849 research outputs found
Quantifying the contamination by old main-sequence stars in young moving groups: the case of the Local Association
The associations and moving groups of young stars are excellent laboratories
for investigating stellar formation in the solar neighborhood. Previous results
have confirmed that a non-negligible fraction of old main-sequence stars is
present in the lists of possible members of young stellar kinematic groups. A
detailed study of the properties of these samples is needed to separate the
young stars from old main-sequence stars with similar space motion, and
identify the origin of these structures. We used stars possible members of the
young (~ 10 - 650 Myr) moving groups from the literature. To determine the age
of the stars, we used several suitable age indicators for young main sequence
stars, i.e., X-ray fluxes and other photometric data. We also used
spectroscopic data, in particular the equivalent width of the lithium line Li I
and Halpha, to constrain the range of ages of the stars. By combining
photometric and spectroscopic data, we were able to separate the young stars
(10 - 650 Myr) from the old (> 1 Gyr) field ones. We found, in particular, that
the Local Association is contaminated by old field stars at the level of ~30%.
This value must be considered as the contamination for our particular sample,
and not of the entire Local Association. For other young moving groups, it is
more difficult to estimate the fraction of old stars among possible members.
However, the level of X-ray emission can, at least, help to separate two age
populations: stars with <200 Myr and stars older than this. Our results are
consistent with a scenario in which the moving groups contain both groups of
young stars formed in a recent star-formation episode and old field stars with
similar space motion. Only by combining X-ray and optical spectroscopic data is
it possible to distinguish between these two age populations.Comment: 7 pages, 7 figures. Accepted for publication in A&
GUIDELINES TO SIMULATE LINEAR VISCOELASTIC MATERIALS WITH AN ARBITRARY NUMBER OF CHARACTERISTIC TIMES IN THE CONTEXT OF ATOMIC FORCE MISCROSCOPY
We provide guidelines for modeling linear viscoelastic materials containing an arbitrary number of characteristic times, under atomic force microscopy (AFM) characterization. Instructions are provided to set up the governing equations that rule the deformation of the material by the AFM tip. Procedures are described in detail in the spirit of providing a simple handbook, which is accompanied by open-access code and workbook (Excel) sheets. These guidelines seek to complement the existing literature and reach out to a larger audience in the awareness of the interdisciplinary nature of science. Examples are given in the context of force-distance curves characterization within AFM, but they can be easily extrapolated to other types of contact characterization techniques at different length scales. Despite the simplified approach of this document, the algorithms described herein are built upon rigorous classical linear viscoelastic theory
The effect of magnetic activity saturation in chromospheric flux-flux relationships
We present a homogeneous study of chromospheric and coronal flux-flux
relationships using a sample of 298 late-type dwarf active stars with spectral
types F to M. The chromospheric lines were observed simultaneously in each star
to avoid spread due to long term variability. Unlike other works, we subtract
the basal chromospheric contribution in all the spectral lines studied. For the
first time, we quantify the departure of dMe stars from the general relations.
We show that dK and dKe stars also deviate from the general trend. Studying the
flux-colour diagrams we demonstrate that the stars deviating from the general
relations are those with saturated X-ray emission and that those stars also
present saturation in the H line. Using several age spectral
indicators, we show that they are younger stars than those following the
general relationships. The non-universality of flux-flux relationships found in
this work should be taken into account when converting between fluxes in
different chromospheric activity indicators.Comment: Accepted for publication in the Monthly Notices of the Royal
Astronomical Societ
Characterization of actin genes in Bonamia ostreae and their application to phylogeny of the Haplosporidia
Bonamia ostreae is a protozoan parasite that infects the European flat oyster Ostrea edulis, causing systemic infections and resulting in massive mortalities in populations of this valuable bivalve species. In this work, we have characterized B. ostreae actin genes and used their sequences for a phylogenetic analysis. Design of different primer sets was necessary to amplify the central coding region of actin genes of B. ostreae. Characterization of the sequences and their amplification in different samples demonstrated the presence of 2 intragenomic actin genes in B. ostreae, without any intron. The phylogenetic analysis placed B. ostreae in a clade with Minchinia tapetis, Minchinia teredinis and Haplosporidium costale as its closest relatives, and demonstrated that the paralogous actin genes found in Bonamia resulted from a duplication of the original actin gene after the Bonamia origi
Chemically tagging the Hyades Supercluster: A homogeneous sample of F6-K4 kinematically selected northern stars
Stellar kinematic groups are kinematical coherent groups of stars that might
have a common origin. These groups are dispersed throughout the Galaxy over
time by the tidal effects of both Galactic rotation and disc heating, although
their chemical content remains unchanged. The aim of chemical tagging is to
establish that the abundances of every element in the analysis are homogeneus
among the members. We study the case of the Hyades Supercluster to compile a
reliable list of members (FGK stars) based on our chemical tagging analysis.
For a total of 61 stars from the Hyades Supercluster, stellar atmospheric
parameters (Teff, logg, xi, and [Fe/H]) are determined using our code called
StePar, which is based on the sensitivity to the stellar atmospheric parameters
of the iron EWs measured in the spectra. We derive the chemical abundances of
20 elements and find that their [X/Fe] ratios are consistent with Galactic
abundance trends reported in previous studies. The chemical tagging method is
applied with a carefully developed differential abundance analysis of each
candidate member of the Hyades Supercluster, using a well-known member of the
Hyades cluster as a reference (vB 153). We find that only 28 stars (26 dwarfs
and 2 giants) are members, i.e. that 46% of our candidates are members based on
the differential abundance analysis. This result confirms that the Hyades
Supercluster cannot originate solely from the Hyades cluster.Comment: A&A, in pres
The Historical Significance of the Welded Tuffs from Arucas, Canary Islands
[Abstract] Arucas Stone (Piedra de Arucas) is a welded tuff quarried in the town of Arucas, on the island of Gran Canaria in the Canary Archipelago. This rock has been quarried for centuries, not only for building but also for many other purposes, such as manufacturing water cisterns or conduits, and especially for carving. The quarrying and economic activity related to this stone have profoundly shaped the history and economy of the city of Arucas, in terms not only of extraction, but also of a highly valued local artisan tradition of stone carving. Immigrants from the Canary Islands even brought this stone to several countries in South America, where it was used to erect numerous important architectural heritage sites. Nowadays, this stone is only quarried in two areas, even though it is often required for the restoration and rehabilitation of historical buildings. Its special characteristics, and, above all, its historical importance, make this stone a prime candidate for Global Heritage Stone designation
XMM-Newton monitoring of the close pre-main-sequence binary AK Sco. Evidence of tide-driven filling of the inner gap in the circumbinary disk
AK Sco stands out among pre-main-sequence binaries because of its prominent ultraviolet excess, the high eccentricity of its orbit, and the strong tides driven by it. AK Sco consists of two F5-type stars that get as close as 11 R-* at periastron passage. The presence of a dense (n(e) similar to 10(11) cm(-3)) extended envelope has been unveiled recently. In this article, we report the results from an XMM-Newton-based monitoring of the system. We show that at periastron, X-ray and UV fluxes are enhanced by a factor of similar to 3 with respect to the apastron values. The X-ray radiation is produced in an optically thin plasma with T similar to 6.4 x 10(6) K and it is found that the N-H column density rises from 0.35 x 10(21) cm(-2) at periastron to 1.11 x 10(21) cm(-2) at apastron, in good agreement with previous polarimetric observations. The UV emission detected in the Optical Monitor band seems to be caused by the reprocessing of the high-energy magnetospheric radiation on the circumstellar material. Further evidence of the strong magnetospheric disturbances is provided by the detection of line broadening of 278.7 km s(-1) in the Nv line with Hubble Space Telescope/Space Telescope Imaging Spectrograph. Numerical simulations of the mass flow from the circumbinary disk to the components have been carried out. They provide a consistent scenario with which to interpret AK Sco observations. We show that the eccentric orbit acts like a gravitational piston. At apastron, matter is dragged efficiently from the inner disk border, filling the inner gap and producing accretion streams that end as ring-like structures around each component of the system. At periastron, the ring-like structures come into contact, leading to angular momentum loss, and thus producing an accretion outburst
ROC evaluation of statistical wavelet-based analysis of brain activation in [15O]-H2O PET scans
This paper presents and evaluates a wavelet-based statistical analysis of PET images for the detection of brain activation areas. Brain regions showing significant activations were obtained by performing Student's t tests in the wavelet domain, reconstructing the final image from only those wavelet coefficients that passed the statistical test at a given significance level, and discarding artifacts introduced during the reconstruction process. Using Receiver Operating Characteristic (ROC) curves, we have compared this statistical analysis in the wavelet domain to the conventional image-domain Statistical Parametric Mapping (SPM) method. For obtaining an accurate assessment of sensitivity and specificity, we have simulated realistic single subject [15O]-H2O PET studies with different hyperactivation levels of the thalamic region. The results obtained from an ROC analysis show that the wavelet approach outperforms conventional SPM in identifying brain activation patterns. Using the wavelet method, activation areas detected were closer in size and shape to the region actually activated in the reference image.Publicad
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