Photometric Observations of Star Formation Activity in Early Type Spirals

We observationally study the current star formation activities of early type spiral galaxies. We construct a complete sample of 15 early type spirals having far-infrared (FIR) to optical B band luminosity ratios, L(FIR)/L(B), larger than the average of the type, and make their CCD imaging of the R and H-alpha bands. The equivalent widths of H-alpha emission increase with increasing L(FIR)/L(B), indicating that L(FIR)/L(B) can be an indicator of star formation for such early type spirals with star formation activities higher than the average. For all of the observed early type spirals, the extended HII regions exist at the central regions with some asymmetric features. H-alpha emission is more concentrated to the galactic center than the R band light, and the degree of the concentration increases with the star formation activity. We also analyze the relation between the star formation activities and the existence of companion galaxies in the sample galaxies and other bright early type spirals. No correlation is found and this suggests that the interaction is not responsible for all of the star formation activities of early type spirals.Comment: LaTex, 23 pages (2 tables included), plus 9 Postscript figures & 1 table. To be published in AJ (November issue

Finite-size Scaling of Correlation Ratio and Generalized Scheme for the Probability-Changing Cluster Algorithm

We study the finite-size scaling (FSS) property of the correlation ratio, the ratio of the correlation functions with different distances. It is shown that the correlation ratio is a good estimator to determine the critical point of the second-order transition using the FSS analysis. The correlation ratio is especially useful for the analysis of the Kosterlitz-Thouless (KT) transition. We also present a generalized scheme of the probability-changing cluster algorithm, which has been recently developed by the present authors, based on the FSS property of the correlation ratio. We investigate the two-dimensional quantum XY model of spin 1/2 with this generalized scheme, obtaining the precise estimate of the KT transition temperature with less numerical effort.Comment: 4 pages, RevTeX4, to appear in Phys. Rev. B, Rapid Communication

Probability-Changing Cluster Algorithm: Study of Three-Dimensional Ising Model and Percolation Problem

We present a detailed description of the idea and procedure for the newly proposed Monte Carlo algorithm of tuning the critical point automatically, which is called the probability-changing cluster (PCC) algorithm [Y. Tomita and Y. Okabe, Phys. Rev. Lett. {\bf 86} (2001) 572]. Using the PCC algorithm, we investigate the three-dimensional Ising model and the bond percolation problem. We employ a refined finite-size scaling analysis to make estimates of critical point and exponents. With much less efforts, we obtain the results which are consistent with the previous calculations. We argue several directions for the application of the PCC algorithm.Comment: 6 pages including 8 eps figures, to appear in J. Phys. Soc. Jp

Near-infrared photoabsorption by C(60) dianions in a storage ring

We present a detailed study of the electronic structure and the stability of C(60) dianions in the gas phase. Monoanions were extracted from a plasma source and converted to dianions by electron transfer in a Na vapor cell. The dianions were then stored in an electrostatic ring, and their near-infrared absorption spectrum was measured by observation of laser induced electron detachment. From the time dependence of the detachment after photon absorption, we conclude that the reaction has contributions from both direct electron tunneling to the continuum and vibrationally assisted tunneling after internal conversion. This implies that the height of the Coulomb barrier confining the attached electrons is at least similar to 1.5 eV. For C(60)(2-) ions in solution electron spin resonance measurements have indicated a singlet ground state, and from the similarity of the absorption spectra we conclude that also the ground state of isolated C(60)(2-) ions is singlet. The observed spectrum corresponds to an electronic transition from a t(1u) lowest unoccupied molecular orbital (LUMO) of C(60) to the t(1g) LUMO+1 level. The electronic levels of the dianion are split due to Jahn-Teller coupling to quadrupole deformations of the molecule, and a main absorption band at 10723 cm(-1) corresponds to a transition between the Jahn-Teller ground states. Also transitions from pseudorotational states with 200 cm(-1) and (probably) 420 cm(-1) excitation are observed. We argue that a very broad absorption band from about 11 500 cm(-1) to 13 500 cm(-1) consists of transitions to so-called cone states, which are Jahn-Teller states on a higher potential-energy surface, stabilized by a pseudorotational angular momentum barrier. A previously observed, high-lying absorption band for C(60)(-) may also be a transition to a cone state

Lifetimes of C-60(2-) and C-70(2-) dianions in a storage ring

C-60(2-) and C-70(2-) dianions have been produced by electrospray of the monoanions and subsequent electron pickup in a Na vapor cell. The dianions were stored in an electrostatic ring and their decay by electron emission was measured up to 1 s after injection. While C-70(2-) ions are stable on this time scale, except for a small fraction of the ions which have been excited by gas collisions, most of the C-60(2-) ions decay on a millisecond time scale, with a lifetime depending strongly on their internal temperature. The results can be modeled as decay by electron tunneling through a Coulomb barrier, mainly from thermally populated triplet states about 120 meV above a singlet ground state. At times longer than about 100 ms, the absorption of blackbody radiation plays an important role for the decay of initially cold ions. The tunneling rates obtained from the modeling, combined with WKB estimates of the barrier penetration, give a ground-state energy 200 +/- 30 meV above the energy of the monoanion plus a free electron and a ground-state lifetime of the order of 20 s. (c) 2006 American Institute of Physics

Probability-Changing Cluster Algorithm for Potts Models

We propose a new effective cluster algorithm of tuning the critical point automatically, which is an extended version of Swendsen-Wang algorithm. We change the probability of connecting spins of the same type, $p = 1 - e^{- J/ k_BT}$, in the process of the Monte Carlo spin update. Since we approach the canonical ensemble asymptotically, we can use the finite-size scaling analysis for physical quantities near the critical point. Simulating the two-dimensional Potts models to demonstrate the validity of the algorithm, we have obtained the critical temperatures and critical exponents which are consistent with the exact values; the comparison has been made with the invaded cluster algorithm.Comment: 4 pages including 5 eps figures, RevTeX, to appear in Phys. Rev. Let

Crossover and self-averaging in the two-dimensional site-diluted Ising model

Using the newly proposed probability-changing cluster (PCC) Monte Carlo algorithm, we simulate the two-dimensional (2D) site-diluted Ising model. Since we can tune the critical point of each random sample automatically with the PCC algorithm, we succeed in studying the sample-dependent $T_c(L)$ and the sample average of physical quantities at each $T_c(L)$ systematically. Using the finite-size scaling (FSS) analysis for $T_c(L)$, we discuss the importance of corrections to FSS both in the strong-dilution and weak-dilution regions. The critical phenomena of the 2D site-diluted Ising model are shown to be controlled by the pure fixed point. The crossover from the percolation fixed point to the pure Ising fixed point with the system size is explicitly demonstrated by the study of the Binder parameter. We also study the distribution of critical temperature $T_c(L)$. Its variance shows the power-law $L$ dependence, $L^{-n}$, and the estimate of the exponent $n$ is consistent with the prediction of Aharony and Harris [Phys. Rev. Lett. {\bf 77}, 3700 (1996)]. Calculating the relative variance of critical magnetization at the sample-dependent $T_c(L)$, we show that the 2D site-diluted Ising model exhibits weak self-averaging.Comment: 6 pages including 6 eps figures, RevTeX, to appear in Phys. Rev.

Magnetization switching in a Heisenberg model for small ferromagnetic particles

We investigate the thermally activated magnetization switching of small ferromagnetic particles driven by an external magnetic field. For low uniaxial anisotropy the spins can be expected to rotate coherently, while for sufficient large anisotropy they should behave Ising-like, i.e., the switching should then be due to nucleation. We study this crossover from coherent rotation to nucleation for the classical three-dimensional Heisenberg model with a finite anisotropy. The crossover is influenced by the size of the particle, the strength of the driving magnetic field, and the anisotropy. We discuss the relevant energy barriers which have to be overcome during the switching, and find theoretical arguments which yield the energetically favorable reversal mechanisms for given values of the quantities above. The results are confirmed by Monte Carlo simulations of Heisenberg and Ising models.Comment: 8 pages, Revtex, 11 Figures include

JHK′JHK' Imaging Photometry of Seyfert 1 AGNs and Quasars II: Observation of Long-Term Variability

Observations of 226 AGNs in the near-infrared $J$, $H$, and $K'$ bands are presented along with the analysis of the observations for variability. Our sample consists mainly of Seyfert 1 AGNs and QSOs. About a quarter of the objects in each category are radio loud. The AGNs in the entire sample have the redshifts spanning the range from $z=0$ to 1, and the absolute magnitudes from $M_B=-29$ to -18. All the objects were observed twice and their variability was measured by differential photometry. A reduction method of differential photometry, optimized to the analysis of extended images, has been developed. The systematic error in variability arising from AGNs of highly extended images is estimated to be less than 0.01 mag in each of the $J$, $H$, and $K'$ bands. The systematic error arising from the flat fielding is negligible for most AGNs, although it is more than 0.1 mag for some particular cases. The overall average flat fielding error is 0.03 mag for the image pairs. We find that these systematic errors are superseded by statistical errors, and the overall average total systematic and statistical errors amounts to 0.05 mag in the measured variability in each band. We find that 58% of all the AGNs in the entire sample show variability of more than $2\sigma$, and 44% of more than $3\sigma$. This result holds independent of the $J$, $H$, and $K'$ bands. The detection rate of variability is higher for a subsample of higher photometric accuracy, and there appears no limit to this tendency. In particular, when we consider a subsample with small photometric errors of $\sigma<0.03$ mag, the rate of $2\sigma$ detection is 80%, and 64% for $3\sigma$ detection. This suggests that most AGNs are variable in the near-infrared

Cosmic Microwave Background anisotropies from second order gravitational perturbations

This paper presents a complete analysis of the effects of second order gravitational perturbations on Cosmic Microwave Background anisotropies, taking explicitly into account scalar, vector and tensor modes. We also consider the second order perturbations of the metric itself obtaining them, for a universe dominated by a collision-less fluid, in the Poisson gauge, by transforming the known results in the synchronous gauge. We discuss the resulting second order anisotropies in the Poisson gauge, and analyse the possible relevance of the different terms. We expect that, in the simplest scenarios for structure formation, the main effect comes from the gravitational lensing by scalar perturbations, that is known to give a few percent contribution to the anisotropies at small angular scales.Comment: 15 pages, revtex, no figures. Version to be published in Phys. Rev.