Close Stellar Encounters in Young, Substructured, Dissolving Star Clusters: Statistics and Effects on Planetary Systems

Abstract

Both simulations and observations indicate that stars form in filamentary, hierarchically clustered associations, most of which disperse into their galactic field once feedback destroys their parent clouds. However, during their early evolution in these substructured environments, stars can undergo close encounters with one another that might have significant impacts on their protoplanetary disks or young planetary systems. We perform N-body simulations of the early evolution of dissolving, substructured clusters with a wide range of properties, with the aim of quantifying the expected number and orbital element distributions of encounters as a function of cluster properties. We show that the presence of substructure both boosts the encounter rate and modifies the distribution of encounter velocities compared to what would be expected for a dynamically relaxed cluster. However, the boost only lasts for a dynamical time, and as a result the overall number of encounters expected remains low enough that gravitational stripping is unlikely to be a significant effect for the vast majority of star-forming environments in the Galaxy. We briefly discuss the implications of this result for models of the origin of the Solar System, and of free-floating planets. We also provide tabulated encounter rates and orbital element distributions suitable for inclusion in population synthesis models of planet formation in a clustered environment.

Stellar orbit evolution in close circumstellar disc encounters

“…Although the orbital decay/capture of the stars is still a sensitive function of disc/envelope mass, the increased encounter rates given by Craig & Krumholz (2013) (which are independent of disc mass) imply that flyby disc encounters could provide a means for truncation early on in the disc lifetime. Extreme environments like Orion nebular cluster may already provide the evidence of this process having taken place, as suggested by de Juan Ovelar et al (2012).…”

“…Despite the increased sophistication of cluster models (Adams et al 2006;Parker & Quanz 2012;Bate 2011Bate , 2012Craig & Krumholz 2013), the dynamical range required on timescales of N -body systems of stars with respect to their planetary systems makes direct simulation of full clusters prohibitive computationally. Instead, we isolate two-body systems in order to study discdisc interactions at high resolution.…”

External photoevaporation of protoplanetary discs in Cygnus OB2: linking discs to star formation dynamical history

“…It is possible that this distribution is truncated at large velocities, and indeed star formation models suggest stars might have a smaller velocity dispersion than the priomordial gas (Offner et al 2009, see Section 4.3.3). A FRAC model is generated using the recipe fully described by Goodwin & Whitworth (2004) which we briefly review here (see also Scally & Clarke 2002;Craig & Krumholz 2013). First we define a cube with side length 2 (in arbitrary units) centred at the origin.…”

Sizes of protoplanetary discs after star-disc encounters

“…Johnstone et al 1998;Scally & Clarke 2001) or gravitational interactions with other cluster members (e.g. Moeckel & Bally 2006;Craig & Krumholz 2013). In this paper we only concentrate on the effects of the latter.…”