659 research outputs found
Absolute magnitudes and kinematics of CP stars from Hipparcos data
The position in the HR diagram and the kinematic characteristics of different
kinds of CP stars of the upper main sequence are obtained using the LM method
(Luri et al., 1996). Most of the CP stars are main sequence stars occupying the
whole width of the sequence. From a kinematic point of view, they belong to the
young disk population (ages < 1.5 Gyr). It has also been found that, on
kinematic grounds, the behaviour of lambda Bootis stars is similar to the one
observed for normal stars of the same spectral range. On the other hand, roAp
and noAp stars show the same kinematic characteristics. The peculiar velocity
distribution function has been decomposed into a sum of three dimensional
gaussians and the presence of Pleiades, Sirius and Hyades moving groups has
been clearly established. Finally, a small number of CP stars are found to be
high-velocity objects.Comment: 8 pages, 1 figure, to appear in: Proc. of the 26th workshop of the
European Working Group on CP stars, eds. P. North, A. Schnell and J.
Ziznovsky, Contrib. Astr. Obs. Skalnate Pleso Vol. 27, No
The Nature of the Gould Belt from a Fractal Analysis of its Stellar Population
The Gould Belt (GB) is a system of gas and young, bright stars distributed
along a plane that is inclined with respect to the main plane of the Milky Way.
Observational evidence suggests that the GB is our closest star formation
complex, but its true nature and origin remain rather controversial. In this
work we analyze the fractal structure of the stellar component of the GB. In
order to do this, we tailor and apply an algorithm that estimates the fractal
dimension in a precise and accurate way, avoiding both boundary and small data
set problems. We find that early OB stars (of spectral types earlier than B4)
in the GB have a fractal dimension very similar to that of the gas clouds in
our Galaxy. On the contrary, stars in the GB of later spectral types show a
larger fractal dimension, similar to that found for OB stars of both age groups
in the local Galactic disk (LGD). This result seems to indicate that while the
younger OB stars in the GB preserve the memory of the spatial structure of the
cloud where they were born, older stars are distributed following a similar
morphology as that found for the LGD stars. The possible causes for these
differences are discussed.Comment: 20 pages including 7 figures and 1 table. ApJ (in press
Effect of patch size on seed removal by harvester ants
The harvester ant Messor barbarus can be responsible for high weed seed losses in dry land cereals in Spain. Because weeds occur in patches, harvester ants have to be able to find and exploit patches. However, seed patches can differ in size and may, therefore, differ in the probability of being discovered and exploited. Here, 90 patches varying in size from 0.25 to 9 m2 were created in three 50 × 50 m subareas in a cereal field. Oat seeds were sown as weed seed surrogates in the patches at 2000 seeds m−2. After 24 h, those remaining were collected and the exploitation rate (the percentage of seeds removed per patch discovered by ants) was estimated. Harvester ant nests and the location of the seed patches were georeferenced and used to estimate distances between them. The patch encounter rate (the proportion of patches discovered by the ants) decreased slightly, but significantly, with decreasing patch size, though not the exploitation rate, which was lowest in the smallest patches (78-94%) and highest in the largest (99-100%). Seed patches that were not found or partially exploited were mostly located in subareas with a lower ant nest density or a longer distance away from the nearest nest than seed patches that were fully exploited. The results of this study indicate that the interaction between the spatial distribution of ant nests and the patchy distribution of seeds can create opportunities for seeds to be subjected to lower levels of predation
Volumetric capnography and chronic obstructive pulmonary disease staging
Spirometry is difficult for some COPD patient to perform. Volumetric capnography could be a second choice test to evaluate the severity of functional disturbances. The aim of this work is to test this hypothesis. A total number of 98 subjects were classified either as normal ex-smokers (N=14) or COPD patients. The latter were staged following GOLD recommendations. Spirometry and volumetric capnography recordings were obtained from each patient. Spirometry parameters, Bohr Dead Space (V(D)Bohr), Airways Dead Space from the pre-interface expirate corrected curve (V(D)aw), Phase III slope (Sl(III)) and Volume of alveolar ejection (V(AE)) were measured. Index of Ventilatory Efficiency (IVE), and Index of Airways Heterogeneity (IAH) were calculated as: IVE = V(AE)/(V(T) - V(D)aw) and IAH = 1-[(V(T)-V(D)Bohr)/(V(T) - V(D)aw)]. In ANOCOVA analysis IAH showed the greatest association with stage (F >40), with no significant covariant dependence on V(T). A receiver operating characteristics curve analysis showed values of the area under the curve greater than 0.9 for IAH and IVE at all stage levels, with a sensitivity = specificity value greater than 80%. We conclude that IAH and IVE can be used when spirometry cannot be reliably performed, as an alternative test to evaluate the degree of functional involvement in COPD patients
Consensus-Based Agglomerative Hierarchical Clustering
Producción CientíficaIn this contribution, we consider that a set of agents assess a set of alternatives
through numbers in the unit interval. In this setting, we introduce a measure
that assigns a degree of consensus to each subset of agents with respect to every
subset of alternatives. This consensus measure is defined as 1 minus the outcome
generated by a symmetric aggregation function to the distances between
the corresponding individual assessments. We establish some properties of the
consensus measure, some of them depending on the used aggregation function.
We also introduce an agglomerative hierarchical clustering procedure that is generated
by similarity functions based on the previous consensus measuresMinisterio de Economía, Industria y Competitividad (ECO2012-32178)Junta de Castilla y León (programa de apoyo a proyectos de investigación – Ref. VA066U13
Galactic Rotation Parameters from Data on Open Star Clusters
Currently available data on the field of velocities Vr, Vl, Vb for open star
clusters are used to perform a kinematic analysis of various samples that
differ by heliocentric distance, age, and membership in individual structures
(the Orion, Carina--Sagittarius, and Perseus arms). Based on 375 clusters
located within 5 kpc of the Sun with ages up to 1 Gyr, we have determined the
Galactic rotation parameters
Wo =-26.0+-0.3 km/s/kpc,
W'o = 4.18+-0.17 km/s/kpc^2,
W''o=-0.45+-0.06 km/s/kpc^3, the system contraction parameter K = -2.4+-0.1
km/s/kpc, and the parameters of the kinematic center Ro =7.4+-0.3 kpc and lo =
0+-1 degrees. The Galactocentric distance Ro in the model used has been found
to depend significantly on the sample age. Thus, for example, it is 9.5+-0.7
kpc and 5.6+-0.3 kpc for the samples of young (50 Myr)
clusters, respectively. Our study of the kinematics of young open star clusters
in various spiral arms has shown that the kinematic parameters are similar to
the parameters obtained from the entire sample for the Carina-Sagittarius and
Perseus arms and differ significantly from them for the Orion arm. The
contraction effect is shown to be typical of star clusters with various ages.
It is most pronounced for clusters with a mean age of 100 Myr, with the
contraction velocity being Kr = -4.3+-1.0 km/s.Comment: 14 pages, 4 figures, 2 table
A New Nearby Candidate Star Cluster in Ophiuchus at d = 170 pc
The recent discoveries of nearby star clusters and associations within a few
hundred pc of the Sun, as well as the order of magnitude difference in the
formation rates of the embedded and open cluster populations, suggests that
additional poor stellar groups are likely to be found at surprisingly close
distances to the Sun. Here I describe a new nearby stellar aggregate found by
virtue of the parallel proper motions, similar trigonometric parallaxes, and
consistent color-magnitude distribution of its early-type members. The 120
Myr-old group lies in Ophiuchus at 170 pc, with its most massive
member being the 4th-magnitude post-MS B8II-III star Oph. The group may
have escaped previous notice due to its non-negligible extinction (
0.9 mag). If the group was born with a normal initial mass function,
and the nine B- and A-type systems represent a complete system of
intermediate-mass stars, then the original population was probably of order
200 systems. The age and space motion of the new cluster are very similar
to those of the Pleiades, Per cluster, and AB Dor Moving Group,
suggesting that these aggregates may have formed in the same star-forming
complex some yr ago.Comment: 23 pages, 3 figs., to appear in Nov. 2006 A
- …