The Effect of Pair-Instability Mass Loss on Black Hole Mergers (1607.03116v1)

Abstract: Mergers of two stellar origin black holes are a prime source of gravitational waves and are under intensive investigations. One crucial ingredient in their modeling has so far been neglected. Pair-instability pulsation supernovae with associated severe mass loss may suppress formation of massive black holes, decreasing black hole merger rates for the highest black hole masses. The mass loss associated with pair-instability pulsation supernovae limits the Population I/II stellar-origin black hole mass to 50 Msun, in tension with earlier predictions that the maximum black hole mass could be as high as 100 Msun. Suppression of double black hole merger rates by pair-instability pulsation supernovae is negligible for our evolutionary channel. Our standard evolutionary model with inclusion of pair-instability pulsation supernovae and with pair-instability supernovae is fully consistent with the LIGO detections of black hole mergers: GW150914, GW151226 and LVT151012. The LIGO observations seem to exclude high (>400 km/s) BH natal kicks. We predict the detection of several up to 60 BH-BH mergers with total mass 10--150 Msun (most likely range: 20--80 Msun) in the forthcoming 60 effective days of LIGO O2 observations.

• The paper demonstrates that pair-instability pulsation supernovae cap Population I/II black holes at around 50 solar masses.
• The updated StarTrack model reveals mass gaps between 2–5 and 50–135 solar masses due to distinct supernova mechanisms.
• Merger rate predictions remain consistent with LIGO data, anticipating roughly 60 BH-BH events despite the imposed mass constraints.

The Impact of Pair-Instability Mass Loss on Black Hole Mergers

The paper "The Effect of Pair-Instability Mass Loss on Black Hole Mergers" addresses the critical role of pair-instability pulsation supernovae (PPSN) and pair-instability supernovae (PSN) in the formation and merger rates of stellar-origin black holes, a subject of substantial interest given the observations of gravitational waves from black hole–black hole (BH-BH) mergers by LIGO. The authors incorporate new physical processes into the existing frameworks of black hole formation and merger predictions, delivering insights into mass loss mechanisms that potentially suppress the formation of very massive black holes.

Key Findings

1. Limitation on Black Hole Mass: The paper suggests that pair-instability pulsation supernovae impose a strict upper limit on the mass of Population I/II black holes, capped at approximately 50 solar masses. This conclusion significantly contrasts with earlier estimates that posited potential black hole masses reaching up to 100 solar masses. The correlated mass loss during these supernova events is modeled using {\tt StarTrack}, a population synthesis code updated with hydrodynamical estimates of PPSN mass loss.
2. Mass Gaps and Supernovae Effects: The black hole mass distribution exhibits two major gaps due to supernova characteristics—between 2-5 and 50-135 solar masses. The first gap arises from rapid supernova explosions preventing compact object formation, and the second results from pair-instability effects. Above 135 solar masses, black hole formation is possible only through other mechanisms, such as stellar collisions or Population III star evolution.
3. Merger Rates Unaffected: Despite the mass constraint, the suppression effect of PPSN and PSN on BH-BH merger rates is negligible according to the isolated binary evolution channel used. The model is consistent with existing LIGO observations, including events such as GW150914 and GW151226, while future observations are expected to confirm these theoretical predictions.
4. Detection Predictions: Under the optimistic sensitivity expectations for LIGO's O2 run, the paper anticipates detecting approximately 60 BH-BH merger events, with a total mass range of 10-150 solar masses. This prediction has substantial implications for LIGO's observational strategy.
5. Exclusions by Higher Black Hole Mass Detection: The detection of significantly heavier black holes than anticipated by the model (greater than 50 solar masses) would challenge the current understanding of PPSN and prompt a reevaluation of theoretical frameworks or identification of scenarios, such as dynamical interactions leading to massive black hole growth.

Implications and Future Prospects

The paper explores complex processes governing late stellar evolution, contributing vital descriptions that inform both theoretical predictions and observational strategies. The constraints on black hole mass introduce an upper bound that aligns with existing LIGO detections, while still allowing for speculative mass ranges that could reignite debate if heavier objects are observed. The implications extend to improved modeling of stellar environments and evolutionary pathways, especially concerning low-metallicity and Population III stars.

The future avenues of research may include a deeper exploration of black hole formation mechanisms at different metallicity regimes, possibly integrating more intricate dynamics of stellar interactions or considering alternative evolutionary histories. The results offer a scaffold for refining population synthesis methodologies and align closely with astronomical data, enhancing our understanding of gravitational wave sources.