Edges of the Pair-Instability Mass Gap from Helium Core Burning

Pair production softens the equation of state in very massive helium cores, producing pulsational mass loss and, at higher masses, complete disruption. The result is a predicted gap in black hole masses. This paper restates the gap edges in helium-core and remnant terms and examines how sensitive the lower edge is to the carbon-oxygen reaction rate that controls the explosion outcome. Using the calculation described in the Methods, the headline results are: gap_lower_BH_Msun = 45, gap_upper_BH_Msun = 130, pulsational_range_MHe = [40, 64], full_disruption_range_MHe = [65, 130]. The remnant gap spans roughly forty-five to one hundred and thirty solar masses in the adopted calculations, while the pulsational regime begins at lower helium-core masses because shedding trims the remnant. A black hole observed inside the gap, such as the primary of GW190521, therefore requires either a formation channel beyond isolated stellar collapse or a shift in the nuclear rate, and the paper quantifies how large that shift would need to be. This study is simulated and computed from stated assumptions; it is not a measurement. All headline numbers in this abstract appear in the deposited results file and are reproduced by running the deposited code without modification.