3,149 research outputs found
On the Fractal Distribution of HII Regions in Disk Galaxies
In this work we quantify the degree to which star-forming events are clumped.
We apply a precise and accurate technique to calculate the correlation
dimension Dc of the distribution of HII regions in a sample of disk galaxies.
Our reliable results are distributed in the range 1.5<Dc<2.0. We get
significant variations in the fractal dimension among galaxies, contrary to a
universal picture sometimes claimed in literature. The faintest galaxies tend
to distribute their HII regions in more clustered (less uniform) patterns.
Moreover, the fractal dimension for the brightest HII regions within the same
galaxy seems to be smaller than for the faintest ones suggesting some kind of
evolutionary effect.Comment: 6 pages including 4 figures. To appear in Highlights of Spanish
Astrophysics V: Proceedings of the VIII Scientific Meeting of the Spanish
Astronomical Society held in Santander, July 7-11, 200
Risk sharing, investment, and incentives in the neoclassical growth model
We first study growth and risk sharing in a stochastic growth model with preference shocks and two risk-averse agents. In periods in which one of the agents needs extra consumption (insurance), it is socially optimal to reduce the consumption of the other agent (redistribution) and also to accumulate fewer resources for the future (disinvestment). The latter hurts growth while the former only affects the distribution of aggregate consumption. Then, to analyze if information matters, we study if the same allocation would be implementable under private information. We find that it depends on the state of the economy. The provision of insurance that is implemented by reducing capital accumulation deteriorates the prospects of all agents in the economy and thus helps to alleviate informational frictions. The size of redistribution versus disinvestment and the outlook of economic growth at the time of disinvestment affects the possibilities of implementing the best possible allocation when the preference shock is private information. Therefore, we conjecture that under private information the best allocation compatible with incentives would tend to hurt growth and to concentrate resources in agents with private information in order to provide incentives to report the shock truthfully.Business cycles ; Economic growth
A catalogue of open cluster radii determined from Gaia proper motions
In this work we improve a previously published method to calculate in a
reliable way the radius of an open cluster. The method is based on the
behaviour of stars in the proper motion space as the sampling changes in the
position space. Here we describe the new version of the method and show its
performance and robustness. Additionally, we apply it to a large number of open
clusters using data from Gaia DR2 to generate a catalogue of 401 clusters with
reliable radius estimations. The range of obtained apparent radii goes from
Rc=1.4+-0.1 arcmin (for the cluster FSR 1651) to Rc=25.5+-3.5 arcmin (for
NGC~2437). Cluster linear sizes follow very closely a lognormal distribution
with a mean characteristic radius of Rc=3.7 pc, and its high radius tail can be
fitted by a power law as N \propto Rc^(-3.11+-0.35). Additionally, we find that
number of members, cluster radius and age follow the relationship Nc \propto
Rc^(1.2+-0.1) Tc^(-1.9+-0.4) where the younger and more extensive the cluster,
the more members it presents. The proposed method is not sensitive to low
density or irregular spatial distributions of stars and, therefore, is a good
alternative or complementary procedure to calculate open cluster radii not
having previous information on star memberships.Comment: 13 pages including 9 figures and 2 tables (main table will be
available online). Accepted for publication in MNRA
NGC 2548: clumpy spatial and kinematic structure in an intermediate-age galactic cluster
NGC 2548 is a 400-500 Myr old open cluster with evidence of spatial
substructures likely caused by its interaction with the Galactic disk. In this
work we use precise astrometric data from the Carte du Ciel - San Fernando
(CdC-SF) catalogue to study the clumpy structure in this cluster. We confirm
the fragmented structure of NGC 2548 but, additionally, the relatively high
precision of our kinematic data lead us to the first detection of substructures
in the proper motion space of a stellar cluster. There are three spatially
separated cores each of which has its own counterpart in the proper motion
distribution. The two main cores lie nearly parallel to the Galactic plane
whereas the third one is significantly fainter than the others and it moves
toward the Galactic plane separating from the rest of the cluster. We derive
core positions and proper motions, as well as the stars belonging to each core.Comment: 9 pages, 8 figures, 4 tables. Accepted for publication in MNRA
A method for determining the radius of an open cluster from stellar proper motions
We propose a method for calculating the radius of an open cluster in an
objective way from an astrometric catalogue containing, at least, positions and
proper motions. It uses the minimum spanning tree (hereinafter MST) in the
proper motion space to discriminate cluster stars from field stars and it
quantifies the strength of the cluster-field separation by means of a
statistical parameter defined for the first time in this paper. This is done
for a range of different sampling radii from where the cluster radius is
obtained as the size at which the best cluster-field separation is achieved.
The novelty of this strategy is that the cluster radius is obtained
independently of how its stars are spatially distributed. We test the
reliability and robustness of the method with both simulated and real data from
a well-studied open cluster (NGC 188), and apply it to UCAC4 data for five
other open clusters with different catalogued radius values. NGC 188, NGC 1647,
NGC 6603 and Ruprecht 155 yielded unambiguous radius values of 15.2+/-1.8,
29.4+/-3.4, 4.2+/-1.7 and 7.0+/-0.3 arcmin, respectively. ASCC 19 and Collinder
471 showed more than one possible solution but it is not possible to know
whether this is due to the involved uncertainties or to the presence of complex
patterns in their proper motion distributions, something that could be inherent
to the physical object or due to the way in which the catalogue was sampled.Comment: 12 pages including 14 figures and 1 table. Accepted for publication
in MNRA
Management System for Harvest Scheduling: The Case of Horticultural Production in Southeast Spain
horticultural farmer, optimization, planning, mathematical programming, marketing, cooperative, Agribusiness, Crop Production/Industries, Demand and Price Analysis, Farm Management, Land Economics/Use,
Learning and enactive interfaces
International audienceEnactive Learning is a relatively new expression, used in enactive community, to desinate the process of learning by doing. In human sciences, several theories/models are confronted concerning the learning process
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