1,197 research outputs found
OPserver: interactive online-computations of opacities and radiative accelerations
Codes to compute mean opacities and radiative accelerations for arbitrary
chemical mixtures using the Opacity Project recently revised data have been
restructured in a client--server architecture and transcribed as a subroutine
library. This implementation increases efficiency in stellar modelling where
element stratification due to diffusion processes is depth dependent, and thus
requires repeated fast opacity reestimates. Three user modes are provided to
fit different computing environments, namely a web browser, a local workstation
and a distributed grid.Comment: 5 pages, 1 figur
Scottish theme towns: have new identities enhanced development?
Three small towns in southwest Scotland have recently been branded as distinct theme towns, based on books, artists and food. This is an attempt to make them more attractive to visitors and thereby improve their economy. The objective of this research is to establish whether the new identities possessed by the towns have enhanced their development. It is argued, using data reviewing the past decade, that they have all developed, albeit at different rates, in terms of the economy and culture. Moreover, it is maintained that social capital has been enhanced and is a factor whose importance has been under-appreciated by planners and observers of this type of process. The relevance of the new identity to the pre-branding identity is also seen as a factor in successful development and ideas of authenticity and heritage are brought to bear on the relationship
Using Three-Body Recombination to Extract Electron Temperatures of Ultracold Plasmas
Three-body recombination, an important collisional process in plasmas,
increases dramatically at low electron temperatures, with an accepted scaling
of T_e^-9/2. We measure three-body recombination in an ultracold neutral xenon
plasma by detecting recombination-created Rydberg atoms using a
microwave-ionization technique. With the accepted theory (expected to be
applicable for weakly-coupled plasmas) and our measured rates we extract the
plasma temperatures, which are in reasonable agreement with previous
measurements early in the plasma lifetime. The resulting electron temperatures
indicate that the plasma continues to cool to temperatures below 1 K.Comment: 5 pages, 3 figure
Ultraviolet Spectra of CV Accretion Disks with Non-Steady T(r) Laws
An extensive grid of synthetic mid- and far-ultraviolet spectra for accretion
disks in cataclysmic variables has been presented by Wade and Hubeny (1998). In
those models, the disk was assumed to be in steady-state, that is T_eff(r) is
specified completely by the mass M_WD and radius R_WD of the accreting white
dwarf star and the mass transfer rate M_dot which is constant throughout the
disk. In these models, T_eff(r) is proportional to r^{-3/4} except as modified
by a cutoff term near the white dwarf.
Actual disks may vary from the steady-state prescription for T_eff(r),
however, e.g. owing to outburst cycles in dwarf novae M_dot not constant with
radius) or irradiation (in which case T_eff in the outer disk is raised above
T_steady). To show how the spectra of such disks might differ from the steady
case, we present a study of the ultraviolet (UV) spectra of models in which
power-law temperature profiles T_eff(r) is proportional to r^{-gamma} with
gamma < 3/4 are specified. Otherwise, the construction of the models is the
same as in the Wade & Hubeny grid, to allow comparison. We discuss both the UV
spectral energy distributions and the appearance of the UV line spectra. We
also briefly discuss the eclipse light curves of the non-standard models.
Comparison of these models with UV observations of novalike variables suggests
that better agreement may be possible with such modified T_eff(r) profiles.Comment: 13 pages, 6 figures (one reduced quality), ApJ in pres
Toward a unified light curve model for multi-wavelength observations of V1974 Cygni (Nova Cygni 1992)
We present a unified model for optical, ultraviolet (UV), and X-ray light
curves of V1974 Cygni (Nova Cygni 1992). Based on an optically thick wind model
of nova outbursts, we have calculated light curves and searched for the best
fit model that is consistent with optical, UV, and X-ray observations. Our best
fit model is a white dwarf (WD) of mass 1.05 M_\sun with a chemical composition
of X=0.46, C+N+O=0.15, and Ne = 0.05 by mass weight. Both supersoft X-ray and
continuum UV 1455 \AA light curves are well reproduced. Supersoft X-rays
emerged on day ~ 250 after outburst, which is naturally explained by our model:
our optically thick winds cease on day 245 and supersoft X-rays emerge from
self-absorption by the winds. The X-ray flux keeps a constant peak value for ~
300 days followed by a quick decay on day ~ 600. The duration of X-ray flat
peak is well reproduced by a steady hydrogen shell burning on the WD. Optical
light curve is also explained by the same model if we introduce free-free
emission from optically thin ejecta. A t^{-1.5} slope of the observed optical
and infrared fluxes is very close to the slope of our modeled free-free light
curve during the optically thick wind phase. Once the wind stops, optical and
infrared fluxes should follow a t^{-3} slope, derived from a constant mass of
expanding ejecta. An abrupt transition from a t^{-1.5} slope to a t^{-3} slope
at day ~ 200 is naturally explained by the change from the wind phase to the
post-wind phase on day ~ 200. The development of hard X-ray flux is also
reasonably understood as shock-origin between the wind and the companion star.
The distance to V1974 Cyg is estimated to be ~ 1.7 kpc with E(B-V)= 0.32 from
the light curve fitting for the continuum UV 1455 \AA.Comment: 8 pages, 4 figures, to appear in the Astrophysical Journa
Driving and damping mechanisms in hybrid pressure-gravity modes pulsators
We study the energetic aspects of hybrid pressure-gravity modes pulsations.
The case of hybrid beta Cephei-SPB pulsators is considered with special
attention. In addition to the already known sensitivity of the driving
mechanism to the heavy elements mixture (mainly the iron abundance), we show
that the characteristics of the propagation and evanescent regions play also a
major role, determining the extension of the stable gap in the frequency domain
between the unstable low order pressure and high order gravity modes. Finally,
we consider the case of hybrid delta Sct-gamma Dor pulsators.Comment: 7 pages, 9 figures, in the proceedings of the Helas II Conference:
"Helioseismology, Asteroseismology and MHD Connections", Goettingen, August
200
A Second Luminous Blue Variable in the Quintuplet Cluster
H and K band moderate resolution and 4 m high resolution spectra have
been obtained for FMM#362, a bright star in the Quintuplet Cluster near the
Galactic Center. The spectral features in these bands closely match those of
the Pistol Star, a luminous blue variable and one of the most luminous stars
known. The new spectra and previously-obtained photometry imply a very high
luminosity for FMM#362, L \Lsun, and a temperature of 10,000 -
13,000 K. Based on its luminosity, temperature, photometric variability, and
similarities to the Pistol Star, we conclude that FMM#362 is a luminous blue
variable.Comment: Accepted for publication in The Astrophysical Journal Letters, 4
PostScript figures, 2 table
Creation and manipulation of Feshbach resonances with radio-frequency radiation
We present a simple technique for studying collisions of ultracold atoms in
the presence of a magnetic field and radio-frequency radiation (rf). Resonant
control of scattering properties can be achieved by using rf to couple a
colliding pair of atoms to a bound state. We show, using the example of 6Li,
that in some ranges of rf frequency and magnetic field this can be done without
giving rise to losses. We also show that halo molecules of large spatial extent
require much less rf power than deeply bound states. Another way to exert
resonant control is with a set of rf-coupled bound states, linked to the
colliding pair through the molecular interactions that give rise to
magnetically tunable Feshbach resonances. This was recently demonstrated for
87Rb [Kaufman et al., Phys. Rev. A 80:050701(R), 2009]. We examine the
underlying atomic and molecular physics which made this possible. Lastly, we
consider the control that may be exerted over atomic collisions by placing
atoms in superpositions of Zeeman states, and suggest that it could be useful
where small changes in scattering length are required. We suggest other species
for which rf and magnetic field control could together provide a useful tuning
mechanism.Comment: 21 pages, 8 figures, submitted to New Journal of Physic
HeII Recombination Lines From the First Luminous Objects
The hardness of the ionizing continuum from the first sources of UV radiation
plays a crucial role in the reionization of the intergalactic medium (IGM).
While usual stellar populations have soft spectra, mini-quasars or metal-free
stars with high effective temperatures may emit hard photons, capable of doubly
ionizing helium and increasing the IGM temperature. Absorption within the
source and in the intervening IGM will render the ionizing continuum of
high-redshift sources inaccessible to direct observation. Here we show that
HeII recombination lines from the first luminous objects are potentially
detectable by the Next Generation Space Telescope. Together with measurements
of the hydrogen Balmer alpha emission line, this detection can be used to infer
the ratio of HeII to HI ionizing photons. A measurement of this ratio would
shed light on the nature and emission mechanism of the first luminous sources,
with important astrophysical consequences for the reheating and reionization of
the IGM.Comment: ApJ published version. Due to an error in one of the references, the
strength of the 1640 A line was underestimated in a previous version; this
line is now brighter by a factor of 1
Rydberg transition frequencies from the Local Density Approximation
A method is given that extracts accurate Rydberg excitations from LDA density
functional calculations, despite the short-ranged potential. For the case of He
and Ne, the asymptotic quantum defects predicted by LDA are in less than 5%
error, yielding transition frequency errors of less than 0.1eV.Comment: 4 pages, 6 figures, submitted to Phys. Rev. Let
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