1Gyr in the Life of the Globular Cluster NGC 6397

TL;DR

This study uses N-body simulations to investigate core fluctuations in the globular cluster NGC 6397 over a billion years, revealing semi-regular oscillations influenced by primordial binaries.

Contribution

It verifies that surface brightness fluctuations observed in Monte Carlo models also occur in N-body models, clarifying their nature and origin.

Findings

Fluctuations are semi-regular oscillations with ~10^8 yr timescale.

Primordial binaries significantly influence core dynamics.

Models match observed core behavior of NGC 6397.

Abstract

M4 and NGC 6397 are two very similar galactic globular clusters, which differ mainly in their surface brightness profile. M4 has a classic King-like profile, whereas NGC 6397 has a more concentrated profile, which is often interpreted as that of a post-core collapse cluster. Heggie & Giersz (2008), however, found that M4 is also a post-core collapse cluster, and Giersz & Heggie (2009) concluded that the main reason for the difference between the two surface brightness profiles is fluctuations. This conclusion was reached on the basis of Monte Carlo models, however, and in the present Letter we verify that similar fluctuations occur in N-body models. The models were initialised by generating initial conditions from the Monte Carlo model of NGC 6397 at the simulated age of 12Gyr, and one was followed for 1Gyr. The new models help to clarify the nature of the fluctuations, which have the nature of semi-regular oscillations with a time scale of order 10^8yr. They are influenced by the dynamical role which is played by primordial binaries in the evolution of the core.