arXiv subjects

Alfie Robinson

Publications and source records attributed to Alfie Robinson.

4 recordsLinked to original sources

arXiv2026

The Influence of Dust Composition on Accretion Outbursts

Context: Episodic accretion outbursts have been shown to occur in protoplanetary discs due to thermal instability at the inner edge of the dead zone. These outbursts periodically heat up the dead zone, significantly altering the composition/chemistry and accretion onto the star. Aims: We investigate how these accretion outbursts affect the inner disc composition and how different dust compositions and properties affect the outbursts' dynamics. Methods: We run vertically integrated axis-symmetric dust and gas evolution models using the TriPoD method, including dust opacity dependent heating and cooling, a dead zone model, and compositional tracking of dust, including the evaporation and condensation of volatiles. Results: When considering dust to be made up of a condensation sequence of silicates and volatiles, the outburst evaporates most of the dust within 0.5 au. The fast re-condensation and subsequent viscous evolution reset the disc between bursts. Additionally, we find that the burst cycle period and maximal accretion rate directly correlate with the dust sublimation temperature.

astro-ph.EP↗

arXiv2026

CO snow lines are stabilised by the vertical transport of volatiles

Volatile evolution in protoplanetary discs determines the compositional evolution of forming planets. Below their sublimation temperatures, volatiles freeze out from the vapour phase onto dust grains in the disc and transition to being dynamically-coupled to the dust component as opposed to the gas. The boundary between the ice and vapour phases is referred to as the snow line, when thought of as the mid-plane radius at which the phase transition occurs, or the snow surface, when viewed as a 2D (radial and vertical) structure in the disc. We investigate whether the CO snow line (and therefore snow surface) is thermally unstable and therefore liable to changes in its location during disc evolution using the disc evolution code cuDisc, to which we have added an ice-vapour chemistry solver. We find that the instability does lead to there being two steady-state stable equilibrium solutions for the snow surface when including the vertical structure. However, in dynamically-evolving simulations, the disc does not enter a limit-cycle - as seen in previous 1D models - due to the shape of the 2D snow surface and the vertical transport of volatiles. We therefore expect that dynamically evolution of snow lines due to instability is limited to transient, stochastic events rather than oscillatory behaviour with a regular period. However, we also expect the snow surface to evolve substantially during the disc lifetime solely due to changes in the thermal structure driven by evolution of the dust spatial structure and grain-size distribution - this we will explore in future models.

astro-ph.EP↗

arXiv2024

The effect of radiation pressure on the dispersal of photoevaporating discs

Observed IR excesses indicate that protoplanetary discs evolve slowly for the majority of their lifetime before losing their near- and mid-IR excesses on short timescales. Photoevaporation models can explain this "two-timescale" nature of disc evolution through the removal of inner regions of discs after a few million years. However, they also predict the existence of a population of non-accreting discs with large cavities. Such discs are scarce within the observed population, suggesting the models are incomplete. We explore whether radiation-pressure-driven outflows are able to remove enough dust to fit observations. We simulate these outflows using cuDisc, including dust dynamics, growth/fragmentation, radiative transfer and a parameterisation of internal photoevaporation. We find that, in most cases, dust mass-loss rates are around 5-10 times too small to meet observational constraints. Particles are launched from the disc inner rim, however grains larger than around a micron do not escape in the outflow, meaning mass-loss rates are too low for the initial dust masses at gap-opening. Only systems that have smooth photoevaporation profiles with gas mass-loss rates $>\sim 5 \times 10^{-9}$ $M_\odot$ yr$^{-1}$ and disc dust masses $<\sim$1 $M_\oplus$ at the time of gap opening can meet observational constraints; in the current models these manifest as EUV winds driven by atypically large high-energy photon fluxes. We also find that the height of the disc's photosphere is controlled by small grains in the outflow as opposed to shadowing from a hot inner rim; the effect of this can be seen in synthetic scattered light observations.

astro-ph.EP↗

arXiv2024

Introducing cuDisc: a 2D code for protoplanetary disc structure and evolution calculations

We present a new 2D axisymmetric code, cuDisc, for studying protoplanetary discs, focusing on the self-consistent calculation of dust dynamics, grain size distribution and disc temperature. Self-consistently studying these physical processes is essential for many disc problems, such as structure formation and dust removal, given that the processes heavily depend on one another. To follow the evolution over substantial fractions of the disc lifetime, cuDisc uses the CUDA language and libraries to speed up the code through GPU acceleration. cuDisc employs a second-order finite-volume Godonuv solver for dust dynamics, solves the Smoluchowski equation for dust growth and calculates radiative transfer using a multi-frequency hybrid ray-tracing/flux-limited-diffusion method. We benchmark our code against current state-of-the-art codes. Through studying steady-state problems, we find that including 2D structure reveals that when collisions are important, the dust vertical structure appears to reach a diffusion-settling-coagulation equilibrium that can differ substantially from standard models that ignore coagulation. For low fragmentation velocities, we find an enhancement of intermediate-sized dust grains at heights of ~ 1 gas scale height due to the variation in collision rates with height, and for large fragmentation velocities, we find an enhancement of small grains around the disc mid-plane due to collisional ''sweeping'' of small grains by large grains. These results could be important for the analysis of disc SEDs or scattered light images, given these observables are sensitive to the vertical grain distribution.

astro-ph.EP↗