Near-Earth Supernova Explosions: Evidence, Implications, and Opportunities

There is now solid experimental evidence of at least one supernova explosion within 100 pc of Earth within the last few million years, from measurements of the short-lived isotope ⁶⁰Fe in widespread deep-ocean samples, as well as in the lunar regolith and cosmic rays. This is the first established example of a specific dated astrophysical event outside the Solar System having a measurable impact on the Earth, offering new probes of stellar evolution, nuclear astrophysics, the astrophysics of the solar neighborhood, cosmic-ray sources and acceleration, multi-messenger astronomy, and astrobiology. Interdisciplinary connections reach broadly to include heliophysics, geology, and evolutionary biology. Objectives for the future include pinning down the nature and location of the established near-Earth supernova explosions, seeking evidence for others, and searching for other short-lived isotopes such as ²⁶Al and ²⁴⁴Pu. The unique information provided by geological and lunar detections of radioactive ⁶⁰Fe to assess nearby supernova explosions make now a compelling time for the astronomy community to advocate for supporting multi-disciplinary, cross-cutting research programs

Analytical Models for the Energetics of Cosmic Accretion Shocks, their Cosmological Evolution, and the Effect of Environment

We present an analytical description of the energetics of the population of cosmic accretion shocks, for a concordance cosmology. We calculate how the shock-processed accretion power and mass current are distributed among different shock Mach numbers, and how they evolve with cosmic time. We calculate the cumulative energy input of cosmic accretion shocks of any Mach number to the intergalactic medium as a function of redshift, and we compare it with the energy output of supernova explosions as well as with the energy input required to reionize the universe. In addition, we investigate and quantify the effect of environmental factors, such as local clustering properties and filament preheating on the statistical properties of these shocks. We find that the energy processed by accretion shocks is higher than the supernova energy output for z<3 and that it becomes more than an order of magnitude higher in the local universe. The energy processed by accretion shocks alone becomes comparable to the energy required to reionize the universe by z~3.5. Finally, we establish both qualitative and quantitatively that both local clustering as well as filament compression and preheating are important factors in determining the statistical properties of the cosmic accretion shock population.Comment: 13 pages, 5 figures, emulateap

Cosmic Chemical Evolution with an Early Population of Intermediate Mass Stars

We explore the consequences of an early population of intermediate mass stars in the 2 - 8 M\odot range on cosmic chemical evolution. We discuss the implications of this population as it pertains to several cosmological and astrophysical observables. For example, some very metal-poor galactic stars show large enhancements of carbon, typical of the C-rich ejecta of low-mass stars but not of supernovae; moreover, halo star carbon and oxygen abundances show wide scatter, which imply a wide range of star-formation and nucleosynthetic histories contributed to the first generations of stars. Also, recent analyses of the 4He abundance in metal-poor extragalactic H II regions suggest an elevated abundance Yp \simeq 0.256 by mass, higher than the predicted result from big bang nucleosynthesis assuming the baryon density determined by WMAP, Yp = 0.249. Although there are large uncertainties in the observational determination of 4He, this offset may suggest a prompt initial enrichment of 4He in early metal-poor structures. We also discuss the effect of intermediate mass stars on global cosmic evolution, the reionization of the Universe, the density of white dwarfs, as well as SNII and SNIa rates at high redshift. We also comment on the early astration of D and 7Li. We conclude that if intermediate mass stars are to be associated with Population III stars, their relevance is limited (primarily from observed abundance patterns) to low mass structures involving a limited fraction of the total baryon content of the Universe.Comment: Submitted to MNRA

LiBeB Production by Nuclei and Neutrinos

The production of LiBeB isotopes by nuclear and neutrino spallation are compared in the framework of galactic evolutionary models. As motivated by $\gamma$-ray observations of Orion, different possible sources of low-energy C and O nuclei are considered, such as supernovae of various masses and WC stars. We confirm that the low energy nuclei (LEN), injected in molecular clouds by stellar winds and type II supernovae originating from the most massive progenitors, can very naturally reproduce the observed Be and B evolution in the early galaxy (halo phase). Assuming the global importance of the LEN component, we compute upper and lower bounds to the neutrino process contribution corresponding to limiting cases of LEN particle spectra. A consistent solution is found with a spectrum of the kind proposed by Ramaty \etal (1995a,b), e.g. flat up to $E_c=30$ MeV/n and decreasing abruptly above. This solution fulfills the challenge of explaining at the same time the general Be and B evolution, and their solar system abundances without overproducing \li7 at very low metallicities, and the meteoritic \b11/\b10 ratio. In this case, neutrino spallation is constrained to play a limited role in the genesis of the solar system \b11. Galactic cosmic rays (GCR) become operative late in the evolution of the disk ([Fe/H]$>$-1), but their contribution to the solar abundances of \be9, \b10 and \b11 is not dominant (35\%, 30\% and 20\% respectively). Thus, with this LEN spectrum, GCR are {\it not}\ the main source of \be9 and B in the Galaxy. The most favorable case for neutrinos, (adopting the same kind of spectrum) has $E_c=20$ MeV/n. Even in this case, the neutrino yields of Woosley and Weaver (1995) must to be reduced by a factor of 5 to avoid \b11 overproduction. Furthermore, this solution leads to a high B/BeComment: 19 pages, 5 postscript figures, uses plain LaTeX, also available at http://www.nd.edu/~bfields/vcfo.htm

Primordial Nucleosynthesis and the Abundances of Beryllium and Boron

The ability to now make measurements of Be and B as well as put constraints on \lisix\ abundances in metal-poor stars has led to a detailed reexamination of Big Bang Nucleosynthesis in the A\groughly6 regime. The nuclear reaction network has been significantly expanded with many new rates added. It is demonstrated that although a number of $A>7$ reaction rates are poorly determined, even with extreme values chosen, the standard homogeneous model is unable to produce significant yields (Be/H and B/H $<10^{-17}$ when $A\le7$ abundances fit) above $A=7$ and the \liseven/\lisix\ ratio always exceeds 500. We also preliminarily explore inhomogeneous models, such as those inspired by a first order quark-hadron phase transition, where regions with high neutron/proton ratios can allow some leakage up to $A>7$. However models that fit the $A\le7$ abundances still seem to have difficulty in obtaining significant $A>7$ yields.Comment: Plain TeX, 28 pages, 8 figures (not included, but available from authors). UMN-TH-1020/9

Resonant Destruction as a Possible Solution to the Cosmological Lithium Problem

We explore a nuclear physics resolution to the discrepancy between the predicted standard big-bang nucleosynthesis (BBN) abundance of 7Li and its observational determination in metal-poor stars. The theoretical 7Li abundance is 3-4 times greater than the observational values, assuming the baryon-to-photon ratio, eta_wmap, determined by WMAP. The 7Li problem could be resolved within the standard BBN picture if additional destruction of A=7 isotopes occurs due to new nuclear reaction channels or upward corrections to existing channels. This could be achieved via missed resonant nuclear reactions, which is the possibility we consider here. We find some potential candidate resonances which can solve the lithium problem and specify their required resonant energies and widths. For example, a 1^- or 2^- excited state of 10C sitting at approximately 15.0 MeV above its ground state with an effective width of order 10 keV could resolve the 7Li problem; the existence of this excited state needs experimental verification. Other examples using known states include 7Be+t \rightarrow 10B(18.80 MeV), and 7Be+d \rightarrow 9B(16.71 MeV). For all of these states, a large channel radius (a > 10 fm) is needed to give sufficiently large widths. Experimental determination of these reaction strengths is needed to rule out or confirm these nuclear physics solutions to the lithium problem.Comment: 37 pages, 9 figures. Additional discussion of channel widths and radii. Matches published versio