Search

The most promising astrophysical sources of kHz gravitational waves (GWs) are the inspiral and merger of binary neutron star(NS)/black hole systems. Maximizing the scientific return of a GW detection will require identifying a coincident electromagnetic (EM) counterpart. One of the most likely sources of isotropic EM emission from compact object mergers is a supernova-like transient powered by the radioactive decay of heavy elements synthesized in ejecta from the merger. We present the first calculations of the optical transients from compact object mergers that self-consistently determine the radioactive heating by means of a nuclear reaction network; using this heating rate, we model the light curve with a one-dimensional Monte Carlo radiation transfer calculation. For an ejecta mass the resulting light-curve peaks on a time-scale ∼1 d at a V-band luminosity [ (−14)]; this corresponds to an effective ‘f’ parameter in the Li–Paczynski toy model. We argue that these results are relatively insensitive to uncertainties in the relevant nuclear physics and to the precise early-time dynamics and ejecta composition. Since NS merger transients peak at a luminosity that is a factor of higher than a typical nova, we propose naming these events ‘kilo-novae’. Because of the rapid evolution and low luminosity of NS merger transients, EM counterpart searches triggered by GW detections will require close collaboration between the GW and astronomical communities. NS merger transients may also be detectable following a short-duration gamma-ray burst or ‘blindly’ with present or upcoming optical transient surveys. Because the emission produced by NS merger ejecta is powered by the formation of rare r-process elements, current optical transient surveys can directly constrain the unknown origin of the heaviest elements in the Universe. [ABSTRACT FROM AUTHOR]

The probability for the production of superheavy elements in the astrophysical r-process is discussed. The dependence of the estimated superheavy-element yields on input data is estimated. Preliminary calculations revealed that the superheavy-element yields at the instant of completion of the r-process may be commensurate with the uranium yield, but the former depend strongly on the models used to forecast the properties of beta-delayed, neutron-induced, and spontaneous fission. This study is dedicated to the 80th anniversary of V.S. Imshennik’s birth. [ABSTRACT FROM AUTHOR]

Rapid neutron capture in stellar explosions is responsible for the heaviest elements in nature, up to Th, U and beyond. This nucleosynthesis process, the r-process, is unique in the sense that a combination of nuclear physics far from stability (masses, half-lives, neutron-capture and photodisintegration, neutron-induced and beta-delayed fission and last but not least neutrino-nucleus interactions) is intimately linked to ejecta from astrophysical explosions (core collapse supernovae or other neutron star related events). The astrophysics and nuclear physics involved still harbor many uncertainties, either in the extrapolation of nuclear properties far beyond present experimental explorations or in the modeling of multidimensional, general relativistic (neutrino-radiation) hydrodynamics with rotation and possibly required magnetic fields. Observational clues about the working of the r-process are mostly obtained from solar abundances and from the abundance evolution of the heaviest elements as a function of galactic age, as witnessed in old extremely metal-poor stars. They contain information whether the r-process is identical for all stellar events, how abundance features develop with galactic time and whether the frequency of r-process events is comparable to that of average core collapse supernovae - producing oxygen through titanium, as well as iron-group nuclei. The theoretical modeling of the r-process has advanced from simple approaches, where the use of static neutron densities and temperatures can aid to test the influence of nuclear properties far from stability on abundance features, to more realistic expansions with a given entropy, global neutron/proton ratio and expansion timescales, as expected from explosive astrophysical events. The direct modeling in astrophysical events such as supernovae still faces the problem whether the required conditions can be met. [ABSTRACT FROM AUTHOR]

We show that for the discussed scenario of a neutron-star merger in highly neutronized ejecta (Ye ∼ 0.1), neutron-induced fission plays a major role in the r-process cycling and is the main obstacle to the formation of superheavy elements. At the final stage of the r-process, when the free-neutron density is already too low to maintain rapid nucleosynthesis and only beta-decay and beta-delayed fission take place, the leading role in forming the final abundances of chemical elements passes to delayed fission. The latter ultimately changes the abundances of individual isotopes in the region before the second peak and heavier than lead, which, in particular, affects the determination of the age of the Galaxy. [ABSTRACT FROM AUTHOR]

Abstract: This article addresses three of the four nucleosynthesis processes involved in producing heavy nuclei beyond Fe (with a main focus on the -process). Opposite to the fourth process (the -process), which operates in stellar evolution during He- and C-burning, they are all related to explosive burning phases, (presumably) linked to core collapse supernova events of massive stars. The (classical) -process is identified with explosive Ne/O-burning in outer zones of the progenitor star. It is initiated by the passage of the supernova shock wave and acts via photodisintegration reactions like a spallation process which produces neighboring (proton-rich) isotopes from pre-existing heavy nuclei. The reproduction of some of the so-called lighter -isotopes with faces problems in this environment. The only recently discovered -process is related to the innermost ejecta, the neutrino wind expelled from the hot proto-neutron star after core collapse and the supernova explosion. This neutrino wind is proton-rich in its early phase, producing nuclei up to 64Ge. Reactions with neutrinos permit to overcome decay/reaction bottlenecks for the flow beyond 64Ge, thus producing light -isotopes, which face problems in the classical -process scenario. The understanding of the -process, being identified for a long time with rapid neutron captures and passing through nuclei far from stability, is still experiencing major problems. These are on the one hand related to nuclear uncertainties far from stability (masses, half-lives, fission barriers), affecting the process speed and abundance peaks. On the other hand the site is still not definitely located, yet. (i) Later, possibly neutron-rich, high entropy phases of the neutrino wind (if they materialize!) could permit its operation. (ii) Other options include the ejection of very neutron-rich neutron star-like matter, occurring possibly in neutron star mergers or core collapse supernova events with jets, related to prior stellar evolution with high rotation rates and magnetic fields. Two different environments are required for a weak and a main/strong -process, witnessed by observations of low metallicity stars and meteoritic inclusions, which could possibly be identified with the two options listed above, i.e. the weak -process could be related to the neutrino wind when changing from -rich to -rich conditions. [Copyright &y& Elsevier]