1,589 research outputs found
HRC-I/Chandra X-ray observations towards sigma Orionis
Aims: We investigated the X-ray emission from young stars and brown dwarfs in
the sigma Orionis cluster (tau~3 Ma, d~385 pc) and its relation to mass,
presence of circumstellar discs, and separation to the cluster centre by taking
advantage of the superb spatial resolution of the Chandra X-ray Observatory.
Methods: We used public HRC-I/Chandra data from a 97.6 ks pointing towards the
cluster centre and complemented them with X-ray data from IPC/Einstein,
HRI/ROSAT, EPIC/XMM-Newton, and ACIS-S/Chandra together with optical and
infrared photometry and spectroscopy from the literature and public catalogues.
On our HRC-I/Chandra data, we measured count rates, estimated X-ray fluxes, and
searched for short-term variability. We also looked for long-term variability
by comparing with previous X-ray observations. Results: Among the 107 detected
X-ray sources, there were 70 cluster stars with known signposts of youth, two
young brown dwarfs, 12 cluster member candidates, four field dwarfs, and two
galaxies with optical-infrared counterpart. The remaining sources had
extragalactic nature. Based on a robust Poisson-chi^2 analysis, nine cluster
stars displayed flares or rotational modulation during the HRC-I observations,
while other eight stars and one brown dwarf showed long-term X-ray flux
variations. We constructed a cluster X-ray luminosity function from O9.5 (~18
Msol) to M6.5 (~0.06 Msol). We found: a tendency of early-type stars in
multiple systems or with spectroscopic peculiarities to display X-ray emission,
that the two detected brown dwarfs and the least-massive star are among the
sigma Orionis objects with the highest L_X/L_J ratios, and that a large
fraction of known classical T Tauri stars in the cluster are absent in this and
other X-ray surveys. We concluded that dozens X-ray sigma Orionis stars and
brown dwarfs are still to be detected [abridged].Comment: A&A, in pres
Deep XMM-Newton observation of the Eta Chamaleontis cluster
The members of the Eta Chamaleontis cluster are in an evolutionary stage in
which disks are rapidly evolving. It also presents some peculiarities, such as
the large fraction of binaries and accretion disks, probably related with the
cluster formation process. Its proximity makes this stellar group an ideal
target for studying the relation between X-ray emission and those stellar
parameters. The main objective of this work is to determine general X-ray
properties of the cluster members in terms of coronal temperature, column
density, emission measure, X-ray luminosity and variability. We also aim to
establish the relation between the X-ray luminosity of these stars and other
stellar parameters, such as binarity and presence of accretion disks. A study
of flare energies for each flare event and their relation with some stellar
parameters is also performed. We used proprietary data from a deep XMM-Newton
observation pointed at the core of the Eta Chamaleontis cluster. Specific
software for the reduction of XMM-Newton data was used for the analysis of our
observation. For the detection of sources, we used the wavelet-based code
PWDetect. General coronal properties were derived from plasma model fitting. We
also determined variability of the Eta Chamaleontis members in the EPIC
field-of-view. A total of six flare-like events were clearly detected in five
different stars. For them, we derived coronal properties during the flare
events and pseudo-quiescent state separately. In our observations, stars that
underwent a flare event have higher X-ray luminosities in the pseudo-quiescent
state than cluster members with similar spectral type with no indications of
flaring, independently whether they have an accretion disk or not. Observed
flare energies are typical of both pre-main and main-sequence M stars. We
detected no difference between flare energies of stars with and without an
accretion disk.Comment: Accepted for publication in Astronomy and Astrophysics. 12 pages, 30
individual figure
The magnetically-active, low-mass, triple system WDS 19312+3607
Aims: We investigated in detail the system WDS 19312+3607, whose primary is
an active M4.5Ve star previously thought to be young (tau ~ 300-500 Ma) based
on high X-ray luminosity. Methods: We collected intermediate- and
low-resolution optical spectra taken with 2 m-class telescopes, photometric
data from the to 8 mum bands, and eleven astrometric epochs with a time
baseline of over 56 years for the two components in the system, G 125-15 and G
125-14. Results: We derived M4.5V spectral types for both stars, confirmed
their common proper motion, estimated the heliocentric distance and projected
physical separation, determined the galactocentric space velocities, and
deduced a most-probable age older than 600 Ma. We discovered that the primary,
G 125-15, is in turn an inflated, double-lined, spectroscopic binary with a
short period of photometric variability of P ~ 1.6 d, which we associated to
orbital synchronisation. The observed X-ray and Halpha emissions, photometric
variability, and abnormal radius and effective temperature of G 125-15 AB
indicate strong magnetic activity, possibly due to fast rotation. Besides, the
estimated projected physical separation between G 125-15 AB and G 125-14 of
about 1200 AU makes WDS 19312+3607 to be one of the widest systems with
intermediate M-type primaries. Conclusions: G 125-15 AB is a nearby (d ~ 26
pc), bright (J ~ 9.6 mag), active spectroscopic binary with a single
proper-motion companion of the same spectral type at a wide separation. They
are thus ideal targets for specific follow-ups to investigate wide and close
multiplicity or stellar expansion and surface cooling due to reduced convective
efficiency.Comment: A&A, in pres
Chemical abundances of late-type pre-main sequence stars in the -Orionis cluster
The young -Orionis cluster is an important location for understanding
the formation and evolution of stars, brown dwarfs, and planetary-mass objects.
Its metallicity, although being a fundamental parameter, has not been well
determined yet. We present the first determination of the metallicity of nine
young late-type stars in -Orionis. Using the optical and near-infrared
broadband photometry available in the literature we derive the effective
temperatures for these nine cluster stars, which lie in the interval 4300--6500
K (1--3 \Msuno). These parameters are employed to compute a grid of synthetic
spectra based on the code MOOG and Kurucz model atmospheres. We employ a
-minimization procedure to derive the stellar surface gravity and
atmospheric abundances of Al, Ca, Si, Fe, Ni and Li, using multi-object optical
spectroscopy taken with WYFFOS+AF2 at at the William Herschel Telescope
(). The average metallicity of the
-Orionis cluster is [Fe/H] (random and
systematic errors). The abundances of the other elements, except lithium, seem
to be consistent with solar values. Lithium abundances are in agreement with
the "cosmic" Li abundance, except for two stars which show a in the range 3.6--3.7 (although almost consistent within
the error bars). There are also other two stars with . We derived an average radial velocity of the
-Orionis cluster of km/s. The -Orionis metallicity is
roughly solar.Comment: Accepted for publication in Astronomy and Astrophysic
Optimizing algorithms for thresholding segmentation applied to weed detection on UAV remote images.
En este trabajo se ha buscado la implementación de una alternativa al método de Otsu (1979) desarrollada por Hui-Fuang Ng (2006), el cual maximiza la diferencia entre varianzas espectrales y realiza una búsqueda multiumbral. En el estudio se emplearon imágenes procedentes de vehículos aéreos no tripulados (UAVs) tomadas en cultivos de maíz y girasol. Con una única ejecución del algoritmo en un entorno de análisis orientado a objetos, se discriminan aquellos objetos correspondientes a la fracción vegetal del suelo desnudo y se estima un umbral diferenciador entre cultivo y malas hierbas que contribuya a un subsiguiente proceso de clasificación. La técnica de Hui-Fuang detectó un mayor porcentaje de vegetación en todos los casos estudiados, oscilando el incremento entre un 3% y un 20%.This works aimed to implement an alternative to Otsu’s method (1979) developed by Hui- Fuang Ng (2006), which maximizes the difference between spectral variances and performs a multithreshold seeking. Unmanned aerial images taken in maize and sunflower crops were used in the research. In a single algorithm execution applied to an Object Based Image Analysis environment, the objects corresponding to both the vegetation fraction and bare soil are discriminated and a threshold to separate crop from weeds was also estimated, making easier a subsequent classification process. Fui-Huang’s technique provides a higher percentage of vegetation detection in all the cases, with an improvement which ranges from 3% to 20%
Insect Diversity of the Lower Montane Evergreen Forest of the Western Andes Mountain Range: Cascada Chilicay and Suncamal
Biological research in the low montane evergreen forests of Ecuador focuses on ecological and botanical aspects, while knowledge of the entomofauna of these areas is almost nil. In February 2022, sampling was carried out during the dry season for 15 days, using direct and indirect capture methods (tapping, sieving, and light traps) in two waterfalls of the low montane evergreen forest of the western Andean Cordillera: Chilicay and Suncamal waterfalls, with the objective of identifying the composition of the terrestrial insect fauna at the family level. Two orders and 21 families were recorded, among which the families Carabidae and Noctuidae represented the highest percentage of the total abundance. Although preliminary, this work constitutes the first contribution to the knowledge of the entomofauna of this ecosystem.
Keywords: biodiversity, conservation, entomofauna, insects.
Resumen
Las investigaciones biológicas en los bosques siempreverdes montanos bajos de Ecuador, se centran en aspectos ecológicos y botánicos, mientras que el conocimiento de la entomofauna de estas zonas es escaso. En febrero de 2022, en la época seca y durante 15 días, utilizando métodos de captura directa e indirecta (golpeteo, tamizado y trampas de luz), se realizaron muestreos en dos cascadas del Bosque siempreverde montano bajo de la cordillera occidental de los Andes: Cascada Chilicay y Suncamal, con el objetivo de identificar la composición de la fauna de insectos terrestres a nivel de familia. Se registraron dos órdenes y 21 familias, entre las cuales, las familias Carabidae y Noctuidae representaron el mayor porcentaje de la abundancia total. Aunque en forma preliminar, este trabajo constituye el primer aporte al conocimiento de la entomofauna de este ecosistema.
Palabras Clave: Biodiversidad, conservación, entomofauna, insectos
Towards an optimum design of wave energy converter arrays through an integrated approach of life cycle performance and operational capacity
Premio ETSI al mejor artículo científico del trimestre. 3a ed. Junio 2018
-Artículo subido a idUS con permiso de su primer autor (López-Ruiz, Alejandro), que proporciona las versiones preprint y postprint.Over the last few decades, several efforts have been made to develop an alternative and sustainable energy source from wind waves. To achieve financial sustainability of this technology, most of the research has focused on analyzing facilities composed of several wave energy converters (WECs) arrays instead of isolated ones. Although the interaction between devices and its implications on the performance of the facilities have been studied previously, these works considered only certain combinations of sea states, limiting the applicability of the results. This work applies a new methodology based on statistical methods to assess the performance of different WEC array distributions during their entire life-cycle in an efficient way, using downscaling techniques and advanced numerical modeling to propagate the wave climate. The results obtained during the hindcasting life-cycle are used to analyze the maintenance and operation capabilities of the different alternatives of arrays defined for the WEC facility. The interactions between devices and their efficiency considering the associated impact are also quantified. The assessment of these efficiencies during the complete life-cycle of the devices is highly valuable tool for promoters and coastal managers to evaluate different WEC array alternatives. The entire process was applied to a hypothetical array location in the Gulf of Cádiz (southwestern Spain), where three different array distributions were defined. The results show that the distance between WECs is a key parameter that controls the potential energy production, the efficiency of the facility and the interactions between several devices.Ministerio de Economía y Competitividad BES-2013-062617Universidad de Sevill
Towards an optimum design of wave energy converter arrays through an integrated approach of life cycle performance and operational capacity
Premio ETSI al mejor artículo científico del trimestre. 3a ed. Junio 2018
-Artículo subido a idUS con permiso de su primer autor (López-Ruiz, Alejandro), que proporciona las versiones preprint y postprint.Over the last few decades, several efforts have been made to develop an alternative and sustainable energy source from wind waves. To achieve financial sustainability of this technology, most of the research has focused on analyzing facilities composed of several wave energy converters (WECs) arrays instead of isolated ones. Although the interaction between devices and its implications on the performance of the facilities have been studied previously, these works considered only certain combinations of sea states, limiting the applicability of the results. This work applies a new methodology based on statistical methods to assess the performance of different WEC array distributions during their entire life-cycle in an efficient way, using downscaling techniques and advanced numerical modeling to propagate the wave climate. The results obtained during the hindcasting life-cycle are used to analyze the maintenance and operation capabilities of the different alternatives of arrays defined for the WEC facility. The interactions between devices and their efficiency considering the associated impact are also quantified. The assessment of these efficiencies during the complete life-cycle of the devices is highly valuable tool for promoters and coastal managers to evaluate different WEC array alternatives. The entire process was applied to a hypothetical array location in the Gulf of Cádiz (southwestern Spain), where three different array distributions were defined. The results show that the distance between WECs is a key parameter that controls the potential energy production, the efficiency of the facility and the interactions between several devices.Ministerio de Economía y Competitividad BES-2013-062617Universidad de Sevill
Rayleigh-wave attenuation by a semi-infinite two-dimensional elastic-band-gap crystal
In this paper, we report experiments on the scattering of surface-elastic waves by a periodic array of cylindrical holes. The experiments were performed in a marble quarry by drilling cylindrical holes in two different configurations: honeycomb and triangular lattices. The attenuation spectra of the surface waves show the existence of absolute band gaps for elastic waves in these semi-infinite two-dimensional crystals. Results are compared with theoretical calculations based on a scalar-wave approach. The scaling property of the underlying theory has led us to explore the possible application of the results obtained to the attenuation of surface waves in seismic movements.This work was partially supported by the Comisión Interministerial de Ciencia y Tecnología of Spain, Contract No. MAT97-0698-C04. We also thank the Consejo Superior de Investigaciones Científicas for the special funding to perform this experiment.Peer reviewe
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