6,799 research outputs found
Deriving star formation histories: Inverting HR diagrams through a variational calculus maximum likelihood method
We introduce a new method for solving maximum likelihood problems through
variational calculus, and apply it to the case of recovering an unknown star
formation history, , from a resulting HR diagram. This approach allows
a totally non-parametric solution which has the advantage of requiring no
initial assumptions on the . As a full maximum likelihood statistical
model is used, we take advantage of all the information available in the HR
diagram, rather than concentrating on particular features such as turn off
points or luminosity functions. We test the method using a series of synthetic
HR diagrams produced from known , and find it to be quite successful
under noise conditions comparable to those present in current observations. At
this point we restrict the analysis to situations where the metallicity of the
system is known, as is the case with the resolved populations of the dwarf
spheroidal companions to the Milky Way or the solar neighbourhood Hipparcos
data. We also include tests to quantify the way uncertainties in the assumed
metallicity, binary fraction and IMF affect our inferences.Comment: 16 pages including 12 figures. Accepted for publication in MNRAS,
appendix adde
Non-parametric star formation histories for 5 dwarf spheroidal galaxies of the local group
We use recent HST colour-magnitude diagrams of the resolved stellar
populations of a sample of local dSph galaxies (Carina, LeoI, LeoII, Ursa Minor
and Draco) to infer the star formation histories of these systems, .
Applying a new variational calculus maximum likelihood method which includes a
full Bayesian analysis and allows a non-parametric estimate of the function one
is solving for, we infer the star formation histories of the systems studied.
This method has the advantage of yielding an objective answer, as one need not
assume {\it a priori} the form of the function one is trying to recover. The
results are checked independently using Saha's statistic. The total
luminosities of the systems are used to normalize the results into physical
units and derive SN type II rates. We derive the luminosity weighted mean star
formation history of this sample of galaxies.Comment: 14 pages including 7 figures. Accepted to MNRA
Objective multiscale analysis of random heterogeneous materials
The multiscale framework presented in [1, 2] is assessed in this contribution for a study of random heterogeneous materials. Results are compared to direct numerical simulations (DNS) and the sensitivity to user-defined parameters such as the domain decomposition type and initial coarse scale resolution is reported. The parallel performance of the implementation is studied for different domain decompositions
Hernia discal traumática en un hurón
La hernia discal es una enfermedad bien definida y estudiada en perros y gatos. En estas especies se han descrito procesos degenerativos que llevan al prolapso del disco, o de su núcleo pulposo, hacia el canal medular, incluso en ausencia de traumatismos. En el caso del hurón doméstico (Mustela putorius furo) no se han descrito cambios degenerativos en el disco qu epuedan predisponer al prolapso, pero las hernias discales son relativamente frecuentes como consecuencia de traumatismos graves. En este artículo se describe un caso de hernia discal en un hurón producida a partir de una caída, su diagnóstico a través de una mielografía, su resolución quirúrgica y la recuperación del paciente tras la cirugía, poniendo especial énfasis en las dificultades diagnósticas y terapéuticas que plantea esta especie, así como las diferencias que presenta cuandose compara perros y gatos.
Oferta de implantes de prótesis total de cadera en Cataluña
El crecimiento exponencial de la oferta de implantes de prótesis total de cadera
hace difícil para el cirujano ortopédico establecer criterios de elección, por este motivo hemos realizado
un análisis de la oferta actual de los mismos. Se han identificado 67 modelos de implantes
de prótesis total de cadera primaria y 19 modelos de revisión comercializados en la comunidad
catalana por 24 empresas. El número de variables, de material o diseño de los distintos modelos,
sin tener en cuenta las diversas medidas de vástagos, cuellos y cótilos, ha sido de 55. Hemos encontrado
referencias bibliográficas de resultados clínicos de un 25% de las prótesis. El rango de
coste de los implantes ha sido de 70.000 a 300.570 pesetas para las prótesis cementadas, con un
precio medio de 172.805 pesetas; de 375.654 a 648.606 pesetas para las no cementadas, con un
precio medio de 480.301 pesetas, y de 412.552 a 713.000 pesetas de las prótesis de revisión, con
una media de 568.138 pesetas. Esta dispersión de modelos y precios es mayor que la de otros países
de nuestro entorno, y el porcentaje de publicaciones de resultados clínicos encontrado es algo
menor que el reportado por algunos autores. Frente a esta situación creemos que serían necesarios
estudios clínicos a largo término y sistemas de evaluación estandarizados para poder decidir
con criterios rigurosos si los nuevos implantes que aparecen en el mercado con costes más elevados
tienen ventajas suficientes para cambiar indicaciones de implantes largamente probados.The exponential growth of the different models of THA actually in offer makes
difficult to the orthopaedic surgeon to find sound criteria for the selection of the proper implant.
This paper is an attempt to analyse the models actually in offer. We have identified 67 primary
THAs types and 19 revision models, commercialised by 24 companies at the catalonian community.
The number of variables, materials or designs between different patterns, independently of the size
of femoral and acetabular components, has been of 55. Only 25% of implants are supported by
reports in the orthopaedic literature. The cost range of implants varied from 70,000 to 300,570 pts.
for cemented prosthesis (average: 172,895); from 375,654 to 648,606 pts. for cementless models
(average: 480,391); the cost range for revision models varied from 412,552 to 713,000 pts., with an
average of 568,138 pts. Such a variety of designs and prices is greater than in other countries
of our environment, and the percentage of reports about clinical results that we have found is
smaller than the noticed by some authors. We believe that it would be necessary some long term
prospective studies and standarized evaluation systems in order to decide if the newer implants,
with more expensive price, have enough advantages to substitute the classic implants widely used
Multiscale analysis of damage using dual and primal domain decomposition techniques
In this contribution, dual and primal domain decomposition techniques
are studied for the multiscale analysis of failure in quasi-brittle materials. The multiscale strategy essentially consists in decomposing the structure into a number of nonoverlapping domains and considering a refined spatial resolution where needed. In multiscale analysis of damage, the spatial refinement is performed where damage nucleation and propagation take place. The domain decomposition approach turns to be a computationally cheaper alternative to the direct numerical solution in which a fine scale model
is considered throughout the complete sample. Dual and primal domain decomposition techniques are appropriate for such concurrent multiscale analyses and provide identical results. Parallel scalability of the multiscale analysis is studied using a moderate number of processors and a parallel direct solver for the system obtained through the assembly of all domains.Postprint (published version
El desarrollo de la profesionalidad docente mediante redes
Desde 2001, un grupo de profesores representantes de distintos centros de educación secundaria del área de Barcelona, se han articulado en red coordinados por el Grupo Observatori d'Oportunitats Educatives de la Universidad Autònoma de Barcelona. El eje del trabajo en red en este período se centró en el análisis de la intervención educativa que reciben los alumnos con dificultades académicas y sociales.
El propósito central de esta comunicación es presentar una validación del trabajo realizado durante este período de tiempo, a partir de las consideraciones de los profesores participantes en dicha red. En este sentido, la referencia de la reflexión al contexto social, administrativo y político de la Educción en Catalunya es un factor esencial para comprender aquella validación en todo su sentido
The recent star formation history of the Hipparcos solar neighbourhood
We use data from the Hipparcos catalogue to construct colour-magnitude
diagrams for the solar neighbourhood, which are then treated using advanced
Bayesian analysis techniques to derive the star formation history, , of
this region over the last 3 Gyr. The method we use allows the recovery of the
underlying without the need of assuming any {\it a priori} structure
or condition on , and hence yields a highly objective result. The
remarkable accuracy of the data permits the reconstruction of the local
with an unprecedented time resolution of Myr. A
having an oscillatory component of period Gyr is found,
superimposed on a small level of constant star formation activity. Problems
arising from the non-uniform selection function of the Hipparcos satellite are
discussed and treated. Detailed statistical tests are then performed on the
results, which confirm the inferred to be compatible with the observed
distribution of stars.Comment: 9 pages including figures, MNRAS in pres
Spin polarized tunneling in ferromagnet/unconventional superconductor junctions
We study tunneling in ferromagnet/unconventional superconductor (F/S)
junctions. We include the effects of spin polarization, interfacial resistance,
and Fermi wavevector mismatch (FWM) between the F and S regions. Andreev
reflection (AR) at the F/S interface, governing tunneling at low bias voltage,
is strongly modified by these parameters. The conductance exhibits a very wide
variety of features as a function of applied voltage.Comment: Revision includes new figures with angular averages and correction of
minor error
A robust statistical estimation of the basic parameters of single stellar populations. I. Method
The colour-magnitude diagrams of resolved single stellar populations, such as
open and globular clusters, have provided the best natural laboratories to test
stellar evolution theory. Whilst a variety of techniques have been used to
infer the basic properties of these simple populations, systematic
uncertainties arise from the purely geometrical degeneracy produced by the
similar shape of isochrones of different ages and metallicities. Here we
present an objective and robust statistical technique which lifts this
degeneracy to a great extent through the use of a key observable: the number of
stars along the isochrone. Through extensive Monte Carlo simulations we show
that, for instance, we can infer the four main parameters (age, metallicity,
distance and reddening) in an objective way, along with robust confidence
intervals and their full covariance matrix. We show that systematic
uncertainties due to field contamination, unresolved binaries, initial or
present-day stellar mass function are either negligible or well under control.
This technique provides, for the first time, a proper way to infer with
unprecedented accuracy the fundamental properties of simple stellar
populations, in an easy-to-implement algorithm.Comment: 17 pages, 12 figures, MNRAS, in pres
- …