Analysis of the loop length distribution for the negative weight percolation problem in dimensions d=2 through 6

We consider the negative weight percolation (NWP) problem on hypercubic lattice graphs with fully periodic boundary conditions in all relevant dimensions from d=2 to the upper critical dimension d=6. The problem exhibits edge weights drawn from disorder distributions that allow for weights of either sign. We are interested in in the full ensemble of loops with negative weight, i.e. non-trivial (system spanning) loops as well as topologically trivial ("small") loops. The NWP phenomenon refers to the disorder driven proliferation of system spanning loops of total negative weight. While previous studies where focused on the latter loops, we here put under scrutiny the ensemble of small loops. Our aim is to characterize -using this extensive and exhaustive numerical study- the loop length distribution of the small loops right at and below the critical point of the hypercubic setups by means of two independent critical exponents. These can further be related to the results of previous finite-size scaling analyses carried out for the system spanning loops. For the numerical simulations we employed a mapping of the NWP model to a combinatorial optimization problem that can be solved exactly by using sophisticated matching algorithms. This allowed us to study here numerically exact very large systems with high statistics.Comment: 7 pages, 4 figures, 2 tables, paper summary available at http://www.papercore.org/Kajantie2000. arXiv admin note: substantial text overlap with arXiv:1003.1591, arXiv:1005.5637, arXiv:1107.174

MSX versus IRAS Two-Color Diagrams and the CSE-Sequence of Oxygen-Rich Late-Type Stars

We present MSX two-color diagrams that can be used to characterize circumstellar environments of sources with good quality MSX colors in terms of IRAS color regions for oxygen-rich stars. With these diagrams we aim to provide a new tool that can be used to study circumstellar environments and to improve detection rates for targeted surveys for circumstellar maser emission similar to the IRAS two-color diagram. This new tool is especially useful for regions in the sky where IRAS was confused, in particular in the Galactic plane and bulge region. Unfortunately, using MSX colors alone does not allow to distinguish between carbon-rich and oxygen-rich objects. An application of this tool on 86 GHz SiO masers shows that for this type of masers an instantaneous detection rate of 60% to 80% can be achieved if target sources are selected according to MSX color (region). Our investigations may have revealed an error in the MSX point source catalog version 2.3. That is, the photometry of the 21.3 $\mu$m (MSX E filter) band for most weak 8.28 $\mu$m (or MSX A filter) band sources seems off by about a factor two (0.5--1 magnitude too bright).Comment: accepted to Astrophysical Journal, 11 pages, 11 figs of which 1 in colo

A highly-collimated SiO jet in the HH212 protostellar outflow

We mapped the HH212 Class 0 outflow in SiO(2--1, 5--4) and continuum using the PdBI in its extended configurations. The unprecedented angular resolution (down to 0.34") allows accurate comparison with a new, deep H2 image obtained at the VLT. The SiO emission is confined to a highly-collimated bipolar jet (width 0.35") along the outflow axis. The jet can be traced down to within 500 AU of the protostar, in a region that is heavily obscured in H2 images. Where both species are detected, SiO shows the same overall kinematics and structure as H2, indicating that both molecules are tracing the same material. We find that the high-velocity SiO gas near the protostar is not tracing a wide-angle wind but is already confined to a flow inside a narrow cone of half-opening angle < 6 deg.Comment: Astronomy and Astrophysics Letter, in pres

Excited-state OH Mainline Masers in AU Geminorum and NML Cygni

Excited-state OH maser emission has previously been reported in the circumstellar envelopes of only two evolved stars: the Mira star AU Geminorum and the hypergiant NML Cygni. We present Very Large Array (VLA) observations of the 1665, 1667, and excited-state 4750 MHz mainline OH transitions in AU Gem and Expanded Very Large Array (EVLA) observations of the excited-state 6030 and 6035 MHz OH mainline transitions in NML Cyg. We detect masers in both mainline transitions in AU Gem but no excited-state emission in either star. We conclude that the excited-state OH emission in AU Gem is either a transient phenomenon (such as for NML Cyg outlined below), or possibly an artifact in the data, and that the excited state OH emission in NML Cyg was generated by an episode of enhanced shock between the stellar mass-loss and an outflow of the Cyg OB2 association. With these single exceptions, it therefore appears that excited-state OH emission indeed should not be predicted nor observable in evolved stars as part of their normal structure or evolution.Comment: ApJ Letter, accepted, 4 pages, 2 figure

Memory difference control of unknown unstable fixed points: Drifting parameter conditions and delayed measurement

Difference control schemes for controlling unstable fixed points become important if the exact position of the fixed point is unavailable or moving due to drifting parameters. We propose a memory difference control method for stabilization of a priori unknown unstable fixed points by introducing a memory term. If the amplitude of the control applied in the previous time step is added to the present control signal, fixed points with arbitrary Lyapunov numbers can be controlled. This method is also extended to compensate arbitrary time steps of measurement delay. We show that our method stabilizes orbits of the Chua circuit where ordinary difference control fails.Comment: 5 pages, 8 figures. See also chao-dyn/9810029 (Phys. Rev. E 70, 056225) and nlin.CD/0204031 (Phys. Rev. E 70, 046205

The magnetic field of IRAS 16293-2422 as traced by shock-induced H2O masers

Shock-induced H2O masers are important magnetic field tracers at very high density gas. Water masers are found in both high- and low-mass star-forming regions, acting as a powerful tool to compare magnetic field morphologies in both mass regimes. In this paper, we show one of the first magnetic field determinations in the low-mass protostellar core IRAS 16293-2422 at volume densities as high as 10^(8-10) cm^-3. Our goal is to discern if the collapsing regime of this source is controlled by magnetic fields or other factors like turbulence. We used the Very Large Array (VLA) to carry out spectro-polarimetric observations in the 22 GHz Zeeman emission of H2O masers. From the Stokes V line profile, we can estimate the magnetic field strength in the dense regions around the protostar. A blend of at least three maser features can be inferred from our relatively high spatial resolution data set (~ 0.1"), which is reproduced in a clear non-Gaussian line profile. The emission is very stable in polarization fraction and position angle across the channels. The maser spots are aligned with some components of the complex outflow configuration of IRAS 16293-2422, and they are excited in zones of compressed gas produced by shocks. The post-shock particle density is in the range of 1-3 x 10^9 cm^-3, consistent with typical water masers pumping densities. Zeeman emission is produced by a very strong line-of-sight magnetic field (B ~ 113 mG). The magnetic field pressure derived from our data is comparable to the ram pressure of the outflow dynamics. This indicates that the magnetic field is energetically important in the dynamical evolution of IRAS 16293-2422.Comment: 7 pages, 6 figures, accepted for publication in A&

Stability of the trapped nonconservative Gross-Pitaevskii equation with attractive two-body interaction

The dynamics of a nonconservative Gross-Pitaevskii equation for trapped atomic systems with attractive two-body interaction is numerically investigated, considering wide variations of the nonconservative parameters, related to atomic feeding and dissipation. We study the possible limitations of the mean field description for an atomic condensate with attractive two-body interaction, by defining the parameter regions where stable or unstable formation can be found. The present study is useful and timely considering the possibility of large variations of attractive two-body scattering lengths, which may be feasible in recent experiments.Comment: 6 pages, 5 figures, submitted to Physical Review