2,580 research outputs found
A fast version of the k-means classification algorithm for astronomical applications
Context. K-means is a clustering algorithm that has been used to classify
large datasets in astronomical databases. It is an unsupervised method, able to
cope very different types of problems. Aims. We check whether a variant of the
algorithm called single-pass k-means can be used as a fast alternative to the
traditional k-means. Methods. The execution time of the two algorithms are
compared when classifying subsets drawn from the SDSS-DR7 catalog of galaxy
spectra. Results. Single-pass k-means turn out to be between 20 % and 40 %
faster than k-means and provide statistically equivalent classifications. This
conclusion can be scaled up to other larger databases because the execution
time of both algorithms increases linearly with the number of objects.
Conclusions. Single-pass k-means can be safely used as a fast alternative to
k-means
Interpretation of HINODE SOT/SP asymmetric Stokes profiles observed in quiet Sun network and internetwork
We present the first interpretation of the Stokes profile asymmetries
measured in the FeI 630 nm lines by SOT/SP, in both quiet Sun internetwork (IN)
and network regions. The inversion is carried out under the hypothesis of
MISMA, where the unresolved structure is assumed to be optically thin. We
analyze a 29.52"x31.70" subfield carefully selected to be representative of the
properties of a 302"x162" quiet Sun field-of-view at disk center. The inversion
code is able to reproduce the observed asymmetries in a very satisfactory way.
The inversion code interprets 25% of inverted profiles as emerging from pixels
in which both positive and negative polarities coexist. kG field strengths are
found at the base of the photosphere in both network and IN; in the case of the
latter, both kG fields and hG fields are admixed. When considering the magnetic
properties at the mid photosphere most kG fields are gone, and the statistics
is dominated by hG fields. We constrain the magnetic field of only 4.5% of the
analyzed photosphere (and this percentage reduces to 1.3% when referred to all
pixels, including those with low polarization not analyzed). The rest of the
plasma is consistent with the presence of weak fields not contributing to the
detected polarization signals. The average flux densities derived in the full
subfield and in IN regions are higher than the ones derived from the same
dataset by Milne-Eddington inversion. The existence of large asymmetries in
SOT/SP polarization profiles is uncovered. These are not negligible in quiet
Sun data. The MISMA inversion code reproduces them in a satisfactory way, and
provides a statistical description of the magnetized IN and network which
partly differs and complements the results obtained so far. From this it
follows the importance of having a complete interpretation of the line profile
shapes.Comment: 11 pages, 9 figures, 1 table - Accepted for publication on A&
Magnetic flux in the inter-network quiet Sun from comparison with numerical simulations
Khomenko et al. estimate the mean magnetic field strength of the quiet Sun to
be 20 G. The figure is smaller than several existing estimates, and it comes
from the comparison between observed Zeeman polarization signals and synthetic
signals from numerical simulations of magneto-convection. The numerical
simulations require an artificially large magnetic diffusivity, which smears
out magnetic structures smaller than the grid scale. Assuming a turbulent
cascade for the unresolved artificially smeared magnetic fields, we find that
their unsigned magnetic flux is at least as important as that explicitly shown
in the simulation. The unresolved fields do not produce Zeeman polarization but
contribute to the unsigned flux.Since they are not considered by Khomenko et
al., their mean magnetic field strength has to be regarded as a lower limit.
This kind of bias is not specific of a particular numerical simulation or a
spectral line. It is to be expected when observed quiet Sun Zeeman signals are
compared with synthetic signals from simulations.Comment: Accepted A&A. 4 pages, 0 figure
Asymmetries of the Stokes V profiles observed by HINODE SOT/SP in the quiet Sun
We present the first classification of SOT/SP circular polarization
measurements with the aim of highlighting exhaustively the whole variety of
Stokes V shapes emerging from the quiet Sun. k-means is used to classify HINODE
SOT/SP Stokes V profiles observed in the quiet Sun network and internetwork
(IN). We analyze a 302 x 162 square arcsec field-of-view (FOV) which can be
considered a complete sample of quiet Sun measurements performed at at the disk
center with 0.32 arcsec angular resolution and 0.001 polarimetric sensitivity.
Such a classification allows us to divide the whole dataset in classes, with
each class represented by a cluster profile, i.e., the average of the profiles
in the class. The set of 35 cluster profiles derived from the analysis
completely characterizes SOT/SP quiet Sun measurements. The separation between
network and IN profile shapes is evident - classes in the network are not
present in the IN, and vice versa. Asymmetric profiles are approximatively 93 %
of the total number of profiles. Among these, approximatively 34 % of the
profiles are strongly asymmetric profiles, and they can be divided in three
families: blue-lobe, red-lobe, and Q-like profiles. The blue-lobe profiles tend
to be associated with upflows (granules), whereas the red-lobe and Q-like ones
appear in downflows (intergranular lanes). Such profiles need to be interpreted
considering model atmospheres different from a uniformly magnetized
Milne-Eddington (ME) atmosphere, i.e., characterized by gradients and/or
discontinuities in the magnetic field and velocity along the line-of-sight
(LOS).Comment: 11 pages, 4 figures, accepted for publication in Astronomy and
Astrophysic
Do galaxies form a spectroscopic sequence?
This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society ©: 2011 RAS © The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.We identify a spectroscopic sequence of galaxies, analogous to the Hubble sequence of morphological types, based on the Automatic Spectroscopic K-means-based (ASK) classification. Considering galaxy spectra as multidimensional vectors, the majority of the spectral classes are distributed along a well-defined curve going from the earliest to the latest types, suggesting that the optical spectra of normal galaxies can be described in terms of a single affine parameter. Optically bright active galaxies, however, appear as an independent, roughly orthogonal branch that intersects the main sequence exactly at the transition between early and late typesThis work has been funded by projects AYA2007-67965-C03-03, AYA 2007-67965-C03-01, AYA2007-67752-C03-01 and CSD 2006-00070 (Spanish Ministry of Science, Technology and Innovation
Convective motions and net circular polarization in sunspot penumbrae
We have employed a penumbral model, that includes the Evershed flow and
convective motions inside penumbral filaments, to reproduce the azimuthal
variation of the net circular polarization (NCP) in sunspot penumbrae at
different heliocentric angles for two different spectral lines. The theoretical
net circular polarization fits the observations as satisfactorily as penumbral
models based on flux-tubes. The reason for this is that the effect of
convective motions on the NCP is very small compared to the effect of the
Evershed flow. In addition, the NCP generated by convective upflows cancels out
the NCP generated by the downflows. We have also found that, in order to fit
the observed NCP, the strength of the magnetic field inside penumbral filaments
must be very close to 1000 G. In particular, field-free or weak-field filaments
fail to reproduce both the correct sign of the net circular polarization, as
well as its dependence on the azimuthal and heliocentric angles.Comment: Accepted for publication in the Astrophysical Journal. 10 pages, 7
figures (3 in color). Uses emulatedap
Search for photospheric footpoints of quiet Sun transition region loops
CONTEXT:The footpoints of quiet Sun Transition Region (TR) loops do not seem
to coincide with the photospheric magnetic structures appearing in traditional
low-sensitivity magnetograms.
AIMS: To look for the so-far unidentified photospheric footpoints of TR loops
using G-band bright points (BPs) as proxies for photospheric magnetic field
concentrations.
METHODS: Comparison of TR measurements with SoHO/SUMER and photospheric
magnetic field observations obtained with the Dutch Open Telescope.
RESULTS: Photospheric BPs are associated with bright TR structures, but they
seem to avoid the brightest parts of the structure. BPs appear in regions that
are globally redshifted, but they avoid extreme velocities. TR explosive events
are not clearly associated with BPs.
CONCLUSIONS: The observations are not inconsistent with the BPs being
footpoints of TR loops, although we have not succeeded to uniquely identify
particular BPs with specific TR loops.Comment: Accepted for publication in A&A. 10 pages, 10 figures. Due to size
limitations, the quality of fig3 is not goo
Análisis de una propuesta formativa para mejorar la capacitación de los “cuidadores informales” de pacientes por un accidente vascular cerebral
El Accidente Vascular Cerebral (AVC) constituye una primera causa de muerte en Portugal. Para aquellos que no mueren, sus secuelas son importantes puesto que el impacto de la atenciĂłn que ese paciente dependiente demanda en el dĂa a dĂa impone una serie de alteraciones sustanciales en la propia dinámica familiar de Ă©ste, generando ansiedad, desorganizaciĂłn o incluso estrĂ©s. Tal cĂşmulo de reacciones se focalizan con más intensidad en aquella persona de sus allegados directos que garantiza su asistencia y atenciĂłn diaria, es decir, el cuidador informal. En esta comunicaciĂłn se trata de aminorar los efectos que esta sobrecarga ejerce sobre la salud y la calidad de vida del propio cuidador/a, evitando que Ă©ste tambiĂ©n precise de atenciĂłn mĂ©dica especializada, y mejorando en Ăşltimo tĂ©rmino el bienestar de la persona dependiente (Martins, 2006). ÂżCĂłmo? Construyendo una herramienta formativa aplicada durante el propio proceso de tránsito entre el hospital donde está internado el paciente AVC y su domicilio. Su planificaciĂłn aborda concepto, caracterĂsticas y tipos del ACV, sus consecuencias en la vida diaria, los problemas de comunicaciĂłn, el posicionamiento, la movilidad, la postura, … en definitiva una propuesta de acciĂłn formativa para mejorar el conocimiento real y funcional del cuidador/a como agente de salud de primer orden.The Cerebral Vascular Accident (CVA) is a leading cause of death in Portugal. For those who do not die, their consequences are important because the impact of dependent care that patient demand in everyday places a number of substantial changes in the family dynamics of it own, creating anxiety, disorganization or even stress. Such cluster reactions are focused more strongly on the person of his direct relatives ensuring attendance and daily care , ie informal caregiver .
This communication is to lessen the effects this overhead have on the health and quality of life of caregivers / a itself, preventing it also require specialized medical care, and ultimately improve the welfare of the dependent (Martins , 2006). How? Building a training tool applied during the very process of transit from the hospital where the patient and his house AVC is hospitalized . Addresses planning concept, characteristics and types of stroke, its impact on daily life, the problems of communication, positioning, mobility, posture, ... ultimately a proposed training activity to improve the actual working knowledge of the caregiver health agent
On the origin of reverse polarity patches found by Hinode in sunspot penumbrae
The satellite Hinode has recently revealed penumbral structures with a
magnetic polarity opposite to the main sunspot polarity. They may be a direct
confirmation of magnetic field lines and mass flows returning to the solar
interior throughout the penumbra, a configuration previously inferred from
interpretation of observed Stokes profile asymmetries. The paper points out the
relationship between the reverse polarity features found by Hinode, and the
model Micro-Structured Magnetic Atmospheres (MISMAs) proposed for sunspots. We
show how the existing model MISMAs produce strongly redshifted reverse polarity
structures as found by Hinode. Ad hoc model MISMAs also explain the asymmetric
Stokes profiles observed by Hinode. The same modeling may be consistent with
magnetograms of dark cored penumbral filaments if the dark cores are associated
with the reverse polarity. Such hypothetical relationship will show up only in
the far red wings of the spectral lines.Comment: To appear in A&A. 8 pages and 7 figure
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