Imaging [CI] around HD 131835: reinterpreting young debris discs with protoplanetary disc levels of CO gas as shielded secondary discs

Abstract

Despite being > 10Myr, there are ∼10 debris discs with as much CO gas as in protoplanetary discs. Such discs have been assumed to be "hybrid", i.e., with secondary dust but primordial gas. Here we show that both the dust and gas in such systems could instead be secondary, with the high CO content caused by accumulation of neutral carbon (C 0 ) that shields CO from photodissociating; i.e., these could be "shielded secondary discs". New ALMA observations are presented of HD131835 that detect ∼ 3 × 10 −3 M ⊕ of C 0 , the majority 40-200au from the star, in sufficient quantity to shield the previously detected CO. A simple semi-analytic model for the evolution of CO, C and O originating in a volatile-rich planetesimal belt shows how CO shielding becomes important when the viscous evolution is slow (low α parameter) and/or the CO production rate is high. Shielding by C 0 may also cause the CO content to reach levels at which CO self-shields, and the gas disc may become massive enough to affect the dust evolution. Application to the HD 131835 observations shows these can be explained if α ∼ 10 −3 ; an inner cavity in C 0 and CO may also mean the system has yet to reach steady state. Application to other debris discs with high CO content finds general agreement for α = 10 −3 to 0.1. The shielded secondary nature of these gas discs can be tested by searching for C 0 , as well as CN, N 2 and CH + , which are also expected to be shielded by C 0 .

Population synthesis of exocometary gas around A stars

“…This choice of parametrization of ν is arbitrary, but it offers a form simple enough to understand the gas evolution. Note that we expect α to be smaller than unity, although its value has only been constrained between 10 −4 − 0.5 Kral & Latter 2016;Kral et al 2019;Moór et al 2019). The input of gas in the system happens through the release of volatile species that escape after the break up of volatile rich bodies in a collisional cascade.…”

“…The CO and carbon evolution presented here is overall consistent with the 0D model presented in Kral et al (2019, see their Figure 18). The main difference is that the predicted masses of carbon and CO (when completely shielded) are overall larger in this study for the same α since in Kral et al (2019) we used the local viscous timescale which is a twelfth of the viscous timescale. Moreover, here we are considering the total gas mass in the system, which can be orders of magnitude higher than the gas mass around the belt location.…”

“…with the estimated CO and neutral carbon mass in four discs (β Pic, 49 Ceti, HD 131835, HD32297, Higuchi et al 2017Cataldi et al 2018;Kral et al 2019;Cataldi et al 2019). Overall we find that the observed carbon masses are lower than predicted by our model.…”

“…So far through this paper we have neglected how gas could affect the dynamics of dust; particularly, how gas could damp the eccentricity of small dust grains in high eccentricity orbits or unbound trajectories due to radiation pressure, and how gas could drag small dust interior or exterior to the planetesimal belt on timescales shorter than collisions or P-R drag. Here we aim to quantitatively check if gas could affect the dust dynamics in the context of shielded discs (see also Kral et al 2019).…”

“…Since secondary gas released from planetesimals is poor in H 2 , it was thought that CO would quickly photodissociate never reaching the high levels observed around the majority of A stars with bright debris discs. Kral et al (2019) showed that neutral carbon produced by CO photodissociation can effectively shield CO and potentially explain the high CO masses around 9 A stars with bright debris discs. Here we present a new model that simulates the gas viscous evolution, accounting for carbon shielding and how the gas release rate decreases with time as the planetesimal disc loses mass.…”