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1. Resolving the Peak of the Black Hole Mass Spectrum

   https://doi.org/10.3847/1538-4357/ac8b83  

   Farag, Ebraheem ; Renzo, Mathieu ; Farmer, Robert ; Chidester, Morgan T. ; Timmes, F. X. ( October 2022 , The Astrophysical Journal)

   Gravitational-wave (GW) detections of binary black hole (BH) mergers have begun to sample the cosmic BH mass distribution. The evolution of single stellar cores predicts a gap in the BH mass distribution due to pair-instability supernovae (PISNe). Determining the upper and lower edges of the BH mass gap can be useful for interpreting GW detections of merging BHs. We useMESAto evolve single, nonrotating, massive helium cores with a metallicity ofZ= 10⁻⁵, until they either collapse to form a BH or explode as a PISN, without leaving a compact remnant. We calculate the boundaries of the lower BH mass gap for S-factors in the range S(300 keV) = (77,203) keV b, corresponding to the ±3σuncertainty in our high-resolution tabulated¹²C(α,γ)¹⁶O reaction rate probability distribution function. We extensively test temporal and spatial resolutions for resolving the theoretical peak of the BH mass spectrum across the BH mass gap. We explore the convergence with respect to convective mixing and nuclear burning, finding that significant time resolution is needed to achieve convergence. We also test adopting a minimum diffusion coefficient to help lower-resolution models reach convergence. We establish a new lower edge of the upper mass gap asM_lower≃${60}_{-14}^{+32}$M_⊙from the ±3σuncertainty in the¹²C(α,γ)¹⁶O rate. We explore the effect of a larger 3αrate on the lower edge of the upper mass gap, findingM_lower≃${69}_{-18}^{+34}$M_⊙. We compare our results with BHs reported in the Gravitational-Wave Transient Catalog.

    

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2. Observing Intermediate-mass Black Holes and the Upper Stellar-mass gap with LIGO and Virgo

   https://doi.org/10.3847/1538-4357/ac3130  

   Mehta, Ajit Kumar ; Buonanno, Alessandra ; Gair, Jonathan ; Miller, M. Coleman ; Farag, Ebraheem ; deBoer, R. J. ; Wiescher, M. ; Timmes, F. X. ( January 2022 , The Astrophysical Journal)

   Abstract Using ground-based gravitational-wave detectors, we probe the mass function of intermediate-mass black holes (IMBHs) wherein we also include BHs in the upper mass gap at ∼60–130 M ⊙ . Employing the projected sensitivity of the upcoming LIGO and Virgo fourth observing run (O4), we perform Bayesian analysis on quasi-circular nonprecessing, spinning IMBH binaries (IMBHBs) with total masses 50–500 M ⊙ , mass ratios 1.25, 4, and 10, and dimensionless spins up to 0.95, and estimate the precision with which the source-frame parameters can be measured. We find that, at 2 σ , the mass of the heavier component of IMBHBs can be constrained with an uncertainty of ∼10%–40% at a signal-to-noise ratio of 20. Focusing on the stellar-mass gap with new tabulations of the 12 C( α , γ ) 16 O reaction rate and its uncertainties, we evolve massive helium core stars using MESA to establish the lower and upper edges of the mass gap as ≃ 59 − 13 + 34 M ⊙ and ≃ 139 − 14 + 30 M ⊙ respectively, where the error bars give the mass range that follows from the ±3 σ uncertainty in the 12 C( α , γ ) 16 O nuclear reaction rate. We find that high resolution of the tabulated reaction rate and fine temporal resolution are necessary to resolve the peak of the BH mass spectrum. We then study IMBHBs with components lying in the mass gap and show that the O4 run will be able to robustly identify most such systems. Finally, we reanalyze GW190521 with a state-of-the-art aligned-spin waveform model, finding that the primary mass lies in the mass gap with 90% credibility. 

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3. On Trapped Modes in Variable White Dwarfs as Probes of the 12 C(α, γ) 16 O Reaction Rate

   https://doi.org/10.3847/1538-4357/ac7ec3  

   Chidester, Morgan T. ; Farag, Ebraheem ; Timmes, F. X. ( August 2022 , The Astrophysical Journal)

   Abstract We seek signatures of the current experimental 12 C α , γ 16 O reaction rate probability distribution function in the pulsation periods of carbon–oxygen white dwarf (WD) models. We find that adiabatic g-modes trapped by the interior carbon-rich layer offer potentially useful signatures of this reaction rate probability distribution function. Probing the carbon-rich region is relevant because it forms during the evolution of low-mass stars under radiative helium-burning conditions, mitigating the impact of convective mixing processes. We make direct quantitative connections between the pulsation periods of the identified trapped g-modes in variable WD models and the current experimental 12 C α , γ 16 O reaction rate probability distribution function. We find an average spread in relative period shifts of Δ P / P ≃ ±2% for the identified trapped g-modes over the ±3 σ uncertainty in the 12 C α , γ 16 O reaction rate probability distribution function—across the effective temperature range of observed DAV and DBV WDs and for different WD masses, helium shell masses, and hydrogen shell masses. The g-mode pulsation periods of observed WDs are typically given to six to seven significant figures of precision. This suggests that an astrophysical constraint on the 12 C α , γ 16 O reaction rate could, in principle, be extractable from the period spectrum of observed variable WDs. 

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4. Impact of the New 65 As(p,γ) 66 Se Reaction Rate on the Two-proton Sequential Capture of 64 Ge, Weak GeAs Cycles, and Type I X-Ray Bursts Such as the Clocked Burster GS 1826−24

   https://doi.org/10.3847/1538-4357/ac4d8b  

   Lam, Yi Hua ; Liu, Zi Xin ; Heger, Alexander ; Lu, Ning ; Jacobs, Adam Michael ; Johnston, Zac ( April 2022 , The Astrophysical Journal)

   Abstract We reassess the 65 As(p, γ ) 66 Se reaction rates based on a set of proton thresholds of 66 Se, S p ( 66 Se), estimated from the experimental mirror nuclear masses, theoretical mirror displacement energies, and full p f -model space shell-model calculation. The self-consistent relativistic Hartree–Bogoliubov theory is employed to obtain the mirror displacement energies with much reduced uncertainty, and thus reducing the proton-threshold uncertainty up to 161 keV compared to the AME2020 evaluation. Using the simulation instantiated by the one-dimensional multi-zone hydrodynamic code, K epler , which closely reproduces the observed GS 1826−24 clocked bursts, the present forward and reverse 65 As(p, γ ) 66 Se reaction rates based on a selected S p ( 66 Se) = 2.469 ± 0.054 MeV, and the latest 22 Mg( α ,p) 25 Al, 56 Ni(p, γ ) 57 Cu, 57 Cu(p, γ ) 58 Zn, 55 Ni(p, γ ) 56 Cu, and 64 Ge(p, γ ) 65 As reaction rates, we find that though the GeAs cycles are weakly established in the rapid-proton capture process path, the 65 As(p, γ ) 66 Se reaction still strongly characterizes the burst tail end due to the two-proton sequential capture on 64 Ge, not found by the Cyburt et al. sensitivity study. The 65 As(p, γ ) 66 Se reaction influences the abundances of nuclei A = 64, 68, 72, 76, and 80 up to a factor of 1.4. The new S p ( 66 Se) and the inclusion of the updated 22 Mg( α ,p) 25 Al reaction rate increases the production of 12 C up to a factor of 4.5, which is not observable and could be the main fuel for a superburst. The enhancement of the 12 C mass fraction alleviates the discrepancy in explaining the origin of the superburst. The waiting point status of and two-proton sequential capture on 64 Ge, the weak-cycle feature of GeAs at a region heavier than 64 Ge, and the impact of other possible S p ( 66 Se) are also discussed. 

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5. Regulated NiCu Cycles with the New 57 Cu(p, γ ) 58 Zn Reaction Rate and the Influence on Type-I X-Ray Bursts: GS 1826–24 Clocked Burster

   https://doi.org/10.1051/epjconf/202226011023  

   Lam, Yi Hua ; Lu, Ning ; Heger, Alexander ; Jacobs, Adam Michael ; Smirnova, Nadezda A. ; Nieto, Teresa Kurtukian ; Johnston, Zac ; Kubono, Shigeru ( January 2022 , EPJ Web of Conferences)

   Liu, W. ; Wang, Y. ; Guo, B. ; Tang, X. ; Zeng, S. (Ed.)

   In Type-I X-ray bursts (XRBs), the rapid-proton capture (rp-) process passes through the NiCu and ZnGa cycles before reaching the region above Ge and Se isotopes that hydrogen burning actively powers the XRBs. The sensitivity study performed by Cyburt et al . [1] shows that the 57 Cu(p, γ ) 58 Zn reaction in the NiCu cycles is the fifth most important rp-reaction influencing the burst light curves. Langer et al . [2] precisely measured some low-lying energy levels of 58 Zn to deduce the 57 Cu(p, γ ) 58 Zn reaction rate. Nevertheless, the order of the 1 + 1 and 2 + 3 resonance states that dominate at 0:2 ≲ T (GK) ≲ 0:8 is not confirmed. The 1 + 2 resonance state, which dominates at the XRB sensitive temperature regime 0:8 ≲ T (GK) ≲ 2 was not detected. Using isobaric-multipletmass equation (IMME), we estimate the order of the 1 + 1 and 2 + 3 resonance states and estimate the lower limit of the 1 + 2 resonance energy. We then determine the 57 Cu(p, γ ) 58 Zn reaction rate using the full pf -model space shell model calculations. The new rate is up to a factor of four lower than the Forstner et al . [3] rate recommended by JINA REACLIBv2.2. Using the present 57 Cu(p, γ ) 58 Zn, the latest 56 Ni(p, γ ) 57 Cu and 55 Ni(p, γ ) 56 Cu reaction rates, and 1D implicit hydrodynamic K epler code, we model the thermonuclear XRBs of the clocked burster GS 1826–24. We find that the new rates regulate the reaction flow in the NiCu cycles and strongly influence the burst-ash composition. The 59 Cu(p, γ ) 56 Ni and 59 Cu(p, α ) 60 Zn reactions suppress the influence of the 57 Cu(p, γ ) 58 Zn reaction. They strongly diminish the impact of the nuclear reaction flow that bypasses the 56 Ni waiting point induced by the 55 Ni(p, γ ) 56 Cu reaction on burst light curve. 

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