In 2015, LIGO observed the emission of gravitational waves (GWs) from the merger and coalescence of a binary black hole (BBH). These LIGO BBHs (LBBHs) are heavier than those previously observed through X-ray binaries and their origin has been much debated over the years. Generally, the methods to discern the origin of LBBHs has focused on their mass, eccentricity, and spin. Our work investigates this mystery through the last of these parameters: spin. LIGO measures spin through a parameter (chi effective) which ranges from -1 to 1. A value of 1 means that both spins of the individual BHs are aligned with the angular momentum of the LBBH, 0 means that you have complete anti-alignment between the individual spins, and -1 means that both spins are aligned opposite the angular momentum of the LBBH. The reason is significant is because the two main proposed formation channels, field formation and dynamical assembly, seem to have characteristic spin alignments, ie. .
The field formation channel is one in which the LBBH originates from an existing stellar binary residing in the galactic field. Through processes such as common envelope evolution or chemically homogenous evolution, the progenitor stars evolve to form a LBBH which should have their spins aligned with their orbital angular momentum (notwithstanding a few caveats). Therefore, this channel can be expected to produce LBBHs with strictly positive values. The dynamical formation channel on the other hand creates LBBHs through stochastic few-body interactions in dense stellar environments such as the cores of globular clusters (GCs). Single BHs frantically exchange partners with other inhabitants of the GCs ad nauseam until the end product is a LBBH. The random nature of these interactions means that the resulting LBBH has isotropically oriented spins and a symmetric distribution of centered around zero. Therefore, if LIGO detects BBHs with a close to 1, then it would seem safe to assume they originated through the field formation channel. Likewise, LBBH sources with zero or negative values appear to rule out the field formation channel and in turn indicate formation through dynamical assembly. While the preceding reasoning seems straightforward we propose that tidal disruption events (TDEs) experienced throughout the lifetime of LBBHs can significantly alter the characteristic values one would expect from each formation channel.
TDEs are phenomena which catastrophically destroy stars when they get too close to a BH. Typically TDEs are studied in the context of supermassive BHs (SMBHs) which have masses greater than one million solar masses. We propose that TDEs can also play an important role in the observed parameters of LBBHs, specifically their spin. In addition, TDEs by LBBHs probe a unique regime of TDEs which exhibit behavior much different than canonical SMBH TDEs. We use hydrodynamic simulations to study the effect of TDEs by LBBHs on the individual spin magnitude and relative directions through mass accretion. Our goal is to investigate whether TDEs can align initially mis-aligned/zero spins. If possible, then no longer can we assume that large positive values automatically point to a field formation origin. We find that the potential to change the natal spin characteristics depend heavily on the mass ratio between the disrupting BH and the disrupted star. Furthermore, we distinguish these interactions through characteristic length scales such as the tidal radius, the binary separation, semi-major axis of bound material, and the Roche Lobe radius of the disrupting BH. Therefore, the final orientation of the spins and whether they can be aligned rely strongly on these parameters.
Watch the simulations: