberg et al (2016) found that an effective α ∼ 0.1 was required to match the accretion rate of HD 163296, while the turbulent mixing in the vertical direction was lower. Furthermore, based on the surface density profile and accretion rate Clarke et al (2018) derive α ∼ 0.01 for CI Tau.…”

Where can a Trappist-1 planetary system be produced?

“…Evidence for enhanced dust to gas mass ratios have been claimed in a number of systems, though currently these are typically all in a higher mass regime (e.g. Meeus & et al 2010;Isella et al 2012;Muto et al 2015;Pinte et al 2016;Boneberg et al 2016;Ansdell et al 2016;Long et al 2017). Our models suggest that external photoevaporation significantly alters the global dust-to-gas mass ratio of protoplanetary discs by more effectively stripping the gas than the dust, at least about low mass stars like Trappist-1.…”