Domain wall displacement triggered by an AC current below threshold

It is theoretically demonstrated that a displacement of a pinned domain wall, typically of order of $\mu$m, can be driven by use of an ac current which is below threshold value. The point here is that finite motion around the pinning center by a low current is enhanced significantly by the resonance if the frequency is tuned close to the pinning frequency as demonstrated by recent experiment

Effect of habitat structural complexity on collembolan communities

We investigated soil microarthropod communities in two physically dissimilar inorganic soil materials and in a mixture of these two materials to examine the effect of the structural complexity of a habitat on microarthropod abundance and communities, teasing it out from that of nutritional factors. Mesh boxes were filled with perlite (a highly porous material), similar size of granite gravels (no pores inside), or their mixture, and placed on a forest floor. The boxes were collected after 8 or 20 months, and the microarthropods were extracted and identified to the species level, with a focus on Collembola. We also evaluated fine-root biomass and the amount of organic matter in the boxes. It was found that the mixture of perlite and granite enhanced microarthropod abundance and root development. A partial redundancy analysis revealed that collembolan communities developed differently among the substrate materials. We also found that variation in the collembolan communities was related to fine-root development and the abundance of other microarthropods, implying that habitat structural complexity affects collembolan communities indirectly by affecting soil food webs

Exact Algorithms for B-Bandwidth Problem with Restricted B

The B-BANDWIDTH problem is a decision problem whether the bandwidth of a given graph is smaller than B, and it is NP-complete even if the graph is a small graph class of trees. Cygan and Pilipczuk proposed exponential time and space algorithms for B-BANDWIDTH with n/3 ≤ B where n is the number of vertices. In this paper, we propose two algorithms for the B-BANDWIDTH problem with n/4 ≤ B < n/3. These algorithms are extension of Cygan and Pilipczuk algorithms with restricted B. One of the algorithms takes O∗(4.5n) time and O∗(1.5n) space when n/4 ≤ B < n / 2 log2 1.5, and the other takes O∗(4.77n) time and O∗(1.59n) space when n / 2 log2 1.5 ≤ B < n/3. Our algorithms are fastest O∗(2n) space algorithms for n/4 ≤B < n/3.The 17th Korea-Japan Joint Workshop on Algorithms and Computation, July 13-15, 2014, Okinawa, Japa

Shock-induced star cluster formation in colliding galaxies

We studied the formation process of star clusters using high-resolution N-body/smoothed particle hydrodynamcs simulations of colliding galaxies. The total number of particles is 1.2x10^8 for our high resolution run. The gravitational softening is 5 pc and we allow gas to cool down to \sim 10 K. During the first encounter of the collision, a giant filament consists of cold and dense gas found between the progenitors by shock compression. A vigorous starburst took place in the filament, resulting in the formation of star clusters. The mass of these star clusters ranges from 10^{5-8} Msun. These star clusters formed hierarchically: at first small star clusters formed, and then they merged via gravity, resulting in larger star clusters.Comment: 4 pages, 3 figures, Proceedings of IAU Symposium 270, Computational Star Formatio

Toward first-principle simulations of galaxy formation: I. How should we choose star formation criteria in high-resolution simulations of disk galaxies?

We performed 3-dimensional N-body/SPH simulations to study how mass resolution and other model parameters such as the star formation efficiency parameter, C* and the threshold density, nth affect structures of the galactic gaseous/stellar disk in a static galactic potential. We employ 10^6 - 10^7 particles to resolve a cold and dense (T 100 cm^{-3}) phase. We found that structures of the ISM and the distribution of young stars are sensitive to the assumed nth. High-nth models with nth = 100 cm^{-3} yield clumpy multi-phase features in the ISM. Young stars are distributed in a thin disk of which half-mass scale height is 10 - 30 pc. In low-nth models with nth = 0.1 cm^{-3}, the stellar disk is found to be several times thicker, and the gas disk appears smoother than the high-nth models. A high-resolution simulation with high-nth is necessary to reproduce the complex structure of the gas disk. The global properties of the model galaxies in low-nth models, such as star formation histories, are similar to those in the high-nth models when we tune the value of C* so that they reproduce the observed relation between surface gas density and surface star formation rate density. We however emphasize that high-nth models automatically reproduce the relation, regardless of the values of C*. The ISM structure, phase distribution, and distributions of young star forming region are quite similar between two runs with values of C* which differ by a factor of 15. We also found that the timescale of the flow from n_H ~1 cm^{-3} to n_H > 100 cm^{-3} is about 5 times as long as the local dynamical time and is independent of the value of C*. The use of a high-nth criterion for star formation in high-resolution simulations makes numerical models fairy insensitive to the modelling of star formation. (Abridged)Comment: 15 pages, 14 figures, accepted for publication in PASJ. Abridged abstract. For high resolution figures, see http://www.cfca.nao.ac.jp/~saitoh/Papers/2008/Saitoh+2008a.pd

Toward First-Principle Simulations of Galaxy Formation: II. Shock-Induced Starburst at a Collision Interface During the First Encounter of Interacting Galaxies

We investigated the evolution of interacting disk galaxies using high-resolution $N$-body/SPH simulations, taking into account the multiphase nature of the interstellar medium (ISM). In our high-resolution simulations, a large-scale starburst occurred naturally at the collision interface between two gas disks at the first encounter, resulting in the formation of star clusters. This is consistent with observations of interacting galaxies. The probability distribution function (PDF) of gas density showed clear change during the galaxy-galaxy encounter. The compression of gas at the collision interface between the gas disks first appears as an excess at $n_{\rm H} \sim 10{\rm cm^{-3}}$ in the PDF, and then the excess moves to higher densities ($n_{\rm H} \gtrsim 100{\rm cm^{-3}}$) in a few times $10^7$ years where starburst takes place. After the starburst, the PDF goes back to the quasi-steady state. These results give a simple picture of starburst phenomena in galaxy-galaxy encounters.Comment: 6 pages, 6 figures, accepted to PASJ. For high resolution figures, see http://www.cfca.nao.ac.jp/~saitoh/Papers/2009/Saitoh+2009a.pd

Switching the centromeres on and off: epigenetic chromatin alterations provide plasticity in centromere activity stabilizing aberrant dicentric chromosomes

Abstract The kinetochore, which forms on a specific chromosomal locus called the centromere, mediates interactions between the chromosome and the spindle during mitosis and meiosis. Abnormal chromosome rearrangements and/or neocentromere formation can cause the presence of multiple centromeres on a single chromosome, which results in chromosome breakage or cell cycle arrest. Analyses of artificial dicentric chromosomes suggested that the activity of the centromere is regulated epigenetically; on some stably maintained dicentric chromosomes, one of the centromeres no longer functions as a platform for kinetochore formation, although the DNA sequence remains intact. Such epigenetic centromere inactivation occurs in cells of various eukaryotes harbouring &apos;regional centromeres&apos;, such as those of maize, fission yeast and humans, suggesting that the position of the active centromere is determined by epigenetic markers on a chromosome rather than the nucleotide sequence. Our recent findings in fission yeast revealed that epigenetic centromere inactivation consists of two steps: disassembly of the kinetochore initiates inactivation and subsequent heterochromatinization prevents revival of the inactivated centromere. Kinetochore disassembly followed by heterochromatinization is also observed in normal senescent human cells. Thus epigenetic centromere inactivation may not only stabilize abnormally generated dicentric chromosomes, but also be part of an intrinsic mechanism regulating cell proliferation