Sizes of protoplanetary discs after star-disc encounters

Abstract

Most stars do not form in isolation, but as part of a star cluster or association. These young stars are initially surrounded by protoplanetary discs. In these cluster environments tidal interactions with other cluster members can alter the disc properties. Besides the disc frequency, its mass, angular momentum, and energy, the disc's size is particularly prone to being changed by a passing star. So far the change in disc size has only been investigated for a small number of very specific encounters. Several studies investigated the effect of the cluster environment on the sizes of planetary systems like our own solar system, based on a generalisation of information from this limited sample. We performed numerical simulations covering the wide parameter space typical of young star clusters, to test the validity of this approach. Here the sizes of discs after encounters are presented, based on a size definition that is comparable to the one used in observational studies. We find that, except for encounters between equal-mass stars, the usually applied estimates are insufficient. They tend to severely overestimate the remaining disc size. We show that the disc size after an encounter can be described by a relatively simple dependence on the periastron distance and the mass ratio of the encounter partners. This knowledge allows us, for example, to pin down the types of encounter possibly responsible for the structure of today's solar system.

Strong effect of the cluster environment on the size of protoplanetary discs?

“…They performed simulations of isolated star-disc encounters for a wide range of periastron distances and mass ratios, choosing the highest contrast in surface density to determine the disc sizes after the encounters. This is similar to the criterion used by observations, and the results by Breslau et al (2014) can easily be applied to cluster simulations. They showed that encounters with large periastron distances and/or low mass ratios do not remove material from the disc but shift it inwards, reducing the disc size but not the mass.…”

“…Third, the discs were modelled consisting of mass-less tracer particles and hydrodynamical effects were excluded, for an indepth discussion see Breslau et al (2014). Rosotti et al (2014) found that the hydrodynamical evolution of the discs is important for high viscosity.…”

“…In contrast to the previous work, we investigate the disc size rather than the disc mass or frequency in clusters. The disc sizes were obtained using the results from Breslau et al (2014).…”

“…Olczak et al 2010;, but fewer studies of the disc size (e.g. Hall et al 1996;Kobayashi & Ida 2001;Breslau et al 2014). Nevertheless, the disc size is an important parameter because it defines the physical extent of the potentially forming planetary system.…”

Origin of the wide-orbit circumbinary giant planet HD 106906

“…3, we discussed the possibility that a stellar fly-by could have stabilized the planet mid-ejection. The effect of such encounters on a disk has been studied intensively (for example in Breslau et al 2014;Jílková et al 2016). This effect is of course very dependent on the mass ratio of the stars and on the encounter periastron and eccentricity.…”