2,999 research outputs found
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 cm^-3 and an incident far-ultraviolet field 10 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
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