Peering over the shoulders of giants?

‘Open’ is a highly-visible cultural trend of the early twenty-first century. A brief scan of anewspaper or quick Internet search reveals it as a prefix to learning, source, standard, data,knowledge, democracy, access, repository, innovation, government, science and probablymore. The expectations of the broadband generation – young people born around the turn ofthe century – for openness and instant, on‑demand access to information affect research asmuch as any other social activity. As the world’s population of digital residents – those whosee the Web as the place where they express opinions, form relationships, develop an identityand belong to a community – grows, the expectation that the Web will be the place whereinformation is created and communicated will grow alongside them

Neutron star mergers and rare core-collapse supernovae as sources of r-process enrichment in simulated galaxies

We use cosmological, magnetohydrodynamical simulations of Milky Way-mass galaxies from the Auriga project to study their enrichment with rapid neutron capture (r-process) elements. We implement a variety of enrichment models from both binary neutron star mergers and rare core-collapse supernovae. We focus on the abundances of (extremely) metal-poor stars, most of which were formed during the first ~Gyr of the Universe in external galaxies and later accreted onto the main galaxy. We find that the majority of metal-poor stars are r-process enriched in all our enrichment models. Neutron star merger models result in a median r-process abundance ratio which increases with metallicity, whereas the median trend in rare core-collapse supernova models is approximately flat. The scatter in r-process abundance increases for models with longer delay times or lower rates of r-process producing events. Our results are nearly perfectly converged, in part due to the mixing of gas between mesh cells in the simulations. Additionally, different Milky Way-mass galaxies show only small variation in their respective r-process abundance ratios. Current (sparse and potentially biased) observations of metal-poor stars in the Milky Way seem to prefer rare core-collapse supernovae over neutron star mergers as the dominant source of r-process elements at low metallicity, but we discuss possible caveats to our models. Dwarf galaxies which experience a single r-process event early in their history show highly enhanced r-process abundances at low metallicity, which is seen both in observations and in our simulations. We also find that the elements produced in a single event are mixed with ~10^8 Msun of gas relatively quickly, distributing the r-process elements over a large region.Comment: Accepted for publication in MNRAS. Revised version: added Figure 13 (on mixing of iron and r-process elements) and an Appendix (on iron and magnesium abundances) and updated the r-process yields (Tables 1 and 2 and normalization of abundances

Renormalization Group Study of the soliton mass on the (lambda Phi^4)_{1+1} lattice model

We compute, on the $(\lambda \Phi^4)_{1+1}$ model on the lattice, the soliton mass by means of two very different numerical methods. First, we make use of a ``creation operator'' formalism, measuring the decay of a certain correlation function. On the other hand we measure the shift of the vacuum energy between the symmetric and the antiperiodic systems. The obtained results are fully compatible. We compute the continuum limit of the mass from the perturbative Renormalization Group equations. Special attention is paid to ensure that we are working on the scaling region, where physical quantities remain unchanged along any Renormalization Group Trajectory. We compare the continuum value of the soliton mass with its perturbative value up to one loop calculation. Both quantities show a quite satisfactory agreement. The first is slightly bigger than the perturbative one; this may be due to the contributions of higher order corrections.Comment: 19 pages, preprint DFTUZ/93/0

Topological aspects of QCD

We review recent results from lattice on topological aspects of QCD: most of the results refer to monopoles and to instantons. We discuss in detail the evidence for condensation of monopoles in the vacuum and confinement of colour by dual superconductivity, and the major role of monopoles in dynamics (monopole dominance). As for instantons we review the $U(1)$ problem, a possible determination of the spin content of the proton, and new lattice data relevant to instanton liquid models.Comment: 8 pages, to be published in the Proceedings of Lattice 95, uuencoded postscript file. IFUP-TH 49/9

The effect of magnetic fields on properties of the circumgalactic medium

We study the effect of magnetic fields on a simulated galaxy and its surrounding gaseous halo, or circumgalactic medium (CGM), within cosmological 'zoom-in' simulations of a Milky Way-mass galaxy as part of the Simulating the Universe with Refined Galaxy Environments (SURGE) project. We use three different galaxy formation models, each with and without magnetic fields, and include additional spatial refinement in the CGM to improve its resolution. The central galaxy's star formation rate and stellar mass are not strongly affected by the presence of magnetic fields, but the galaxy is more disc dominated and its central black hole is more massive when B > 0. The physical properties of the CGM change significantly. With magnetic fields, the circumgalactic gas flows are slower, the atomic hydrogen-dominated extended discs around the galaxy are more massive and the densities in the inner CGM are therefore higher, the temperatures in the outer CGM are higher, and the pressure in the halo is higher and smoother. The total gas fraction and metal mass fraction in the halo are also higher when magnetic fields are included, because less gas escapes the halo. Additionally, we find that the CGM properties depend on azimuthal angle and that magnetic fields reduce the scatter in radial velocity, whilst enhancing the scatter in metallicity at fixed azimuthal angle. The metals are thus less well-mixed throughout the halo, resulting in more metal-poor halo gas. These results together show that magnetic fields in the CGM change the flow of gas in galaxy haloes, making it more difficult for metal-rich outflows to mix with the metal-poor CGM and to escape the halo, and therefore should be included in simulations of galaxy formation

Neutron star mergers and rare core-collapse supernovae as sources of r-process enrichment in simulated galaxies

We use cosmological, magnetohydrodynamical simulations of Milky Way-mass galaxies from the Auriga project to study their enrichment with rapid neutron capture (r-process) elements. We implement a variety of enrichment models from both binary neutron star mergers and rare core-collapse supernovae. We focus on the abundances of (extremely) metal-poor stars, most of which were formed during the first ∼Gyr of the Universe in external galaxies and later accreted on to the main galaxy. We find that the majority of metal-poor stars are r-process enriched in all our enrichment models. Neutron star merger models result in a median r-process abundance ratio, which increases with metallicity, whereas the median trend in rare core-collapse supernova models is approximately flat. The scatter in r-process abundance increases for models with longer delay times or lower rates of r-process-producing events. Our results are nearly perfectly converged, in part due to the mixing of gas between mesh cells in the simulations. Additionally, different Milky Way-mass galaxies show only small variation in their respective r-process abundance ratios. Current (sparse and potentially biased) observations of metal-poor stars in the Milky Way seem to prefer rare core-collapse supernovae over neutron star mergers as the dominant source of r-process elements at low metallicity, but we discuss possible caveats to our models. Dwarf galaxies that experience a single r-process event early in their history show highly enhanced r-process abundances at low metallicity, which is seen both in observations and in our simulations. We also find that the elements produced in a single event are mixed with ≈108 M≥ of gas relatively quickly, distributing the r-process elements over a large region

Color confinement and dual superconductivity of the vacuum. III

It is demonstrated that monopole condensation in the confined phase of SU(2) and SU(3) gauge theories is independent of the specific Abelian projection used to define the monopoles. Hence the dual excitations which condense in the vacuum to produce confinement must have magnetic U(1) charge in all the Abelian projections. Some physical implications of this result are discussed.Comment: 6 pages, 5 postscript figure

Free energy of an SU(2) monopole-antimonopole pair

We present a high-statistic numerical study of the free energy of a monopole-antimonopole pair in pure SU(2) theory. We find that the monopole-antimonopole interaction potential exhibits a screened behavior, as one would expect in presence of a monopole condensate. Screening occurs both in the low-temperature, confining phase of the theory, and in the high-temperature deconfined phase, with no evidence of a discontinuity of the screening mass across the transition. The mass of the object responsible for the screening at low temperature is approximately twice the established value for the lightest glueball, indicating a prevalent coupling to glueball excitations. At high temperature, the screening mass increases. We contrast the behavior of the quantum system with that of the corresponding classical system, where the monopole-antimonopole potential is of the Coulomb type.Comment: Latex, 22 pages, 8 figures. A mistake in the computer program implementing the multihistogram method has been corrected and all the affected numerical data have been revised. The main conclusions of the paper are unchanged, but the screening masses turn out somehow larger. (We thank Philippe de Forcrand for correspondence which helped us find the error.

Dual superconductivity in the SU(2) pure gauge vacuum: a lattice study

We investigate the dual superconductivity hypothesis in pure SU(2) lattice gauge theory. We focus on the dual Meissner effect by analyzing the distribution of the color fields due to a static quark-antiquark pair. We find evidence of the dual Meissner effect both in the maximally Abelian gauge and without gauge fixing. We measure the London penetration length. Our results suggest that the London penetration length is a physical gauge-invariant quantity. We put out a simple relation between the penetration length and the square root of the string tension. We find that our extimation is quite close to the extrapolated continuum limit available in the literature. A remarkable consequence of our study is that an effective Abelian theory can account for the long range properties of the SU(2) confining vacuum.Comment: 38 pages, uuencoded compressed (using GNU's gzip) tar file containing 1 LaTeX2e file (to be processed 3 times) + 16 encapsulated Postscript figures. A full Postscript version of this paper is available at http://www.ba.infn.it/disk$gruppo_4/cosmai/www/papers/195-95.P