4,140 research outputs found
THE ROLE OF IMPORTS IN THE CHANGING NATURE OF THE U.S. FARM MACHINERY INDUSTRY
Agribusiness, International Relations/Trade,
Adobe Flash as a medium for online experimentation: a test of reaction time measurement capabilities
Adobe Flash can be used to run complex psychological experiments over the Web. We examined the reliability of using Flash to measure reaction times (RTs) using a simple binary-choice task implemented both in Flash and in a Linux-based system known to record RTs with millisecond accuracy. Twenty-four participants were tested in the laboratory using both implementations; they also completed the Flash version on computers of their own choice outside the lab. RTs from the trials run on Flash outside the lab were approximately 20 msec slower than those from trials run on Flash in the lab, which in turn were approximately 10 msec slower than RTs from the trials run on the Linux-based system (baseline condition). RT SDs were similar in all conditions, suggesting that although Flash may overestimate RTs slightly, it does not appear to add significant noise to the data recorded
Pyrochlore Antiferromagnet: A Three-Dimensional Quantum Spin Liquid
The quantum pyrochlore antiferromagnet is studied by perturbative expansions
and exact diagonalization of small clusters. We find that the ground state is a
spin-liquid state: The spin-spin correlation functions decay exponentially with
distance and the correlation length never exceeds the interatomic distance. The
calculated magnetic neutron diffraction cross section is in very good agreement
with experiments performed on Y(Sc)Mn2. The low energy excitations are
singlet-singlet ones, with a finite spin gap.Comment: 4 pages, 4 figure
Classical heisenberg antiferromagnet away from the pyrochlore lattice limit: entropic versus energetic selection
The stability of the disordered ground state of the classical Heisenberg
pyrochlore antiferromagnet is studied within extensive Monte Carlo simulations
by introducing an additional exchange interaction that interpolates
between the pyrochlore lattice () and the face-centered cubic lattice
(). It is found that for as low as , the system is
long range ordered : the disordered ground state of the pyrochlore
antiferromagnet is unstable when introducing very small deviations from the
pure limit. Furthermore, it is found that the selected phase is a
collinear state energetically greater than the incommensurate phase suggested
by a mean field analysis. To our knowledge this is the first example where
entropic selection prevails over the energetic one.Comment: 5 (two-column revtex4) pages, 1 table, 7 ps/eps figures. Submitted to
Phys. Rev.
X-ray Observations of Gravitationally Lensed Quasars; Evidence for a Hidden Quasar Population
The large magnification factors of gravitationally lensed (GL) systems allow
us to investigate the properties of quasars with X-ray luminosities that are
substantially lower than those of unlensed ones and also provide an independent
means of estimating the contribution of faint quasars to the hard X-ray
component of the cosmic X-ray background. Our spectral analysis indicate a
flattening of the spectral index in the hard band (4-20keV restframe) for 2
radio-loud quasars in the GL quasar sample for which the data have moderate
signal-to-noise ratio. We have identified a large fraction of Broad Absorption
Line (BAL) quasars amongst the GL quasar population. We find that approximately
35% of radio-quiet GL quasars contain BAL features which is significantly
larger than the 10% fraction of BAL quasars presently found in optically
selected flux limited quasar samples. We present a simple model that estimates
the effects of attenuation and lens magnification on the luminosity function of
quasars and that explains the observed fraction of GL BAL quasars. These
observations suggest that a large fraction of BAL quasars are missed from flux
limited optical surveys. Modeling of several X-ray observations of the GL BAL
quasar PG1115+080 suggests that the observed large X-ray variability may be
caused in part by a variable intrinsic absorber consistent with previously
observed variability of the BAL troughs in the UV band. The observed large
X-ray flux variations in PG1115+080 offer the prospect of considerably reducing
errors in determining the time delay with future X-ray monitoring of this
system and hence constraining the Hubble constant H.Comment: 19 pages, 10 figures, 9 Tables, accepted for publication in Ap
Contribution of White Dwarfs to Cluster Masses
I present a literature search through 31 July 1997 of white dwarfs (WDs) in
open and globular clusters. There are 36 single WDs and 5 WDs in binaries known
among 13 open clusters, and 340 single WDs and 11 WDs in binaries known among
11 globular clusters. From these data I have calculated WD mass fractions for
four open clusters (the Pleiades, NGC 2168, NGC 3532, and the Hyades) and one
globular cluster (NGC 6121). I develop a simple model of cluster evolution that
incorporates stellar evolution but not dynamical evolution to interpret the WD
mass fractions. I augment the results of my simple model with N-body
simulations incorporating stellar evolution (Terlevich 1987; de la Feunte
Marcos 1996; Vesperini & Heggie 1997). I find that even though these clusters
undergo moderate to strong kinematical evolution the WD mass fraction is
relatively insensitive to kinematical evolution. By comparing the cluster mass
functions to that of the Galactic disk, and incorporating plausibility
arguments for the mass function of the Galactic halo, I estimate the WD mass
fraction in these two populations. I assume the Galactic disk is ~10 Gyrs old
(Winget et al. 1987; Liebert, Dahn, & Monet 1988; Oswalt et al. 1996) and that
the Galactic halo is ~12 Gyrs old (Reid 1997b; Gratton et al. 1997; Chaboyer et
al. 1998), although the WD mass fraction is insensitive to age in this range. I
find that the Galactic halo should contain 8 to 9% (alpha = -2.35) or perhaps
as much as 15 to 17% (alpha = -2.0) of its stellar mass in the form of WDs. The
Galactic disk WD mass fraction should be 6 to 7% (alpha = -2.35), consistent
with the empirical estimates of 3 to 7% (Liebert, Dahn, & Monet 1988; Oswalt et
al. 1996). (abridged)Comment: 20 pages, uuencoded gunzip'ed latex + 3 postscrip figures, to be
published in AJ, April, 199
On the primordial scenario for abundance variations within globular clusters. The isochrone test
Self-enrichment processes occurring in the early stages of a globular cluster
lifetime are generally invoked to explain the observed CNONaMgAl abundance
anticorrelations within individual Galactic globulars.We have tested, with
fully consistent stellar evolution calculations, if theoretical isochrones for
stars born with the observed abundance anticorrelations satisfy the
observational evidence that objects with different degrees of these anomalies
lie on essentially identical sequences in the Color-Magnitude-Diagram (CMD). To
this purpose, we have computed for the first time low-mass stellar models and
isochrones with an initial metal mixture that includes the extreme values of
the observed abundance anticorrelations, and varying initial He mass fractions.
Comparisons with 'normal' alpha-enhanced isochrones and suitable Monte Carlo
simulations that include photometric errors show that a significant broadening
of the CMD sequences occurs only if the helium enhancement is extremely large
(in this study, when Y=0.35) in the stars showing anomalous abundances. Stellar
luminosity functions up to the Red Giant Branch tip are also very weakly
affected, apart from - depending on the He content of the polluting material -
the Red Giant Branch bump region. We also study the distribution of stars along
the Zero Age Horizontal Branch, and derive general constraints on the relative
location of objects with and without abundance anomalies along the observed
horizontal branches of globular clusters.Comment: Accepted for publication in Ap
A Study of the Reionization History of Intergalactic Helium with FUSE and VLT
We obtained high-resolution VLT and FUSE spectra of the quasar HE2347-4342 to
study the properties of the intergalactic medium between redshifts z=2.0-2.9.
The high-quality optical spectrum allows us to identify approximately 850 HeII
absorption components with column densities between N~5X10^11 and $ 10^18
cm^-2. The reprocessed FUSE spectrum extends the wavelength coverage of the
HeII absorption down to an observed wavelength of 920 A. Approximately 1400
HeII absorption components are identified, including 917 HeII Ly-alpha systems
and some of their HeII Ly-beta, Ly-gamma, and Ly-delta counterparts. The
ionization structure of HeII is complex, with approximately 90 components that
are not detected in the hydrogen spectrum. These components may represent the
effect of soft ionizing sources. The ratio Eta=N(HeII)/N(HI) varies
approximately from unity to more than a thousand, with a median value of 62 and
a distribution consistent with the intrinsic spectral indices of quasars. This
suggests that the dominant ionizing field is from the accumulated quasar
radiation, with contributions from other soft sources such as star-forming
regions and obscured AGN, which do not ionize helium. We find an evolution in
Eta toward smaller values at lower redshift, with the gradual disappearance of
soft components. At redshifts z>2.7, the large but finite increase in the HeII
opacity, Tau=5+/-1, suggests that we are viewing the end stages of a
reionization process that began at an earlier epoch. Fits of the absorption
profiles of unblended lines indicate comparable velocities between hydrogen and
He^+ ions. At hydrogen column densities N<3X10^12 cm^-2 the number of forest
lines shows a significant deficit relative to a power law, and becomes
negligible below N=10^11 cm^-2.Comment: 40 pages, 10 Postscript figures, uses Aastex.sty The Astrophysical
Journal, in pres
The UV spectrum of HS1700+6416 I. Predicting the metal line content of the far UV spectrum
In preparation of a study of the HeII/HI ratio towards the bright QSO
HS1700+6416, we predict the metal line content of the far-UV spectral range by
modelling 18 metal absorption line systems with redshifts 0.2<z<2.6 identified
in the spectrum of this quasar. For that purpose, we investigate the spectral
energy distribution of the metagalactic ionizing radiation field. Simple
photoionization models based on 8 different shapes of the ionizing background
are tested for each system. The adopted energy distributions comprise the
Haardt & Madau (2001, HM) model of metagalactic UV background as well as
typical spectra of AGN and starburst galaxies. The models are evaluated and the
favoured one is estimated. We find that the majority of the considered systems
is best reproduced with a HM-like ionizing radiation, where the HeII break,
formally located at 4 Ryd, is shifted to lower energies (~3 Ryd), probably due
to the opacity of the higher HeII Lyman series lines. The remaining systems can
be reasonably described with models based on the unmodified HM background or
the spectra of AGN or starburst galaxies. This finding supports the idea that
the UV background is spatially variable due to both IGM opacity variations and
to local sources. In comparison to an unmodified HM background, the resulting
ionizing spectrum leads to carbon abundances lower by ~0.5 dex. Furthermore, if
the ionizing radiation field as determined from metal line systems was typical
for the IGM, the expected HeII/HI ratio would be 150 to 190.Comment: 25 pages, 8 figures and 17 figures online material, accepted for
publication in A&
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