A Data Exchange Standard for Optical (Visible/IR) Interferometry

This paper describes the OI Exchange Format, a standard for exchanging calibrated data from optical (visible/infrared) stellar interferometers. The standard is based on the Flexible Image Transport System (FITS), and supports storage of the optical interferometric observables including squared visibility and closure phase -- data products not included in radio interferometry standards such as UV-FITS. The format has already gained the support of most currently-operating optical interferometer projects, including COAST, NPOI, IOTA, CHARA, VLTI, PTI, and the Keck Interferometer, and is endorsed by the IAU Working Group on Optical Interferometry. Software is available for reading, writing and merging OI Exchange Format files.Comment: 26 pages, 1 figur

Generalized Centrifugal Force Model for Pedestrian Dynamics

A spatially continuous force-based model for simulating pedestrian dynamics is introduced which includes an elliptical volume exclusion of pedestrians. We discuss the phenomena of oscillations and overlapping which occur for certain choices of the forces. The main intention of this work is the quantitative description of pedestrian movement in several geometries. Measurements of the fundamental diagram in narrow and wide corridors are performed. The results of the proposed model show good agreement with empirical data obtained in controlled experiments.Comment: 10 pages, 14 figures, accepted for publication as a Regular Article in Physical Review E. This version contains minor change

Constraining Disk Parameters of Be Stars using Narrowband H-alpha Interferometry with the NPOI

Interferometric observations of two well-known Be stars, gamma Cas and phi Per, were collected and analyzed to determine the spatial characteristics of their circumstellar regions. The observations were obtained using the Navy Prototype Optical Interferometer equipped with custom-made narrowband filters. The filters isolate the H-alpha emission line from the nearby continuum radiation, which results in an increased contrast between the interferometric signature due to the H-alpha-emitting circumstellar region and the central star. Because the narrowband filters do not significantly attenuate the continuum radiation at wavelengths 50 nm or more away from the line, the interferometric signal in the H-alpha channel is calibrated with respect to the continuum channels. The observations used in this study represent the highest spatial resolution measurements of the H-alpha-emitting regions of Be stars obtained to date. These observations allow us to demonstrate for the first time that the intensity distribution in the circumstellar region of a Be star cannot be represented by uniform disk or ring-like structures, whereas a Gaussian intensity distribution appears to be fully consistent with our observations.Comment: 23 pages, 14 figures, accepted for publication in A

A measure of centrality based on the spectrum of the Laplacian

We introduce a family of new centralities, the k-spectral centralities. k-Spectral centrality is a measurement of importance with respect to the deformation of the graph Laplacian associated with the graph. Due to this connection, k-spectral centralities have various interpretations in terms of spectrally determined information. We explore this centrality in the context of several examples. While for sparse unweighted networks 1-spectral centrality behaves similarly to other standard centralities, for dense weighted networks they show different properties. In summary, the k-spectral centralities provide a novel and useful measurement of relevance (for single network elements as well as whole subnetworks) distinct from other known measures.Comment: 12 pages, 6 figures, 2 table

ISO observations of the Galactic center Interstellar Medium: neutral gas and dust

The 500 central pc of the Galaxy (hereafter GC) exhibit a widespread gas component with a kinetic temperature of 100-200 K. The bulk of this gas is not associated to the well-known thermal radio continuum or far infrared sources like Sgr A or Sgr B. How this gas is heated has been a longstanding problem. With the aim of studying the thermal balance of the neutral gas and dust in the GC, we have observed 18 molecular clouds located at projected distances far from thermal continuum sources with the Infrared Space Observatory (ISO). In this paper we present observations of several fine structure lines and the full continuum spectra of the dust between 40 and 190 microns. A warm dust component with a temperature between 27 and 42 K is needed to fit the spectra. We have compared the gas and the dust emission with the predictions from J-type and C-type shocks and photodissociation region (PDRs) models. We conclude that the dust and the fine structure lines observations are best explained by a PDR with a density of 10$^3$ cm^-3 and an incident far-ultraviolet field 10$^3$ times higher than the local interstellar radiation field. PDRs can naturally explain the discrepancy between the gas and the dust temperatures. However, these PDRs can only account for 10-30% of the total H2 column density with a temperature of ~ 150 K. We discuss other possible heating mechanisms (short version).Comment: Accepted for publication by A&

Measurements of binary stars with coherent integration of NPOI data

In this paper we use coherently integrated visibilities (see separate paper in these proceedings, Jorgensen et al. 2008) to measure the properties of binary stars. We use only the phase of the complex visibility and not the amplitude. The reason for this is that amplitudes suffer from the calibration effect (the same for coherent and incoherent averages) and thus effectively provide lower accuracy measurements. We demonstrate that the baseline phase alone can be used to measure the separation, orientation and brightness ratio of a binary star, as a function of wavelength.Comment: 2008 SPIE Astronomical Telescopes and Instrumentatio

Solving the Imaging Problem with Coherently Integrated Multiwavelength Data

Recovering images from optical interferometric observations is one of the major challenges in the field. Unlike the case of observations at radio wavelengths, in the optical the atmospheric turbulence changes the phases on a very short time scale, which results in corrupted phase measurements. In order to overcome these limitations, several groups developed image reconstruction techniques based only on squared visibility and closure phase information, which are unaffected by atmospheric turbulence. We present the results of two techniques used by our group, which employed coherently integrated data from the Navy Prototype Optical Interferometer. Based on these techniques we were able to recover complex visibilities for several sources and image them using standard radio imaging software. We describe these techniques, the corrections applied to the data, present the images of a few sources, and discuss the implications of these results.Comment: 8 pages, 2008 SPIE Astronomical Telescopes and Instrumentation, Eds. M. Schoeller, W. C. Danchi, F. Delplanck

A VLA H92alpha Study of the Arched Filament Complex Near the Galactic Center

The VLA has been used at 8.3 GHz in the DnC and CnB array configurations to carry out an H92alpha recombination line study (at 8.3 GHz) of the ionized gas in the Arched Filaments H II complex, which defines the western edge of the Galactic center Radio Arc. The H92alpha line properties of the ionized gas are consistent with photoionization from hot stars,and consistent with the physical properties of other Galactic center H II regions. The LTE electron temperatures vary only slightly across the entire extent of the source, and have an average value of 6200 K. The velocity field is very complex, with velocities ranging from +15 to - 70 km/s and the majority of velocities having negative values. Large velocity gradients (2-7 km/s/pc, with gradients in some regions >10 km/s/pc) occur along each of the filaments, with the velocities becoming increasingly negative with decreasing distance from the Galactic center. The magnitudes of the velocity gradient are consistent with the cloud residing on an inner, elongated orbit which is due to the Galaxy's stellar bar, or with a radially infalling cloud. The ionization of the Arched Filaments can be accounted for completely by the massive Arches stellar cluster, which consists of > 150 O-stars. This cluster is likely to belocated 10-20 pc from the Arched Filaments, which can explain the uniformity of ionization conditions in the ionized gas.Comment: 29 pages, 22 figures embedded (some poor quality), accepted to the Astronomical Journal (May 2001 issue), higher resolution figures available from [email protected]

The HgMn Binary Star Phi Herculis: Detection and Properties of the Secondary and Revision of the Elemental Abundances of the Primary

Observations of the Mercury-Manganese star Phi Herculis with the Navy Prototype Optical Interferometer (NPOI) conclusively reveal the previously unseen companion in this single-lined binary system. The NPOI data were used to predict a spectral type of A8V for the secondary star Phi Her B. This prediction was subsequently confirmed by spectroscopic observations obtained at the Dominion Astrophysical Observatory. Phi Her B is rotating at 50 +/-3 km/sec, in contrast to the 8 km/sec lines of Phi Her A. Recognizing the lines from the secondary permits one to separate them from those of the primary. The abundance analysis of Phi Her A shows an abundance pattern similar to those of other HgMn stars with Al being very underabundant and Sc, Cr, Mn, Zn, Ga, Sr, Y, Zr, Ba, Ce, and Hg being very overabundant.Comment: Accepted to ApJ, 45 pages, 11 figure

The Mid-Infrared Colors of the ISM and Extended Sources at the Galactic Center

A mid-infrared (3.6-8 um) survey of the Galactic Center has been carried out with the IRAC instrument on the Spitzer Space Telescope. This survey covers the central 2x1.4 degree (~280x200 pc) of the Galaxy. At 3.6 and 4.5 um the emission is dominated by stellar sources, the fainter ones merging into an unresolved background. At 5.8 and 8 um the stellar sources are fainter, and large-scale diffuse emission from the ISM of the Galaxy's central molecular zone becomes prominent. The survey reveals that the 8 to 5.8 um color of the ISM emission is highly uniform across the surveyed region. This uniform color is consistent with a flat extinction law and emission from polycyclic aromatic hydrocarbons (PAHs). Models indicate that this broadband color should not be expected to change if the incident radiation field heating the dust and PAHs is <10^4 times that of the solar neighborhood. The few regions with unusually red emission are areas where the PAHs are underabundant and the radiation field is locally strong enough to heat large dust grains to produce significant 8 um emission. These red regions include compact H II regions, Sgr B1, and wider regions around the Arches and Quintuplet Clusters. In these regions the radiation field is >10^4 times that of the solar neighborhood. Other regions of very red emission indicate cases where thick dust clouds obscure deeply embedded objects or very early stages of star formation.Comment: 37 pages, 15 Postscript figures (low resolution). Accepted for publication in the Ap