7,224 research outputs found
Multi-wavelength differential astrometry of the S5 polar cap sample
We report on the status of our S5 polar cap astrometry program. Since 1997 we
have observed all the 13 radio sources of the complete S5 polar cap sample at
the wavelengths of 3.6 cm, 2 cm and 7 mm. Images of the radio sources at 3.6
and 2 cm have already been published reporting morphological changes.
Preliminary astrometric analyses have been carried out at three frequencies
with precisions in the relative position determination ranging from 80 to 20
microarcseconds. We report also on the combination of our phase-delay global
astrometry results with the microarcsecond-precise optical astrometry that will
be provided by future space-based instruments.Comment: 2 pages. 1 figure. Proceedings of the 7th European VLBI Network
Symposium held in Toledo, Spain on October 12-15, 2004. Editors: R.
Bachiller, F. Colomer, J.-F. Desmurs, P. de Vicente (Observatorio Astronomico
Nacional), p. 323-324. Needs evn2004.cl
Eternity and the cosmological constant
The purpose of this paper is to analyze the stability of interacting matter
in the presence of a cosmological constant. Using an approach based on the heat
equation, no imaginary part is found for the effective potential in the
presence of a fixed background, which is the n-dimensional sphere or else an
analytical continuation thereof, which is explored in some detail.Comment: 45 pages, 6 figure
1.6 GHz VLBI Observations of SN 1979C: almost-free expansion
We report on 1.6 GHz Very-Long-Baseline-Interferometry (VLBI) observations of
supernova SN 1979C made on 18 November 2002. We derive a model-dependent
supernova size. We also present a reanalysis of VLBI observations made by us on
June 1999 and by other authors on February 2005. We conclude that, contrary to
our earlier claim of strong deceleration in the expansion, SN 1979C has been
undergoing almost-free expansion (; ) for over
25 years.Comment: 4 pages, 4 figures; submitted to A&A on 14 May 2009. Accepted on 7
Jul 200
Journals’ policies of storage and reuse of raw research data and their impact in five scientific areas
Analisys of journals' policies about storage and re-use of raw research data
Labor Force and Economic Changes in Cuba
The labor adjustments and the economic changes present limitations, and if left unchanged, are likely to hinder Cuba’s prospects for successful economic reforms. The present article examines recent labor adjustments in Cuba and presents a balanced analysis of the new economic measures. This article demonstrates that while recent labor force and economic changes in Cuba represent a step in the right direction, these reform measures face a series of challenges that will require profound policy transformations and greater flexibility on the part of the Government
Tensor network states and geometry
Tensor network states are used to approximate ground states of local
Hamiltonians on a lattice in D spatial dimensions. Different types of tensor
network states can be seen to generate different geometries. Matrix product
states (MPS) in D=1 dimensions, as well as projected entangled pair states
(PEPS) in D>1 dimensions, reproduce the D-dimensional physical geometry of the
lattice model; in contrast, the multi-scale entanglement renormalization ansatz
(MERA) generates a (D+1)-dimensional holographic geometry. Here we focus on
homogeneous tensor networks, where all the tensors in the network are copies of
the same tensor, and argue that certain structural properties of the resulting
many-body states are preconditioned by the geometry of the tensor network and
are therefore largely independent of the choice of variational parameters.
Indeed, the asymptotic decay of correlations in homogeneous MPS and MERA for
D=1 systems is seen to be determined by the structure of geodesics in the
physical and holographic geometries, respectively; whereas the asymptotic
scaling of entanglement entropy is seen to always obey a simple boundary law --
that is, again in the relevant geometry. This geometrical interpretation offers
a simple and unifying framework to understand the structural properties of, and
helps clarify the relation between, different tensor network states. In
addition, it has recently motivated the branching MERA, a generalization of the
MERA capable of reproducing violations of the entropic boundary law in D>1
dimensions.Comment: 18 pages, 18 figure
A search for debris disks in the Herschel-ATLAS
Original article can be found at: http://www.aanda.org/ Copyright The European Southern Observatory (ESO)Aims. We aim to demonstrate that the Herschel-ATLAS (H-ATLAS) is suitable for a blind and unbiased survey for debris disks by identifying candidate debris disks associated with main sequence stars in the initial science demonstration field of the survey. We show that H-ATLAS reveals a population of far-infrared/sub-mm sources that are associated with stars or star-like objects on the SDSS main-sequence locus. We validate our approach by comparing the properties of the most likely candidate disks to those of the known population. Methods. We use a photometric selection technique to identify main sequence stars in the SDSS DR7 catalogue and a Bayesian Likelihood Ratio method to identify H-ATLAS catalogue sources associated with these main sequence stars. Following this photometric selection we apply distance cuts to identify the most likely candidate debris disks and rule out the presence of contaminating galaxies using UKIDSS LAS K-band images. Results. We identify 78 H-ATLAS sources associated with SDSS point sources on the main-sequence locus, of which two are the most likely debris disk candidates: H-ATLAS J090315.8 and H-ATLAS J090240.2. We show that they are plausible candidates by comparing their properties to the known population of debris disks. Our initial results indicate that bright debris disks are rare, with only 2 candidates identified in a search sample of 851 stars. We also show that H-ATLAS can derive useful upper limits for debris disks associated with Hipparcos stars in the field and outline the future prospects for our debris disk search programme.Peer reviewe
Efficient model for modular multi-level converter simulation
Detailed models of modular multi-level converters (MMCs) are cumbersome for electromagnetic transient simulation programs because of their high number of components which involve large simulation times. In this work, the modeling of an MMC is addressed with the objective of reducing the simulation time. First, the MMC structure is presented, including the modulation technique, the voltage balancing algorithm and the circulating current control used to validate the model. Next, an efficient simplified model is proposed. This is formed by just one variable voltage source, one variable capacitor and one variable resistor per arm, regardless of the number of submodules. This simplified model allows the simulation time to be reduced while keeping the dynamics of the MMC. The comparison through several PSCAD simulations with a detailed 5-level MMC model proves its validity during both steady-state and transient conditions (ac and dc short-circuits).The authors acknowledge the financial support of the Spanish Ministry of Economy and EU FEDER Funds under Grant DPI2014-53245-R and the Universitat Jaume I under Grants P1.1B2013-51 and E-2014-24.Vidal-Albalate, R.; Belenguer, E.; Beltrán, H.; Blasco Gimenez, R. (2015). Efficient model for modular multi-level converter simulation. Mathematics and Computers in Simulation. 99(1):1-15. https://doi.org/10.1016/j.matcom.2015.10.001S11599
Journals’ policies of storage and reuse of raw research data and their impact in five scientific areas
Analisys of journals' policies about storage and re-use of raw research data
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