5,486 research outputs found
Constraining the orbit of the possible companion to Beta Pictoris: New deep imaging observations
We recently reported on the detection of a possible planetary-mass companion
to Beta Pictoris at a projected separation of 8 AU from the star, using data
taken in November 2003 with NaCo, the adaptive-optics system installed on the
Very Large Telescope UT4. Eventhough no second epoch detection was available,
there are strong arguments to favor a gravitationally bound companion rather
than a background object. If confirmed and located at a physical separation of
8 AU, this young, hot (~1500 K), massive Jovian companion (~8 Mjup) would be
the closest planet to its star ever imaged, could be formed via core-accretion,
and could explain the main morphological and dynamical properties of the dust
disk. Our goal was to return to Beta Pic five years later to obtain a
second-epoch observation of the companion or, in case of a non-detection,
constrain its orbit. Deep adaptive-optics L'-band direct images of Beta Pic and
Ks-band Four-Quadrant-Phase-Mask (4QPM) coronagraphic images were recorded with
NaCo in January and February 2009. We also use 4QPM data taken in November
2004. No point-like signal with the brightness of the companion candidate
(apparent magnitudes L'=11.2 or Ks ~ 12.5) is detected at projected distances
down to 6.5 AU from the star in the 2009 data. As expected, the non-detection
does not allow to rule out a background object; however, we show that it is
consistent with the orbital motion of a bound companion that got closer to the
star since first observed in 2003 and that is just emerging from behind the
star at the present epoch. We place strong constraints on the possible orbits
of the companion and discuss future observing prospects.Comment: 8 pages, 8 figures, 1 table, accepted for publication in Astronomy
and Astrophysic
Elastic Lattice Polymers
We study a model of "elastic" lattice polymer in which a fixed number of
monomers is hosted by a self-avoiding walk with fluctuating length . We
show that the stored length density scales asymptotically
for large as , where is the
polymer entropic exponent, so that can be determined from the analysis
of . We perform simulations for elastic lattice polymer loops with
various sizes and knots, in which we measure . The resulting estimates
support the hypothesis that the exponent is determined only by the
number of prime knots and not by their type. However, if knots are present, we
observe strong corrections to scaling, which help to understand how an entropic
competition between knots is affected by the finite length of the chain.Comment: 10 page
Polymer desorption under pulling: a novel dichotomic phase transition
We show that the structural properties and phase behavior of a self-avoiding
polymer chain on adhesive substrate, subject to pulling at the chain end, can
be obtained by means of a Grand Canonical Ensemble (GCE) approach. We derive
analytical expressions for the mean length of the basic structural units of
adsorbed polymer, such as loops and tails, in terms of the adhesive potential
and detachment force, and determine values of the universal exponents which
govern their probability distributions. Most notably, the hitherto
controversial value of the critical adsorption exponent is found to
depend essentially on the interaction between different loops. The chain
detachment transition turns out to be of the first order, albeit dichotomic,
i.e., no coexistence of different phase states exists. These novel theoretical
predictions and the suggested phase diagram of the adsorption-desorption
transformation under external pulling force are verified by means of extensive
Monte Carlo simulations.Comment: 10 pages, 4 figure
Individual Entanglements in a Simulated Polymer Melt
We examine entanglements using monomer contacts between pairs of chains in a
Brownian-dynamics simulation of a polymer melt. A map of contact positions with
respect to the contacting monomer numbers (i,j) shows clustering in small
regions of (i,j) which persists in time, as expected for entanglements. Using
the ``space''-time correlation function of the aforementioned contacts, we show
that a pair of entangled chains exhibits a qualitatively different behavior
than a pair of distant chains when brought together. Quantitatively, about 50%
of the contacts between entangled chains are persistent contacts not present in
independently moving chains. In addition, we account for several observed
scaling properties of the contact correlation function.Comment: latex, 12 pages, 7 figures, postscript file available at
http://arnold.uchicago.edu/~ebn
Development ethics through the lenses of caring, gender, and human security.
Thinking about ethics of development and human development must both treat development
in a global perspective and yet reflect on the content of human. This paper explores some faces
of globalization by using a gender perspective, in order to consider reproduction
(psychological and emotional as well as biological) and the activities and attitudes
of care that give moral resources for response to systemic tragedy,
not only for identifying and understanding it. There now exist globally
interconnected systems of vulnerability and capability, for which matching systems of human security,
care and responsibility are needed in order to protect human dignity. The discourse of human security
helps here by better grounding an agenda of basic human needs, in an ethnography of ordinary lives
rather than only an abstracted accounting of deficiencies or an elevated language of opportunities.
It must be emotionally and existentially grounded too. The authors examine the potential contributions
the tradition of Mahayana Buddhism; the work of philosopher-anthropologist Ananta Giri; and feminist care ethics
Introductory Remarks
Driven by diverse forces – economic pressures and opportunities, climate change, war, conquest, and transformation of political regimes – human migration has been central to circulation of knowledge and values, goods and labour. Yet, it has been subject to mainly disciplinary inquiries and the existing body of studies has lacked a comprehensive perspective. This volume essays precisely such a more comprehensive historical and experiential perspective, and as a result leads us to reconsider the meanings of ‘human’, ‘movement’, and ‘borders’
Excitonic ferromagnetism in the hexaborides
A ferromagnet with a small spontaneous moment but with a high Curie
temperature can be obtained by doping an excitonic insulator made from a spin
triplet exciton condensate. Such a condensate can occur in a semimetal with a
small overlap or a semiconductor with a small bandgap. We propose that it is
responsible for the unexpected ferromagnetism in the doped hexaboride material
Ca_{1-x}La_xB_6.Comment: 4 pages, 3 figure
The first tidally disrupted ultra-faint dwarf galaxy? : a spectroscopic analysis of the Tucana III stream
We present a spectroscopic study of the tidal tails and core of the Milky Way satellite Tucana III, collectively referred to as the Tucana III stream, using the 2dF+AAOmega spectrograph on the Anglo-Australian Telescope and the IMACS spectrograph on the Magellan Baade Telescope. In addition to recovering the brightest nine previously known member stars in the Tucana III core, we identify 22 members in the tidal tails.We observe strong evidence for a velocity gradient of 8.0 0.4 km s-1 deg-1 over at least 3° on the sky. Based on the continuity in velocity, we confirm that the Tucana III tails are real tidal extensions of Tucana III. The large velocity gradient of the stream implies that Tucana III is likely on a radial orbit. We successfully obtain metallicities for four members in the core and 12 members in the tails. We find that members close to the ends of the stream tend to be more metal-poor than members in the core, indicating a possible metallicity gradient between the center of the progenitor halo and its edge. The spread in metallicity suggests that the progenitor of the Tucana III stream is likely a dwarf galaxy rather than a star cluster. Furthermore, we find that with the precise photometry of the Dark Energy Survey data, there is a discernible color offset between metal-rich disk stars and metal-poor stream members. This metallicity-dependent color offers a more efficient method to recognize metal-poor targets and will increase the selection efficiency of stream members for future spectroscopic follow-up programs on stellar streams
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