3/22/2021

The investigation of Zero Age Main Sequence (ZAMS) massive binaries with orbital periods of a few days is necessary to answer an important question—what fraction of binaries survive the main sequence without merging? As close stellar binaries in low-metallicity environments are necessary to produce black hole pairs that merge in the Hubble time, constraining their main sequence population is a crucial first step. Given such short initial periods, these binaries go through a contact phase on the main sequence, wherein both stars overflow their Roche lobes simultaneously to share a common surface. They can experience one of two outcomes- either the stars merge while burning hydrogen in their cores OR or they avoid merging and potentially collapse to black hole binaries. We compute a large grid of short-period massive star binaries spanning a range of initial parameters and compare with the observational sample available for these systems. The results of this first investigation provide constraints on this important evolutionary phase and its physics, along with having significant implications for the prediction of compact binary objects residing in distant galaxies.