Are the observed black hole mergers spins consistent with field binary progenitors?

TL;DR

This paper investigates whether the low observed spins of black hole mergers align with predictions from field binary progenitor models, considering uncertainties in stellar evolution and binary interactions.

Contribution

It provides a detailed comparison between observed black hole merger spins and theoretical expectations from different stellar evolution scenarios, highlighting uncertainties and potential for future observational tests.

Findings

Expected high spin fraction is ≥25% under typical parameters.

Uncertainties in stellar wind and common envelope phases allow for low spin outcomes.

Negative spin observations could challenge the field binary progenitor scenario.

Abstract

One of the puzzles in the recent observations of gravitational waves from binary black hole mergers is the observed low (projected) spins of the progenitor black holes. In two of the four events, GW150914, and the recent GW170104, the observed spins are most likely negative (but consistent with zero). In the third case LVT151012 it is practically zero and only in the forth case, GW151226, the spin is positive but low. These observations are puzzling within the field binary scenario in which positive higher spins are expected. Considering the most favorable Wolfe Rayet (WR) progenitors we estimate the expected spin distribution for different evolution scenarios and compare it to the observations. With typical parameters one expects a significant fraction ($\ge 25\%$) of the mergers to have high effective spin values. However due to uncertainties in the outcome of the common envelope phase (typical separation and whether the stars are rotating or not) and in the late stages of massive star evolution (the strength of the winds) one cannot rule our scenarios in which the expected spins are low. While observations of high effective spin events will support this scenario, further observations of negative spin events would rule it out.