Gamma-ray astronomy explores the most energetic photons in nature to address some of the most pressing puzzles in contemporary astrophysics. It encompasses a wide range of objects and phenomena: stars, supernovae, novae, neutron stars, stellar-mass black holes, nucleosynthesis, the interstellar medium, cosmic rays and relativistic-particle acceleration, and the evolution of galaxies. MeV gamma-rays provide a unique probe of nuclear processes in astronomy, directly measuring radioactive decay, nuclear de-excitation, and positron annihilation. The substantial information carried by gamma-ray photons allows us to see deeper into these objects, the bulk of the power is often emitted at gamma-ray energies, and radioactivity provides a natural physical clock that adds unique information. New science will be driven by time-domain population studies at gamma-ray energies. This science is enabled by next-generation gamma-ray instruments with one to two orders of magnitude better sensitivity, larger sky coverage, and faster cadence than all previous gamma-ray instruments. This transformative capability permits: (a) the accurate identification of the gamma-ray emitting objects and correlations with observations taken at other wavelengths and with other messengers; (b) construction of new gamma-ray maps of the Milky Way and other nearby galaxies where extended regions are distinguished from point sources; and (c) considerable serendipitous science of scarce events -- nearby neutron star mergers, for example. Advances in technology push the performance of new gamma-ray instruments to address a wide set of astrophysical questions.

We present results from general relativistic calculations of the tidal disruption of white dwarf stars from near encounters with intermediate-mass black holes. We follow the evolution of 0.2 M-circle dot and 0.6 M-circle dot stars on parabolic trajectories that approach 10(3) - 10(4) M-circle dot black holes as close as a few Schwarzschild radii at periapsis, paying particular attention to the effect that tidal disruption has on thermonuclear reactions and the synthesis of intermediate-mass to heavy elements. These encounters create diverse thermonuclear environments that are characteristic of Type I supernovae and capable of producing both intermediate-mass and heavy elements in arbitrary ratios, depending on the strength (or proximity) of the interaction. Nuclear ignition is triggered in all of our calculations, even at weak tidal strengths beta similar to 2.6 and large periapsis radius RP similar to 28 Schwarzschild radii. A strong inverse correlation exists between the mass ratio of calcium-group to iron-group elements and tidal strength, with beta less than or similar to 5 producing predominantly calcium-rich debris. At these moderate to weak interactions, nucleosynthesis is not especially efficient, limiting the total mass and outflows of calcium-group elements to < 15% of available nuclear fuel. Iron-group elements, however, continue to be produced in greater quantity and ratio with increasing tidal strength, peaking at similar to 60% mass conversion efficiency in our closest encounter cases. These events generate short bursts of gravitational waves with characteristic frequencies 0.1-0.7 Hz and strain amplitudes from 0.5. x. 10(-22) to 3.5. x. 10(-22) at a source distance of 10 Mpc.

We present results from the JINA REACLIB project, an ongoing effort to maintain a current and accurate library of thermonuclear reaction rates for astrophysical applications. Ongoing updates are transparently documented and version tracked, and any set of rates is publicly available and can be downloaded via a Web interface at http://groups.nscl.msu.edu/jina/reaclib/db/. We discuss here our library V1.0, a snapshot of recommended rates for stable and explosive hydrogen and helium burning. We show that the updated reaction rates lead to modest but significant changes in full network, one-dimensional X-ray burst model calculations, compared with calculations with previously used reaction rate sets. The late time behavior of X-ray burst light curves shows significant changes, suggesting that the previously found small discrepancies between model calculations and observations may be solved with a better understanding of the nuclear input. Our X-ray burst model calculations are intended to serve as a benchmark for future model comparisons and sensitivity studies, as the complete underlying nuclear physics is fully documented and publicly available.

We evaluate two dominant nuclear reaction rates and their uncertainties that affect 44Ti production in explosive nucleosynthesis. Experimentally we develop thick-target yields for the 40Ca(alpha,gamma)44Ti reaction at E(alpha) = 4.13, 4.54, and 5.36 MeV using gamma-ray spectroscopy. At the highest beam energy, we also performed an activation measurement that agrees with the thick target result. From the measured yields a stellar reaction rate was developed that is smaller than current statistical-model calculations and recent experimental results, which would suggest lower 44Ti production in scenarios for the alpha-rich freeze out. Special attention has been paid to assessing realistic uncertainties of stellar rates produced from a combination of experimental and theoretical cross sections, which we use to develop a re-evaluation of the 44Ti(alpha,p)47V reaction rate. Using these we carry out a sensitivity survey of 44Ti synthesis in eight expansions representing peak temperature and density conditions drawn from a suite of recent supernova explosion models. Our results suggest that the current uncertainty in these two reaction rates could lead to as large an uncertainty in 44Ti synthesis as that produced by different treatments of stellar physics.

Today we understand, to reasonable accuracy, the origin of most of the abundant elements in the sun and similar Population if stars. Given our relatively primitive ability to model supernova explosion mechanisms, stellar mass loss, and stellar mixing, this is a remarkable achievement. Tins understanding is possible, in part, because supernovae are highly constrained by their spectra, light curves and the sorts of remnants they leave. This same understanding extends to the major abundances seen in primitive metal-poor stars down to [Fe/H] > -4. In particular, one finds no compelling evidence for exotic energies or unusual stellar properties. There are exceptions, however. About half of the isotopes above iron, the r-process and the p-process with A < 130, still have an uncertain origin, both in the sun and in metal-poor stars. The abundances in the hyper-iron-poor stars ([Fe/H] < -4) also require a special explanation. We suggest that they represent; the operation of a first generation of massive stars that produced almost exclusively C, N, and O and black holes, a generation in which 100 M-circle dot were abundant, but stars over about 150 M-circle dot and under 30 M-circle dot were almost absent.

Nucleosynthesis in early proton-rich neutrino winds is investigated.The inclusion of charged-current neutrino reactions on free nucleons indicates that in conditions extracted from a current supernova model nucleosynthesis will proceed some 40 mass units higher than if they are neglected.An analytical estimate of how high in Z the nucleosynthesis might proceed as a function of the neutrino flux and the usual parameters defining the expansion (entropy, Y e , expansion timescale) is presented as well as the sensitivity of the nucleosynthesis to variations in late time neutron addition and increases to the entropy predicted in the model due to various physical processes that were not included.

The dynamic synergy between observation, theory, and experiment developed over many years around the field of γ-ray astronomy has as its ultimate goal observations of specific radionuclides informing our understanding of stellar explosions and the theoretical models that predict nucleosynthesis. Observations of Ni and their decay products Co are used in many ways to constrain our current models of the core collapse mechanism. The radionuclide Ti (τ1/2 = 58.9 +0.3 yr), made in the same explosive environment but in much lower amounts compared to the very abundant nickel isotopes, is hoped to one day serve as an even more sensitive diagnostic and a valuable probe to the conditions extant in some of the deepest layers to be ejected. We [1] investigate Ti nucleosynthesis in adiabatic expansions from peak conditions drawn from a model for Cassiopia A and determine variations due to experimental uncertainties in two key reaction rates. We find that the current uncertainty in these two rates could lead to as large a variation in Ti synthesis as that produced by different treatments of stellar physics in historical models of SNII.

The thick-target yield for the {sup 40}Ca({alpha},{gamma}){sup 44}Ti reaction has been measured for E{sub beam} = 4.13, 4.54, and 5.36 MeV using both an activation measurement and online {gamma}-ray spectroscopy. The results of the two measurements agree. From the measured yield a reaction rate is deduced that is smaller than statistical model calculations. This implies a smaller {sup 44}Ti production in supernova compared to recently measured {sup 40}Ca({alpha},{gamma}){sup 44}Ti reaction rates.

We have studied detailed nucleosynthesis in the shocked surface layers of an oxygen-neon-magnesium core-collapse supernova with an eye to determining whether the conditions are suitable for r-process nucleosynthesis. We find no such conditions in an unmodified model, but do find overproduction of N = 50 nuclei (previously seen in early neutron-rich neutrino winds) in amounts that, if ejected, would pose serious problems for Galactic chemical evolution.

Nucleosynthesis in early neutrino winds is investigated. Presented is a brief overview of two recent problems of supernova nucleosynthesis. In the first part we investigate the effect of nuclear parameters on the synthesis of Mo92 and Mo94. Based on recent experimental results, we find that the proton rich winds of the model investigated here can not be the source of solar Mo92 and Mo94. In the second part we investigate the nucleosynthesis from neutron rich bubbles and show that they do not contribute to the overall nucleosynthesis.

One of the most important challenges confronting laser-driven capsule implosion experiments will be a quantitative evaluation of the implosion dynamics. Since these experiments will encounter extreme conditions of pressure and temperature, establishing robust, sensitive diagnostics will be difficult. Radiochemical signatures provide insight into material mixing and laser drive asymmetry and complement x-ray and other nuclear diagnostics, since the relevant nuclear reactions sample core implosion conditions directly. Simulations of an ignition double shell target indicate that several experimentally accessible isomeric ratios will be suitable monitors of mix.