[Paper Review] On the Dynamical Evolution of the Arches Cluster

Sourav Chatterjee, Sanghamitra Goswami+4 more|ArXiv.org|Nov 8, 2009

Material Science and Thermodynamics3 references3 citations

TL;DR

This study uses N-body simulations to investigate whether the Arches cluster's observed mass segregation and top-heavy initial mass function (IMF) are primordial or dynamically evolved. It finds that a standard Kroupa IMF without primordial segregation cannot reproduce the current mass segregation, but a primordially mass-segregated IMF leads to collisional runaway by 2–3 Myr, potentially forming very massive stars (VMS) or intermediate-mass black holes (IMBHs), depending on stellar wind mass loss.

ABSTRACT

We study the dynamical evolution of the young star cluster Arches and its dependence on the assumed initial stellar mass function (IMF). We perform many direct $N$-body simulations with various initial conditions and two different choices of IMFs. One is a standard Kroupa IMF without any mass segregation. The other is a radially dependent IMF, as presently observed in the Arches. We find that it is unlikely for the Arches to have attained the observed degree of mass segregation at its current age starting from a standard non-segregated Kroupa IMF. We also study the possibility of a collisional runaway developing in the first $\sim 2-3 m{Myr}$ of dynamical evolution. We find that the evolution of this cluster is dramatically different depending on the choice of IMF: if a primordially mass segregated IMF is chosen, a collisional runaway should always occur between $2-3 m{Myr}$ for a broad range of initial concentrations. In contrast, for a standard Kroupa IMF no collisional runaway is predicted. We argue that if Arches was created with a mass segregated IMF similar to what is observed today then at the current cluster age a very unusual, high-mass star should be created. However, whether a collisional runaway leads to the formation of an intermediate-mass black hole (IMBH) depends strongly on the mass loss rate via winds from massive stars. Growth of stellar mass through collisions can be quenched by strong wind mass loss. In that case, the inter-cluster as well as intra-cluster medium are expected to have a significant Helium enrichment which may be observed via Helium recombination lines. The excess amount of gas lost in winds may also be observed via X-ray observations as diffused X-ray sources.

Motivation & Objective

• To determine whether the observed mass segregation in the Arches cluster is a result of dynamical evolution or primordial formation conditions.
• To assess the impact of different initial mass functions (IMFs), particularly a radially dependent IMF observed in the Arches, on dynamical evolution and collisional runaway.
• To investigate the likelihood of collisional runaway forming very massive stars (VMS) or intermediate-mass black holes (IMBHs) in the Arches cluster within its estimated age of 2±1 Myr.
• To evaluate the observational signatures of such runaway processes, including Helium enrichment and X-ray emission from stellar winds.

Proposed method

• Conducting direct N-body simulations of the Arches cluster with varying initial conditions and two IMFs: a standard non-segregated Kroupa IMF and a radially dependent, mass-segregated IMF matching current observations.
• Using a parametric model to simulate the formation of stars from pre-stellar cores (PSCs), where higher core density in the cluster center leads to PSC collisions and preferential formation of massive stars.
• Tracking the evolution of stellar collisions and mass growth over time, particularly focusing on the onset and progression of collisional runaways.
• Applying wind mass loss prescriptions from Glebbeek et al. (2009) to model the evolution of collision products, estimating mass loss rates and Helium enrichment.
• Simulating the potential formation of very massive stars (VMS), intermediate-mass black holes (IMBHs), and associated observational signatures such as recombination lines and X-ray emission.
• Analyzing the statistical fluctuations in collision sequences to assess the robustness of double collisional runaways and binary formation.

Experimental results

Research questions

• RQ1 Can the current degree of mass segregation in the Arches cluster be explained by dynamical evolution from a standard, non-segregated Kroupa IMF?
• RQ2 Does a primordially mass-segregated IMF lead to collisional runaway within the first 2–3 Myr, consistent with the cluster’s estimated age?
• RQ3 What is the likelihood of forming very massive stars (VMS) or intermediate-mass black holes (IMBHs) via collisional runaway in the Arches cluster?
• RQ4 How do strong stellar winds affect the growth of massive stars through collisions, and what observational signatures (e.g., Helium enrichment, X-ray emission) would result?
• RQ5 Can the presence of a Pistol-like star or a double collisional runaway be predicted from the initial conditions and IMF of the Arches cluster?

Key findings

• A standard non-segregated Kroupa IMF cannot reproduce the observed level of mass segregation in the Arches cluster at its current age.
• With a primordially mass-segregated IMF, collisional runaway consistently begins between 2 and 3 Myr across a range of initial concentrations.
• Double collisional runaways occur in simulations with the mass-segregated IMF, even without primordial binaries, due to the enhanced reservoir of high-mass stars.
• If stellar wind mass loss is strong, VMS formation is quenched, and instead, Helium-rich stars with $X_{He,S} > 0.9$ are formed, enriching the cluster and surrounding medium.
• The ejected Helium-rich gas could be detectable via recombination lines at distances beyond 3–7 pc from the cluster center.
• X-ray observations may reveal diffused X-ray emission from the wind material, providing indirect evidence of collisional runaway.