1,910 research outputs found
The Lowest Mass White Dwarf
Extremely low mass white dwarfs are very rare objects likely formed in
compact binary systems. We present MMT optical spectroscopy of 42 low mass
white dwarf candidates serendipitously discovered in a survey for hypervelocity
B-type stars. One of these objects, SDSS J0917+46, has Teff= 11,288 \pm 72 K
and log g = 5.48 \pm 0.03; with an estimated mass of 0.17 M_sun, it is the
lowest gravity/mass white dwarf currently known. However, 40 of the low mass
candidates are normal DA white dwarfs with apparently inaccurate SDSS g
magnitudes. We revisit the identification of low mass white dwarf candidates
previously found in the SDSS, and conclude that four objects have M < 0.2
M_sun. None of these white dwarfs show excess emission from a binary companion,
and radial velocity searches will be necessary to constrain the nature of the
unseen companions.Comment: ApJ, accepted versio
Defect-Mediated Emulsification in Two Dimensions
We consider two dimensional dispersions of droplets of isotropic phase in a
liquid with an XY-like order parameter, tilt, nematic, and hexatic symmetries
being included. Strong anchoring boundary conditions are assumed. Textures for
a single droplet and a pair of droplets are calculated and a universal
droplet-droplet pair potential is obtained. The interaction of dispersed
droplets via the ordered phase is attractive at large distances and repulsive
at short distances, which results in a well defined preferred separation for
two droplets and topological stabilization of the emulsion. This interaction
also drives self-assembly into chains. Preferred separations and energy
barriers to coalescence are calculated, and effects of thermal fluctuations and
film thickness are discussed.Comment: revtex4, 13 pages, 12 figure
A Deep Multicolor Survey V: The M Dwarf Luminosity Function
We present a study of M dwarfs discovered in a large area, multicolor survey.
We employ a combination of morphological and color criteria to select M dwarfs
to a limiting magnitude in V of 22, the deepest such ground-based survey for M
dwarfs to date. We solve for the vertical disk stellar density law and use the
resulting parameters to derive the M dwarf luminosity and mass functions from
this sample. We find the stellar luminosity function peaks at M_V = 12 and
declines thereafter. Our derived mass function for stars with M < 0.6 M_sun is
inconsistent with a Salpeter function at the 3 sigma level; instead, we find
the mass function is relatively flat for 0.6 M_sun > M > 0.1 M_sun.Comment: Accepted for publication in AJ. 19 pages including 4 embedded
postscript figures (AASTEX
Point Source Extraction with MOPEX
MOPEX (MOsaicking and Point source EXtraction) is a package developed at the
Spitzer Science Center for astronomical image processing. We report on the
point source extraction capabilities of MOPEX. Point source extraction is
implemented as a two step process: point source detection and profile fitting.
Non-linear matched filtering of input images can be performed optionally to
increase the signal-to-noise ratio and improve detection of faint point
sources. Point Response Function (PRF) fitting of point sources produces the
final point source list which includes the fluxes and improved positions of the
point sources, along with other parameters characterizing the fit. Passive and
active deblending allows for successful fitting of confused point sources.
Aperture photometry can also be computed for every extracted point source for
an unlimited number of aperture sizes. PRF is estimated directly from the input
images. Implementation of efficient methods of background and noise estimation,
and modified Simplex algorithm contribute to the computational efficiency of
MOPEX. The package is implemented as a loosely connected set of perl scripts,
where each script runs a number of modules written in C/C++. Input parameter
setting is done through namelists, ASCII configuration files. We present
applications of point source extraction to the mosaic images taken at 24 and 70
micron with the Multiband Imaging Photometer (MIPS) as part of the Spitzer
extragalactic First Look Survey and to a Digital Sky Survey image. Completeness
and reliability of point source extraction is computed using simulated data.Comment: 20 pages, 13 Postscript figures, accepted for publication in PAS
Precise Variational Calculation For The Doubly Excited State (2p^2)^3P^e of Helium
Highly precise variational calculations of non-relativistic energies of the
(2p^2)^3P^e state of Helium atom are presented.We get an upper bound energy
E=-0.71050015565678 a.u.,the lowest yet obtained.Comment: 5 pages, 1 tabl
Phase-resolved far-ultraviolet HST spectroscopy of the peculiar magnetic white dwarf RE J0317-853
We present phase resolved FUV HST FOS spectra of the rapidly rotating, highly
magnetic white dwarf RE J0317-853. Using these data, we construct a new model
for the magnetic field morphology across the stellar surface. From an expansion
into spherical harmonics, we find the range of magnetic field strengths present
is 180-800MG. For the first time we could identify an absorption feature
present at certain phases at 1160A as a ``forbidden'' 1s_0 -> 2s_0 component,
due to the combined presence of an electric and magnetic field.Comment: 15 pages including 4 figures. Accepted for publication in ApJ Letter
General lower bounds for evolutionary algorithms
Evolutionary optimization, among which genetic optimization, is a general framework for optimization. It is known (i) easy to use (ii) robust (iii) derivative-free (iv) unfortunately slow. Recent work [8] in particular show that the convergence rate of some widely used evolution strategies (evolutionary optimization for continuous domains) can not be faster than linear (i.e. the logarithm of the distance to the optimum can not decrease faster than linearly), and that the constant in the linear convergence (i.e. the constant C such that the distance to the optimum after n steps is upp er b ounded by C n ) unfortunately converges quickly to 1 as the dimension increases to infinity. We here show a very wide generalization of this result: al l comparison-based algorithms have such a limitation. Note that our result also concerns methods like the Hooke & Jeeves algorithm, the simplex method, or any direct search method that only compares the values to previously seen values of the fitness. But it does not cover methods that use the value of the fitness (see [5] for cases in which the fitness-values are used), even if these methods do not use gradients. The former results deal with convergence with respect to the number of comparisons performed, and also include a very wide family of algorithms with resp ect to the number of function-evaluations. However, there is still place for faster convergence rates, for more original algorithms using the full ranking information of the population and not only selections among the population. We prove that, at least in some particular cases, using the full ranking information can improve these lower bounds, and ultimately provide sup erlinear convergence results
Molecular Line Profile Fitting with Analytic Radiative Transfer Models
We present a study of analytic models of starless cores whose line profiles
have ``infall asymmetry,'' or blue-skewed shapes indicative of contracting
motions. We compare the ability of two types of analytical radiative transfer
models to reproduce the line profiles and infall speeds of centrally condensed
starless cores whose infall speeds are spatially constant and range between 0
and 0.2 km s-1. The model line profiles of HCO+ (J=1-0) and HCO+ (J=3-2) are
produced by a self-consistent Monte Carlo radiative transfer code. The analytic
models assume that the excitation temperature in the front of the cloud is
either constant (``two-layer'' model) or increases inward as a linear function
of optical depth (``hill'' model). Each analytic model is matched to the line
profile by rapid least-squares fitting.
The blue-asymmetric line profiles with two peaks, or with a blue shifted peak
and a red shifted shoulder, can be well fit by the ``HILL5'' model (a five
parameter version of the hill model), with an RMS error of 0.02 km s-1. A peak
signal to noise ratio of at least 30 in the molecular line observations is
required for performing these analytic radiative transfer fits to the line
profiles.Comment: 48 pages, 20 figures, accepted for publication in Ap
Chondrodystrophic dwarfism and multiple malformations in two sisters.
A genetic skeletal dysplasia with dwarfism, scoliosis and multiple skeletal defects was observed in two sisters. Only nine cases with similar features have been reported in the literature
Evacuation decisions in a chemical air pollution incident: cross sectional survey.
OBJECTIVE: To compare the health outcomes in sheltered and evacuated populations after a chemical incident in a plastics factory. DESIGN: Cross sectional survey. SETTING: Urban area in southwest England. PARTICIPANTS: 1750 residents from the area exposed to the chemical smoke, of which 472 were evacuated and the remaining 1278 were advised to shelter indoors. MAIN OUTCOME MEASURE: Number of adverse health symptoms. A case was defined by the presence of four or more symptoms. MAIN RESULTS: 1096 residents (63%; 299 evacuated, 797 sheltered) provided data for analyses. The mean symptom score and proportion of cases were higher in evacuated people than in the sheltered population (evacuated: symptom score 1.9, cases 19.7% (n = 59); sheltered: symptom score 1.0, cases 9.5% (n = 76); P < 0.001 for both). The difference between the two groups attenuated markedly at the end of two weeks from the start of the incident. The two main modifiable risk factors for the odds of becoming a case were evacuation (odds ratio 2.5, 95% confidence interval 1.7 to 3.8) and direct exposure to smoke for more than two hours on the first day of the incident (2.0, 1.7 to 2.3). The distance of residence from the factory or level of exposure before intervention (first six hours) had little effect on the odds of a person becoming a case. CONCLUSIONS: Sheltering may have been a better protective action than evacuation in this chemical incident, which is consistent with the prevailing expert view. Although this study has limitations, it is based on a real event. Evacuations carry their own risks and resource implications; increased awareness may help to reduce unnecessary evacuations in the future
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