6,575 research outputs found
Embedded Diagrams of Kerr and Non-Kerr black Holes
Embedded diagrams are drawn for investigating the black hole of Kerr and
non-Kerr metric. Kerr black holes are characterized by masses and spin
parameters . Non-Kerr black holes also are characterized by the deformation
parameters , which is related to shape of the black hole, in addition
to their masses and spins. In this paper, we investigate the behavior of
non-Kerr black holes compared with Kerr black holes in the parameter space of
(,) using embedded diagrams. The event horizons and the naked
singularity of non-Kerr BHs are discussed in detail.Comment: 4 pages, 6 figure
Age Estimation of Extragalactic Globular Cluster Systems Using H beta Index
After taking into account, for the first time, the detailed systematic
variation of horizontal-branch (HB) morphology with age and metallicity, our
population synthesis models show that the integrated H beta index is
significantly affected by the presence of blue HB stars. Our models indicate
that the strength of the H beta index increases as much as 0.75 A due to blue
HB stars. According to our models, a systematic difference between the globular
cluster system in the Milky Way Galaxy and that in NGC 1399 in the H beta vs.
Mg2 diagram is understood if globular cluster systems in giant elliptical
galaxies are couple of billion years older, in the mean, than the Galactic
counterpart.Comment: 3 pages, 2 figures. Contribution to `Astrophysical Ages and
Timescales' ASP Conf. Ser., Eds. T. von Hippel, N. Manset, C. Simpson, EPS
files are available at http://csaweb.yonsei.ac.kr/~hclee/Hawai
Nonlinear Color-Metallicity Relations of Globular Clusters. VIII. Reproducing Color Distributions of Individual Globular Cluster Systems in the Virgo and Fornax Galaxy Clusters
The color distributions of globular clusters (GCs) in individual early-type
galaxies show great diversity in their morphology. Based on the conventional
"linear" relationship between the colors and metallicities of GCs, the GC
metallicity distributions inferred from colors and in turn their formation
histories, should be as diverse as they appear. In contrast, here we show that
an alternative scenario rooted in the "nonlinear" nature of the
metallicity-to-color transformation points to a simpler and more coherent
picture. Our simulations of the color distributions for 80 GC systems in
early-type galaxies from the ACS Virgo and Fornax Cluster Surveys suggest that
the majority (70) of early-type galaxies have old (13 Gyr) and
coeval GCs. Their variety in the color distribution morphology stems mainly
from one parameter, the mean metallicity of a GC system. Furthermore, the color
distributions of the remaining (30) GC systems are also explained by
the nonlinearity scenario, assuming additional young or intermediate-age GCs
with a number fraction of 20 of underlying old GCs. Our results
reinforce the nonlinearity explanation for the GC color bimodality and provide
a new perspective on early-type galaxy formation in the cluster environment,
such as the Virgo and Fornax galaxy clusters.Comment: 33 pages, 7 figures, 3 tables, accepted for publication in ApJ
A general framework for compressed sensing and parallel MRI using annihilating filter based low-rank Hankel matrix
Parallel MRI (pMRI) and compressed sensing MRI (CS-MRI) have been considered
as two distinct reconstruction problems. Inspired by recent k-space
interpolation methods, an annihilating filter based low-rank Hankel matrix
approach (ALOHA) is proposed as a general framework for sparsity-driven k-space
interpolation method which unifies pMRI and CS-MRI. Specifically, our framework
is based on the fundamental duality between the transform domain sparsity in
the primary space and the low-rankness of weighted Hankel matrix in the
reciprocal space, which converts pMRI and CS-MRI to a k-space interpolation
problem using structured matrix completion. Using theoretical results from the
latest compressed sensing literatures, we showed that the required sampling
rates for ALOHA may achieve the optimal rate. Experimental results with in vivo
data for single/multi-coil imaging as well as dynamic imaging confirmed that
the proposed method outperforms the state-of-the-art pMRI and CS-MRI
Yonsei evolutionary population synthesis (YEPS). II. Spectro-photometric evolution of helium-enhanced stellar populations
The discovery of multiple stellar populations in Milky Way globular clusters
(GCs) has stimulated various follow-up studies on helium-enhanced stellar
populations. Here we present the evolutionary population synthesis models for
the spectro-photometric evolution of simple stellar populations (SSPs) with
varying initial helium abundance (). We show that
brings about {dramatic} changes in spectro-photometric properties of SSPs. Like
the normal-helium SSPs, the integrated spectro-photometric evolution of
helium-enhanced SSPs is also dependent on metallicity and age for a given
. {We discuss the implications and prospects for the
helium-enhanced populations in relation to the second-generation populations
found in the Milky Way GCs.} All of the models are available at
\url{http://web.yonsei.ac.kr/cosmic/data/YEPS.htm}.Comment: Accepted for publication in ApJ, 21 pages, 12 figures, 3 table
Which Contrast Does Matter? Towards a Deep Understanding of MR Contrast using Collaborative GAN
Thanks to the recent success of generative adversarial network (GAN) for
image synthesis, there are many exciting GAN approaches that successfully
synthesize MR image contrast from other images with different contrasts. These
approaches are potentially important for image imputation problems, where
complete set of data is often difficult to obtain and image synthesis is one of
the key solutions for handling the missing data problem. Unfortunately, the
lack of the scalability of the existing GAN-based image translation approaches
poses a fundamental challenge to understand the nature of the MR contrast
imputation problem: which contrast does matter? Here, we present a systematic
approach using Collaborative Generative Adversarial Networks (CollaGAN), which
enable the learning of the joint image manifold of multiple MR contrasts to
investigate which contrasts are essential. Our experimental results showed that
the exogenous contrast from contrast agents is not replaceable, but other
endogenous contrast such as T1, T2, etc can be synthesized from other contrast.
These findings may give important guidance to the acquisition protocol design
for MR in real clinical environment.Comment: 32 pages, 6 figure
Thermally activated switching of perpendicular magnet by spin-orbit spin torque
We theoretically investigate the threshold current for thermally activated
switching of a perpendicular magnet by spin-orbit spin torque. Based on the
Fokker-Planck equation, we obtain an analytic expression of the switching
current, in agreement with numerical result. We find that thermal energy
barrier exhibits a quasi-linear dependence on the current, resulting in an
almost linear dependence of switching current on the log-scaled current
pulse-width even below 10 ns. This is in stark contrast to standard spin torque
switching, where thermal energy barrier has a quadratic dependence on the
current and the switching current rapidly increases at short pulses. Our
results will serve as a guideline to design and interpret switching experiments
based on spin-orbit spin torqueComment: 18 pages, 4 figure
Threshold current for switching of a perpendicular magnetic layer induced by spin Hall effect
We theoretically investigate the switching of a perpendicular magnetic layer
by in-plane charge current due to the spin Hall effect. We find that, in the
high damping regime, the threshold switching current is independent of the
damping constant, and is almost linearly proportional to both effective
perpendicular magnetic anisotropy field and external in-plane field applied
along the current direction. We obtain an analytic expression of the threshold
current, in excellent agreement with numerical results. This expression can be
used to determine the physical quantities associated with spin Hall effect, and
to design relevant magnetic devices based on the switching of perpendicular
magnetic layers.Comment: 16 pages, 3 figure
Nonlinear Color--Metallicity Relations of Globular Clusters. VI. On Calcium II Triplet Based Metallicities of Globular Clusters in Early-type Galaxies
The metallicity distribution function of globular clusters (GCs) in galaxies
is a key to understanding galactic formation and evolution. The calcium II
triplet (CaT) index has recently become a popular metal abundance indicator
thanks to its sensitivity to GC metallicity. Here we revisit and assess the
reliability of CaT as a metallicity indicator using our new stellar population
synthesis simulations based on empirical, high-resolution fluxes. The model
shows that the CaT strength of old ( 10 Gyr) GCs is proportional to below . In the modest metal-rich regime, however, CaT does not
increase anymore with due to the little contribution from
coolest red giant stars to the CaT absorption. The nonlinear nature of the
color-- relation is confirmed by the observations of GCs in nearby
early-type galaxies. This indicates that the CaT should be used carefully when
deriving metallicities of metal-rich stellar populations. Our results offer an
explanation for the observed sharp difference between the color and
distributions of GCs in the same galaxies. We take this as an analogy to the
view that metallicity--color and metallicity--Lick index nonlinearity of GCs is
primarily responsible for their observed "bimodal" distributions of colors and
absorption indices.Comment: 27 pages, 10 figures, 2 tables, accepted for publication in Ap
Horizontal-Branch Stars as an Age Indicator
Surface temperature distribution of horizontal-branch (HB) stars is very
sensitive to age in old stellar systems, which makes it an attractive age
indicator. In this paper, we present the recent revision of our model
calculations for the HB morphology of globular clusters. The result is more
updated version of our earlier models (Lee, Demarque, & Zinn 1994), which
suggests that the HB morphology is more sensitive (\sim 40%) to age. We also
present our new model calculations on the effect of HB stars in dating old
stellar systems using the H index. Our results indicate that the effect
of HB stars is rather strong, and suggest a possibility for the systematic
difference in age between the globular clusters in the Milky Way Galaxy and
those in giant elliptical galaxies. Finally, we compare these results with the
relative ages estimated from our far-UV dating technique for giant elliptical
galaxies.Comment: 10 pages, 2 figures, Invited paper presented at the workshop on
"Spectrophotometric Dating of Stars & Galaxies", held at Annapolis, MD, USA,
in April 25-29, 199
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