25,345 research outputs found
The Star Clusters in the Irregular Galaxy NGC 4449
We examine the star clusters in the irregular galaxy NGC 4449. We use a
near-infrared spectrum and broad-band images taken with the HST to place a
limit of 8--15 Myrs on the age of the bright central ojbect in NGC 4449. Its
luminosity and size suggest that it is comparable to young super star clusters.
However, there is a peculiar nucleated-bar structure at the center of this star
cluster, and we suggest that this structure is debris from the interaction that
has produced the counter-rotating gas systems and extended gas streamers in the
galaxy.
From the images we identify 60 other candidate compact star clusters in NGC
4449. Fourteen of these could be background elliptical galaxies or old globular
star clusters. Of the star clusters, three, in addition to the central object,
are potentially super star clusters, and many others are comparable to the
populous clusters found in the LMC. The star clusters span a large range in
ages with no obvious peak in cluster formation that might be attributed to the
interaction that the galaxy has experienced.Comment: To be published in PASP, Feb. 2001; also attainable from
ftp.lowell.edu, cd pub/dah/n4449pape
The Stellar and Gas Kinematics of Several Irregular Galaxies
We present long-slit spectra of three irregular galaxies from which we
determinethe stellar kinematics in two of the galaxies (NGC 1156 and NGC 4449)
and ionized-gas kinematics in all three (including NGC 2366). We compare this
to the optical morphology and to the HI kinematics of the galaxies. In the
ionized gas, we see a linear velocity gradient in all three galaxies. In NGC
1156 we also detect a weak linear velocity gradient in the stars of (5+/-1/sin
i) km/s/kpc to a radius of 1.6 kpc. The stars and gas are rotating about the
same axis, but this is different from the major axis of the stellar bar which
dominates the optical light of the galaxy. In NGC 4449 we do not detect
organized rotation of the stars and place an upper limit of (3/sin i) km/s/kpc
to a radius of 1.2 kpc. For NGC 4449, which has signs of a past interaction
with another galaxy, we develop a model to fit the observed kinematics of the
stars and gas. In this model the stellar component is in a rotating disk seen
nearly face-on while the gas is in a tilted disk with orbits whose planes
precess in the gravitational potential. This model reproduces the apparent
counter-rotation of the inner gas of the galaxy. The peculiar orbits of the gas
are presumed due to acquisition of gas in the past interaction.Comment: To be published in ApJ, November 20, 200
Strategic argumentation dialogues for persuasion: Framework and experiments based on modelling the beliefs and concerns of the persuadee
Persuasion is an important and yet complex aspect of human intelligence. When undertaken through dialogue, the deployment of good arguments, and therefore counterarguments, clearly has a significant effect on the ability to be successful in persuasion. Two key dimensions for determining whether an argument is 'good' in a particular dialogue are the degree to which the intended audience believes the argument and counterarguments, and the impact that the argument has on the concerns of the intended audience. In this paper, we present a framework for modelling persuadees in terms of their beliefs and concerns, and for harnessing these models in optimizing the choice of move in persuasion dialogues. Our approach is based on the Monte Carlo Tree Search which allows optimization in real-time. We provide empirical results of a study with human participants that compares an automated persuasion system based on this technology with a baseline system that does not take the beliefs and concerns into account in its strategy
Liquid friction on charged surfaces: from hydrodynamic slippage to electrokinetics
Hydrodynamic behavior at the vicinity of a confining wall is closely related
to the friction properties of the liquid/solid interface. Here we consider,
using Molecular Dynamics simulations, the electric contribution to friction for
charged surfaces, and the induced modification of the hydrodynamic boundary
condition at the confining boundary. The consequences of liquid slippage for
electrokinetic phenomena, through the coupling between hydrodynamics and
electrostatics within the electric double layer, are explored. Strong
amplification of electro-osmotic effects is revealed, and the non-trivial
effect of surface charge is discussed. This work allows to reconsider existing
experimental data, concerning Zeta potentials of hydrophobic surfaces and
suggest the possibility to generate ``giant'' electro-osmotic and
electrophoretic effects, with direct applications in microfluidics
Critical Casimir interaction of ellipsoidal colloids with a planar wall
Based on renormalization group concepts and explicit mean field calculations
we study the universal contribution to the effective force and torque acting on
an ellipsoidal colloidal particle which is dissolved in a critical fluid and is
close to a homogeneous planar substrate. At the same closest distance between
the substrate and the surface of the particle, the ellipsoidal particle prefers
an orientation parallel to the substrate and the magnitude of the fluctuation
induced force is larger than if the orientation of the particle is
perpendicular to the substrate. The sign of the critical torque acting on the
ellipsoidal particle depends on the type of boundary conditions for the order
parameter at the particle and substrate surfaces, and on the pivot with respect
to which the particle rotates
Light elements in massive single and binary stars
We highlight the role of the light elements (Li, Be, B) in the evolution of
massive single and binary stars, which is largely restricted to a diagnostic
value, and foremost so for the element boron. However, we show that the boron
surface abundance in massive early type stars contains key information about
their foregoing evolution which is not obtainable otherwise. In particular, it
allows to constrain internal mixing processes and potential previous mass
transfer event for binary stars (even if the companion has disappeared). It may
also help solving the mystery of the slowly rotating nitrogen-rich massive main
sequence stars.Comment: 10 pages, 8 figures, to appear in proc. IAU-Symp. 268. C. Charbonnel
et al., eds
DDO 88: A Galaxy-Sized Hole in the Interstellar Medium
We present an HI and optical study of the gas-rich dwarf irregular galaxy DDO
88. Although DDO 88's global optical and HI parameters are normal for its
morphological type, it hosts a large (3 kpc diameter) and unusually complete
ring of enhanced HI emission. The gas ring is located at approximately
one-third of the total HI radius and one-half the optically-defined Holmberg
radius, and contains 30% of the total HI of the galaxy. The ring surrounds a
central depression in the HI distribution, so it may be a shell formed by a
starburst episode. However, the UBV colors in the HI hole are not bluer than
the rest of the galaxy as would be expected if an unusual star-forming event
had taken place there recently, but there is an old (~1-3 Gyr), red cluster
near the center of the hole that is massive enough to have produced the hole in
the HI. An age estimate for the ring, however, is uncertain because it is not
observed to be expanding. An expansion model produces a lower estimate of 0.5
Gyr, but the presence of faint star formation regions associated with the ring
indicate a much younger age. We also estimate that the ring could have
dispersed by now if it is older than 0.5 Gyr. This implies that the ring is
younger than 0.5 Gyr. A younger age would indicate that the red cluster did not
produce the hole and ring. If this ring and the depression in the gas which it
surrounds were not formed by stellar winds and supernovae, this would indicate
that some other, currently unidentified, mechanism is operating.Comment: 44 pages; 16 figures. To appear in AJ, January 2005. Available from
ftp.lowell.edu, cd pub/dah/papers/d88 and http://www.fiu.edu/~simpsonc/d8
Validity and practical utility of accelerometry for the measurement of in-hand physical activity in horses
Background:
Accelerometers are valid, practical and reliable tools for the measurement of habitual physical activity (PA). Quantification of PA in horses is desirable for use in research and clinical settings. The objective of this study was to evaluate a triaxial accelerometer for objective measurement of PA in the horse by assessment of their practical utility and validity.
Horses were recruited to establish both the optimal site of accelerometer attachment and questionnaire designed to explore owner acceptance. Validity and cut-off values were obtained by assessing PA at various gaits. Validation study- 20 horses wore the accelerometer while being filmed for 10Â min each of rest, walking and trotting and 5 mins of canter work. Practical utility study- five horses wore accelerometers on polls and withers for 18Â h; compliance and relative data losses were quantified.
Results:
Accelerometry output differed significantly between the four PA levels (P <0•001) for both wither and poll placement. For withers placement, ROC analyses found optimal sensitivity and specificity at a cut-off of <47 counts per minute (cpm) for rest (sensitivity 99.5 %, specificity 100 %), 967–2424 cpm for trotting (sensitivity 96.7 %, specificity 100 %) and ≥2425 cpm for cantering (sensitivity 96.0 %, specificity 97.0 %). Attachment at the poll resulted in optimal sensitivity and specificity at a cut-off of <707 counts per minute (cpm) for rest (sensitivity 97.5 %, specificity 99.6 %), 1546–2609 cpm for trotting (sensitivity 90.33 %, specificity 79.25 %) and ≥2610 cpm for cantering (sensitivity 100 %, specificity 100 %) In terms of practical utility, accelerometry was well tolerated and owner acceptance high.
Conclusion:
Accelerometry data correlated well with varying levels of in-hand equine activity. The use of accelerometers is a valid method for objective measurement of controlled PA in the horse
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