Showing papers on "Continuum (set theory) published in 2009"

Energy resolution and discretization artifacts in the numerical renormalization group

Abstract: We study the limits of the energy resolution that can be achieved in the calculations of spectral functions of quantum impurity models using the numerical renormalization group (NRG) technique with interleaving ($z$ averaging). We show that overbroadening errors can be largely eliminated, that higher-moment spectral sum rules are satisfied to a good accuracy, and that positions, heights and widths of spectral features are well reproduced; the NRG approximates very well the spectral-weight distribution. We find, however, that the discretization of the conduction-band continuum nevertheless introduces artifacts. We present a modified discretization scheme which removes the band-edge discretization artifacts of the conventional approach and significantly improves the convergence to the continuum $(\ensuremath{\Lambda}\ensuremath{\rightarrow}1)$ limit. Sample calculations of spectral functions with high energy resolution are presented. We follow in detail the emergence of the Kondo resonance in the Anderson impurity model as the electron-electron repulsion is increased, and the emergence of the phononic side peaks and the crossover from the spin Kondo effect to the charge Kondo effect in the Anderson-Holstein impurity model as the electron-phonon coupling is increased. We also compute the spectral function of the Hubbard model within the dynamical mean-field theory, confirming the presence of fine structure in the Hubbard bands.

The Far-Infrared--Radio Correlation at High Redshifts: Physical Considerations and Prospects for the Square Kilometer Array

Abstract: (Abridged) I present a predictive analysis for the behavior of the FIR--radio correlation as a function of redshift in light of the deep radio continuum surveys which may become possible using the SKA. To keep a fixed ratio between the FIR and predominantly non-thermal radio continuum emission of a normal star-forming galaxy requires a nearly constant ratio between galaxy magnetic field and radiation field energy densities. While the additional term of IC losses off of the cosmic microwave background (CMB) is negligible in the local Universe, the rapid increase in the strength of the CMB energy density (i.e. $\sim(1+z)^{4})$ suggests that evolution in the FIR-radio correlation should occur with infrared (IR; $8-1000 \micron$)/radio ratios increasing with redshift. At present, observations do not show such a trend with redshift; $z\sim6$ radio-quiet QSOs appear to lie on the local FIR-radio correlation while a sample of $z\sim4.4$ and $z\sim2.2$ SMGs exhibit ratios that are a factor of $\sim$2.5 {\it below} the canonical value. I also derive a 5$\sigma$ point-source sensitivity goal of $\approx$20 nJy (i.e. $\sigma_{\rm RMS} \sim 4$ nJy) requiring that the SKA specified be $A_{\rm eff}/T_{\rm sys}\approx 15000$ m$^{2}$ K$^{-1}$; achieving this sensitivity should enable the detection of galaxies forming stars at a rate of $\ga25 M_{\sun} {\rm yr}^{-1}$, at all redshifts if present. By taking advantage of the fact that the non-thermal component of a galaxy's radio continuum emission will be quickly suppressed by IC losses off of the CMB, leaving only the thermal (free-free) component, I argue that deep radio continuum surveys at frequencies $\ga$10 GHz may prove to be the best probe for characterizing the high-$z$ star formation history of the Universe unbiased by dust.

Microlensing-Based Estimate of the Mass Fraction in Compact Objects in Lens

Abstract: We estimate the fraction of mass that is composed of compact objects in gravitational lens galaxies. This study is based on microlensing measurements (obtained from the literature) of a sample of 29 quasar image pairs seen through 20 lens galaxies. We determine the baseline for no microlensing magnification between two images from the ratios of emission line fluxes. Relative to this baseline, the ratio between the continua of the two images gives the difference in microlensing magnification. The histogram of observed microlensing events peaks close to no magnification and is concentrated below 0.6 magnitudes, although two events of high magnification, $\Delta m \sim 1.5$, are also present. We study the likelihood of the microlensing measurements using frequency distributions obtained from simulated microlensing magnification maps for different values of the fraction of mass in compact objects, $\alpha$. The concentration of microlensing measurements close to $\Delta m \sim 0$ can be explained only by simulations corresponding to very low values of $\alpha$ (10% or less). A maximum likelihood test yields $\alpha=0.05_{-0.03}^{+0.09}$ (90% confidence interval) for a quasar continuum source of intrinsic size $r_{s_0}\sim 2.6 \cdot 10^{15} \rm cm$. Regarding the current controversy about Milky Way/LMC and M31 microlensing studies, our work supports the hypothesis of a very low content in MACHOS (Massive Compact Halo Objects).

Plasmon dispersion on epitaxial graphene studied using high-resolution electron energy-loss spectroscopy

Abstract: We have investigated the momentum-space-dependent behavior of plasmons on epitaxial graphene (EG) using high-resolution electron energy-loss spectroscopy. There are significant differences in the $\ensuremath{\pi}$ plasmon behavior for single, bilayer, and 3--4 layer graphene which originate from differences in the in-plane and out-of-plane modes, as well as the different band structures between single-layer and few-layer graphene. The $\ensuremath{\pi}$ and $\ensuremath{\sigma}+\ensuremath{\pi}$ surface plasmon modes in single-layer EG are recorded at 5.1 and 14.5 eV at small momentum transfer $(q)$; these are redshifted from the values in multilayer EG. In single-layer graphene, a linear dispersion of the plasmon mode is observed, in contrast to the parabolic dispersion in multilayer EG. The overall linear $\ensuremath{\pi}$ plasmon dispersion between 4.8--6.7 eV is attributed to the mixing of electronic transitions caused by local field effects, which includes the linear dispersion features resulting from transitions within the ``Dirac cone.'' We also observe that the intensity of the Fuches-Kliewer phonon of SiC and loss continuum of EG varies with the thickness of epitaxial graphene.

Effects of breakup couplings on B 8 + Ni 58 elastic scattering

Abstract: We use the continuum discretized coupled channel (CDCC) method to investigate the effects of breakup coupling on $^{8}\mathrm{B}+^{58}\mathrm{Ni}$ elastic scattering. We evaluate angular distributions at several collision energies and show that our theoretical results are in excellent agreement with the recent data of Aguilera et al. [Phys. Rev. C 79, 021601(R) (2009)]. We show that nuclear excitations of the target have a weak influence on the elastic angular distributions but that the inclusion of continuum-continuum couplings is essential to reproduce the data.

A chandra x-ray view of stephan's quintet: shocks and star formation

Abstract: We use a deep Chandra observation to examine the structure of the hot intra-group medium of the compact group of galaxies Stephan's Quintet. The group is thought to be undergoing a strong dynamical interaction as an interloper, NGC 7318b, passes through the group core at {approx}850kms{sup -1}. Previous studies have interpreted a bright ridge of X-ray and radio continuum emission as the result of shock heating, with support from observations at other wavelengths. We find that gas in this ridge has a similar temperature ({approx}0.6 keV) and abundance ({approx}0.3 Z {sub sun}) to the surrounding diffuse emission, and that a hard emission component is consistent with that expected from high-mass X-ray binaries associated with star formation in the ridge. The cooling rate of gas in the ridge is consistent with the current star formation rate, suggesting that radiative cooling is driving the observed star formation. The lack of a high-temperature gas component is used to place constraints on the nature of the interaction and shock, and we find that an oblique shock heating a pre-existing filament of H I may be the most likely explanation of the X-ray gas in the ridge. The mass of hot gas in the groupmore » is roughly equal to the deficit in observed H I mass compared to predictions, but only {approx}2% of the gas is contained in the ridge. The hot gas component is too extended to have been heated by the current interaction, strongly suggesting that it must have been heated during previous dynamical encounters.« less

Four-body continuum-discretized coupled-channels calculations

Abstract: The development of a continuum-bin scheme of discretization for three-body projectiles is necessary for studies of reactions of Borromean nuclei such as $^{6}\mathrm{He}$ within the continuum-discretized coupled-channels approach. Such a procedure, for constructing bin states on selected continuum energy intervals, is formulated and applied for the first time to reactions of a three-body projectile. The continuum representation uses the eigenchannel expansion of the three-body $S$ matrix. The method is applied to the challenging case of the $^{6}\mathrm{He}+^{208}\mathrm{Pb}$ reaction at 22 MeV, where an accurate treatment of both the Coulomb and the nuclear interactions with the target is necessary.

On Action-Minimizing Retrograde and Prograde Orbits of the Three-Body Problem

Abstract: A retrograde orbit of the planar three-body problem is a relative periodic solution with two adjacent masses revolving around each other in one direction while their mass center revolves around the third mass in the other direction. The orbit is said to be prograde or direct if both revolutions follow the same direction. Let T > 0 and \({\phi\in[0,2\pi)}\) be fixed, and consider the rotating frame which rotates the inertia frame about the origin with angular velocity \({\frac{\phi}{T}}\) . In a recent work of K.-C.Chen [5], the existence of action-minimizing retrograde orbits which are T-periodic on this rotation frame were proved to exist for a large class of masses and a continuum of \({\phi}\) . In this paper we generalize the main result in [5], provide some quantitative estimates for admissible masses and mutual distances, and show miscellaneous examples of action-minimizing retrograde orbits. We also show the existence of some prograde and retrograde solutions with additional symmetries.

A ring/disk/outflow system associated with W51 North: a very massive star in the making

Abstract: Sensitive and high angular resolution ($\sim$ 0.4\arcsec) SO$_2$[22$_{2,20}$ $\to$ 22$_{1,21}$] and SiO[5$\to$4] line and 1.3 and 7 mm continuum observations made with the Submillimeter Array (SMA) and the Very Large Array (VLA) towards the young massive cluster W51 IRS2 are presented. We report the presence of a large (of about 3000 AU) and massive (40 M$_\odot$) dusty circumstellar disk and a hot gas molecular ring around a high-mass protostar or a compact small stellar system associated with W51 North. The simultaneous observations of the silicon monoxide molecule, an outflow gas tracer, further revealed a massive (200 M$_\odot$) and collimated ($\sim14^\circ$) outflow nearly perpendicular to the dusty and molecular structures suggesting thus the presence of a single very massive protostar with a bolometric luminosity of more than 10$^5$ L$_\odot$. A molecular hybrid LTE model of a Keplerian and infalling ring with an inner cavity and a central stellar mass of more than 60 M$_\odot$ agrees well with the SO$_2$[22$_{2,20}$ $\to$ 22$_{1,21}$] line observations. Finally, these results suggest that mechanisms, such as mergers of low- and intermediate- mass stars, might be not necessary for forming very massive stars.

Continuum phase shifts and partial cross sections for photoionization from excited states of atomic helium measured by high-order harmonic optical pump-probe velocity map imaging

Abstract: Phase shift differences and ratios of radial dipole matrix elements of the outgoing $S$ and $D$ continuum waves from state-selected helium atoms are directly measured from the photoelectron angular distributions using pump-probe velocity map imaging. Aligned $1s3p$ $^{1}P_{1}$ and $1s4p$ $^{1}P_{1}$ states in helium are prepared by high-order harmonics and ionized with either 800, 400, or $267\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ light. The results allow for the determination of energy-dependent quantum defect differences and ratios of partial cross sections and agree favorably with theoretical calculations on electron scattering and photoionization.

State-selective differential cross sections for single and double electron capture in He+,2+-He and p-He collisions

Abstract: Using the cold target recoil ion momentum spectroscopy technique, we have measured state-selective projectile scattering angles for single and double electron captures in collisions of protons and ${\text{He}}^{1,2+}$ projectiles with a helium target for incident energies of 60--630 keV/u. We also report theoretical results obtained by means of four-body one-channel distorted-wave models (continuum distorted-wave Born final state, continuum distorted-wave Born initial state, and Born distorted wave) and find mixed agreement with the measured data.

Elastic proton scattering on tritium below the n − He 3 threshold

Abstract: Elastic proton scattering on the $^{3}\mathrm{H}$ nucleus is studied between $p\text{\ensuremath{-}}^{3}\mathrm{H}$ and $n\text{\ensuremath{-}}^{3}\mathrm{He}$ thresholds, in the energy region where the first excited state of the $\ensuremath{\alpha}$ particle is embedded in the continuum. Faddeev-Yakubovski equations are solved in configuration space by fully considering effects from isospin breaking and rigorously treating the Coulomb interaction. Different realistic nuclear Hamiltonians are tested, elucidating open problems in the description of the nuclear interaction.

Precision check on the triviality of the ϕ 4 theory by a new simulation method

Abstract: We report precise simulations of {phi}{sup 4} theory in the Ising limit. A recent technique to stochastically evaluate the all-order strong coupling expansion is combined with exact identities in the closely related Aizenman random current representation. In this way estimates of the renormalized coupling close to the continuum limit become possible with unprecedented precision and yet low CPU cost. As a sample application we present results for the unbroken phase of the Ising model in dimensions 3, 4 and 5 and investigate the question of triviality by studying a finite size scaling continuum limit.

Physical properties of Southern infrared dark clouds

Abstract: It is commonly assumed that cold and dense Infrared Dark Clouds (IRDCs) likely represent the birth sites massive stars. Therefore, this class of objects gets increasing attention. To enlarge the sample of well-characterised IRDCs in the southern hemisphere, we have set up a program to study the gas and dust of southern IRDCs. The present paper aims at characterizing the continuuum properties of this sample of objects. We cross-correlated 1.2 mm continuum data from SIMBA@SEST with Spitzer/GLIMPSE images to establish the connection between emission sources at millimeter wavelengths and the IRDCs we see at 8 $\mu$m in absorption against the bright PAH background. Analysing the dust emission and extinction leads to a determination of masses and column densities, which are important quantities in characterizing the initial conditions of massive star formation. The total masses of the IRDCs were found to range from 150 to 1150 $\rm M_\odot$ (emission data) and from 300 to 1750 $\rm M_\odot$ (extinction data). We derived peak column densities between 0.9 and 4.6 $\times 10^{22}$ cm$^{-2}$ (emission data) and 2.1 and 5.4 $\times 10^{22}$ cm$^{-2}$ (extinction data). We demonstrate that the extinction method fails for very high extinction values (and column densities) beyond A$_{\rm V}$ values of roughly 75 mag according to the Weingartner & Draine (2001) extinction relation $R_{\rm V} = 5.5$ model B. The derived column densities, taking into account the spatial resolution effects, are beyond the column density threshold of 3.0 $\times 10^{23}$ cm$^{-2}$ required by theoretical considerations for massive star formation. We conclude that the values for column densities derived for the selected IRDC sample make these objects excellent candidates for objects in the earliest stages of massive star formation.

Role of the final-state interaction and three-body force on the longitudinal response function of 4He.

Abstract: We present an ab initio calculation of the longitudinal electron scattering response function off $^{4}\mathrm{He}$ with two- and three-nucleon forces and compare to experimental data. The full four-body continuum dynamics is considered via the Lorentz integral transform method. The importance of the final-state interaction is shown at various energies and momentum transfers $q$. The three-nucleon force reduces the quasielastic peak by 10% for $q$ between 300 and $500\text{ }\text{ }\mathrm{MeV}/c$. Its effect increases significantly at lower $q$, up to about 40% at $q=100\text{ }\text{ }\mathrm{MeV}/c$. At very low $q$, however, data are missing.

Four-body calculation of 6He breakup with the Coulomb-corrected eikonal method

Abstract: The elastic breakup of a three-body projectile on a target is studied within the eikonal approximation with full account of final-state interactions. Bound and scattering states are calculated in hyperspherical coordinates on a Lagrange mesh. A correction is introduced to avoid the divergence of breakup cross sections due to the Coulomb interaction. The eikonal approximation allows the direct calculation of various cross sections, and in particular multidifferential cross sections can be obtained. The model is applied to the breakup of {sup 6}He on {sup 208}Pb. The {sup 6}He halo nucleus is described within a three-body {alpha}+n+n model involving effective {alpha}n and nn interactions. The eikonal phase is obtained from optical potentials between {alpha} and n, and the target. Around 0.8 MeV, the total breakup cross sections exhibit a narrow 2{sup +} resonant peak superimposed over a broad bump corresponding to a 1{sup -} resonance. These results suffer from a disagreement with experimental data at 240 MeV/nucleon, where cross sections are much smaller at low energies. The obtained E1 strength distribution resembles other theoretical results and reopens a long-standing problem about the existence of a 1{sup -} low-energy resonance in the {sup 6}He continuum.

Growth rate for beta-expansions

Abstract: Let $\beta>1$ and let $m>\be$ be an integer. Each $x\in I_\be:=[0,\frac{m-1}{\beta-1}]$ can be represented in the form \[ x=\sum_{k=1}^\infty \epsilon_k\beta^{-k}, \] where $\epsilon_k\in\{0,1,...,m-1\}$ for all $k$ (a $\beta$-expansion of $x$). It is known that a.e. $x\in I_\beta$ has a continuum of distinct $\beta$-expansions. In this paper we prove that if $\beta$ is a Pisot number, then for a.e. $x$ this continuum has one and the same growth rate. We also link this rate to the Lebesgue-generic local dimension for the Bernoulli convolution parametrized by $\beta$. When $\beta<\frac{1+\sqrt5}2$, we show that the set of $\beta$-expansions grows exponentially for every internal $x$.

Critical random graphs: limiting constructions and distributional properties

Abstract: We consider the Erdos-Renyi random graph G(n,p) inside the critical window, where p = 1/n + lambda * n^{-4/3} for some lambda in R. We proved in a previous paper (arXiv:0903.4730) that considering the connected components of G(n,p) as a sequence of metric spaces with the graph distance rescaled by n^{-1/3} and letting n go to infinity yields a non-trivial sequence of limit metric spaces C = (C_1, C_2, ...). These limit metric spaces can be constructed from certain random real trees with vertex-identifications. For a single such metric space, we give here two equivalent constructions, both of which are in terms of more standard probabilistic objects. The first is a global construction using Dirichlet random variables and Aldous' Brownian continuum random tree. The second is a recursive construction from an inhomogeneous Poisson point process on R_+. These constructions allow us to characterize the distributions of the masses and lengths in the constituent parts of a limit component when it is decomposed according to its cycle structure. In particular, this strengthens results of Luczak, Pittel and Wierman by providing precise distributional convergence for the lengths of paths between kernel vertices and the length of a shortest cycle, within any fixed limit component.

Gamow-Teller unit cross sections of the (p,n) reaction at 198 and 297 MeV on medium-heavy nuclei

Abstract: Gamow-Teller (GT) unit cross sections, ${\stackrel{\ifmmode \hat{}\else \^{}\fi{}}{\ensuremath{\sigma}}}_{\mathrm{GT}}$, are obtained at 198 and 297 MeV by measuring the double differential cross sections at ${0}^{\ifmmode^\circ\else\textdegree\fi{}}$ for the $(p,n)$ reaction on $^{58}\mathrm{Ni}$, $^{70}\mathrm{Zn}$, $^{114}\mathrm{Cd}$, $^{118}\mathrm{Sn}$, and $^{120}\mathrm{Sn}$. The mass dependence of ${\stackrel{\ifmmode \hat{}\else \^{}\fi{}}{\ensuremath{\sigma}}}_{\mathrm{GT}}$ and the ratio of ${\stackrel{\ifmmode \hat{}\else \^{}\fi{}}{\ensuremath{\sigma}}}_{\mathrm{GT}}$ to the Fermi unit cross section, ${\stackrel{\ifmmode \hat{}\else \^{}\fi{}}{\ensuremath{\sigma}}}_{\mathrm{F}}$, $({R}^{2})$ are also derived in the mass region of $58\ensuremath{\leqslant}A\ensuremath{\leqslant}120$. The ${\stackrel{\ifmmode \hat{}\else \^{}\fi{}}{\ensuremath{\sigma}}}_{\mathrm{GT}}$ value for $^{90}\mathrm{Zr}$ at 297 MeV interpolated using the $A$-dependence obtained herein agrees with that used in a previous analysis where the GT transition strength over a wide energy region up to the continuum was discussed. However, the deduced $^{64}\mathrm{Ni}$${\stackrel{\ifmmode \hat{}\else \^{}\fi{}}{\ensuremath{\sigma}}}_{\mathrm{GT}}$ value at 198 MeV is 20% smaller than that obtained from the analysis of a previous $(n,p)$ measurement. The present ${R}^{2}$ values in the mass region heavier than $^{42}\mathrm{Ca}$ are larger than those in the region up to $^{42}\mathrm{Ca}$ and increase as a function of $A$.

Breakup coupling effects on near-barrier quasi-elastic scattering of Li 6 , 7 on Sm 144

Abstract: Excitation functions of quasi-elastic scattering at backward angles have been measured for the $^{6,7}\mathrm{Li}+^{144}\mathrm{Sm}$ systems at near-barrier energies, and fusion barrier distributions have been extracted from the first derivatives of the experimental cross sections with respect to the bombarding energies. The data have been analyzed in the framework of continuum discretized coupled-channel calculations, and the results have been obtained in terms of the influence exerted by the inclusion of different reaction channels, with emphasis on the role played by the projectile breakup.

Continuum Hartree-Fock-Bogoliubov theory for weakly bound deformed nuclei using the coordinate-space Green's function method

Abstract: We formulate a new scheme of the Hartree-Fock-Bogoliubov mean-field theory applicable to weakly bound and pair-correlated deformed nuclei using the coordinate-space Green's function technique. On the basis of a coupled-channel representation of the quasiparticle wave function expanded in terms of the partial waves, we impose the correct boundary condition of the asymptotically out-going waves on the continuum quasiparticle states. We perform numerical analysis for $^{38}\mathrm{Mg}$ to illustrate properties of the continuum quasiparticle states and the pair correlation in deformed nuclei near the neutron drip line.

The loop-erased random walk and the uniform spanning tree on the four-dimensional discrete torus

Abstract: Let x and y be points chosen uniformly at random from \({\mathbb {Z}_n^4}\), the four-dimensional discrete torus with side length n. We show that the length of the loop-erased random walk from x to y is of order n2(log n)1/6, resolving a conjecture of Benjamini and Kozma. We also show that the scaling limit of the uniform spanning tree on \({\mathbb {Z}_n^4}\) is the Brownian continuum random tree of Aldous. Our proofs use the techniques developed by Peres and Revelle, who studied the scaling limits of the uniform spanning tree on a large class of finite graphs that includes the d-dimensional discrete torus for d ≥ 5, in combination with results of Lawler concerning intersections of four-dimensional random walks.

The accretion disk corona and disk atmosphere of 4U 1624-490 as viewed by the Chandra-HETGS

Abstract: We present a detailed spectral study (photoionization modelling and variability) of the "Big Dipper" 4U 1624-490 based on a \chandra-High Energy Transmission Gratings Spectrometer (HETGS) observation over the $\sim76$ ks binary orbit of 4U 1624-490. While the continuum spectrum can be modeled using a blackbody plus power-law, a slightly better fit is obtained using a single $\Gamma=2.25$ power-law partially (71%) covered by a local absorber of column density $N_{\rm H, Local}=8.1_{-0.6}^{+0.7}\times 10^{22} \rm cm^{-2}$. The data show a possible quasi-sinusoidal modulation with period $43_{-9}^{+13}$ ks that might be due to changes in local obscuration. Photoionization modeling with the {\sc xstar} code and variability studies of the observed strong \ion{Fe}{25} and \ion{Fe}{26} absorption lines point to a two-temperature plasma for their origin: a highly ionized component of ionization parameter $\xi_{\rm hot} \approx 10^{4.3} {\rm ergs cm s^{-1}}$ ($T\sim 3.0\times 10^{6}$ K) associated with an extended accretion disk corona of radius $R \sim3\times10^{10}$ cm, and a less ionized more variable component of $\xi \approx 10^{3.4} {\rm ergs cm s^{-1}}$ ($T\sim 1.0\times 10^{6}$ K) and $\rm \xi \approx 10^{3.1} ergs cm s^{-1}$ ($T\sim 0.9\times 10^{6}$ K) coincident with the accretion disk rim. We use this, with the observed \ion{Fe}{25} and \ion{Fe}{26} absorption line variations (in wavelength, strength, and width) to construct a viewing geometry that is mapped to changes in plasma conditions over the 4U 1624-490 orbital period.

Spitzer spectroscopy of the galactic supernova remnant g292.0+1.8: structure and composition of the oxygen-rich ejecta

Abstract: We present mid-infrared (mid-IR; 5-40 {mu}m) spectra of shocked ejecta in the Galactic oxygen-rich supernova remnant (SNR) G292.0+1.8, acquired with the Infrared Spectrograph onboard the Spitzer Space Telescope. The observations targeted two positions within the brightest oxygen-rich feature in G292.0+1.8. Emission lines of [Ne II] {lambda}12.8, [Ne III] {lambda}{lambda}15.5,36.0, [Ne V] {lambda}24.3, and [O IV] {lambda}25.9 {mu}m are detected from the shocked ejecta. In marked contrast to what is observed in Cassiopeia A, no discernible mid-IR emission from heavier species such as Mg, Si, S, Ar, or Fe is detected in G292.0+1.8. We also detect a broad emission bump between 15 and 28 {mu}m in spectra of the radiatively shocked O-rich ejecta in G292.0+1.8. We suggest that this feature arises from either shock-heated Mg{sub 2}SiO{sub 4} (forsterite) dust in the radiatively shocked O-rich ejecta or collisional excitation of polycyclic aromatic hydrocarbons in the blast wave of the SNR. If the former interpretation is correct, this would be the first mid-IR detection of ejecta dust in G292.0+1.8. A featureless dust continuum is also detected from nonradiative shocks in the circumstellar medium around G292.0+1.8. The mid-IR continuum from these structures, which lack mid-IR line emission, is seen in Chandra images as brightmore » X-ray filaments, is well described by a two-component silicate dust model. The temperature of the hot dust component (M {sub d} {approx} 2 x 10{sup -3} M {sub sun}) is {approx}115 K, while that of the cold component (roughly constrained to be {approx} 61, and M {sub O}/M {sub S} {approx} 50. These ratios (especially the large O/Ne mass ratio) are difficult to reproduce with standard nucleosynthesis models of well-mixed supernova ejecta. This reinforces the conclusions of existing X-ray studies that the reverse shock in G292.0+1.8 is currently propagating into the hydrostatic nucleosynthetic layers of the progenitor star, and has not yet penetrated the layers dominated by explosive nucleosynthetic products.« less

Expansion Parallax of the Planetary Nebula IC 418

Abstract: In this paper, we present radio continuum observations of the planetary nebula IC 418 obtained at two epochs separated by more than 20 years. These data allow us to show that the angular expansion rate of the ionization front in IC 418 is 5.8 {+-} 1.5 mas yr{sup -1}. If the expansion velocity of the ionization front is equal to the expansion velocity of the gas along the line of sight as measured by optical spectroscopy, then the distance to IC 418 must be 1.1 {+-} 0.3 kpc. Recent theoretical predictions appropriate for the case of IC 418, however, suggest that the ionization front may be expanding about 20% faster than the material. Under this assumption, the distance to IC 418 would increase to 1.3 {+-} 0.4 kpc.

Double-slit, confinement, and non-Franck-Condon effects in photoionization of H 2 at high photon energy

Abstract: A detailed theoretical study of ${\mathrm{H}}_{2}$ ionization by photons of a few hundreds of eV is presented. Bound and continuum states are accurately evaluated by using $B$-spline basis functions. The nuclear degrees of freedom are also included. A striking feature is observed when one analyzes the vibrational distribution of the residual $\mathrm{H}_{2}{}^{+}$ ion: this vibrational distribution does not follow the usual Franck-Condon behavior when ${\mathrm{H}}_{2}$ is parallel to the polarization direction. The origin of this anomaly is related to interference effects. Such effects are more clearly seen in the fully differential electron angular distributions associated with specific vibrational states of $\mathrm{H}_{2}{}^{+}$: for ${\mathrm{H}}_{2}$ molecules perpendicular to the polarization direction, the distributions clearly resemble those obtained in Young's double-slit experiment, while, for molecules parallel to the polarization direction, they show that the electron is sometimes prevented to escape in the direction of the radiation field due to the suppression (or confinement) of a given partial wave. The calculations also show that, at these high photon energies, the nuclear asymmetry parameter exhibits a reminiscence of Cohen-Fano oscillations when it is plotted as a function of photon energy.

Theory of (d,p) and (p,d) reactions including breakup: Comparison of methods

Abstract: There is an increasing interest in studying transfer reactions to probe the nuclear structure of exotic nuclei. For these loosely bound systems, the role of the continuum needs to be well understood. In this study, we concentrate on $(p,d)$ and $(d,p)$ reactions and compare two formulations for the transfer process that take into account breakup states. Applications to $^{11}\mathrm{Be}$$(p,d)$$^{10}\mathrm{Be}$ at ${E}_{\mathrm{lab}}=38.4$ MeV/nucleon and $^{10}\mathrm{Be}$$(d,p)$$^{11}\mathrm{Be}$ at ${E}_{\mathrm{lab}}=12.5$ MeV/nucleon are presented, as is a detailed discussion of convergence rates.

Search for three-nucleon force effects on the longitudinal response function of He 4

Abstract: A detailed study of the $^{4}\mathrm{He}$ longitudinal response function ${R}_{L}(\ensuremath{\omega},q)$ is performed at different kinematics, with particular emphasis on the role of three-nucleon forces. The effects reported are the results of an $\mathrm{ab} \mathrm{initio}$ calculation where the full four-body continuum dynamics is considered via the Lorentz integral transform method. The contributions of the various multipoles to the longitudinal response function are analyzed, and integral properties of the response are discussed as well. The Argonne V18 nucleon-nucleon interaction and two three-nucleon force models (Urbana IX and Tucson-Melbourne\ensuremath{'}) are used. At lower momentum transfers ($q\ensuremath{\leqslant}200 \mathrm{MeV}/c$) three-nucleon forces play an important role. One even finds a dependence of ${R}_{L}$ on the three-nucleon force model itself, with differences of up to 10%. Thus a Rosenbluth separation of the inclusive electron scattering cross section of $^{4}\mathrm{He}$ at low momentum transfers would be of great value for differentiating among force models.

Energetic ion, atom, and molecule reactions and excitation in low-current H2 discharges: Spatial distributions of emissions

Abstract: Spatial distributions of H alpha , H beta , and the near-uv continuum emission from the H2 a ;{3}Sigma g;+ state are measured and compared with a model for low-current electrical discharges in H2 at high E/N and low Nd , where E is the spatially uniform electric field, N is the gas density, and d is the electrode separation. Data are analyzed for 300 Td

Continuum percolation of isotropically oriented circular cylinders.

Abstract: The continuum percolation of circular cylinders has been studied for various values of the aspect ratio ${b}^{\ensuremath{'}}$. The percolation threshold is shown to have a maximum for ${b}^{\ensuremath{'}}\ensuremath{\approx}2$ when the cylinder length is equal to its diameter. Other quantities such as the average intersection volume and the porosity also possess a maximum for this value.