13,504 research outputs found
Heat exchange mediated by a quantum system
We consider heat transfer between two thermal reservoirs mediated by a
quantum system using the generalized quantum Langevin equation. The thermal
reservoirs are treated as ensembles of oscillators within the framework of the
Drude-Ullersma model. General expressions for the heat current and thermal
conductance are obtained for arbitrary coupling strength between the reservoirs
and the mediator and for different temperature regimes. As an application of
these results we discuss the origin of Fourier's law in a chain of large, but
finite subsystems coupled to each other by the quantum mediators. We also
address a question of anomalously large heat current between the STM tip and
substrate found in a recent experiment. The question of minimum thermal
conductivity is revisited in the framework of scaling theory as a potential
application of the developed approach.Comment: 16 pages, 6 figure
Attention mechanisms in the CHREST cognitive architecture
In this paper, we describe the attention mechanisms in CHREST, a computational architecture of human visual expertise. CHREST organises information acquired by direct experience from the world in the form of chunks. These chunks are searched for, and verified, by a unique set of heuristics, comprising the attention mechanism. We explain how the attention mechanism combines bottom-up and top-down heuristics from internal and external sources of information. We describe some experimental evidence demonstrating the correspondence of CHREST’s perceptual mechanisms with those of human subjects. Finally, we discuss how visual attention can play an important role in actions carried out by human experts in domains such as chess
Validation of the Fibromyalgia Survey Questionnaire within a Cross-Sectional Survey
The Fibromyalgia Survey Questionnaire (FSQ) assesses the key symptoms of fibromyalgia syndrome. The FSQ can be administrated in survey research and settings where the use of interviews to evaluate the number of pain sites and extent of somatic symptom intensity and tender point examination would be difficult. We validated the FSQ in a cross-sectional survey with FMS patients. In a cross-sectional survey, participants with physician diagnosis of FMS were recruited by FMS-self help organisations and nine clinical institutions of different levels of care. Participants answered the FSQ (composed by the Widespread Pain Index [WPI] and the Somatic Severity Score [SSS]) assessing the Fibromyalgia Survey Diagnostic Criteria (FSDC) and the Patient Health Questionnaire PHQ 4. American College of Rheumatology 1990 classification criteria were assessed in a subgroup of participants. 1,651 persons diagnosed with FMS were included into analysis. The acceptance of the FSQ-items ranged between 78.9 to 98.1% completed items. The internal consistency of the items of the SSS ranged between 0.75–0.82. 85.5% of the study participants met the FSDC. The concordance rate of the FSDC and ACR 1990 criteria was 72.7% in a subsample of 128 patients. The Pearson correlation of the SSS with the PHQ 4 depression score was 0.52 (p<0.0001) and with the PHQ anxiety score was 0.51 (p<0.0001) (convergent validity). 64/202 (31.7%) of the participants not meeting the FSDC criteria and 152/1283 (11.8%) of the participants meeting the FSDC criteria reported an improvement (slightly too very much better) in their health status since FMS-diagnosis (Chi2 = 55, p<0.0001) (discriminant validity). The study demonstrated the feasibility of the FSQ in a cross-sectional survey with FMS-patients. The reliability, convergent and discriminant validity of the FSQ were good. Further validation studies of the FSQ in clinical and general population settings are necessary
The Deuterium to Hydrogen Abundance Ratio Towards a Fourth QSO: HS0105+1619
We report the measurement of the primordial D/H abundance ratio towards QSO
\object. The column density of the hydrogen in the Lyman limit
system is high, \lnhi \cmm, allowing for the deuterium to
be seen in 5 Lyman series transitions. The measured value of the D/H ratio
towards QSO \object is found to be D/H. The
metallicity of the system showing D/H is found to be solar,
indicating that the measured D/H is the primordial D/H within the measurement
errors. The gas which shows D/H is neutral, unlike previous D/H systems which
were more highly ionized. Thus, the determination of the D/H ratio becomes more
secure since we are measuring it in different astrophysical environments, but
the error is larger because we now see more dispersion between measurements.
Combined with prior measurements of D/H, the best D/H ratio is now D/H, which is 10% lower than the previous value. The new
values for the baryon to photon ratio, and baryonic matter density derived from
D/H are and \ob
respectively.Comment: Minor text and reference changes. To appear in the May 10, 2001 issue
of the Astrophysical Journa
Magnetic White Dwarfs from the SDSS II. The Second and Third Data Releases
Fifty-two magnetic white dwarfs have been identified in spectroscopic
observations from the Sloan Digital Sky Survey (SDSS) obtained between mid-2002
and the end of 2004, including Data Releases 2 and 3. Though not as numerous
nor as diverse as the discoveries from the first Data Release, the collection
exhibits polar field strengths ranging from 1.5MG to ~1000MG, and includes two
new unusual atomic DQA examples, a molecular DQ, and five stars that show
hydrogen in fields above 500MG. The highest-field example, SDSSJ2346+3853, may
be the most strongly magnetic white dwarf yet discovered. Analysis of the
photometric data indicates that the magnetic sample spans the same temperature
range as for nonmagnetic white dwarfs from the SDSS, and support is found for
previous claims that magnetic white dwarfs tend to have larger masses than
their nonmagnetic counterparts. A glaring exception to this trend is the
apparently low-gravity object SDSSJ0933+1022, which may have a history
involving a close binary companion.Comment: 20 pages, 4 figures Accepted for publication in the Astronomical
Journa
Imaging the cool gas, dust, star formation, and AGN in the first galaxies
When, and how, did the first galaxies and supermassive black holes (SMBH)
form, and how did they reionization the Universe? First galaxy formation and
cosmic reionization are among the last frontiers in studies of cosmic structure
formation. We delineate the detailed astrophysical probes of early galaxy and
SMBH formation afforded by observations at centimeter through submillimeter
wavelengths. These observations include studies of the molecular gas (= the
fuel for star formation in galaxies), atomic fine structure lines (= the
dominant ISM gas coolant), thermal dust continuum emission (= an ideal star
formation rate estimator), and radio continuum emission from star formation and
relativistic jets. High resolution spectroscopic imaging can be used to study
galaxy dynamics and star formation on sub-kpc scales. These cm and mm
observations are the necessary compliment to near-IR observations, which probe
the stars and ionized gas, and X-ray observations, which reveal the AGN.
Together, a suite of revolutionary observatories planned for the next decade
from centimeter to X-ray wavelengths will provide the requisite panchromatic
view of the complex processes involved in the formation of the first generation
of galaxies and SMBHs, and cosmic reionization.Comment: 8 pages total. White paper submitted to the Astro 2010 Decadal Surve
The Lyman-alpha Forest and Heavy Element Systems of GB1759+7539
(abridged) We present observations of the high-redshift QSO GB1759+7539
(z=3.05) obtained with HIRES on the Keck 10m telescope. The spectrum has a
resolution of FWHM = 7 km/s, and a typical signal-to-noise ratio per 2 km/s
pixel of ~25 in the Ly-alpha forest region, and ~60 longward of the Ly-alpha
emission. The observed Ly-alpha forest systems have a mean redshift of z=2.7.
There is marginal evidence of clustering along the line of sight over the
velocity range 100 < v < 250 km/s. The 1-point and 2-point joint probability
distributions of the transmitted flux for the Ly-alpha forest were calculated,
and shown to be very insensitive to the heavy element contamination. We could
find no evidence of Voigt profile departures due to infalling gas, as observed
in the simulated forest spectra. Twelve heavy-element absorption systems were
identified, including damped Lyman-alpha systems at z=2.62 and 2.91. The C, N,
O, Al, Si, P, S, Mg, Fe, and Ni absorption features of these systems were
studied, and the elemental abundances calculated for the weak unsaturated
lines. The systems have metallicities of Z(2.62) ~1/20 Z(solar) and Z(2.91)
\~1/45 Z(solar). Both systems appear to have a low dust content. They show an
over-abundance of alpha-elements relative to Fe-peak elements, and an
under-abundance of odd atomic number elements relative to even. Nitrogen was
observed, and found to be under-abundant relative to oxygen, in line with the
time delay model of primary nitrogen production. C II* was also seen, allowing
us to determine an upper limit to the cosmic microwave background temperature
at z=2.62 of T(CMB)<12.9K.Comment: Added reference, and new S & N abundance determinations for z=2.91
syste
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