40,756 research outputs found
A precessing jet model for the PN K 3-35: simulated radio-continuum emission
The bipolar morphology of the planetary nebula (PN) K 3-35 observed in
radio-continuum images was modelled with 3D hydrodynamic simulations with the
adaptive grid code yguazu-a. We find that the observed morphology of this PN
can be reproduced considering a precessing jet evolving in a dense AGB
circumstellar medium, given by a mass loss rate
\dot{M}_{csm}=5x10^{-5}M_{\odot}/yr and a terminal velocity v_{w}=10 km/s.
Synthetic thermal radio-continuum maps were generated from numerical results
for several frequencies. Comparing the maps and the total fluxes obtained from
the simulations with the observational results, we find that a model of
precessing dense jets, where each jet injects material into the surrounding CSM
at a rate \dot{M}_j=2.8x10^{-4} {M_{\odot}/yr (equivalent to a density of
8x10^{4} {cm}^{-3}, a velocity of 1500 km/s, a precession period of 100 yr, and
a semi-aperture precession angle of 20 degrees agrees well with the
observations.Comment: 6 pages, 4 figures, accepted to MNRA
Heat Fluctuations in Brownian Transducers
Heat fluctuation probability distribution function in Brownian transducers
operating between two heat reservoirs is studied. We find, both analytically
and numerically, that the recently proposed Fluctuation Theorem for Heat
Exchange [C. Jarzynski and D. K. Wojcik, Phys. Rev. Lett. 92, 230602 (2004)]
has to be modified when the coupling mechanism between both baths is
considered. We also extend such relation when external work is present. Our
work fixes the domain of applicability of the theorem in more realistic
operating systems.Comment: Comments are welcom
Gravitational Field of Spherical Branes
The warped solution of Einstein's equations corresponding to the spherical
brane in five-dimensional AdS is considered. This metric represents interiors
of black holes on both sides of the brane and can provide gravitational
trapping of physical fields on the shell. It is found the analytic form of the
coordinate transformations from the Schwartschild to co-moving frame that
exists only in five dimensions. It is shown that in the static coordinates
active gravitational mass of the spherical brane, in agreement with Tolman's
formula, is negative, i.e. such objects are gravitationally repulsive.Comment: Minor corrections, 8 pages, the version accepted by Mod. Phys. Lett.
Stability of Influence Maximization
The present article serves as an erratum to our paper of the same title,
which was presented and published in the KDD 2014 conference. In that article,
we claimed falsely that the objective function defined in Section 1.4 is
non-monotone submodular. We are deeply indebted to Debmalya Mandal, Jean
Pouget-Abadie and Yaron Singer for bringing to our attention a counter-example
to that claim.
Subsequent to becoming aware of the counter-example, we have shown that the
objective function is in fact NP-hard to approximate to within a factor of
for any .
In an attempt to fix the record, the present article combines the problem
motivation, models, and experimental results sections from the original
incorrect article with the new hardness result. We would like readers to only
cite and use this version (which will remain an unpublished note) instead of
the incorrect conference version.Comment: Erratum of Paper "Stability of Influence Maximization" which was
presented and published in the KDD1
The age structure of stellar populations in the solar vicinity. Clues of a two-phase formation history of the Milky Way disk
We analyze high quality abundances data of solar neighborhood stars and show
that there are two distinct regimes of [alpha/Fe] versus age which we identify
as the epochs of the thick and thin disk formation. A tight correlation between
metallicity and [alpha/Fe] versus age is clearly identifiable on thick disk
stars, implying that this population formed from a well mixed ISM, over a time
scale of 4-5 Gyr. Thick disk stars vertical velocity dispersion correlate with
age, with the youngest objects having as small scale heights as those of thin
disk stars. A natural consequence of these two results is that a vertical
metallicity gradient is expected in this population. We suggest that the thick
disk set the initial conditions for the formation of the inner thin disk. This
provides also an explanation of the apparent coincidence between the step in
metallicity at 7-10 kpc in the thin disk and the confinment of the thick disk
at about R<10 kpc. We suggest that the outer thin disk developped outside the
influence of the thick disk, but also that the high alpha-enrichment of the
outer regions may originate from a primordial pollution by the gas expelled
from the thick disk. Local metal-poor thin disk stars, whose properties are
best explained by an origin in the outer disk, are shown to be as old as the
youngest thick disk (9-10 Gyr), implying that the outer thin disk started to
form while the thick disk formation was still on-going in the inner Galaxy. We
point out that, given the tight age-abundance relations in the thick disk, an
inside-out process would give rise to a radial gradient in abundances in this
population which is not observed. Finally, we argue that the data discussed
here leave little room for radial migration, either to have contaminated the
solar vicinity, or, to have redistributed stars in significant proportion
across the solar annulus.Comment: Accepted in A&A, Revised version with new figures and extended
discussio
The Micro Slit Gas Detector
We describe the first tests with a new proportional gas detector. Its
geometry consists in slits opened in a copper metallized kapton foil with 30
micron anode strips suspended in these openings. In this way the multiplication
process is similar to a standard MSGC. The fundamental difference is the
absence of an insulating substrate around the anode. Also the material budget
is significantly reduced, and the problems related to charging-up or
polarization are removed. Ageing properties of this detector are under study.Comment: 13 pages tex file, 10 figures ep
From the Two Faces of Unionism to the Facebook Society: Union Voice in a 21st Century Context, Manpower Human Resources Lab Discussion Paper No. 6
Union membership has declined precipitously in the US over the past 40 years. Can anything be done to stem this decline? This paper argues that union voice is an attribute (among others) of union membership that is experiential in nature and that unlike the costs of unionisation, can be discerned only after joining a union. This makes the act of âsellingâ unionism
to workers (and to some extent firms as well) rather difficult. Supportive social trends and social customs are required in order to make union membershipâs many hard-to-observe benefits easier to discern. Most
membership based institutions face the same dilemma. However, recent social networking organizations such as Facebook and other on-line communities have been rather successful in attracting millions of members in a relatively short period of time. The question of whether the union movement can appropriate some of these lessons is discussed with reference to historical and contemporary examples
- âŠ