7,476 research outputs found
The city as one thing
This paper summarises the latest theories in the field of space syntax. It opens with a discussion of the relationship between the form of urban grids and the process of how cities are formed by human activity; this is done by a comprehensive review of space syntax theory from its starting point in the1970s. The paper goes on to present research into how cities balance the micro-economic factors which shape the spatial structure of cities with the cultural factors that shape the underlying form of residential areas. It goes on to discuss the relationship between activity and space and how this relationship is formed by the way different activities make different demands on movement and co-presence. The paper ends with a discussion regarding the manner in which patterns of spatial integration influence the location of different classes and social groups in the city and contribute to the pathology of housing estates. The paper concludes that spatial form needs to be understood as a contributing factor in forming the patterns of integration and segregation in cities
Radio-Optical Galaxy Shape and Shear Correlations in the COSMOS Field using 3 GHz VLA Observations
We present a weak lensing analysis of the 3 GHz VLA radio survey of the
COSMOS field, which we correlate with overlapping HST-ACS optical observations
using both intrinsic galaxy shape and cosmic shear correlation statistics.
After cross-matching sources between the two catalogues, we measure the
correlations of galaxy position angles and find a Pearson correlation
coefficient of . This is a marked improvement from previous
studies which found very weak, or non-existent correlations, and gives insight
into the emission processes of radio and optical galaxies. We also extract
power spectra of averaged galaxy ellipticities (the primary observable for
cosmic shear) from the two catalogues, and produce optical-optical,
radio-optical and radio-radio spectra. The optical-optical auto-power spectrum
was measured to a detection significance of 9.80 and is consistent with
previous observations of the same field. For radio spectra (which we do not
calibrate, given the unknown nature of their systematics), although we do not
detect significant radio-optical (1.50) or radio-radio (1.45)
-mode power spectra, we do find the -mode spectra to be more consistent
with the shear signal expected from previous studies than with a null signal,
and vice versa for -mode and cross-correlation spectra. Our results
give promise that future radio weak lensing surveys with larger source number
densities over larger areas will have the capability to measure significant
weak lensing signals.Comment: 19 pages, 17 figures, accepted for publication in MNRA
Quantitative spectroscopic analysis of and distance to SN1999em
This work presents a detailed quantitative spectroscopic analysis of, and the
determination of the distance to, the type II supernovae (SN) SN1999em with
CMFGEN (Dessart & Hillier 2005a), based on spectrophotometric observations at
eight dates up to 40 days after discovery. We use the same iron-group metal
content for the ejecta, the same power-law density distribution (with exponent
n~10), and a Hubble-velocity law at all times. We adopt a H/He/C/N/O abundance
pattern compatible with CNO-cycle equilibrium values for a RSG/BSG progenitor,
with C/O enhanced and N depleted at later times. Based on our synthetic fits to
spectrophotometric observations of SN1999em, we obtain a distance of 11.5Mpc,
similar to that of Baron et al. (2004) and the Cepheid distance to the galaxy
host of 11.7Mpc (Leonard et al. 2003). Similarly, based on such models, the
Expanding Photosphere Method (EPM) delivers a distance of 11.6Mpc, with
negligible scatter between photometric bandpass sets; there is thus nothing
wrong with the EPM as such. Previous determinations using the tabulated
correction factors of Eastman et al. (1996) all led to 30-50% underestimates:
we find that this is caused by 1) an underestimate of the correction factors
compared to the only other study of the kind by Dessart & Hillier (2005b), 2) a
neglect of the intrinsic >20% scatter of correction factors, and 3) the use of
the EPM at late times when severe line blanketing makes the method inaccurate.
The need of detailed model computations for reliable EPM distance estimates
thus defeats the appeal and simplicity of the method. However, detailed fits to
SN optical spectra, based on tailored models for individual SN observations,
offers a promising approach to obtaining distances with 10-20% accuracy, either
through the EPM or a la Baron et al. (2004).Comment: 20 pages, 13 figures, accepted for publication in A&
The Atomic Physics Underlying the Spectroscopic Analysis of Massive Stars and Supernovae
We have developed a radiative transfer code, CMFGEN, which allows us to model
the spectra of massive stars and supernovae. Using CMFGEN we can derive
fundamental parameters such as effective temperatures and surface gravities,
derive abundances, and place constraints on stellar wind properties. The last
of these is important since all massive stars are losing mass via a stellar
wind that is driven from the star by radiation pressure, and this mass loss can
substantially influence the spectral appearance and evolution of the star.
Recently we have extended CMFGEN to allow us to undertake time-dependent
radiative transfer calculations of supernovae. Such calculations will be used
to place constraints on the supernova progenitor, to place constraints on the
supernova explosion and nucleosynthesis, and to derive distances using a
physical approach called the "Expanding Photosphere Method". We describe the
assumptions underlying the code and the atomic processes involved. A crucial
ingredient in the code is the atomic data. For the modeling we require accurate
transition wavelengths, oscillator strengths, photoionization cross-sections,
collision strengths, autoionization rates, and charge exchange rates for
virtually all species up to, and including, cobalt. Presently, the available
atomic data varies substantially in both quantity and quality.Comment: 8 pages, 2 figures, Accepted for publication in Astrophysics & Space
Scienc
Mass fluxes for O stars
The theory of moving reversing layers for hot stars is updated to include an
extensive line list, a radiative boundary condition from static model
atmospheres, line transfer by scattering, and continuation to supersonic
velocities. A Monte Carlo technique determines the theory's eigenvalue J, the
mass flux, and the derived J's are in good agreement with the wind models of
Pauldrach et al. (2001). The solutions' sensitivity to the photospheric
microturbulent velocity reveals that this parameter has a throttling effect on
J: turbulent line-broadening in the quasi-static layers reduces the radiation
force available to accelerate matter through the sonic point. If photospheric
turbulence approaches sonic velocities, this mechanism reduces mass loss rates
by factors > 3, which would partly account for the reduced rates found
observationally for clumpy winds.Comment: Accepted by A&A; 9 pages, 4 figure
2D non-LTE Modeling for Axi-symmetric Winds. II. A Short Characteristic Solution for Radiative Transfer in Rotating Winds
We present a new radiative transfer code for axi-symmetric stellar
atmospheres and compare test results against 1D and 2D models with and without
velocity fields. The code uses the short characteristic method with
modifications to handle axi-symmetric and non-monotonic 3D wind velocities, and
allows for distributed calculations. The formal solution along a characteristic
is evaluated with a resolution that is proportional to the velocity gradient
along the characteristic. This allows us to accurately map the variation of the
opacities and emissivities as a function of frequency and spatial coordinates,
but avoids unnecessary work in low velocity regions. We represent a
characteristic with an impact-parameter vector p (a vector that is normal to
the plane containing the characteristic and the origin) rather than the
traditional unit vector in the direction of the ray. The code calculates the
incoming intensities for the characteristics by a single latitudinal
interpolation without any further interpolation in the radiation angles. Using
this representation also provides a venue for distributed calculations since
the radiative transfer can be done independently for each p.Comment: 18 pages, 12 figures, accepted for publication in A&
High Rate Detection of Volatile Products Using Differential Electrochemical Mass Spectrometry: Combining an Electrode-Coated Membrane with Hydrodynamic Flow in a Wall-Tube Configuration
We present an experimental system that combines differential electrochemical mass spectrometry with hydrodynamic flow consisting of an impinging jet in a wall-tube configuration. This assembly allows simultaneous detection of electrochemical signals along with monitoring of dissolved gas species using differential electrochemical mass spectrometry under well-defined hydrodynamic conditions and over a wide range of mass transfer rates. The working electrode is deposited directly onto a thin, hydrophobic membrane, which also serves as the inlet to the mass spectrometer. This inlet provides extremely rapid mass detection as well as a high flux of products from the electrode surface into the mass spectrometer. The impinging jet is designed in a wall-tube configuration, in which the jet diameter is large compared to the electrode diameter, thus providing uniform and rapid mass transfer conditions over the entirety of the electrode surface. This combination of rapid detection and controllable flow conditions allows a wide range of hydrodynamic conditions to be accessed with simultaneous electrochemical and mass spectrometric detection of dissolved gas species, which is important in the analysis of a range of electrochemical reactions. The capabilities of this configuration are illustrated using a platinum-coated electrode and several electrochemical reactions, including ferrocyanide oxidation, proton reduction, and oxalic acid oxidation
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Photometric modeling of a cometary nucleus: taking Hapke modeling to the limit
Detection of a Hot Binary Companion of Carinae
We report the detection of a hot companion of Carinae using high
resolution spectra (905 - 1180 \AA) obtained with the Far Ultraviolet
Spectroscopic Explorer (\fuse) satellite. Observations were obtained at two
epochs of the 2024-day orbit: 2003 June during ingress to the 2003.5 X-ray
eclipse and 2004 April several months after egress. These data show that
essentially all the far-UV flux from \etacar shortward of \lya disappeared at
least two days before the start of the X-ray eclipse (2003 June 29), implying
that the hot companion, \etaB, was also eclipsed by the dense wind or extended
atmosphere of \etaA. Analysis of the far-UV spectrum shows that \etaB is a
luminous hot star. The \nii \wll1084-1086 emission feature suggests that it may
be nitrogen-rich. The observed far-UV flux levels and spectral features,
combined with the timing of their disappearance, is consistent with \etacar\
being a massive binary system
O-star mass-loss rates at low metallicity
Mass fluxes J are computed for the extragalactic O stars investigated by
Tramper et al. (2011; TSKK). For one early-type O star, computed and observed
rates agree within errors. However, for two late-type O stars, theoretical
mass-loss rates underpredict observed rates by ~ 1.6 dex, far exceeding
observational errors. A likely cause of the discrepancy is overestimated
observed rates due to the neglect of wind-clumping. A less likely but
intriguing possibility is that, in observing O stars with Z/Z_sun ~ 1/7, TSKK
have serendipitously discovered an additional mass-loss mechanism not evident
in the spectra of Galactic O stars with powerful radiation-driven winds.
Constraints on this unknown mechanism are discussed.
In establishing that the discrepancies, if real, are inescapable for purely
radiation-driven winds, failed searches for high-J solutions are reported and
the importance of a numerical technique that cannot spuriously create or
destroy momentum stressed.
The Z-dependences of the computed rates for Z/Z_sun in the interval (1/30, 2)
show significant departures from a single power law, and these are attributed
to curve-of-growth effects in the differentially-expanding reversing layers.
The best-fitting power-law exponents range from 0.68-0.97.Comment: 6 pages, 2 figure
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