Gravitational influence of the globular cluster NGC 7078 (M 15) flyby of the Oort cloud system

¹ Main Astronomical Observatory, National Academy of Sciences of Ukraine, 27 Akademika Zabolotnoho St, 03143 Kyiv, Ukraine
² Nicolaus Copernicus Astronomical Centre Polish Academy of Sciences, ul. Bartycka 18, 00-716 Warsaw, Poland
³ Fesenkov Astrophysical Institute, Observatory 23, 050020 Almaty, Kazakhstan
⁴ Konkoly Observatory, Research Centre for Astronomy and Earth Sciences, Eötvös Loránd Research Network (ELKH), MTA Centre of Excellence, Konkoly Thege Miklós út 15–17, 1121 Budapest, Hungary

^★ Corresponding author: marina@mao.kiev.ua

Received: 3 April 2025
Accepted: 11 June 2025

Abstract

Context. It is crucial to understand the interaction between globular clusters (GCs) and the Oort cloud, as close flybys of such massive objects can significantly disturb the cloud’s structure and redirect comets towards the inner Solar System. This increases the risk of impacts on Earth. Studying such events can teach us about the evolution and stability of the Solar System, as well as the effect of external gravitational forces on its dynamics over time.

Aims. In our study of the gravitational effects of the flyby of the NGC 7078 or M 15 GC on the Oort cloud, we focus on two types of approximation. First, we investigate the impact on the Sun’s orbit during close passages, treating the GC as a point mass. At the second stage, we use a complete N-body system representation of the GC comprising over one million particles. The ultimate goal of the research is to quantify the number of particles stripped from the Oort cloud, and to understand the conditions under which this occurs.

Methods. We carried out a dynamical study of the gravitational interaction between Oort cloud particles and galactic GCs within the time-varying galactic external potential. Initially, the GCs are represented as point masses orbiting the Galaxy alongside the Sun and the Oort cloud system. This study was also extended to include the case of NGC 7078, for which full N-body long-term dynamical modelling of the GC itself was used.

Results. Our study reveals significant variations in the impact of NGC 7078 on the Oort cloud, depending on whether it is modelled as a point mass or a complete N-body system. The N-body system results in much greater stripping of Oort cloud particles, with over 52° stripped during a close pass, compared to a few percent in the point mass model for a flyby at a large distance (>200 pc) and 36° for a closer 10 pc point mass flyby. The N-body system also causes substantial expansion, with particles spreading over 50 pc from the Sun within 30 Myr after the GC’s crossing. This creates a twisted and flattened cloud structure with extended outer tails. These stripped cloud particles (more than 10°) spread across the galaxy, reaching distances of up to 16 kpc from the Sun. These differences emphasise the importance of using detailed N-body simulations to accurately evaluate the gravitational influence of GCs on the Oort cloud and shed light on the varying effects of simple versus complex system representations.