REVIEW
The most ordinary formation of the most unusual double black hole merger
High Energy Astrophysical Phenomena · Submitted 28 Sep 2020 · 51 references
Pith's verdict: A transparent, well-scoped population-synthesis argument that standard isolated binaries can form GW190521-like mergers, but only if you accept a non-standard 12C+12C rate and a specific dredge-up episode.
T0 · AI review
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Referee report
The paper
The paper claims
GW190521's 85 and 66 solar-mass black holes can form through ordinary isolated binary evolution, without dynamical capture or exotic physics, if pair-instability mass loss stops below about 90 solar masses.
The reading
What carries the argument
The central object is the shifted pair-instability upper mass gap: helium cores with mass below 90 solar masses are assumed to collapse directly to black holes, with pulsational pair-instability mass loss and pair-instability disruption acting only above that threshold. This is implemented in a population-synthesis model extended to 200 solar-mass initial stars, and it converts a very massive low-metallicity binary into two heavy black holes through stable Roche-lobe overflow, a common-envelope episode, and two direct core collapses with no natal kicks. The carbon fusion rate being two standard deviations below the standard value, plus a dredge-up episode during core helium burning, is what allows the low-metallicity helium cores to stay below the pair-instability threshold.
Extended reading notes
Core claim
The paper establishes that a GW190521-like merger can be produced by classical isolated binary evolution if the pair-instability upper mass gap is raised to about 90 solar masses. The demonstration is a specific binary: 187.1 + 143.2 solar masses at metallicity Z = 0.0001 on a wide, nearly circular orbit. The primary fills its Roche lobe as a Hertzsprung-gap star; stable mass transfer widens the orbit and strips the envelope, and the primary collapses directly into an 83.6 solar-mass black hole. The secondary later initiates a common envelope, the orbit shrinks to about 12 solar radii, the primary accretes to 84.9 solar masses, and the secondary collapses to 64.6 solar masses. The pair merges by gravitational radiation about 7.5 million years after formation, with effective spin in the range 0.09-0.29, consistent with the LIGO/Virgo estimate, while the predicted precession spin is zero. Population synthesis gives merger rates of 63 Gpc$^{-3}$ yr$^{-1}$ for light black-hole mergers (both components below 50 solar masses) and 0.04 Gpc$^{-3}$ yr$^{-1}$ for heavy mergers (both above 50 solar masses), the latter falling inside the 90% credible interval inferred from GW190521.
Load-bearing premise
Everything rests on helium cores below 90 solar masses being able to collapse straight into black holes, which requires the carbon fusion rate to be about two standard deviations lower than currently standard and a dredge-up episode during core helium burning.
Editorial extensions
If this is right
- Heavy black-hole mergers should occur at about 0.04 Gpc$^{-3}$ yr$^{-1}$, within the 0.02-0.43 range LIGO/Virgo inferred from GW190521 alone.
- GW190521-like systems form and merge on a timescale of about 7.5 million years, so their detection redshift should track low-metallicity star formation.
- The heavy mergers in this model have mass ratios $q \gtrsim 0.6$ and no precession, so future gravitational-wave measurements of mass ratio and precession can test the channel.
- The same calculation reproduces the observed light black-hole merger rate, so the heavy branch does not require adding a separate formation mechanism.
Reading between the lines
- If improved measurements pin the carbon fusion rate to within less than two standard deviations of the standard value, this model's heavy branch disappears, and a detected heavy merger would have to come from dynamical capture, repeated mergers, or primordial black holes.
- The same shifted mass gap implies that heavy black-hole mergers should have total masses between roughly 100 and 180 solar masses; an event above that range would be evidence for physics beyond this model.
- Because direct collapse gives no natal kicks, the simplest version of this channel predicts aligned, non-precessing spins; a confident future detection of in-plane precession in a heavy merger would force either a different formation site or asymmetric neutrino-emission kicks.
Original abstract
LIGO/Virgo Collaboration reported the detection of the most massive black hole - black hole (BH-BH) merger up to date with component masses of 85 Msun and 66 Msun (GW190521). Motivated by recent observations of massive stars in the 30 Doradus cluster in the Large Magellanic Cloud (>200 Msun; e.g. R136a) and employing newly estimated uncertainties on pulsational pair-instability mass-loss (that allow for possibility of forming BHs with mass up to 90Msun) we show that it is trivial to form such massive BH-BH mergers through the classical isolated binary evolution (with no assistance from either dynamical interactions or exotica). A binary consisting of two massive (180+150 Msun) Population II stars (Z=0.0001) evolves through a stable Roche lobe overflow and common envelope episode. Both exposed stellar cores undergo direct core-collapse and form massive BHs while avoiding pair-instability pulsation mass-loss or total disruption. LIGO/Virgo observations show that the merger rate density of light BH-BH mergers (both components: <50 Msun) is of the order of 10-100 Gpc^-3 yr^-1, while GW190521 indicates that the rate of heavier mergers is 0.02-0.43 Gpc^-3 yr^-1. Our model (with standard assumptions about input physics) but extended to include 200 Msun stars and allowing for the possibility of stellar cores collapsing to 90 Msun BHs produces the following rates: 63 Gpc^-3 yr^-1 for light BH-BH mergers and 0.04 Gpc^-3 yr^-1 for heavy BH-BH mergers. We do not claim that GW190521 was formed by an isolated binary, but it appears that such a possibility can not be excluded.
The compiled record
The paper's own sentences, arranged, kept beside the review and separate from it. Each label is drawn from a closed vocabulary; the number beside it is the probability the classifier gave that label. Numbers in the tables are the authors', checked by code. No sentence in this record was written by a model.
The abstract, sentence by sentence
LIGO/Virgo Collaboration reported the detection of the most massive black hole - black hole (BH-BH) merger up to date with component masses of 85 Msun and 66 Msun (GW190521). Motivated by recent observations of massive stars in the 30 Doradus cluster in the Large Magellanic Cloud (>200 Msun; e.g. R136a) and employing newly estimated uncertainties on pulsational pair-instability mass-loss (that allow for possibility of forming BHs with mass up to 90Msun) we show that it is trivial to form such massive BH-BH mergers through the classical isolated binary evolution (with no assistance from either dynamical interactions or exotica). A binary consisting of two massive (180+150 Msun) Population II stars (Z=0.0001) evolves through a stable Roche lobe overflow and common envelope episode. Both exposed stellar cores undergo direct core-collapse and form massive BHs while avoiding pair-instability pulsation mass-loss or total disruption. LIGO/Virgo observations show that the merger rate density of light BH-BH mergers (both components: <50 Msun) is of the order of 10-100 Gpc^-3 yr^-1, while GW190521 indicates that the rate of heavier mergers is 0.02-0.43 Gpc^-3 yr^-1. Our model (with standard assumptions about input physics) but extended to include 200 Msun stars and allowing for the possibility of stellar cores collapsing to 90 Msun BHs produces the following rates: 63 Gpc^-3 yr^-1 for light BH-BH mergers and 0.04 Gpc^-3 yr^-1 for heavy BH-BH mergers. We do not claim that GW190521 was formed by an isolated binary, but it appears that such a possibility can not be excluded.
background contribution method result interpretation main claim
| # | sentence | role | the authors' own work | states the main conclusion | strength |
|---|---|---|---|---|---|
| s0 | LIGO/Virgo Collaboration reported the detection of the most massive black hole - black hole (BH-BH) merger up to date with component masses of 85 Msun and 66 Msun (GW190521). | background 1.00 | 0.04 | 0.03 | asserted 1.00 |
| s1 | Motivated by recent observations of massive stars in the 30 Doradus cluster in the Large Magellanic Cloud (>200 Msun; e.g. R136a) and employing newly estimated uncertainties on pulsational pair-instability mass-loss (that allow for possibility of forming BHs with mass up to 90Msun) we show that it is trivial to form such massive BH-BH mergers through the classical isolated binary evolution (with no assistance from either dynamical interactions or exotica). | contribution 0.85 | 0.74 | 0.85 | asserted 0.99 |
| s2 | A binary consisting of two massive (180+150 Msun) Population II stars (Z=0.0001) evolves through a stable Roche lobe overflow and common envelope episode. | method 0.93 | 0.56 | 0.04 | asserted 0.99 |
| s3 | Both exposed stellar cores undergo direct core-collapse and form massive BHs while avoiding pair-instability pulsation mass-loss or total disruption. | method 0.82 | 0.61 | 0.10 | asserted 1.00 |
| s4 | LIGO/Virgo observations show that the merger rate density of light BH-BH mergers (both components: <50 Msun) is of the order of 10-100 Gpc^-3 yr^-1, while GW190521 indicates that the rate of heavier mergers is 0.02-0.43 Gpc^-3 yr^-1. | background 0.97 | 0.07 | 0.03 | asserted 0.82 |
| s5 | Our model (with standard assumptions about input physics) but extended to include 200 Msun stars and allowing for the possibility of stellar cores collapsing to 90 Msun BHs produces the following rates: 63 Gpc^-3 yr^-1 for light BH-BH mergers and 0.04 Gpc^-3 yr^-1 for heavy BH-BH mergers. | result 1.00 | 0.93 | 0.21 | asserted 1.00 |
| s6 | We do not claim that GW190521 was formed by an isolated binary, but it appears that such a possibility can not be excluded. | limitation 0.50 | 0.48 | 0.28 | hedged possibility 1.00 |
What was examined, and what was not
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Figures
Figures reproduced from arXiv:2009.13526 by the authors.
References
How later papers used it
68 later papers cite it
1 paper by the same authors is listed below: 1 used its method or tool.
Uses its method or tool 1 paper · 1 by the same authors
By the same authors
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The second approach is revised prescription from [ref] in which star experience disruption in PSN if the final mass of its helium core is in the range $90<M_He <175$.
The role of supernova convection for the lower mass gap in the isolated binary formation of gravitational wave sources (2022) (same authors) · StarTrack code · 0.93
How the literature uses it
29 arXiv papers cite this in 32 sentences
Papers citing it · 26 sentences without a confident label
Uses its data or values 1 paper
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However, the angular momentum at binary black hole formation can be a factor of a few less in some cases, e.g., it is $∼50M^2$ in the isolated binary formation scenarios for GW170729, GW190412, and GW190521 given in Fig. 9 of [ref] , Fig. 1 of [ref] , and Fig. 2 of [ref] , respectively.
Inferring spin tilts at formation from gravitational wave observations of binary black holes: Interfacing precession-averaged and orbit-averaged spin evolution · Uncertainties in approximating the tilts at formation by those at infinity · 0.98
Cites it as background 5 papers
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These include: reduction by $≳2σ$ of the $^12C(α,γ)^16O$ reaction rate [ref] , modifications to the standard model [ref] , and population III stars [ref] .
The Stellar Merger Scenario for Black Holes in the Pair-instability Gap · Introduction · 0.84
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Pre-explosion mechanisms include mass loss suppression in low-metallicity ($Z≲10^-3$) progenitors [ref] ; collisions between massive stars in dense clusters [ref] ;
Cosmologically coupled compact objects: a single parameter model for LIGO--Virgo mass and redshift distributions · Introduction · 0.87
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The precise lower limit of the PISN mass gap is an area of active reseach; see [ref] , and references therein.
GW190521: orbital eccentricity and signatures of dynamical formation in a binary black hole merger signal · Introduction · 0.96
What it cites · 11 sentences without a confident label
Uses the method or tool of 1 paper
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We have also adopted the most favorable (in terms of forming massive BHs from stars) model from [ref] that avoids PPSN mass loss for helium core masses: $M_He<90$, but allows for disruption of stars above this mass threshold.
Constraints from gravitational wave detections of binary black hole mergers on the $^{12}\rm{C}\left(\alpha,\gamma\right)^{16}\!\rm{O}$ rate · Calculations · 0.83
Cites as background 6 papers
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These proposals include dynamical formation scenarios of repeated BH mergers in dense clusters [ref] , repeated stellar mergers in dense clusters [ref] , primordial black holes [ref] .
On the Origin of GW190521-like events from repeated black hole mergers in star clusters · Introduction · 0.96
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These proposals include dynamical formation scenarios of repeated BH mergers in dense clusters [ref] , repeated stellar mergers in dense clusters [ref] , primordial black holes [ref] .
Intermediate Mass Black Hole Formation in compact Young Massive Star Clusters · Introduction · 0.96
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These proposals include dynamical formation scenarios of repeated BH mergers in dense clusters [ref] , repeated stellar mergers in dense clusters [ref] , primordial black holes [ref] .
The GW190521 Mass Gap Event and the Primordial Black Hole Scenario · Introduction · 0.96
Discussion
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