LISA sources from young massive and open stellar clusters

TL;DR

This paper investigates how young massive and open star clusters contribute to the population of stellar-mass binary black holes detectable by LISA, highlighting their significant role in producing gravitational-wave sources.

Contribution

It presents detailed models showing YMCs and OCs as major sources of LISA-detectable binary black holes, with specific predictions on their properties and merger rates.

Findings

YMCs and OCs produce dozens to hundreds of LISA BBH sources.

Most LISA BBHs are eccentric and have similar component masses.

A small fraction of these BBHs will merge within LISA's mission timeframe.

Abstract

I study the potential role of young massive (YMCs) and open star clusters (OCs) in assembling stellar-mass binary black holes (BBHs) which would be detectable as persistent gravitational-wave (GW) sources by the forthcoming LISA mission. The energetic dynamical interactions inside star clusters make them factories of assembling BBHs and other types of double-compact binaries that undergo general-relativistic (GR) inspiral and merger. The initial phase of such inspirals would, typically, sweep through the LISA GW band. Here, such LISA sources are studied from a set of evolutionary models of star clusters with masses ranging over $10^4M_\odot-10^5M_\odot$ that represent YMCs and intermediate-aged OCs in metal-rich and metal-poor environments of the Local Universe. These models are evolved with long-term, direct, relativistic many-body computations incorporating state-of-the-art stellar-evolutionary and remnant-formation models. Based on models of Local Universe constructed with such model clusters, it is shown that YMCs and intermediate-aged OCs would yield several 10s to 100s of LISA BBH sources at the current cosmic epoch with GW frequency within $10^{-3}{\rm~Hz} - 10^{-1}{\rm~Hz}$ and signal-to-noise-ratio (S/N) $>5$, assuming a mission lifetime of 5 or 10 years. Such LISA BBHs would have a bimodal distribution in total mass, be generally eccentric ($\lesssim0.7$), and typically have similar component masses although mass-asymmetric systems are possible. Intrinsically, there would be 1000s of present-day, LISA-detectable BBHs from YMCs and OCs. That way, YMCs and OCs would provide a significant and the dominant contribution to the stellar-mass BBH population detectable by LISA. A small fraction, $<5$%, of these BBHs would undergo GR inspiral to make it to LIGO-Virgo GW frequency band and merge, within the mission timespan; $<15$% would do so within twice the timespan.