How Do Disks and Planetary Systems in High-mass Open Clusters Differ from Those around Field Stars?

Abstract

Only star clusters that are sufficiently compact and massive survive largely unharmed beyond 10 Myr. However, their compactness means a high stellar density which can lead to strong gravitational interactions between the stars. As young stars are often initially surrounded by protoplanetary disks and later on potentially by planetary systems, the question arises to what degree these strong gravitational interactions influence planet formation and the properties of planetary systems. Here, we perform simulations of the evolution of compact high-mass clusters like Trumpler 14 and Westerlund 2 from the embedded to the gas-free phase and study the influence of stellar interactions. We concentrate on the development of the mean disk size in these environments. Our simulations show that in high-mass open clusters 80 − 90% of all disks/planetary systems should be smaller than 50 AU just due to the strong stellar interactions in these environments. Already in the initial phases, 3-4 close fly-bys lead to typical disk sizes within the range of 18 − 27 AU. Afterwards, the disk sizes are altered only to a small extent. Our findings agree with the recent observation that the disk sizes in the once dense environment of the Upper Scorpio OB association, NGC 2362, and h/χPersei are at least three times smaller in size than, for example, in Taurus. We conclude that the observed planetary systems in highmass open clusters should also be on average smaller than those found around field stars; in particular, planets on wide orbits are expected to be extremely rare in such environments.

On the survivability of planets in young massive clusters and its implication of planet orbital architectures in globular clusters

“…There are several dedicated studies on the effects of stellar environments to the protoplanetary discs. For example, a number of authors suggest that the protoplanetary disc can be truncated, or even disrupted by close stellar flybys (e.g., Olczak et al 2012;Portegies Zwart 2016;Vincke & Pfalzner 2016;Wijnen et al 2017;Richert et al 2018;Vincke & Pfalzner 2018), although the viscous evolution of the disc may be helpful in damping the eccentricity and eventually establishing a new equilibrium (Concha-Ramírez et al 2019). On the other hand, the initial mass function dictates that there will be a small fraction of O/B stars that emit energetic FUV photons.…”

A Hypothesis for the Rapid Formation of Planets

“…The observed higher frequency of planets around high-metallicity stars than around low-metallicity stars (Fischer & Valenti 2005) and indications that planet properties depend on metallicity (Narang et al 2018) are natural outcomes of our scenario. Equally, the non-detection of planets in globular clusters can be interpreted as a lower planet formation rate during that epoch, rather than the potential alternatives of a bias against detection due to the high stellar density, hindered planet formation (Vincke & Pfalzner 2018) or increased planet ejection due to the high stellar density.…”

DESTINY: Database for the Effects of STellar encounters on dIsks and plaNetary sYstems

“…In star cluster environments stellar fly-bys can reduce the disk mass, change the disk's angular momentum, lead to additional accretion or truncate the protoplanetary disk [34,35,[38][39][40][41][42][43][44][45]. Depending on the local stellar density even already formed planetary systems might be influenced by the gravitational interactions with neighbouring stars, even though this is much less frequent [30,[46][47][48][49].…”

“…(Near) parabolic encounters dominate for typical clusters in the solar neighborhood [40,50]. The situation is different in very dense clusters like Westerlund 2 and Arches, where hyperbolic encounters are much more common [44,58]. However, since disks are less affected by hyperbolic encounters in comparison to the parabolic ones, the effect of parabolic encounters can be regarded as an upper limit.…”

“…The work presented herein is an outcome of a collective effort of continuously increasing the accuracy and expanding parameter space over the last 15 years [28,31,33,44,50,59].…”