2,693 research outputs found
Space use by passerine birds : a study of territory economics in robins Erithacus rubecula and dippers Cinclus cinclus
1. Cost constraints in models of territory size are based on time/activity/laboratory
estimates that predict birds using larger territories will incur higher energy costs. The predicted form of the cost constraint may be linear, accelerating or decelerating depending on assumptions inherent in the models. The aim of this study was to assess the reality and form of the cost constraint by making direct measurements of the energy costs of territory use in birds that occupy territories of different size and shape; polygonal territories represented by the
robin Erithacus rubecula, and linear by the dipper Cinclus cinclus. Free-living energy
expenditure was measured using the doubly-labelled water technique, whilst simultaneously recording patterns of territory use by radio-tracking.
2. Territorial robins concentrated their activity in one or more foraging patches located in bushes. Range polygons containing all the foraging patches used by an individual provided estimates of territory area, and were generally of high eccentricity. A small proportion of robins
was classified as non-territorial based on range polygon areas. Furthermore, while territorial robins showed high fidelity to ranges over the short term (days), non-territorial individuals were nomadic. Over the longer term (months), however, some territorial robins showed range drift.
Dippers similarly used preferred core regions within ranges, although there was no selection for particular habitat features.
3. Because robins occupied territory polygons which varied from polygonal to highly linear, work was focused on this species to allow intra-specific comparison. Robins tended to commute between foraging patches by flying. It was appropriate, therefore, to describe territories in terms of a number of patches linked by a network of flight paths. This generated two further measures of territory size; the number of patches used and the total flight distance
between patches.
4. The robins exploited a renewing food supply. Predictions were tested concerning the temporal scheduling of visits to foraging patches within territories. Patches tended to be separated by flight paths of similar lengths, and were visited in a regular sequence. Although the number of foraging patches used varied, all territories had similar total core areas. Robins using many small foraging patches commuted between patches more often and covered a larger total flight distance during each foraging circuit of the territory. The configurations of foraging
patches were used in a highly linear manner. This was true even if the territory containing them was of low eccentricity.
5. Changes in structure and pattern of use varied predictably with territory size, and could be described mathematically. Based on this and published time/activity budgets, a suite of models was developed to predict how energy costs would vary with number of patches used and total flight distance between patches. Models were tested by directly measuring the energy expenditure of robins using different territories. The number of patches used and total flight
distance between patches were both significantly correlated with energy expenditure, while territory area was not. One of the models showed a significant fit to the observed data, and suggested that the form of the energy cost constraint on territory size was linear. The effect of
territory shape on energy costs was minimal. The implications of these results for models of territory size are discussed.
6. The slope and elevation of the energy cost constraint varied with the morphology of
territory occupants. Based on this, an association of morphology with territory size was predicted; robins of lower mass and wing-loading using larger territories. The observed data supported these predictions, and suggested a possible genetic predisposition to particular patterns of territory occupancy in the robin
Stellar Winds on the Main-Sequence I: Wind Model
Aims: We develop a method for estimating the properties of stellar winds for
low-mass main-sequence stars between masses of 0.4 and 1.1 solar masses at a
range of distances from the star.
Methods: We use 1D thermal pressure driven hydrodynamic wind models run using
the Versatile Advection Code. Using in situ measurements of the solar wind, we
produce models for the slow and fast components of the solar wind. We consider
two radically different methods for scaling the base temperature of the wind to
other stars: in Model A, we assume that wind temperatures are fundamentally
linked to coronal temperatures, and in Model B, we assume that the sound speed
at the base of the wind is a fixed fraction of the escape velocity. In Paper II
of this series, we use observationally constrained rotational evolution models
to derive wind mass loss rates.
Results: Our model for the solar wind provides an excellent description of
the real solar wind far from the solar surface, but is unrealistic within the
solar corona. We run a grid of 1200 wind models to derive relations for the
wind properties as a function of stellar mass, radius, and wind temperature.
Using these results, we explore how wind properties depend on stellar mass and
rotation.
Conclusions: Based on our two assumptions about the scaling of the wind
temperature, we argue that there is still significant uncertainty in how these
properties should be determined. Resolution of this uncertainty will probably
require both the application of solar wind physics to other stars and detailed
observational constraints on the properties of stellar winds. In the final
section of this paper, we give step by step instructions for how to apply our
results to calculate the stellar wind conditions far from the stellar surface.Comment: 24 pages, 13 figures, 2 tables, Accepted for publication in A&
Effect of stellar wind induced magnetic fields on planetary obstacles of non-magnetized hot Jupiters
We investigate the interaction between the magnetized stellar wind plasma and
the partially ionized hydrodynamic hydrogen outflow from the escaping upper
atmosphere of non- or weakly magnetized hot Jupiters. We use the well-studied
hot Jupiter HD 209458b as an example for similar exoplanets, assuming a
negligible intrinsic magnetic moment. For this planet, the stellar wind plasma
interaction forms an obstacle in the planet's upper atmosphere, in which the
position of the magnetopause is determined by the condition of pressure balance
between the stellar wind and the expanded atmosphere, heated by the stellar
extreme ultraviolet (EUV) radiation. We show that the neutral atmospheric atoms
penetrate into the region dominated by the stellar wind, where they are ionized
by photo-ionization and charge exchange, and then mixed with the stellar wind
flow. Using a 3D magnetohydrodynamic (MHD) model, we show that an induced
magnetic field forms in front of the planetary obstacle, which appears to be
much stronger compared to those produced by the solar wind interaction with
Venus and Mars. Depending on the stellar wind parameters, because of the
induced magnetic field, the planetary obstacle can move up to ~0.5-1 planetary
radii closer to the planet. Finally, we discuss how estimations of the
intrinsic magnetic moment of hot Jupiters can be inferred by coupling
hydrodynamic upper planetary atmosphere and MHD stellar wind interaction models
together with UV observations. In particular, we find that HD 209458b should
likely have an intrinsic magnetic moment of 10-20% that of Jupiter.Comment: 8 pages, 6 figures, 2 tables, accepted to MNRA
A Critique of Current Magnetic-Accretion Models for Classical T-Tauri Stars
Current magnetic-accretion models for classical T-Tauri stars rely on a
strong, dipolar magnetic field of stellar origin to funnel the disk material
onto the star, and assume a steady-state. In this paper, I critically examine
the physical basis of these models in light of the observational evidence and
our knowledge of magnetic fields in low-mass stars, and find it lacking.
I also argue that magnetic accretion onto these stars is inherently a
time-dependent problem, and that a steady-state is not warranted.
Finally, directions for future work towards fully-consistent models are
pointed out.Comment: 2 figure
Measurements of the electric quadrupole moment of Nb and Zr isotopes with modulated adiabatic fast passage after recoil implantation into hcp Co
Hitting Time of Quantum Walks with Perturbation
The hitting time is the required minimum time for a Markov chain-based walk
(classical or quantum) to reach a target state in the state space. We
investigate the effect of the perturbation on the hitting time of a quantum
walk. We obtain an upper bound for the perturbed quantum walk hitting time by
applying Szegedy's work and the perturbation bounds with Weyl's perturbation
theorem on classical matrix. Based on the definition of quantum hitting time
given in MNRS algorithm, we further compute the delayed perturbed hitting time
(DPHT) and delayed perturbed quantum hitting time (DPQHT). We show that the
upper bound for DPQHT is actually greater than the difference between the
square root of the upper bound for a perturbed random walk and the square root
of the lower bound for a random walk.Comment: 9 page
Emergence of fractal behavior in condensation-driven aggregation
We investigate a model in which an ensemble of chemically identical Brownian
particles are continuously growing by condensation and at the same time undergo
irreversible aggregation whenever two particles come into contact upon
collision. We solved the model exactly by using scaling theory for the case
whereby a particle, say of size , grows by an amount over the
time it takes to collide with another particle of any size. It is shown that
the particle size spectra of such system exhibit transition to dynamic scaling
accompanied by the emergence of fractal of
dimension . One of the remarkable feature of this
model is that it is governed by a non-trivial conservation law, namely, the
moment of is time invariant regardless of the choice of the
initial conditions. The reason why it remains conserved is explained by using a
simple dimensional analysis. We show that the scaling exponents and
are locked with the fractal dimension via a generalized scaling relation
.Comment: 8 pages, 6 figures, to appear in Phys. Rev.
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