Sculpting protoplanetary discs -- modelling circumbinary cavities at observable scales with radiation hydrodynamics

TL;DR

Circumbinary disc cavities exhibit diverse shapes that correlate with system scale, suggesting the need to model radiative processes. The authors perform 27 two-dimensional radiation-hydrodynamic simulations across three binary separations and eccentricities, comparing thermodynamic models to locally isothermal cases. They find that radially varying cooling timescales govern cavity size and eccentricity: intermediate-scale systems (a_bin ≈ a few au) tend toward small, circular cavities due to efficient cooling, while very small or very large separations can produce larger, more eccentric cavities unless in-plane radiative diffusion facilitates cooling. The results reproduce observed features in Cs Cha and GG Tau with a single physical framework, underscoring radiative cooling as a central driver of circumbinary disc evolution and morphology.

Abstract

Observations of circumbinary discs reveal inner cavities, with their shape and size varying strongly between different systems. The structure of the cavity is determined by the complex interplay between spirals induced by tidal forcing from the binary and the viscous and radiative damping of the spirals at the cavity edge. To fully understand what determines the properties of observed cavities, it is therefore necessary to capture the effect of radiative processes in modelling. To this end, we run 27 simulations of circumbinary discs in 2D using the PLUTO code. These simulations include various size scales, binary eccentricities and thermodynamic models. We find that the diverse cavity shapes are a natural outcome of the radially-varying cooling timescale, as different radiative processes mediate cooling at different disc size regimes. For binaries with separation of a few au, where the cooling timescale is comparable to the orbital timescale at the cavity edge, we recover much more circular cavities than for quickly- or slowly-cooling discs. Our results show that the cavity structure around several binary systems such as Cs Cha and GG Tau can be explained with one physical model, and highlight the importance of radiative cooling in modelling the dynamical evolution of circumbinary discs.

Figures (10)

• Figure 1: Mass-weighted azimuthally averaged aspect ratios against radius. The scale is readjusted to irradiative flaring and a constant line represents a $R^{2/7}$ radial flaring. The dark blue lines represent locally isothermal models, green lines models that include all heating terms, radiative cooling through the surface and in-plane cooling and light red are models with heating and cooling but without in-plane cooling. Different line style represent different separations (1 au dashed; 5 au potted; 25 au dash-potted). Regions inside the cavity are in lighter colours.
• Figure 2: Azimuthally averaged cooling timescale profiles. The green lines represent models and simulations with in-plane cooling, the light red lines without in-plane cooling. Line styles indicate model with a binary separation of ($a_\mathrm{bin} = 1$ au: dashed; $a_\mathrm{bin} = 5$ au: dotted; $a_\mathrm{bin} = 25$ au: dashed-dotted). Inside the cavity, lines turn pale.
• Figure 3: 2D cooling timescale map. The maps show a snapshot after 29 k$T_{\mathrm{bin}}$ evolution. The colour denotes the orbital cooling time scale $\beta$. The white dashed lines marks the location of the inner cavity. The upper models include all heating and cooling terms with only surface cooling, the lower models in addition include in-plane cooling. The binary orbit has $e_\mathrm{bin}=0.3$ in all models, and $a_\mathrm{bin}=1~$au in the left panel, $a_\mathrm{bin}=5~$au in the middle panel and $a_\mathrm{bin}=25~$au in the right panel.
• Figure 4: Two-dimensional surface density maps for different thermodynamical prescriptions (different coloured sets), $a_\mathrm{bin}=[1,5,25]~$au (top to bottom in each set) and $e_\mathrm{bin}=[0,0.15,0.3]$ (left to right). The cavity edge is marked by a white dashed line.
• Figure 5: Evolution of the cavity size and eccentricity for models with $a_\mathrm{bin}=1~$au. Thick lines mark the cavity semi-major axis, thin lines mark the cavity eccentricity. The dark blue models are locally isothermal, the green models include in-plane cooling and the light red models do not include in-plane cooling. The binary eccentricity of all model in a panel is from top to bottom $e_\mathrm{bin}=[0,0.15,0.3]$.