54,631 research outputs found
Large cities are less green
We study how urban quality evolves as a result of carbon dioxide emissions as
urban agglomerations grow. We employ a bottom-up approach combining two
unprecedented microscopic data on population and carbon dioxide emissions in
the continental US. We first aggregate settlements that are close to each other
into cities using the City Clustering Algorithm (CCA) defining cities beyond
the administrative boundaries. Then, we use data on emissions at a
fine geographic scale to determine the total emissions of each city. We find a
superlinear scaling behavior, expressed by a power-law, between
emissions and city population with average allometric exponent
across all cities in the US. This result suggests that the high productivity of
large cities is done at the expense of a proportionally larger amount of
emissions compared to small cities. Furthermore, our results are substantially
different from those obtained by the standard administrative definition of
cities, i.e. Metropolitan Statistical Area (MSA). Specifically, MSAs display
isometric scaling emissions and we argue that this discrepancy is due to the
overestimation of MSA areas. The results suggest that allometric studies based
on administrative boundaries to define cities may suffer from endogeneity bias
Fracturing the optimal paths
Optimal paths play a fundamental role in numerous physical applications
ranging from random polymers to brittle fracture, from the flow through porous
media to information propagation. Here for the first time we explore the path
that is activated once this optimal path fails and what happens when this new
path also fails and so on, until the system is completely disconnected. In fact
numerous applications can be found for this novel fracture problem. In the
limit of strong disorder, our results show that all the cracks are located on a
single self-similar connected line of fractal dimension .
For weak disorder, the number of cracks spreads all over the entire network
before global connectivity is lost. Strikingly, the disconnecting path
(backbone) is, however, completely independent on the disorder.Comment: 4 pages,4 figure
Three dimensional Lifshitz black hole and the Korteweg-de Vries equation
We consider a solution of three dimensional New Massive Gravity with a
negative cosmological constant and use the AdS/CTF correspondence to inquire
about the equivalent two dimensional model at the boundary. We conclude that
there should be a close relation with the Korteweg-de Vries equation.Comment: 4 page
Near-infrared study of new embedded clusters in the Carina complex
We analyse the nature of a sample of stellar overdensities that we found
projected on the Carina complex. This study is based on 2MASS photometry and
involves the photometry decontamination of field stars, elaboration of
intrinsic colour-magnitude diagrams J(J-Ks), colour-colour diagrams
(J-H)(H-Ks) and radial density profiles, in order to determine the
structure and the main astrophysical parameters of the best candidates. The
verification of an overdensity as an embedded cluster requires a CMD consistent
with a PMS content and MS stars, if any. From these results, we are able to
verify if they are, in fact, embedded clusters. The results were, in general,
rewarding: in a sample of 101 overdensities, the analysis provided 15
candidates, of which three were previously catalogued as clusters
(CCCP-Cl16, Treasure Chest and FSR1555), and the 12 remaining are
discoveries that provided significant results, with ages not above 4.5Myr
and distances compatible with the studied complex. The resulting values for the
differential reddening of most candidates were relatively high, confirming that
these clusters are still (partially or fully) embedded in the surrounding gas
and dust, as a rule within a shell. Histograms with the distribution of the
masses, ages and distances were also produced, to give an overview of the
results. We conclude that all the 12 newly found embedded clusters are related
to the Carina complex.Comment: 10 pages, 14 figures, accepted for publication in MNRA
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