Dynamically-Coupled Oscillators -- Cooperative Behavior via Dynamical Interaction --

We propose a theoretical framework to study the cooperative behavior of dynamically coupled oscillators (DCOs) that possess dynamical interactions. Then, to understand synchronization phenomena in networks of interneurons which possess inhibitory interactions, we propose a DCO model with dynamics of interactions that tend to cause 180-degree phase lags. Employing an approach developed here, we demonstrate that although our model displays synchronization at high frequencies, it does not exhibit synchronization at low frequencies because this dynamical interaction does not cause a phase lag sufficiently large to cancel the effect of the inhibition. We interpret the disappearance of synchronization in our model with decreasing frequency as describing the breakdown of synchronization in the interneuron network of the CA1 area below the critical frequency of 20 Hz.Comment: 10 pages, 3 figure

Starspot Jitter in Photometry, Astrometry and Radial Velocity Measurements

Analytical relations are derived for the amplitude of astrometric, photometric and radial velocity perturbations caused by a single rotating spot. The relative power of the star spot jitter is estimated and compared with the available data for $\kappa^1$ Ceti and HD 166435, as well as with numerical simulations for $\kappa^1$ Ceti and the Sun. A Sun-like star inclined at i=90\degr at 10 pc is predicted to have a RMS jitter of 0.087 \uas in its astrometric position along the equator, and 0.38 m s$^{-1}$ in radial velocities. If the presence of spots due to stellar activity is the ultimate limiting factor for planet detection, the sensitivity of SIM Lite to Earth-like planets in habitable zones is about an order of magnitude higher that the sensitivity of prospective ultra-precise radial velocity observations of nearby stars.Comment: accepted in ApJ Letters, Nov. 200

First Keck Nulling Observations of a Young Stellar Object: Probing the Circumstellar Environment of the Herbig Ae star MWC 325

We present the first N-band nulling plus K- and L-band V2 observations of a young stellar object, MWC325, taken with the 85 m baseline Keck Interferometer. The Keck nuller was designed for the study of faint dust signatures associated with debris disks, but it also has a unique capability for studying the temperature and density distribution of denser disks found around young stellar objects. Interferometric observations of MWC 325 at K, L and N encompass a factor of five in spectral range and thus, especially when spectrally dispersed within each band, enable characterization of the structure of the inner disk regions where planets form. Fitting our observations with geometric models such as a uniform disk or a Gaussian disk show that the apparent size increases monotonically with wavelength in the 2-12 um wavelength region, confirming the widely held assumption based on radiative transfer models, now with spatially resolved measurements over broad wavelength range, that disks are extended with a temperature gradient. The effective size is a factor of about 1.3 and 2 larger in the L-band and N-band, respectively, compared to that in the K-band. The existing interferometric measurements and the spectral energy distribution can be reproduced by a flat disk or a weakly-shadowed nearly flat-disk model, with only slight flaring in the outer regions of the disk, consisting of representative "sub-micron" (0.1 um) and "micron" (2 um) grains of a 50:50 ratio of silicate and graphite. This is marked contrast with the disks previously found in other Herbig Ae/Be stars suggesting a wide variety in the disk properties among Herbig Ae/Be stars.Comment: Accepted for publication in the Ap

Detectability of Terrestrial Planets in Multi-Planet Systems: Preliminary Report

We ask if Earth-like planets (terrestrial mass and habitable-zone orbit) can be detected in multi-planet systems, using astrometric and radial velocity observations. We report here the preliminary results of double-blind calculations designed to answer this question.Comment: 10 pages, 0 figure

Measurement of HO2 and other trace gases in the stratosphere using a high resolution far-infrared spectrometer at 28 km

This report covers the time period 1 January 1993 to 30 June 1993. During this reporting period we had our third Upper Atmosphere Research Satellite (UARS) correlative balloon flight and submitted the results from this flight to the Central Data Handling Facility (CDHF). We made a number of improvements in our data processing software in preparation for a new analysis of our old balloon data sets. Finally, we continue to analyze the data obtained during the second Airborne Arctic Stratospheric Expedition (AASE 2)

SMART precision interferometry at 794 nm

Single-mode fibers have been used in the near-infrared to dramatically reduce calibration error for long-baseline interferometry. We have begun an effort to apply the advantages of spatial filtering at visible wavelengths for precision measurements of pulsating Cepheids using the IOTA interferometer. Rather than employing photometric taps to calibrate fluctuating coupling efficiency, we are using an "asymmetric" coupler which allows this calibration to be done without losing photons. The Single-Mode Asymmetric Recombination Technique (SMART) experiment has finished lab-testing, and has been installed at IOTA for full commissioning in Summer 2002. We report the results of lab characterization and first sky tests, as well as first fringes on a star using a visible-wavelength single-mode coupler. With both lab and sky experience using unpolarized light, we have found that circular silica fibers are quite practical for precision interferometric measurements. We conclude that circular fibers (as opposed to polarization maintaining fibers) have an undeserved poor reputation and that birefringence effects pose no significant difficulty

Neural Network Model for Apparent Deterministic Chaos in Spontaneously Bursting Hippocampal Slices

A neural network model that exhibits stochastic population bursting is studied by simulation. First return maps of inter-burst intervals exhibit recurrent unstable periodic orbit (UPO)-like trajectories similar to those found in experiments on hippocampal slices. Applications of various control methods and surrogate analysis for UPO-detection also yield results similar to those of experiments. Our results question the interpretation of the experimental data as evidence for deterministic chaos and suggest caution in the use of UPO-based methods for detecting determinism in time-series data.Comment: 4 pages, 5 .eps figures (included), requires psfrag.sty (included