Homogeneous SPC/E water nucleation in large molecular dynamics simulations

We perform direct large molecular dynamics simulations of homogeneous SPC/E water nucleation, using up to $\sim 4\cdot 10^6$ molecules. Our large system sizes allow us to measure extremely low and accurate nucleation rates, down to $\sim 10^{19}\,\textrm{cm}^{-3}\textrm{s}^{-1}$, helping close the gap between experimentally measured rates $\sim 10^{17}\,\textrm{cm}^{-3}\textrm{s}^{-1}$. We are also able to precisely measure size distributions, sticking efficiencies, cluster temperatures, and cluster internal densities. We introduce a new functional form to implement the Yasuoka-Matsumoto nucleation rate measurement technique (threshold method). Comparison to nucleation models shows that classical nucleation theory over-estimates nucleation rates by a few orders of magnitude. The semi-phenomenological nucleation model does better, under-predicting rates by at worst, a factor of 24. Unlike what has been observed in Lennard-Jones simulations, post-critical clusters have temperatures consistent with the run average temperature. Also, we observe that post-critical clusters have densities very slightly higher, $\sim 5\%$, than bulk liquid. We re-calibrate a Hale-type $J$ vs. $S$ scaling relation using both experimental and simulation data, finding remarkable consistency in over $30$ orders of magnitude in the nucleation rate range, and $180\,$K in the temperature range.Comment: Accepted for publication in the Journal of Chemical Physic

Synchrotron X-ray emission from old pulsars

We study the synchrotron radiation as the observed non-thermal X-ray emission from old pulsars ($\gtrsim1-10$Myr) to investigate the particle acceleration in their magnetospheres. We assume that the power-law component of the observed X-ray spectra is caused by the synchrotron radiation from electrons and positrons in the magnetosphere. We consider two pair production mechanisms of X-ray emitting particles, the magnetic and the photon-photon pair productions. High-energy photons, which ignite the pair production, are emitted via the curvature radiation of the accelerated particles. We use the analytical description for the radiative transfer and estimate the luminosity of the synchrotron radiation. We find that for pulsars with the spin-down luminosity $L_{\rm sd}\lesssim10^{33}$ erg s$^{-1}$, the locations of the particle acceleration and the non-thermal X-ray emission are within $\lesssim10^7$cm from the centre of the neutron star, where the magnetic pair production occurs. For pulsars with the spin-down luminosity $L_{\rm sd}\lesssim10^{31}$ erg s$^{-1}$ such as J0108-1431, the synchrotron radiation is difficult to explain the observed non-thermal component even if we consider the existence of the strong and small-scale surface magnetic field structures.Comment: 25 pages, 7 figures, 2 tables, accepted for publication in MNRA

Loopy belief propagation and probabilistic image processing

Estimation of hyperparameters by maximization of the marginal likelihood in probabilistic image processing is investigated by using the cluster variation method. The algorithms are substantially equivalent to generalized loopy belief propagation

Absorption spectrum of (H2O)-O-18 in the range 12 400-14 520 cm(-1)

Fourier transform spectra recorded using (a) natural abundance water vapor, (b) (H2O)-O-18-enriched water vapor, and (c) (H2O)-O-17-enriched water vapor are analyzed. The ratio of intensities in three spectra is used to identify 927 lines due to absorption by (H2O)-O-18. Intensities and self-broadening parameters are derived for these lines. Using theoretical linelists, comparisons with previously assigned (H2O)-O-16 spectra, and automatic searches for combination differences, 747 lines are assigned. These lines belong to 14 vibrational states in the 3nu + delta and 4nu polyads. Newly determined (H2O)-O-18 vibrational band origins include 4nu(1) at 13 793.09 cm(-1), 3nu(1) + nu(3) at 13 795.40 cm(-1), 2nu(1) + 2nu(3) at 14 188.82 cm(-1), nu(1) + 3nu(3) at 14 276.34 cm(-1), and 2nu(2) + 2nu(2) + nu(3) at 13 612.71 cm(-1). These results are compared with data in HITRAN. (C) 2002 Elsevier Science (USA)