76,140 research outputs found
Rotation and activity in the solar-type stars of NGC 2547
We present high resolution spectroscopy of a sample of 24 solar-type stars in
the young (15-40 Myr), open cluster, NGC 2547. We use our spectra to confirm
cluster membership in 23 of these stars, determine projected equatorial
velocities and chromospheric activity, and to search for the presence of
accretion discs. We have found examples of both fast (vsini>50kms) and slow
(vsini<10kms) rotators, but find no evidence for active accretion in any of the
sample. The distribution of projected rotation velocities is indistinguishable
from the slightly older IC 2391 and IC 2602 clusters, implying similar initial
angular momentum distributions and circumstellar disc lifetimes. The presence
of very slow rotators indicates that either long (10-40 Myr) disc lifetimes or
internal differential rotation are needed, or that NGC 2547 (and IC 2391/2602)
were born with more slowly rotating stars than are presently seen in even
younger clusters and associations. The solar-type stars in NGC 2547 follow a
similar rotation-activity relationship to that seen in older clusters. X-ray
activity increases until a saturation level is reached for vsini>15-20kms. We
are unable to explain why this saturation level, of log (L_x/L_bol)~-3.3, is a
factor of two lower than in other clusters, but rule out anomalously slow
rotation rates or uncertainties in X-ray flux calculations.Comment: Accepted by MNRA
The Dynamics and Light Curves of Beamed Gamma Ray Burst Afterglows
The energy requirements of gamma ray bursts have in past been poorly
constrained because of three major uncertainties: The distances to bursts, the
degree of burst beaming, and the efficiency of gamma ray production. The first
of these has been resolved, with both indirect evidence (the distribution of
bursts in flux and position) and direct evidence (redshifted absorption
features in the afterglow spectrum of GRB 970508) pointing to cosmological
distances. We now wish to address the second uncertainty. Afterglows allow a
statistical test of beaming, described in an earlier paper. In this paper, we
modify a standard fireball afterglow model to explore the effects of beaming on
burst remnant dynamics and afterglow emission. If the burst ejecta are beamed
into angle zeta, the burst remnant's evolution changes qualitatively once its
bulk Lorentz factor Gamma < 1/zeta: Before this, Gamma declines as a power law
of radius, while afterwards, it declines exponentially. This change results in
a broken power law light curve whose late-time decay is faster than expected
for a purely spherical geometry. These predictions disagree with afterglow
observations of GRB 970508. We explored several variations on our model, but
none seems able to change this result. We therefore suggest that this burst is
unlikely to have been highly beamed, and that its energy requirements were near
those of isotropic models. More recent afterglows may offer the first practical
applications for our beamed models.Comment: 18 pages, uses emulateapj.sty, four embedded postscript figures.
Submitted to The Astrophysical Journal, 199
Chemiluminescent Tags for Tracking Insect Movement in Darkness: Application to Moth Photo-Orientation
The flight tracks of Manduca sexta (Lepidoptera: Sphingidae) flying toward a 5 watt incandescent light bulb were recorded under low light conditions with the aid of a camera-mounted photomultiplier and a glowing marker technique. Small felt pads bearing a chemiluminescent (glowi maerial, Cyalume®, were affixed to the abdomens of free-flying moths. insects orienting to a dim incandescent bulb were easily visible to the naked eye and were clearly captured on videotape. On their initial approach to the light source, M. sexta were found to orient at a mean angle of -0.220 ± 2.70 (mean ± SEM). The speed of the initial approach flight (OA ± 0.03 m/s) was significantly faster than the speed immediately after passing the light (0.29 ± 0.02 m/s; t =6.4, PM. sexta initially fly approximately at a light source and only after passing it, do they engage in circular flight around the source. M. sexta flight to lights does not entirely match any paths predicted by several light orientation mechanisms, including the commonly invoked light compass theory
A deep, wide-field search for substellar members in NGC 2264
We report the first results of our ongoing campaign to discover the first
brown dwarfs (BD) in NGC 2264, a young (3 Myr), populous star forming region
for which our optical studies have revealed a very high density of potential
candidates - 236 in 1 deg - from the substellar limit down to at least
20 M for zero reddening. Candidate BD were first selected
using wide field () band imaging with CFH12K, by reference to current
theoretical isochrones. Subsequently, 79 (33%) of the sample were found
to have near-infrared 2MASS photometry ( 0.3 mag. or better),
yielding dereddened magnitudes and allowing further investigation by comparison
with the location of NextGen and DUSTY isochrones in colour-colour and
colour-magnitude diagrams involving various combinations of ,, and
. We discuss the status and potential substellarity of a number of
relatively unreddened (A 5) likely low-mass members in our
sample, but in spite of the depth of our observations in , we are as yet
unable to unambiguously identify substellar candidates using only 2MASS data.
Nevertheless, there are excellent arguments for considering two faint (observed
18.4 and 21.2) objects as cluster candidates with masses
respectively at or rather below the hydrogen burning limit. More current
candidates could be proven to be cluster members with masses around 0.1
M {\it via} gravity-sensitive spectroscopy, and deeper near-infrared
imaging will surely reveal a hitherto unknown population of young brown dwarfs
in this region, accessible to the next generation of deep near-infrared
surveys.Comment: 10 pages, 12 figures, accepted by A&
Melting and Rippling Phenomenan in Two Dimensional Crystals with localized bonding
We calculate Root Mean Square (RMS) deviations from equilibrium for atoms in
a two dimensional crystal with local (e.g. covalent) bonding between close
neighbors. Large scale Monte Carlo calculations are in good agreement with
analytical results obtained in the harmonic approximation. When motion is
restricted to the plane, we find a slow (logarithmic) increase in fluctuations
of the atoms about their equilibrium positions as the crystals are made larger
and larger. We take into account fluctuations perpendicular to the lattice
plane, manifest as undulating ripples, by examining dual layer systems with
coupling between the layers to impart local rigidly (i.e. as in sheets of
graphene made stiff by their finite thickness). Surprisingly, we find a rapid
divergence with increasing system size in the vertical mean square deviations,
independent of the strength of the interplanar coupling. We consider an
attractive coupling to a flat substrate, finding that even a weak attraction
significantly limits the amplitude and average wavelength of the ripples. We
verify our results are generic by examining a variety of distinct geometries,
obtaining the same phenomena in each case.Comment: 17 pages, 28 figure
The effect of cultural and environmental factors on potato seed tuber morphology and subsequent sprout and stem development
Seed crops of the variety Estima were grown in each of 2 years using two planting dates, two harvest dates, two plant densities and two irrigation regimes to produce seed tubers which had experienced different cultural and environmental conditions. The effects of these treatments on tuber characteristics, sprout production and stem development in the ware crop were then determined in subsequent experiments using storage regimes of 3 and 10 °C. Time of planting the seed crop affected numbers of eyes, sprouts and above ground stems in the subsequent ware crop because environmental conditions around the time of tuber initiation appeared to alter tuber shape. Cooler, wetter conditions in the 7 days after tuber initiation were associated with tubers which were longer, heavier and had more eyes, sprouts and above ground stems. In contrast, the time of harvesting the seed crop did not affect tuber shape or numbers of above ground stems and there was no interaction with tuber size. The density of the seed crop had no effect on any character measured and irrigation well after tuber initiation did not affect tuber shape, numbers of sprouts or numbers of stems. Seed production treatments, which resulted in earlier dormancy break, were associated with tubers that produced more sprouts and above ground stems, in contrast to the conventional understanding of apical dominance. Storage at 3 °C gave fewer sprouts, a lower proportion of eyes with sprouts and fewer stems than storage at 10 °C. The major effects on stem production appear to result from environmental conditions at the time of tuber initiation of the seed crop and sprouting temperature
Synchronization and Control in Intrinsic and Designed Computation: An Information-Theoretic Analysis of Competing Models of Stochastic Computation
We adapt tools from information theory to analyze how an observer comes to
synchronize with the hidden states of a finitary, stationary stochastic
process. We show that synchronization is determined by both the process's
internal organization and by an observer's model of it. We analyze these
components using the convergence of state-block and block-state entropies,
comparing them to the previously known convergence properties of the Shannon
block entropy. Along the way, we introduce a hierarchy of information
quantifiers as derivatives and integrals of these entropies, which parallels a
similar hierarchy introduced for block entropy. We also draw out the duality
between synchronization properties and a process's controllability. The tools
lead to a new classification of a process's alternative representations in
terms of minimality, synchronizability, and unifilarity.Comment: 25 pages, 13 figures, 1 tabl
Helical structures from an isotropic homopolymer model
We present Monte Carlo simulation results for square-well homopolymers at a
series of bond lengths. Although the model contains only isotropic pairwise
interactions, under appropriate conditions this system shows spontaneous chiral
symmetry breaking, where the chain exists in either a left- or a right-handed
helical structure. We investigate how this behavior depends upon the ratio
between bond length and monomer radius.Comment: 10 pages, 3 figures, accepted for publication by Physical Review
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