Not just winds: why models find binary black hole formation is metallicity dependent, while binary neutron star formation is not

Understand

Both detailed and rapid population studies alike predict that binary black hole (BHBH) formation is orders of magnitude more efficient at low metallicity than high metallicity, while binary neutron star (NSNS) formation remains mostly flat with metallicity, and black hole-neutron star (BHNS) mergers show intermediate behavior.

• This finding is a key input to employ double compact objects as tracers of low-metallicity star formation, as spectral sirens, and for merger rate calculations.
• Yet, the literature offers various (sometimes contradicting) explanations for these trends.
• We investigate the dominant cause for the metallicity dependence of double compact object formation.