Core-Collapse Supernovae to Nuclear Electron Capture

For millions of years stars like our Sun, but much more massive, peacefully burn the hydrogen and helium that is the fuel of their existence. However, these stars live for just a short time—one percent or less the total life span of the Sun. In the last few days of their lives, a heavy iron core is formed which cannot be burned for further energy generation. At such a time, the core of these massive stars begins to recede inward, but is stopped due to a pressure from one of nature’s smallest particles, the electron.

This outward electron pressure is quantum mechanical in nature, and for a time, evenly balances the inward force of gravity. However, more and more iron-group nuclei are formed in the layers just outside the core, which quickly sink inward adding to the core’s mass, and therefore the inward force of gravity. At the same time, atomic nuclei in the core are removing electrons from the material through a nuclear reaction known as electron capture.

The combination of an ever-increasing core mass and the reduction of electrons— which provide the primary support of the core in these final moments—result in one of our universes most violent and energetic deaths: a core-collapse supernovae.

The weak-rate library is available here: https://groups.nscl.msu.edu/charge_exchange/weakrates.html

The rate of electron captures of individual nuclei is a topic of focus for the NSCL's Charge-Exchange research group. Utilizing the Coupled-Cyclotron Facility (and in the future FRIB), they are able to extract key information for determining how quickly electrons are captured by nuclei in stellar environments. Due to the importance of electron capture to core-collapse supernovae, the Charge-Exchange group has recently completed a computational investigation into the sensitivity of these events to the electron-capture rates inferred from their measurements, and theoretical estimates which rely on experimental data.

Which electron capturing nuclei are most important to core-collapse supernovae--and therefore, which measurements should be made--is a question that their work sought to answer. Also of interest was how strongly uncertainties in the electron-capture rates impact core-collapse supernovae simulations. Building on prior work of Hix, Langanke and Martinez-Pinedo et al., the Charge-Exchange group have sought to answer these questions by developing an open-source library of weak interaction rate datasets and approximations and by implementing it into the core-collapse supernovae code GR1D, developed by Evan O’Connor (NCSU).

By performing nearly 150 simulations across many initial stellar models and equations of state, they found that the simulations were most sensitive to neutron rich nuclei with atomic masses in excess of A=65. Specifically, nuclei with neutron numbers around N=50, and atomic mass of approximately A=80, contribute most to the collapse and bounce phases of the supernovae. Furthermore, they found large structural changes in the stellar core when the electron-capture rates were adjusted by factors consistent with uncertainties in the electron capture rates. For example, the inner-core mass of the forming protoneutron star changed by as much as 20% with these variations.

Compared to the range of inner-core mass seen when keeping the electron-capture rates constant but performing core-collapse simulations on 32 stars of different mass, they found that the sensitivity to the electron-capture rates was nearly 5 times that of the sensitivity of the supernova to the initial stellar model.

These, and other results, motivate the need for new experimental and theoretical efforts to constrain the electron-capture rates for the nuclei that contribute most strongly to the observed changes. Because the set of important nuclei is large, robust theories that agree with experimental data in the region of interest are critical.  And for experimenters, this means that presently feasible measurements on neutron-rich nuclei at and near stability with masses between 60 and 120 should add to the few cases that have been measured in this region.

Moving to the future where higher beam intensities will be available at next generation rare isotope facilities, experimental programs should focus on the neutron-rich component of the primary electron-capture channel highlighted in this recent work by NSCL’s Charge-Exchange group.

Chris Sullivan, Evan O’Connor, Remco G. T. Zegers, Thomas Grubb, Sam M. Austin ApJ, 816, 44 (2015) - http://iopscience.iop.org/article/10.3847/0004-637X/816/1/44