A self-consistent model for the evolution of the gas produced in the debris disc of β Pictoris

Abstract

This paper presents a self-consistent model for the evolution of gas produced in the debris disc of β Pictoris. Our model proposes that atomic carbon and oxygen are created from the photodissociation of CO, which is itself released from volatile-rich bodies in the debris disc due to grain-grain collisions or photodesorption. While the CO lasts less than one orbit, the atomic gas evolves by viscous spreading resulting in an accretion disc inside the parent belt and a decretion disc outside. The temperature, ionisation fraction and population levels of carbon and oxygen are followed with the photodissociation region model Cloudy, which is coupled to a dynamical viscous α model. We present new gas observations of β Pic, of C I observed with APEX and O I observed with Herschel, and show that these along with published C II and CO observations can all be explained with this new model. Our model requires a viscosity α > 0.1, similar to that found in sufficiently ionised discs of other astronomical objects; we propose that the magnetorotational instability is at play in this highly ionised and dilute medium. This new model can be tested from its predictions for high resolution ALMA observations of C I. We also constrain the water content of the planetesimals in β Pic. The scenario proposed here might be at play in all debris discs and this model could be used more generally on all discs with C, O or CO detections.

Detection of Submillimeter-wave [C i] Emission in Gaseous Debris Disks of 49 Ceti and β Pictoris

“…Rough estimations were (4.0-4.4)×10 −3 M ⊕ and (1.5-1.8)×10 −4 M ⊕ for 49 Ceti and β Pictoris, respectively, assuming the above temperature range. Our result is an order of magnitude lower than previous estimates made from [C II] observations for β Pictoris: 7.1×10 −3 M ⊕ (Cataldi et al 2014) and 2×10 −3 M ⊕ (Kral et al 2016). Under the local thermodynamic equilibrium and optically thin condition, the CO column density was calculated as (e.g., Oka et al 2001)…”

Rocky Planet Formation: Quick and Neat

“…Among A-type stars with debris disks, several have neutral or ionized gas close to the star (e.g., Hobbs et al 1985;Ferlet et al 1987;Welsh et al 1998;Redfield 2007;Montgomery & Welsh 2012;Kiefer et al 2014;Cataldi et al 2014, and references therein). Evaporation of comets (Lagrange et al 1987;Beust et al 1990) and vaporization of colliding particles (e.g., Czechowski & Mann 2007) might supply some of this material (see also Kral et al 2016). …”

The magnetorotational instability in debris-disc gas

“…The CO input rate and location of the debris belt are determined by ALMA observations of β Pic. Once these are fixed, Kral et al (2016) find that all other observations are reconciled if α 0.1. Thus the anomolous transport required is very efficient.…”

“…The atomic gas then evolves viscously, due to turbulence parametrised by an α viscosity. As the viscosity depends on temperature, Kral et al (2016) use a PDR-like code (called 'Cloudy') to follow the thermal evolution of the gas at the same time as its dynamics. The temperature is fixed by a competition between the carbon photoionisation heating and the cooling by the C II fine structure line.…”

“…We use the model presented in Kral et al (2016) to determine the parameters of its gas disc and then determine its linear stability, following the formalism of Kunz & Balbus (2004, hereafter KB04). We find that, on the whole, non-ideal MHD effects are not crucial, nor is the dust-gas coupling.…”