Gravitational wave spectral synthesis

TL;DR

This paper models gravitational wave spectra from stellar populations in star clusters, analyzing how age and metallicity influence the types and detectability of sources with LISA, highlighting the importance of binary evolution and cluster properties.

Contribution

It introduces a spectral synthesis approach for LISA sources based on BPASS stellar population models, emphasizing the impact of metallicity and binary evolution on GW signals.

Findings

WD-WD binaries dominate at late times

NS-WD and BH-WD binaries are significant at 10^8-10^9 years

Metallicity affects GW spectrum and binary type dominance

Abstract

We study the LISA sources that arise from isolated binary evolution, and how these depend on age and metallicity, using model stellar populations from BPASS. We model these as single-aged populations which are analogous to star clusters. We calculate the combined GW spectrum of all the binaries within these model clusters, including all types of compact binaries as well as those with living stars. These results allow us to evaluate the detectability of star clusters with LISA. We find at late times the dominant sources are WD-WD binaries by factors of 50-200, but at times between $10^8$ and $10^9$ years we find a significant population of NS-WD and BH-WD binaries (2-40 per $10^6$ M$_{\odot}$), which is related to the treatment of mass transfer and common envelope events in BPASS, wherein mass transfer is relatively likely to be stable. Metallicity also has an effect on the GW spectrum and on the relative dominance of different types of binaries. Using the information about known star clusters will aid the identification of sky locations where one could expect LISA to find GW sources.