Shell Model Calculation of the β- and β+ Partial Half-Lives of 54Mn and Other Unique Second Forbidden β Decays

The nucleus 54Mn, observed in cosmic rays, decays there dominantly by the β- branch with an unknown rate. The branching ratio of its β+ decay was determined recently. We use the shell model with only a minimal truncation and calculate both β+ and β- decay rates. Good agreement for the β+ branch suggests that the calculated partial half-life of the β- decay, 4.94×10^5 yr, should be reliable. However, this half-life is noticeably shorter than the range 1–2×10^6 yr indicated by the fit based on the 54Mn abundance in cosmic rays. We also evaluate other known unique second forbidden β decays from the p and sd shells and show that the shell model can describe them with reasonable accuracy as well

Calculation of nuclear matrix elements in neutrinoless double electron capture

We compute nuclear matrix elements for neutrinoless double electron capture on $^{152}$Gd, $^{164}$Er and $^{180}$W nuclei. Recent precise mass measurements for these nuclei have shown a large resonance enhancement factor that makes them the most promising candidates for observing this decay mode. We use an advanced energy density functional method which includes beyond mean-field effects such as symmetry restoration and shape mixing. Our calculations reproduce experimental charge radii and $B(E2)$ values predicting a large deformation for all these nuclei. This fact reduces significantly the values of the NMEs leading to half-lives larger than $10^{29}$ years for the three candidates

Linking neutrino oscillations to the nucleosynthesis of elements

Neutrino interactions with matter play an important role in determining the nucleosynthesis outcome in explosive astrophysical environments such as core-collapse supernovae or mergers of compact objects. In this article, we first discuss our recent work on the importance of studying the time evolution of collective neutrino oscillations among active flavors in determining their effects on nucleosynthesis. We then consider the possible active-sterile neutrino mixing and demonstrate the need of a consistent approach to evolve neutrino flavor oscillations, matter composition, and the hydrodynamics when flavor oscillations can happen very deep inside the supernovae.Comment: 6 pages, 2 figures, OMEG 2015 conference proceedings, to appear in EPJ WOC proceeding

Pairing and the structure of the pf-shell N ~ Z nuclei

The influence of the isoscalar and isovector L=0 pairing components of the effective nucleon-nucleon interaction is evaluated for several isobaric chains, in the framework of full pf shell model calculations. We show that the combined effect of both isospin channels of the pairing force is responsible for the appearance of T=1 ground states in N=Z odd-odd nuclei. However, no evidence is found relating them to the Wigner energy. We study the dependence of their contributions to the total energy on the rotational frecuency in the deformed nucleus 48Cr. Both decrease with increasing angular momentum and go to zero at the band termination. Below the backbending their net effect is a reduction of the moment of inertia, more than half of which comes from the proton-neutron channel.Comment: 5 pages, RevTeX, 5 figure

Effects of neutrino oscillations on nucleosynthesis and neutrino signals for an 18 M supernova model

In this paper, we explore the effects of neutrino flavor oscillations on supernova nucleosynthesis and on the neutrino signals. Our study is based on detailed information about the neutrino spectra and their time evolution from a spherically-symmetric supernova model for an 18 M progenitor. We find that collective neutrino oscillations are not only sensitive to the detailed neutrino energy and angular distributions at emission, but also to the time evolution of both the neutrino spectra and the electron density profile. We apply the results of neutrino oscillations to study the impact on supernova nucleosynthesis and on the neutrino signals from a Galactic supernova. We show that in our supernova model, collective neutrino oscillations enhance the production of rare isotopes 138La and 180Ta but have little impact on the nu p-process nucleosynthesis. In addition, the adiabatic MSW flavor transformation, which occurs in the C/O and He shells of the supernova, may affect the production of light nuclei such as 7Li and 11B. For the neutrino signals, we calculate the rate of neutrino events in the Super-Kamiokande detector and in a hypothetical liquid argon detector. Our results suggest the possibility of using the time profiles of the events in both detectors, along with the spectral information of the detected neutrinos, to infer the neutrino mass hierarchy.Comment: 19 pages, 21 figure

Neutrinos and nucleosynthesis of elements

Neutrinos are known to play important roles in many astrophysical scenarios from the early period of the big bang to current stellar evolution being a unique messenger of the fusion reactions occurring in the center of our sun. In particular, neutrinos are crucial in determining the dynamics and the composition evolution in explosive events such as core-collapse supernovae and the merger of two neutron stars. In this paper, we review the current understanding of supernovae and binary neutron star mergers by focusing on the role of neutrinos therein. Several recent improvements on the theoretical modeling of neutrino interaction rates in nuclear matter as well as their impact on the heavy element nucleosynthesis in the supernova neutrino-driven wind are discussed, including the neutrino-nucleon opacity at the mean field level taking into account the relativistic kinematics of nucleons, the effect due to the nucleon-nucleon correlation, and the nucleon-nucleon bremsstrahlung. We also review the framework used to compute the neutrino-nucleus interactions and the up-to-date yield prediction for isotopes from neutrino nucleosynthesis occurring in the outer envelope of the supernova progenitor star during the explosion. Here improved predictions of energy spectra of supernova neutrinos of all flavors have had significant impact on the nucleosynthesis yields. Rapid progresses in modeling the flavor oscillations of neutrinos in these environments, including several novel mechanisms for collective neutrino oscillations and their potential impacts on various nucleosynthesis processes are summarized.Comment: Review paper submitted to PPN

Advanced burning stages and fate of 8-10 Mo stars

The stellar mass range 8<M/Mo<12 corresponds to the most massive AGB stars and the most numerous massive stars. It is host to a variety of supernova progenitors and is therefore very important for galactic chemical evolution and stellar population studies. In this paper, we study the transition from super-AGB star to massive star and find that a propagating neon-oxygen burning shell is common to both the most massive electron capture supernova (EC-SN) progenitors and the lowest mass iron-core collapse supernova (FeCCSN) progenitors. Of the models that ignite neon burning off-center, the 9.5Mo model would evolve to an FeCCSN after the neon-burning shell propagates to the center, as in previous studies. The neon-burning shell in the 8.8Mo model, however, fails to reach the center as the URCA process and an extended (0.6 Mo) region of low Ye (0.48) in the outer part of the core begin to dominate the late evolution; the model evolves to an EC-SN. This is the first study to follow the most massive EC-SN progenitors to collapse, representing an evolutionary path to EC-SN in addition to that from SAGB stars undergoing thermal pulses. We also present models of an 8.75Mo super-AGB star through its entire thermal pulse phase until electron captures on 20Ne begin at its center and of a 12Mo star up to the iron core collapse. We discuss key uncertainties and how the different pathways to collapse affect the pre-supernova structure. Finally, we compare our results to the observed neutron star mass distribution.Comment: 20 pages, 14 figures, 1 table. Submitted to ApJ 2013 February 19; accepted 2013 June