Experimentally Well-Constrained Masses Of P-27 And S-27: Implications For Studies Of Explosive Binary Systems

The mass of P-27 is expected to impact the X-ray burst (XRB) model predictions of burst light curves and the composition of the burst ashes, but large uncertainties and inconsistencies still exist in the reported P-27 masses. We have used the beta-decay spectroscopy of S-27 to determine the most precise mass excess of P-27 to date to be -659(9) keV, which is 63 keV (2.3 sigma) higher and a factor of 3 more precise than the value recommended in the 2016 Atomic Mass Evaluation. Based on the new P-27 mass, the Si-26(p, gamma)P-27 reaction rate and its uncertainty were recalculated using Monte Carlo techniques. We also estimated the previously unknown mass excess of S-27 to be 17678(77) keV, based on the measured beta-delayed two-proton energy and the Coulomb displacement energy relations. The impact of these well-constrained masses and reaction rates on the modeling of the explosive astrophysical scenarios has been investigated by post-processing XRB and hydrodynamic nova models. Compared to the model calculations based on the masses and rates from databases, the abundance of A = 26 in the burst ashes is increased by a factor of 2.4, while no substantial change was found in the XRB energy generation rate or the light curve. Our calculation also suggests that S-27 is not a significant waiting point in the rapid proton capture process, and the change of the Si-26(p, gamma)P-27 reaction rate is not sufficiently large to affect the conclusion previously drawn on the nova contribution to the synthesis of galactic Al-26. (C) 2020 The Author. Published by Elsevier B.V.