The Importance of Photoevaporation in the Evolution of Protoplanetary Discs

Protoplanetary discs consist of gas and dust leftover from the star formation process orbiting a central star. Lasting a few million years, these are the birth environments of planets, from small rocky worlds to the large gaseous ones, which represent one possible fate for the material in the disc.

However, the planet formation process competes with other processes for this material. For example, if the material in the disc is heated, it can be blown off of the disc in the manner of a wind. This process is known as photoevaporation. The source of the heating may be other nearby stars - "external photoevaporation" - or the central star itself - "internal photoevaporation". Understanding photoevaporation is key because if it acts quickly enough, it may limit the material available for the formation of planets; other theories suggest it could instead promote it. The project seeks to understand the relative importance of photoevaporation processes to the evolution of these discs from two directions: a) simulations of the process itself, b) comparisons of observational trends to photoevaporation models.

a) The rates at which winds driven by photoevaporation remove mass from a disc are still uncertain and vary between models, for example, whether the heating is driven by radiation in the ultraviolet or x-ray part of the spectrum. Understanding how different physical processes act to set the mass loss rates will help resolve this tension and determine the strength of the photoevaporation process. I am tackling this by running numerical hydrodynamical simulations, building up from the simplest models with just the most important features. While simple models have been run before, we are planning a more diverse set that explore, for example, the angle that the wind originates at. These will be useful for interpreting direct observations of the winds. Moreover, at each stage as we add complexity (the first level would be gravity and centrifugal force, more novel aspects would be improved chemistry or relaxing the assuming of radiative equilibrium) to our models, we will compare our results to those from the most advanced models to see which changes have an important effect; we can also apply consistent analysis across these models, such as tracing the radiation, to understand why they may disagree.
b) Existing observational surveys, particularly those conducted using the Atacama Large Millimetre Array (ALMA), are providing data that show a number of correlations between the properties of protoplanetary discs within clusters of young stars. Changes can also be seen with age from cluster to cluster, giving an insight into how the discs evolve. By combining the end results of photoevaporation models with models for the evolution of the disc's dust, we will investigate how the different processes that occur in discs interact. Hence, we will assess whether different photoevaporation models are necessary or helpful in explaining the observed properties, or whether other processes such as planet formation or the "radial drift" of dust are more important in shaping them. Hopefully, this may help place limits on various models of the photoevaporation process or on the intrinsic or initial properties of these discs.