Forming short period sub-stellar companions in 47 Tuc: I. Dynamical model and brown dwarf tidal capture rates

TL;DR

This study models the dynamical formation of brown dwarf companions in 47 Tuc, revealing lower capture rates than previously thought and highlighting the potential for future surveys to constrain sub-stellar populations in globular clusters.

Contribution

The paper introduces a Monte Carlo dynamical model to estimate brown dwarf tidal capture rates in 47 Tuc, showing reduced rates and the influence of mass segregation on capture efficiency.

Findings

Capture rates are lower than earlier estimates.

Mass segregation reduces capture efficiency for brown dwarfs.

Current constraints on sub-stellar companions are marginally consistent with equal initial populations.

Abstract

Stars in globular clusters formed and evolved in the most extreme environment: high density and low metallicity. If the formation of stars and planets are at all sensitive to environmental conditions, this should therefore be evident in globular clusters. Observations have indicated that hot Jupiters are at least an order of magnitude less prevalent in the central region of the globular cluster 47 Tucanae than in the field. In this work, we explore the claims in the literature for additional consequences for the low mass stellar initial mass function. Tidal capture, the mechanism that produces X-ray binaries in globular clusters, applies also to brown dwarfs (BDs). This process produces tight stellar-BD binaries that would be detectable by transit surveys. Applying a Monte Carlo dynamical evolution model, we compute the overall BD capture rates. We find that the number of captures is lower than previous estimates. Capture efficiency increases steeply with stellar mass, which means that mass segregation reduces capture efficiency as BDs and low mass stars occupy the same regions. The result of this effect is that the current constraints on the short period companion fraction remains marginally consistent with initially equal numbers of BDs and stars. However, our findings suggest that expanding the sample in 47 Tuc or surveying other globular clusters for close sub-stellar companions can yield constraints on the sub-stellar initial mass function in these environments. We estimate the capture rates in other globular clusters and suggest that 47 Tuc remains a promising target for future transit surveys.