In order to understand certain observed features of arc-like giant radio relics such as the rareness, uniform surface brightness, and curved integrated spectra, we explore a diffusive shock acceleration (DSA) model for radio relics in which a spheric
al shock impinges on a magnetized cloud containing fossil relativistic electrons. Toward this end, we perform DSA simulations of spherical shocks with the parameters relevant for the Sausage radio relic in cluster CIZA J2242.8+5301, and calculate the ensuing radio synchrotron emission from re-accelerated electrons. Three types of fossil electron populations are considered: a delta-function like population with the shock injection momentum, a power-law distribution, and a power-law with an exponential cutoff. The surface brightness profile of radio-emitting postshock region and the volume-integrated radio spectrum are calculated and compared with observations. We find that the observed width of the Sausage relic can be explained reasonably well by shocks with speed $u_s sim 3times 10^3 kms$ and sonic Mach number $M_s sim 3$. These shocks produce curved radio spectra that steepen gradually over $(0.1-10) u_{rm br}$ with break frequency $ u_{rm br}sim 1$ GHz, if the duration of electron acceleration is $sim 60 - 80$ Myr. However, the abrupt increase of spectral index above $sim 1.5$ GHz observed in the Sausage relic seems to indicate that additional physical processes, other than radiative losses, operate for electrons with $gamma_e gtrsim 10^4$.
An ionization front (IF) surrounding an H II region is a sharp interface where a cold neutral gas makes transition to a warm ionized phase by absorbing UV photons from central stars. We investigate the instability of a plane-parallel D-type IF thread
ed by parallel magnetic fields, by neglecting the effects of recombination within the ionized gas. We find that weak D-type IFs always have the post-IF magnetosonic Mach number $mathcal{M}_{rm M2} leq 1$. For such fronts, magnetic fields increase the maximum propagation speed of the IFs, while reducing the expansion factor $alpha$ by a factor of $1+1/(2beta_1)$ compared to the unmagnetized case, with $beta_1$ denoting the plasma beta in the pre-IF region. IFs become unstable to distortional perturbations due to gas expansion across the fronts, exactly analogous to the Darrieus-Landau instability of ablation fronts in terrestrial flames. The growth rate of the IF instability is proportional linearly to the perturbation wavenumber as well as the upstream flow speed, and approximately to $alpha^{1/2}$. The IF instability is stabilized by gas compressibility and becomes completely quenched when the front is D-critical. The instability is also stabilized by magnetic pressure when the perturbations propagate in the direction perpendicular to the fields. When the perturbations propagate in the direction parallel to the fields, on the other hand, it is magnetic tension that reduces the growth rate, completely suppressing the instability when $mathcal{M}_{rm M2}^2 < 2/(beta_1 - 1)$. When the front experiences an acceleration, the IF instability cooperates with the Rayleigh-Taylor instability to make the front more unstable.
The neutral component of the interstellar medium is segregated into the cold neutral medium (CNM) and warm neutral medium (WNM) as a result of thermal instability. It was found that a plane-parallel CNM-WNM evaporation interface, across which the CNM
undergoes thermal expansion, is linearly unstable to corrugational disturbances, in complete analogy with the Darrieus-Landau instability (DLI) of terrestrial flames. We perform a full linear stability analysis as well as nonlinear hydrodynamic simulations of the DLI of such evaporation fronts in the presence of thermal conduction. We find that the DLI is suppressed at short length scales by conduction. The length and time scales of the fastest growing mode are inversely proportional to the evaporation flow speed of the CNM and its square, respectively. In the nonlinear stage, the DLI saturates to a steady state where the front deforms to a finger-like shape protruding toward the WNM, without generating turbulence. The evaporation rate at nonlinear saturation is larger than the initial plane-parallel value by a factor of 2.4 when the equilibrium thermal pressure is 1800 k_B cm^-3 K. The degrees of front deformation and evaporation-rate enhancement at nonlinear saturation are determined primarily by the density ratio between the CNM and WNM. We demonstrate that the Field length in the thermally unstable medium should be resolved by at least four grid points to obtain reliable numerical outcomes involving thermal instability.
Nuclear rings in barred galaxies are sites of active star formation. We use hydrodynamic simulations to study temporal and spatial behavior of star formation occurring in nuclear rings of barred galaxies where radial gas inflows are triggered solely
by a bar potential. The star formation recipes include a density threshold, an efficiency, conversion of gas to star particles, and delayed momentum feedback via supernova explosions. We find that star formation rate (SFR) in a nuclear ring is roughly equal to the mass inflow rate to the ring, while it has a weak dependence on the total gas mass in the ring. The SFR typically exhibits a strong primary burst followed by weak secondary bursts before declining to very small values. The primary burst is associated with the rapid gas infall to the ring due to the bar growth, while the secondary bursts are caused by re-infall of the ejected gas from the primary burst. While star formation in observed rings persists episodically over a few Gyr, the duration of active star formation in our models lasts for only about a half of the bar growth time, suggesting that the bar potential alone is unlikely responsible for gas supply to the rings. When the SFR is low, most star formation occurs at the contact points between the ring and the dust lanes, leading to an azimuthal age gradient of young star clusters. When the SFR is large, on the other hand, star formation is randomly distributed over the whole circumference of the ring, resulting in no apparent azimuthal age gradient. Since the ring shrinks in size with time, star clusters also exhibit a radial age gradient, with younger clusters found closer to the ring. The cluster mass function is well described by a power law, with a slope depending on the SFR. Giant gas clouds in the rings have supersonic internal velocity dispersions and are gravitationally bound.
We investigate the direct determination of expansion history using redshift distortions without plugging into detailed cosmological parameters. The observed spectra in redshift space include a mixture of information: fluctuations of density-density a
nd velocity-velocity spectra, and distance measures of perpendicular and parallel components to the line of sight. Unfortunately it is hard to measure all the components simultaneously without any specific prior assumption. Common prior assumptions include a linear/quasi-linear model of redshift distortions or a model for the shape of the power spectra, which eventually breaks down on small scales at later epochs where nonlinear structure formation disturbs coherent growth. The degeneracy breaking, between the effect of cosmic distances and redshift distortions for example, depends on the prior we assume. An alternative approach is to utilize the cosmological principle inscribed in the heart of the Friedmann-Lematre-Robertson-Walker (hereafter FLRW) universe, that is, the specific relation between the angular diameter distance and the Hubble parameter, in this degeneracy breaking. We show that utilizing this FLRW prior early in the step of distinguishing the distance effect from redshift distortions helps us improve the detectability of power spectra and distance measures with no leaning on a combination of other experiments.
We discuss the impact for light neutralinos in an effective Minimal Supersymmetric extension of the Standard Model of the recent results presented by the CMS and ATLAS Collaborations at the CERN Large Hadron Collider for a search of supersymmetry in
proton-proton collisions at a center-of-mass energy of 7 TeV with an integrated luminosity of 35 inverse pb. We find that, in the specific case of light neutralinos, efficiencies for the specific signature searched by ATLAS (jets+missing transverse energy and an isolated lepton) imply a lower sensitivity compared to CMS (which searches for jets +missing transverse energy). Focusing on the CMS bound, if squark soft masses of the three families are assumed to be degenerate, the combination of the ensuing constraint on squark and gluino masses with the experimental limit on the b to s + gamma decay imply a lower bound on the neutralino mass that can reach the value of 11.9 GeV, depending on the gluino mass. On the other hand, when the universality condition among squark soft parameters is relaxed, the lower bound on the neutralino mass is not constrained by the CMS measurement and then remains at the value 7.5 GeV derived in previous papers.
We present arcsecond-resolution Submillimeter Array (SMA) polarimetric observations of the 880 um continuum emission from the protoplanetary disks around two nearby stars, HD 163296 and TW Hydrae. Although previous observations and theoretical work h
ave suggested that a 2-3% polarization fraction should be common for the millimeter continuum emission from such disks, we detect no polarized continuum emission above a 3-sigma upper limit of 7 mJy in each arcsecond-scale beam, or <1% in integrated continuum emission. We compare the SMA upper limits with the predictions from the exploratory Cho & Lazarian (2007) model of polarized emission from T Tauri disks threaded by toroidal magnetic fields, and rule out their fiducial model at the ~10-sigma level. We explore some potential causes for this discrepancy, focusing on model parameters that describe the shape, magnetic field alignment, and size distribution of grains in the disk. We also investigate related effects like the magnetic field strength and geometry, scattering off of large grains, and the efficiency of grain alignment, including recent advances in grain alignment theory, which are not considered in the fiducial model. We discuss the impact each parameter would have on the data and determine that the suppression of polarized emission plausibly arises from rounding of large grains, reduced efficiency of grain alignment with the magnetic field, and/or some degree of magnetic field tangling (perhaps due to turbulence). A poloidal magnetic field geometry could also reduce the polarization signal, particularly for a face-on viewing geometry like the TW Hya disk. The data provided here offer the most stringent limits to date on the polarized millimeter-wavelength emission from disks around young stars.
We have performed mid-IR photometry of the young open cluster NGC 2264 using the images obtained with the Spitzer Space Telescope IRAC and MIPS instruments and present a normalized classification scheme of young stellar objects in various color-color
diagrams to make full use of the information from multicolor photometry. These results are compared with the classification scheme based on the slope of the spectral energy distribution (SED). From the spatial distributions of Class I and II stars, we have identified two subclusterings of Class I objects in the CONE region of Sung et al. The disked stars in the other star forming region S MON are mostly Class II objects. These three regions show a distinct difference in the fractional distribution of SED slopes as well as the mean value of SED slopes. The fraction of stars with primordial disks is nearly flat between log m = 0.2 -- -0.5, and that of transition disks is very high for solar mass stars. In addition, we have derived a somewhat higher value of the primordial disk fraction for NGC 2264 members located below the main pre-main sequence locus (so-called BMS stars). This result supports the idea that BMS stars are young stars with nearly edge-on disks. We have also found that the fraction of primordial disks is very low near the most massive star S Mon and increases with distance from S Mon.
We compute in order alpha_s the nonrelativistic QCD (NRQCD) short-distance coefficients that match quark-antiquark operators of all orders in the heavy-quark velocity v to the electromagnetic current. We employ a new method to compute the one-loop NR
QCD contribution to the matching condition. The new method uses full-QCD expressions as a starting point to obtain the NRQCD contribution, thus greatly streamlining the calculation. Our results show that, under a mild constraint on the NRQCD operator matrix elements, the NRQCD velocity expansion for the quark-antiquark-operator contributions to the electromagnetic current converges. The velocity expansion converges rapidly for approximate J/psi operator matrix elements.
A binary poset code of codimension M (of cardinality 2^{N-M}, where N is the code length) can correct maximum M errors. All possible poset metrics that allow codes of codimension M to be M-, (M-1)- or (M-2)-perfect are described. Some general conditi
ons on a poset which guarantee the nonexistence of perfect poset codes are derived; as examples, we prove the nonexistence of R-perfect poset codes for some R in the case of the crown poset and in the case of the union of disjoin chains. Index terms: perfect codes, poset codes
