Electron-capture supernovae as sources of 60Fe

We investigate the nucleosynthesis of the radionuclide 60Fe in electron-capture supernovae (ECSNe). The nucleosynthetic results are based on a self-consistent, two-dimensional simulation of an ECSN as well as models in which the densities are systematically increased by some factors (low-entropy models). 60Fe is found to be appreciably made in neutron-rich ejecta during the nuclear quasi-equilibrium phase with greater amounts being produced in the lower-entropy models. Our results, combining them with the yields of core-collapse supernovae (CCSNe) in the literature, suggest that ECSNe account for at least 4-30% of live 60Fe in the Milky Way. ECSNe co-produce neutron-rich isotopes, 48Ca, 50Ti, 54Cr, some light trans-iron elements, and possibly weak r-process elements including some radionuclides such as 93Zr, 99Tc, and 107Pd, whose association with 60Fe might have been imprinted in primitive meteorites or in the deep ocean crust on the Earth.Comment: 6 pages, 2 figures, accepted for publication in ApJ

Electron-capture supernovae as origin of 48Ca

We report that electron-capture supernovae (ECSNe), arising from collapsing oxygen-neon-magnesium cores, are a possible source of 48Ca, whose origin has remained a long-standing puzzle. Our two-dimensional, self-consistent explosion model of an ECSN predicts ejection of neutron-rich matter with electron fractions Ye = 0.40-0.42 and relatively low entropies, s = 13-15 kB per nucleon (kB is the Boltzmann constant). Post-processing nucleosynthesis calculations result in appreciable production of 48Ca in such neutron-rich and low-entropy matter during the quasi-nuclear equilibrium and subsequent freezeout phases. The amount of ejected 48Ca can account for that in the solar inventory when we consider possible uncertainties in the entropies or ejecta-mass distribution. ECSNe could thus be a site of 48Ca production in addition to a hypothetical, rare class of high-density Type Ia supernovae.Comment: 6 pages, 5 figures, accepted for publication in ApJ

A Ulysses Detection of Secondary Helium Neutrals

The Interstellar Boundary EXplorer (IBEX) mission has recently studied the flow of interstellar neutral He atoms through the solar system, and discovered the existence of a secondary He flow likely originating in the outer heliosheath. We find evidence for this secondary component in Ulysses data. By coadding hundreds of Ulysses He beam maps together to maximize signal-to-noise, we identify a weak signal that is credibly associated with the secondary component. Assuming a laminar flow from infinity, we infer the following He flow parameters: V=12.8+/-1.9 km/s, lambda=74.4+/-1.8 deg, beta=-10.5+/-4.1 deg, and T=3000+/-1100 K; where lambda and beta are the ecliptic longitude and latitude direction in J2000 coordinates. The secondary component has a density that is 4.9+/-0.9% that of the primary component. These measurements are reasonably consistent with measurements from IBEX, with the exception of temperature, where our temperature is much lower than IBEX's T=9500 K. Even the higher IBEX temperature is suspiciously low compared to expectactions for the outer heliosheath source region. The implausibly low temperatures are due to the incorrect assumption of a laminar flow instead of a diverging one, given that the flow in the outer heliosheath source region will be deflecting around the heliopause. As for why the IBEX and Ulysses T values are different, difficulties with background subtraction in the Ulysses data are a potential source of concern, but the discrepancy may also be another effect of the improper laminar flow assumption, which could affect the IBEX and Ulysses analyses differently.Comment: 9 pages, 4 figures, to appear in The Astrophysical Journa

Internet Governance: the State of Play

The Global Forum on Internet Governance held by the UNICT Task Force in New York on 25-26 March concluded that Internet governance issues were many and complex. The Secretary-General's Working Group on Internet Governance will have to map out and navigate this complex terrain as it makes recommendations to the World Summit on an Information Society in 2005. To assist in this process, the Forum recommended, in the words of the Deputy Secretary-General of the United Nations at the closing session, that a matrix be developed "of all issues of Internet governance addressed by multilateral institutions, including gaps and concerns, to assist the Secretary-General in moving forward the agenda on these issues." This paper takes up the Deputy Secretary-General's challenge. It is an analysis of the state of play in Internet governance in different forums, with a view to showing: (1) what issues are being addressed (2) by whom, (3) what are the types of consideration that these issues receive and (4) what issues are not adequately addressed

Revisiting Ulysses Observations of Interstellar Helium

We report the results of a comprehensive reanalysis of Ulysses observations of interstellar He atoms flowing through the solar system, the goal being to reassess the interstellar He flow vector and to search for evidence of variability in this vector. We find no evidence that the He beam seen by Ulysses changes at all from 1994-2007. The direction of flow changes by no more than ~0.3 deg and the speed by no more than ~0.3 km/s. A global fit to all acceptable He beam maps from 1994-2007 yields the following He flow parameters: V_ISM=26.08+/-0.21 km/s, lambda=75.54+/-0.19 deg, beta=-5.44+/-0.24 deg, and T=7260+/-270 K; where lambda and beta are the ecliptic longitude and latitude direction in J2000 coordinates. The flow vector is consistent with the original analysis of the Ulysses team, but our temperature is significantly higher. The higher temperature somewhat mitigates a discrepancy that exists in the He flow parameters measured by Ulysses and the Interstellar Boundary Explorer, but does not resolve it entirely. Using a novel technique to infer photoionization loss rates directly from Ulysses data, we estimate a density of n_He=0.0196+/-0.0033 cm^-3 in the interstellar medium.Comment: to appear in The Astrophysical Journa

Characterizing SASI- and Convection-Dominated Core-Collapse Supernova Explosions in Two Dimensions

The success of the neutrino mechanism of core-collapse supernovae relies on the supporting action of two hydrodynamic instabilities: neutrino-driven convection and the Standing Accretion Shock Instability (SASI). Depending on the structure of the stellar progenitor, each of these instabilities can dominate the evolution of the gain region prior to the onset of explosion, with implications for the ensuing asymmetries. Here we examine the flow dynamics in the neighborhood of explosion by means of parametric two-dimensional, time-dependent hydrodynamic simulations for which the linear stability properties are well understood. We find that systems for which the convection parameter is sub-critical (SASI-dominated) develop explosions once large-scale, high-entropy bubbles are able to survive for several SASI oscillation cycles. These long-lived structures are seeded by the SASI during shock expansions. Finite-amplitude initial perturbations do not alter this outcome qualitatively, though they can lead to significant differences in explosion times. Supercritical systems (convection-dominated) also explode by developing large-scale bubbles, though the formation of these structures is due to buoyant activity. Non-exploding systems achieve a quasi-steady state in which the time-averaged flow adjusts itself to be convectively sub-critical. We characterize the turbulent flow using a spherical Fourier-Bessel decomposition, identifying the relevant scalings and connecting temporal and spatial components. Finally, we verify the applicability of these principles on the general relativistic, radiation-hydrodynamic simulations of Mueller, Janka, & Heger (2012), and discuss implications for the three-dimensional case.Comment: accepted by MNRAS with minor change

Supernova deleptonization asymmetry: Impact on self-induced flavor conversion

During the accretion phase of a core-collapse supernova (SN), the deleptonization flux has recently been found to develop a global dipole pattern (LESA---Lepton Emission Self-sustained Asymmetry). The $\nu_e$ minus $\bar\nu_e$ flux essentially vanishes in one direction, potentially facilitating self-induced flavor conversion. On the other hand, below the stalled shock wave, self-induced flavor conversion is typically suppressed by multi-angle matter effects, preventing any impact of flavor conversion on SN explosion dynamics. In a schematic model of SN neutrino fluxes, we study the impact of modified $\bar\nu_e$-$\nu_e$ flux asymmetries on collective flavor conversion. In the parameter space consisting of matter density and effective neutrino density, the region of instability with regard to self-induced flavor conversion is much larger for a vanishing lepton number flux, yet this modification does not intersect a realistic SN profile. Therefore, it appears that, even in the presence of LESA, self-induced flavor conversion remains suppressed below the shock front.Comment: 14 pages, 6 figures; v2: significant change in presentation, results and conclusion unchanged, appendix adde