arXiv subjects

S. Kim

Publications and source records attributed to S. Kim.

At least 199 records · Page 11Linked to original sources

arXiv2007

Topology of HI in the Large Magellanic Cloud

We have analyzed the HI aperture synthesis image of the Large Magellanic Cloud (LMC), using an objective and quantitative measure of topology to understand the HI distribution hosting a number of holes and clumps of various sizes in the medium. The HI distribution shows different topology at four different chosen scales. At the smallest scales explored (19-29 pc), the HI mass is distributed in such a way that numerous clumps are embedded on top of a low density background. At the larger scales from 73 to 194 pc, it shows a generic hole topology. These holes might have been formed mainly by stellar winds from hot stars. At the scales from 240 to 340 pc, slightly above the disk scale-height of the gaseous disk, major clumps in the HI map change the distribution to have a slight clump topology. These clumps include the giant cloud associations in the spiral arms and the thick filaments surrounding superholes. At the largest scales studied (390-485 pc), the hole topology is present again. Responsible to the hole topology at this scale are a few superholes which seem mainly associated with supernova explosions in the outer disk. The gaps between the bar and spiral arms have a minor effect on the topology at this scale.

astro-ph↗

arXiv2007

IRAC Observations of CO J=4-3 High-Velocity Cloud in the 30 Doradus Complex in the Large Magellanic Cloud

We compare the CO 2-1 observations against previously taken CO 4-3 observations and analyze the spatial distribution of young stellar objects (YSO's) within the cloud using the Spitzer IRAC observations of the 30 Doradus complex. Both peaks of CO 2-1 and 4-3 emitting clouds coincide with the densest region of the filaments where multiple shells are colliding. We find that the YSO's are clustered in the southern ridge of the warm and dense molecular gas clouds traced by CO J=4-3, indicating a filamentary structure of star formation throughout the 30 Doradus. We also find that some of Class I YSO's candidates which are likely to be associated with a high-velocity component of CO 4-3 emitting clouds are present. This is a bona fide place where the triggered star formation had happened and newly formed stars may have produced such a high-velocity outflow interacting with the surrounding molecular cloud material.

astro-ph↗

arXiv2007

Optical BVI Imaging and HI Synthesis Observations of the Dwarf Irregular Galaxy ESO 364-G 029

As part of an effort to enlarge the number of well-studied Magellanic-type galaxies, we obtained broadband optical imaging and neutral hydrogen radio synthesis observations of the dwarf irregular galaxy ESO 364-G 029. The optical morphology characteristically shows a bar-like main body with a one-sided spiral arm, an approximately exponential light distribution, and offset photometric and kinematic centers. The HI distribution is mildly asymmetric and, although slightly offset from the photometric center, roughly follows the optical brightness distribution, extending to over 1.2 Holmberg radii (where mu_B = 26.5 mag/arcsec^2). In particular, the highest HI column densities closely follow the bar, one-arm spiral, and a third optical extension. The rotation is solid-body in the inner parts but flattens outside of the optical extent. The total HI flux F_HI = 23.1 pm 1.2 Jy km/s, yielding a total HI mass M_HI= (6.4 pm 1.7) x 10^8 Msun (for a distance D = 10.8 pm 1.4 Mpc) and a total HI mass-to-blue-luminosity ratio M_HI/L_B = (0.96 pm 0.14) Msun / Lsun,B (distance independent). The HI data suggest a very complex small-scale HI structure, with evidence of large shells and/or holes, but deeper observations are required for a detailed study. Follow-up observations are also desirable for a proper comparison with the Large Magellanic Cloud, where despite an optical morphology very similar to ESO 364-G 029 the HI bears little resemblance to the optical.

astro-ph↗

arXiv2006

Role of anisotropy in the spin-dimer compound BaCuSi2O6

We present results of magnetisation and electron paramagnetic resonance experiments on the spin-dimer system BaCuSi2O6. Evidence indicates that the origin of anisotropic terms in the spin Hamiltonian is from magnetic dipolar interactions. Axial symmetry-breaking is on a very small energy scale of ~11 mK, confirming Bose Einstein condensation critical scaling over an extended temperature range in the vicinity of the quantum critical point.

cond-mat.str-el↗

arXiv2006

The Carina-Near Moving Group

We identify a group of ~20 co-moving, mostly southern hemisphere, ~200 Myr old stars near Earth. Of the stars likely to be members of this Carina-Near Moving Group, in either its nucleus (~30 pc from Earth) or surrounding stream, all but 3 are plausible members of a multiple star system. The nucleus is (coincidentally) located quite close to the nucleus of the AB Doradus moving group notwithstanding that the two groups have substantially different ages and Galactic space motions, UVW.

astro-ph↗

arXiv2006

Application of Unit Time Block Entropy to Fetal Distress Heart Rate

Recently, multiple time scale characteristics of heart dynamics have received much attention for distinguishing healthy and pathologic cardiac systems. Despite structural peculiarities of the fetal cardiovascular system, the fetal heart rate(FHR) displays multiple time scale characteristics similar to the adult heart rate due to the autorhythmicity of its different oscillatory tissues and its interaction with other neural controllers. In this paper, we investigate the event and time scale characteristics of the normal and two pathologic fetal heart rate groups with the help of the new measure, called the Unit Time Block Entropy(UTBE), which approximates the entropy at each event and time scale based on symbolic dynamics. This method enables us to match the measurement time and the number of words between fetal heart rate data sets simultaneously. We find that in the small event scale and the large time scale, the normal fetus and the two pathologic fetus are completely distinguished. We also find that in the large event scale and the small time scale, the presumed distress fetus and the acidotic distress fetus are significantly distinguished.

physics.med-ph↗

arXiv2005

A Neutral Hydrogen Survey of the Large Magellanic Cloud: Aperture Synthesis and Multibeam Data Combined

Using a Fourier-plane technique, we have merged HI ATCA and Parkes data sets providing an accurate set of images of the LMC sensitive to structure on scales of 15 pc (for an LMC distance of 55 kpc) upwards. The spatial dynamic range (2.8 orders of magnitude), velocity resolution (1.649 km/s), brightness temperature sensitivity (2.4 K) and column density sensitivity ($8.9 \times 10^ {18}$ cm$^{-2}$ per 1.649 km/s channel) allow for the studies of phenomena ranging from the galaxy-wide interaction of the LMC with its close neighbours to the small-scale injection of energy from supernovae and stellar associations into the ISM of the LMC. We find that a previous analysis of the holes and shells (Kim et al. 1999) in the HI distribution shows the same power- law behavior with that of clumps. The measured perimeter P and measured enclosed area A of each HI cloud in a log-log plot give us a set of points lying along with a slope of D/2 = 0.73 $\pm$ 0.1. The relation between area and perimeter of each identified HI cloud, P $\propto$ $A^{D/2}$, can determine the fractal dimension D=1.47 $\pm$ 0.2 of a cloud boundary (Vogelaar & Wakker 1994; Williams, Blitz, & McKee 2000). The measured fractal dimension of HI clouds in the LMC is similar to the dimension, D=1.4, found in many studies of the molecular ISM.

astro-ph↗

arXiv2005

Properties of hadron screening masses at small baryonic density

The properties of hadron screening masses around the deconfinement phase transition at finite baryonic density can be studied by evaluating the Taylor coefficients with respect to the iso-scalar and iso-vector chemical potentials. We simulate 2-flavour lattice QCD with staggered fermions on a 12*12*24*6 lattice with ma = 0.05 and 0.10 and report investigations of nucleon, pseudo-scalar and vector mesons. We present new, strong evidence that in the confining phase, the screening masses at vanishing chemical potential have significant temperature dependence, but the effect of the iso-scalar chemical potential is very small. Above the critical temperature, the second derivative terms of mesons rapidly increase as contrasted to the case of baryon. We also study the responses of the screening masses to an iso-vector chemical potential and discuss some of the issues related to the properties of hadron masses at finite density.

hep-lat↗

arXiv2003

The CDFII Time-Of-Flight Detector and Impact on Beauty Flavor Tagging

The new CDFII detector incorporates a Time-of-Flight detector (TOF), employing plastic scintillator bars and fine-mesh photomultipliers. Since August 2001 the TOF system has been fully instrumented and integrated into the CDFII data acquisition system. With a design goal of $100 ps$ resolution the TOF system will provide at least two standard deviations separation between $K^{\pm}$ and $π^{\pm}$ for momenta $p<1.6 GeV/c$, complementing low momentum particle identification by means of the specific ionization energy loss measured in the drift chamber. We describe the design of the TOF detector and discuss the current status of its calibration and initial performances. Finally we review the expected impact of the TOF detector in the flavor tagging of neutral $B_S$ meson.

hep-ex↗

arXiv2002

A New Look at the Large-Scale HI Structure of the LMC

We present a Parkes multibeam \HI survey of the Large Magellanic Cloud (LMC). This survey, which is sensitive to spatial structure in the range 200 pc to 10 kpc, complements the Australia Telescope Compact survey, which is sensitive to structure in the range 15 pc to 500 pc. With an rms column density sensitivity of 8 x 10^16/cm^2 for narrow lines and 4 x 10^17/cm^2 for typical linewidths of 40 km/s, emission is found to be extensive well beyond the main body of the LMC. Arm-like features extend from the LMC to join the Magellanic Bridge and the Leading Arm, a forward counterpart to the Magellanic Stream. These features, whilst not as dramatic as those in the SMC, appear to have a common origin in the Galactic tidal field, in agreement with recent 2MASS and DENIS results for the stellar population. The diffuse gas which surrounds the LMC, particularly at pa's 90 to 330 deg, appears to be loosely associated with tidal features, but loosening by the ram pressure of tenuous Galactic halo gas against the outer parts of the LMC cannot be discounted. High-velocity clouds, which lie between the Galaxy and the LMC in velocity and which appear in the UV spectra of some LMC stars, are found to be associated with the LMC if their heliocentric velocity exceeds about +100 km/s. They are possibly the product of energetic outflows from the LMC disk. The HI mass of the LMC is found to be (4.8+/-0.2) x 10^8 Msun (for an assumed distance of 50 kpc), substantially more than previous recent measurements.

astro-ph↗

arXiv2002

QCD simulations at small chemical potential

Within the reweighting approach, one has the freedom to choose the Monte Carlo action so that it provides a good overlap with the finite-μmeasure but remains simple to simulate. We explore several choices of action in the regime of small μ. Simulating with a finite isospin chemical potential μ_I=μgives a better overlap than the standard choice μ=0, with no computational overhead.

hep-lat↗

arXiv2002

Bottomonium Decay Matrix Elements from Lattice QCD with Two Light Quarks

We calculate the long-distance matrix elements for the decays of the Upsilon (eta_b) and chi_b (h_b) states in lattice QCD with two flavors of light dynamical quarks. We relate the lattice matrix elements to their continuum counterparts through one-loop order in perturbation theory. In the case of the leading S-wave matrix element, we compare our result with a phenomenological value that we extract from the experimental leptonic decay rate by using the theoretical expression for the decay rate, accurate through relative order alpha_s. Whereas estimates of the leading S-wave matrix element from quenched QCD are 40--45% lower than the phenomenological value, the two-flavor estimate of the same matrix element is close to the phenomenological value. Extrapolating to the real world of 2+1 light flavors, we find that this matrix element is approximately 6% higher than the phenomenological value, but that the phenomenological value lies within our error bars. We also compute the color-singlet and color-octet matrix elements for P-wave decays. We find the value of the color-singlet matrix element for 2+1 flavors to be approximately 70% larger than the quenched value and the value of the color-octet matrix element for 2+1 flavors to be approximately 40% larger than the quenched value.

hep-lat↗

arXiv2001

A Geometric Fractal Growth Model for Scale Free Networks

We introduce a deterministic model for scale-free networks, whose degree distribution follows a power-law with the exponent $γ$. At each time step, each vertex generates its offsprings, whose number is proportional to the degree of that vertex with proportionality constant m-1 (m>1). We consider the two cases: first, each offspring is connected to its parent vertex only, forming a tree structure, and secondly, it is connected to both its parent and grandparent vertices, forming a loop structure. We find that both models exhibit power-law behaviors in their degree distributions with the exponent $γ=1+\ln (2m-1)/\ln m$. Thus, by tuning m, the degree exponent can be adjusted in the range, $2 <γ< 3$. We also solve analytically a mean shortest-path distance d between two vertices for the tree structure, showing the small-world behavior, that is, $d\sim \ln N/\ln {\bar k}$, where N is system size, and $\bar k$ is the mean degree. Finally, we consider the case that the number of offsprings is the same for all vertices, and find that the degree distribution exhibits an exponential-decay behavior.

cond-mat.stat-mech↗

arXiv2001

Gauged Nambu-Jona Lasinio Studies of the Triviality of Quantum Electrodynamics

By adding a small, irrelevant four fermi interaction to the action of noncompact lattice Quantum Electrodynamics (QED), the theory can be simulated with massless quarks in a vacuum free of lattice monopoles. The lattice theory possesses a second order chiral phase transition which we show is logarithmically trivial, with the same systematics as the Nambu-Jona Lasinio model. The irrelevance of the four fermi coupling is established numerically. The widths of the scaling windows are examined in both the coupling constant and bare fermion mass directions in parameter space. For vanishing fermion mass we find a broad scaling window in coupling. By adding a small bare fermion mass to the action we find that the width of the scaling window in the fermion mass direction is very narrow. Only when a subdominant scaling term is added to the leading term of the equation of state are adequate fits to the data possible. The failure of past studies of lattice QED to produce equation of state fits with adequate confidence levels to seriously address the question of triviality is explained. The vacuum state of the lattice model is probed for topological excitations, such as lattice Monopoles and Dirac strings, and these objects are shown to be non-critical along the chiral transition line as long as the four fermi coupling is nonzero.

hep-lat↗

arXiv2000

On the Triviality of Textbook Quantum Electrodynamics

By adding a small, irrelevant four fermi interaction to the action of lattice Quantum Electrodynamics (QED), the theory can be simulated with massless quarks in a vacuum free of lattice monopoles. This allows an ab initio high precision, controlled study of the existence of "textbook" Quantum Electrodynamics with several species of fermions. The lattice theory possesses a second order chiral phase transition which we show is logarithmically trivial. The logarithms of triviality, which modify mean field scaling laws, are pinpointed in several observables. The result supports Landau's contention that perturbative QED suffers from complete screening and would have a vanishing fine structure constant in the absence of a cutoff.

hep-lat↗

arXiv2000

HI Imaging of the Large Magellanic Cloud

We present results from the combined Australia Telescope Compact Array (ATCA) aperture synthesis mosaic survey of HI emission in the Large Magellanic Cloud with 64m Parkes single dish telescope observations.

astro-ph↗

arXiv2000

Pinholes May Mimic Tunneling

Interest in magnetic-tunnel junctions has prompted a re-examination of tunneling measurements through thin insulating films. In any study of metal-insulator-metal trilayers, one tries to eliminate the possibility of pinholes (small areas over which the thickness of the insulator goes to zero so that the upper and lower metals of the trilayer make direct contact). Recently, we have presented experimental evidence that ferromagnet-insulator-normal trilayers that appear from current-voltage plots to be pinhole-free may nonetheless in some cases harbor pinholes. Here, we show how pinholes may arise in a simple but realistic model of film deposition and that purely classical conduction through pinholes may mimic one aspect of tunneling, the exponential decay in current with insulating thickness.

cond-mat.mtrl-sci↗

arXiv1998

Source Behavior for ATM ABR Traffic Management: An Explanation

The Available Bit Rate (ABR) service has been developed to support data applications over Asynchronous Transfer Mode (ATM) networks. The network continuously monitors its traffic and provides feedback to the source end systems. This paper explains the rules that the sources have to follow to achieve a fair and efficient allocation of network resources.

cs.NI↗