2,915 research outputs found
Exploiting Query Structure and Document Structure to Improve Document Retrieval Effectiveness
In this paper we present a systematic analysis of document
retrieval using unstructured and structured queries within
the score region algebra (SRA) structured retrieval framework. The behavior of di®erent retrieval models, namely
Boolean, tf.idf, GPX, language models, and Okapi, is tested
using the transparent SRA framework in our three-level structured retrieval system called TIJAH. The retrieval models are implemented along four elementary retrieval aspects: element and term selection, element score computation, score combination, and score propagation.
The analysis is performed on a numerous experiments
evaluated on TREC and CLEF collections, using manually
generated unstructured and structured queries. Unstructured queries range from the short title queries to long title
+ description + narrative queries. For generating structured
queries we exploit the knowledge of the document structure
and the content used to semantically describe or classify
documents. We show that such structured information can
be utilized in retrieval engines to give more precise answers to user queries then when using unstructured queries
Shrubs in the cold : interactions between vegetation, permafrost and climate in Siberian tundra
The Arctic is experiencing strong increases in air temperature during the last decades. High-latitude tundra regions are very responsive to changes in temperature and may cause a shift in tundra vegetation composition towards greater dominance of deciduous shrubs. With increasing deciduous shrub cover, the surface albedo (proportion of sunlight that is reflected to the atmosphere) may be reduced and lead to air warming by trapping more solar radiation into the Arctic ecosystem. As a result of this warming, thawing of carbon-rich permafrost soils may increase and cause a large greenhouse gas flux to the atmosphere, thus contributing to global warming. In my thesis I studied how climate influences shrub growth in the Siberian tundra and how climate-induced changes in shrub cover affect summer permafrost thaw and surface albedo. I investigated these interactions between climate, permafrost and Arctic shrub growth using a combination of shrub ring width analysis, field experiments and remote sensing techniques. I measured and compared growth ring widths with meteorological station data and observed that shrub growth is stimulated by higher summer air temperatures. By performing a shrub removal experiment, I demonstrated that a temperature-induced increase in shrub cover may reduce summer permafrost thaw. Shading by the shrub canopy reduced the transfer of energy to the soil. A denser shrub cover thus effectively reduces summer permafrost thaw, despite leading at the same time to a lower surface albedo. These results indicate it is important to incorporate feedbacks between shrub growth, climate and permafrost thaw in model predictions on the Arctic climate and stability of permafrost in a future warmer world. </p
A-dependence of hadronization in nuclei
The A-dependence of models for the attenuation of hadron production in
semi-inclusive deep-inelastic scattering on a nucleus is investigated for
realistic matter distributions. It is shown that the dependence for a pure
partonic (absorption) mechanism is more complicated than a simple
() behavior, commonly found when using rectangular or Gaussian
distributions, but that the A-dependence may still be indicative for the
dominant mechanism of hadronization.Comment: 4 pages, 4 figure
Mass Density Profiles of LSB Galaxies
We derive the mass density profiles of dark matter halos that are implied by
high spatial resolution rotation curves of low surface brightness galaxies. We
find that at small radii, the mass density distribution is dominated by a
nearly constant density core with a core radius of a few kpc. For rho(r) ~ r^a,
the distribution of inner slopes a is strongly peaked around a = -0.2. This is
significantly shallower than the cuspy a < -1 halos found in CDM simulations.
While the observed distribution of alpha does have a tail towards such extreme
values, the derived value of alpha is found to depend on the spatial resolution
of the rotation curves: a ~ -1 is found only for the least well resolved
galaxies. Even for these galaxies, our data are also consistent with constant
density cores (a = 0) of modest (~ 1 kpc) core radius, which can give the
illusion of steep cusps when insufficiently resolved. Consequently, there is no
clear evidence for a cuspy halo in any of the low surface brightness galaxies
observed.Comment: To be published in ApJ Letters. 6 pages. Uses aastex and
emulateapj5.sty Typo in Eq 1 fixe
Immediate Effects of Cervical Spine Thrust Joint Manipulation on Gait Parameters in Individuals with Neck Pain
Purpose/Hypotheses: 1. To investigate for any change in gait parameters in individuals with neck pain while walking with different functional neck conditions immediately following cervical thrust joint manipulation (TJM) versus a sham intervention. 2. To investigate any association between Global Rating of Change (GROC) scores and gait parameters immediately following cervical thrust joint manipulation versus a sham intervention. The hypotheses were that, a) cervical TJM would have an immediate effect on gait parameters during walking with the neck in at least one of three conditions (neutral, flexion/extension and rotation) among individuals with neck pain; and b) higher scores on the GROC would be associated with improved gait parameters post-intervention. Subjects: Convenience sample of 40 individuals (30 female; mean age 24.5 ± 6.78 years) with neck pain. To qualify, subjects had to have a score \u3e0 on the question of pain intensity in the neck on the Neck Disability Index (NDI) questionnaire and have no contraindications or precautions for cervical TJM. Materials/Methods: Subjects walked on a Zeno Walkway under the following conditions: 1) head in neutral; 2) head rotating from side-to-side, and 3) head nodding up and down. After completing 30 practice trials (10 in each condition), pre-intervention trial 1 gait parameters were recorded for each of the three neck conditions in a randomized order. After a 5-minute rest period, pre-intervention trial 2 was conducted for each condition in same order as trial 1. Subjects then received one of two randomly assigned interventions: cervical spine TJM or active cervical rotation. Immediately after the intervention, the subject returned to the Zeno Walkway for the post-intervention trial 3 in each of the three conditions, in the same order as their previous trials. Gait parameters of average step length, stride length, stride width, velocity, and cadence were analyzed using a 2x2 repeated measures ANOVA (of trials 2 and 3), as well as independent and paired t-tests, to determine if there were any significant changes based on intervention when comparing TJM to sham groups. Results: The results of the 2x2 ANOVA revealed significant interactions between group and time on average gait velocity (p=0.008), step length (p=\u3c0.001), and stride length (p=0.009) when the head was in a neutral position. The TJM group experienced significant increases from pre to post-intervention as shown by paired samples t-test for average gait velocity (p=0.003), step length (p\u3c0.001), and stride length (p=0.008). The sham group however, experienced no significant change in gait velocity (p = 0.290), average step length (p = 0.299), and stride length (p = 0.292). There was also a significant decrease in the Numeric Pain Rating Scale (NPRS) (mean decrease of 1.25; p=0.003) and the group that received cervical TJM reported an improved perception of change demonstrated by an average increase in GROC score by 2.85 (p=0.001). Conclusions: Although our results demonstrate a statistically significant improvement in three gait parameters following TJM while walking with the neck in a neutral position, the improvements are not clinically significant. At this time, there is no evidence-based indication for the clinical use of cervical TJM to improve gait parameters in individuals with neck pain. Our findings cannot confirm clinical significance for reduction of neck pain with cervical TJM based on NDI, NPRS, or GROC questionnaires
A High-Resolution Rotation Curve of NGC 6822: A Test-case for Cold Dark Matter
We present high resolution rotation curves of the local group dwarf irregular
galaxy NGC 6822 obtained with the Australia Telescope Compact Array. Our best
curves have an angular resolution of 8'' or 20 pc and contain some 250
independent points. The stellar and gas components of NGC 6822 cannot explain
the shape of the curve, except for the very inner regions, and NGC 6822 is
consequently very dark matter dominated. There is no evidence for the presence
of a steep density cusp down to scales of ~20 pc, contrary to the predictions
of Cold Dark Matter.Comment: Accepted for publication in MNRA
Free-Floating HI Clouds in the M 81 Group
Recent VLA observations pointed at dwarf spheroidal (dSph) galaxies in the M
81 group reveal a hitherto hidden population of extremely low mass (~1e5 Msol)
HI clouds with no obvious optical counterparts. We have searched 10 fields in
the M81 group totalling 2.2 square degree, both targeting known dwarf
spheroidal galaxies and blank fields around the central triplet. Our
observations show that the new population of low-mass HI clouds appears to be
confined to a region toward the South-East of the central triplet (at distances
of ~100 kpc from M 81). Possible explanations for these free-floating HI clouds
are that they are related to the dSphs found to the South-East of M 81, that
they belong to the galaxies of the M 81 triplet (equivalent to HVCs), that they
are of primordial nature and provide fresh, unenriched material falling into
the M 81 group, or that they are tidal debris from the 3-body interaction
involving M 81-M 82-NGC 3077. Based on circumstantial evidence, we currently
favour the latter explanation.Comment: To appear in the Proceedings of IAU Symp. 244 on "Dark Galaxies and
Lost Baryons", eds. J. I. Davies & M. J. Disne
A search for Low Surface Brightness galaxies in the near-infrared I. Selection of the sample
A sample of about 3,800 Low Surface Brightness (LSB) galaxies was selected
using the all-sky near-infrared (J, H and K_s-band) 2MASS survey. The selected
objects have a mean central surface brightness within a 5 arcsec radius around
their centre fainter than 18 mag/sq.arcsec in the K_s band, making them the
lowest surface brightness galaxies detected by 2MASS. A description is given of
the relevant properties of the 2MASS survey and the LSB galaxy selection
procedure, as well as of basic photometric properties of the selected objects.
The latter properties are compared to those of other samples of galaxies, of
both LSBs and `classical' high surface brightness (HSB) objects, which were
selected in the optical. The 2MASS LSBs have a (B_T_c)-(K_T) colour which is on
average 0.9 mag bluer than that of HSBs from the NGC. The 2MASS sample does not
appear to contain a significant population of red objects.Comment: accepted for publication in Astronomy and Astrophysics on 24/2/2003;
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