Are beryllium abundances anomalous in stars with giant planets?

In this paper we present beryllium (Be) abundances in a large sample of 41 extra-solar planet host stars, and for 29 stars without any known planetary-mass companion, spanning a large range of effective temperatures. The Be abundances were derived through spectral synthesis done in standard Local Thermodynamic Equilibrium, using spectra obtained with various instruments. The results seem to confirm that overall, planet-host stars have ``normal'' Be abundances, although a small, but not significant, difference might be present. This result is discussed, and we show that this difference is probably not due to any stellar ``pollution'' events. In other words, our results support the idea that the high-metal content of planet-host stars has, overall, a ``primordial'' origin. However, we also find a small subset of planet-host late-F and early-G dwarfs that might have higher than average Be abundances. The reason for the offset is not clear, and might be related either to the engulfment of planetary material, to galactic chemical evolution effects, or to stellar-mass differences for stars of similar temperature.Comment: 15 pages, 9 figures, accepted for publication in Astronomy & Astrophysic

Reconstrucción de imagen en tomografía por emisión de positrones

Las imágenes de tomografía de emisión se generan mediante un algoritmo de reconstrucción, a partir de un conjunto de proyecciones adquiridas del objeto o paciente bajo examen. El procedimiento clásico de reconstrucción de imagen es la retroproyección filtrada (FBP). Este método es rápido y sencillo, pero no utiliza información estadística. Es un buen método para aplicaciones en las que el número de cuentas es alto (como tomografía de rayos X o CT), pero es peor cuando hay un bajo número de cuentas, como en imágenes de medicina nuclear. Los métodos iterativos de reconstrucción de imagen se han propuesto como alternativas a FBP. Estas técnicas tienen un coste computacional más alto que FBP pero producen imágenes de mejor contraste y relación señal-ruido. Los métodos iterativos eliminan los artefactos de líneas presentes en las imágenes FBP, reduciendo los falsos positivos y los falsos negativos cuando las lesiones están en la proximidad de órganos calientes. Este artículo presenta una visión de conjunto de los principios de la reconstrucción de imágenes para tomografía por emisión de positrones (PET); se introducen brevemente las bases matemáticas del método FBP para seguidamente presentar los métodos estadísticos de reconstrucción iterativa, principalmente los basados en la estimación de la máxima verosimilitud. También se comenta la técnica de subconjuntos ordenados para acelerar su cómputo, así como el uso de probabilidades a priori bayesianas, lo que permite la incorporación de información a priori (tal como restricciones de suavidad o información topológica parcialmente especificada) y así mejorar la calidad de la imagen. Finalmente se muestran ejemplos con fantomas y con estudios de pacientes para comparar los métodos presentadosIn emission tomography images are obtained from a reconstruction process using a set of measured projections of the object or the patient examined. The classical method for image reconstruction is filtered backprojection (FBP). This method is fast and simple, but it does not use any statistical information about the measurements. It is a good method for applications when the number of measured counts is high (e.g., in X-ray computed tomography, CT) but insufficient when applied for low-count projection data in nuclear molecular imaging. Iterative image reconstruction methods have been proposed as alternatives to FBP. Statistical iterative image reconstruction techniques show higher computational cost than FBP but are shown to produce images of better contrast and signalto- noise ratio. The elimination of streak artifacts present in FBP images with the use of iterative methods minimizes false-positive as well as false-negative results, when lesions are situated in the vicinity of hot organs. This paper presents an overview of the principles of image reconstruction for positron emission tomography (PET) and introduces briefly the mathematical background of the FBP method before entering into details of statistical iterative image reconstruction methods,mainly based on maximum likelihood estimation. A discussion is included about the ordered subsets technique for the acceleration of these methods and the Bayesian prior approach, which allows the incorporation of a priori information (such as smoothness constraints or partial specified topological information) and therefore further improves image quality. Some examples are included from phantom and patient studies to compare some of the methods presentedPublicad

Beryllium anomalies in solar-type field stars

We present a study of beryllium (Be) abundances in a large sample of field solar-type dwarfs and sub-giants spanning a large range of effective temperatures. The analysis shows that Be is severely depleted for F stars, as expected by the light-element depletion models. However, we also show that Beryllium abundances decrease with decreasing temperature for stars cooler than $\sim$6000 K, a result that cannot be explained by current theoretical models including rotational mixing, but that is, at least in part, expected from the models that take into account internal wave physics. In particular, the light element abundances of the coolest and youngest stars in our sample suggest that Be, as well as lithium (Li), has already been burned early during their evolution. Furthermore, we find strong evidence for the existence of a Be-gap for solar-temperature stars. The analysis of Li and Be abundances in the sub-giants of our sample also shows the presence of one case that has still detectable amounts of Li, while Be is severely depleted. Finally, we compare the derived Be abundances with Li abundances derived using the same set of stellar parameters. This gives us the possibility to explore the temperatures for which the onset of Li and Be depletion occurs.Comment: 16 pages, 13 figures, accepted for publication in Astronomy & Astrophysic

Abrupt Changes in the Dynamics of Quantum Disentanglement

Entanglement evolution in high dimensional bipartite systems under dissipation is studied. Discontinuities for the time derivative of the lower bound of entanglement of formation is found depending on the initial conditions for entangled states. This abrupt changes along the evolution appears as precursors of entanglement sudden death.Comment: 4 pages and 6 figures, submitted for publicatio

Light elements in stars with exoplanets

It is well known that stars orbited by giant planets have higher abundances of heavy elements when compared with average field dwarfs. A number of studies have also addressed the possibility that light element abundances are different in these stars. In this paper we will review the present status of these studies. The most significant trends will be discussed.Comment: 10 pages, 6 figures. Submitted to the proceedings of IAU symposium 268: Light elements in the universe

Nitrogen abundances in Planet-harbouring stars

We present a detailed spectroscopic analysis of nitrogen abundances in 91 solar-type stars, 66 with and 25 without known planetary mass companions. All comparison sample stars and 28 planet hosts were analysed by spectral synthesis of the near-UV NH band at 3360 \AA observed at high resolution with the VLT/UVES,while the near-IR NI 7468 \AA was measured in 31 objects. These two abundance indicators are in good agreement. We found that nitrogen abundance scales with that of iron in the metallicity range -0.6 <[Fe/H]< +0.4 with the slope 1.08 \pm 0.05. Our results show that the bulk of nitrogen production at high metallicities was coupled with iron. We found that the nitrogen abundance distribution in stars with exoplanets is the high [Fe/H] extension of the curve traced by the comparison sample of stars with no known planets. A comparison of our nitrogen abundances with those available in the literature shows a good agreement.Comment: 15 pages, 7 figures, Accepted for publication in A&

Beryllium abundances in stars hosting giant planets

We have derived beryllium abundances in a wide sample of stars hosting planets, with spectral types in the range F7V-K0V, aimed at studying in detail the effects of the presence of planets on the structure and evolution of the associated stars. Predictions from current models are compared with the derived abundances and suggestions are provided to explain the observed inconsistencies. We show that while still not clear, the results suggest that theoretical models may have to be revised for stars with Teff<5500K. On the other hand, a comparison between planet host and non-planet host stars shows no clear difference between both populations. Although preliminary, this result favors a ``primordial'' origin for the metallicity ``excess'' observed for the planetary host stars. Under this assumption, i.e. that there would be no differences between stars with and without giant planets, the light element depletion pattern of our sample of stars may also be used to further investigate and constraint Li and Be depletion mechanisms.Comment: A&A in press -- accepted on the 22/02/2002 (11 pages, 6 figures included

Star-formation histories of local luminous infrared galaxies

We present the analysis of the integrated spectral energy distribution (SED) from the ultraviolet (UV) to the far-infrared and H$\alpha$ of a sample of 29 local systems and individual galaxies with infrared (IR) luminosities between 10^11 Lsun and 10^11.8 Lsun. We have combined new narrow-band H$\alpha$+[NII] and broad-band g, r optical imaging taken with the Nordic Optical Telescope (NOT), with archival GALEX, 2MASS, Spitzer, and Herschel data. The SEDs (photometry and integrated H$\alpha$ flux) have been fitted with a modified version of the MAGPHYS code using stellar population synthesis models for the UV-near-IR range and thermal emission models for the IR emission taking into account the energy balance between the absorbed and re-emitted radiation. From the SED fits we derive the star-formation histories (SFH) of these galaxies. For nearly half of them the star-formation rate appears to be approximately constant during the last few Gyrs. In the other half, the current star-formation rate seems to be enhanced by a factor of 3-20 with respect to that occured ~1 Gyr ago. Objects with constant SFH tend to be more massive than starbursts and they are compatible with the expected properties of a main-sequence (M-S) galaxy. Likewise, the derived SFHs show that all our objects were M-S galaxies ~1 Gyr ago with stellar masses between 10^10.1 and 10^11.5 Msun. We also derived from our fits the average extinction (A_v=0.6-3 mag) and the polycyclic aromatic hydrocarbons (PAH) luminosity to L(IR) ratio (0.03-0.16). We combined the A_v with the total IR and H$\alpha$ luminosities into a diagram which can be used to identify objects with rapidly changing (increasing or decreasing) SFR during the last 100 Myr.Comment: 16 pages + online material, accepted for publication in A&

A branch-point approximant for the equation of state of hard spheres

Using the first seven known virial coefficients and forcing it to possess two branch-point singularities, a new equation of state for the hard-sphere fluid is proposed. This equation of state predicts accurate values of the higher virial coefficients, a radius of convergence smaller than the close-packing value, and it is as accurate as the rescaled virial expansion and better than the Pad\'e [3/3] equations of state. Consequences regarding the convergence properties of the virial series and the use of similar equations of state for hard-core fluids in $d$ dimensions are also pointed out.Comment: 6 pages, 4 tables, 3 figures; v2: enlarged version, extension to other dimensionalities; v3: typos in references correcte

Design of a high resolution small animal octagonal PET scanner: preliminary studies

[Abstract] AMI International Conference 2003, September 21 - 27, Madrid, Spain: "High Resolution Molecular Imaging: from Basic Science to Clinical Applications"We present a preliminary study on the design of a high resolution small animal octagonal positron emission tomography (PET) scanner, based on Monte Carlo simulations. The purpose of this study is to evaluate the impact of several critical design parameters on the reconstructed image quality, as well as the calculation of the system matrix for iterative image reconstruction based on statistical modelsPublicad