Establishing binarity amongst Galactic RV Tauri stars with a disc

This study is a contribution in comprehending the role of binarity upon late stages of stellar evolution. We determine the binary status of six Galactic RV Tauri stars, namely DY Ori, EP Lyr, HP Lyr, IRAS 17038-4815, IRAS 09144-4933 and TW Cam, which are surrounded by a dusty disc. We also place them on the HR diagram, thereby establishing their evolutionary nature. All the six Galactic RV Tauri stars included in this study are binaries with orbital periods ranging between $\sim$ 650 and 1700 days and with eccentricities between 0.2 and 0.6. The mass functions range between 0.08 to 0.55 M$_\odot$ which points to an unevolved low mass companion. In the photometric time series we detect a long-term variation on the time-scale of the orbital period for IRAS 17038-4815, IRAS 09144-4933 and TW Cam. Our derived stellar luminosities obtained from a calibrated PLC relation indicates that all except DY Ori and EP Lyr, are post-AGB stars. DY Ori and EP Lyr are likely examples of the recently discovered dusty post-RGB stars. The orbital parameters strongly suggest that the evolution of these stars was interrupted by a strong phase of binary interaction during or even prior to the AGB. The observed eccentricities and long orbital periods among these stars provides a challenge to the standard theory of binary evolution.Comment: 18 pages, 19 figures, Accepted to be published in A&

The post-common-envelope X-ray binary nucleus of the planetary nebula NGC 2392

The Chandra X-ray Observatory has detected relatively hard X-ray emission from the central stars of several planetary nebulae (PNe). A subset have no known late-type companions, making it very difficult to isolate which of several competing mechanisms may be producing the X-ray emission. The central star of NGC 2392 is one of the most vexing members, with substantial indirect evidence for a hot white dwarf (WD) companion. Here we report on the results of a radial velocity (RV) monitoring campaign of its central star with the HERMES echelle spectrograph of the Flemish 1.2 m Mercator telescope. We discover a single-lined spectroscopic binary with an orbital period of $1.902208\pm0.000013$ d and a RV semi-amplitude of $9.96\pm0.13$ km/s. The high degree of nebula ionisation requires a WD companion ($M\gtrsim0.6 M_\odot$), which the mass-function supports at orbital inclinations $\lesssim$7 deg, in agreement with the nebula orientation of 9 deg. The hard component of the X-ray spectrum may be explained by the companion accreting mass from the wind of the Roche lobe filling primary, while the softer component may be due to colliding winds. A companion with a stronger wind than the primary could produce the latter and would be consistent with models of the observed diffuse X-ray emission detected in the nebula. The diffuse X-rays may also be powered by the jets of up to 180 km/s and active accretion would imply that they could be the first active jets of a post-common-envelope PN, potentially making NGC 2392 an invaluable laboratory to study jet formation physics. The 1.9 d orbital period rules out a double-degenerate merger leading to a Type Ia supernova and the weak wind of the primary likely also precludes a single-degenerate scenario. We suggest that a hard X-ray spectrum, in the absence of a late-type companion, could be a powerful tool to identify accreting WD companions.Comment: 12 pages, 7 figures, 2 tables. Accepted for publication in PAS

The complex fossil magnetic field of the δ\delta Scuti star HD\,41641

Only three magnetic $\delta$ Scuti stars are known as of today. HD 41641 is a $\delta$ Scuti star showing chemical peculiarities and rotational modulation of its light-curve, making it a good magnetic candidate. We acquired spectropolarimetric observations of this star with Narval at TBL to search for the presence of a magnetic field and characterize it. We indeed clearly detect a magnetic field in HD 41641, making it the fourth known magnetic $\delta$ Scuti star. Our analysis shows that the field is of fossil origin, like magnetic OBA stars, but with a complex field structure rather than the much more usual dipolar structure.Comment: 9 pages, 6 figures, 5 tables plus 1 in appendi

Protocolo para analizar Diffusion Tensor Imaging (DTI) del cerebro humano

El cerebro está compuesto por una gran variedad de células nerviosas que pueden clasificarse, de forma muy general, en neuronas de proyección, cuyos axones integran la sustancia blanca (SB) cerebral, e interneuronas, con axones confinados a la sustancia gris. Por lo tanto, la SB está formada por los haces de fibras nerviosas que facilitan la comunicación entre regiones cerebrales distantes y la integración de la información en un sistema de procesamiento distribuido. El estudio de la conectividad cerebral ha estado siempre muy presente en la investigación neurocientífica, ya que ha permitido delimitar aquellos circuitos cerebrales que subyacen a cada una de las funciones cognitivas y motoras que caracterizan a los humanos. Desde los primeros estudios de tinción neuronal realizados por Camillo Golgi y Santiago Ramón y Cajal, se han diseñado una gran variedad de técnicas dirigidas a estudiar la conectividad cerebral. El nivel máximo de detalle estructural se obtiene a partir de los análisis de microscopía electrónica que permiten identificar los contactos sinápticos de un circuito local y el número de espinas dendríticas de cada neurona. Sin embargo, esta aproximación no permite estudiar la conectividad cerebral mediante técnicas in vivo y, por lo tanto, tiene un recorrido muy limitado en el diagnóstico de enfermedades del sistema nervioso. Actualmente, la aproximación más utilizada para estudiar la conectividad cerebral se basa en el estudio de las propiedades derivadas de las imágenes de difusión (Diffusion weighted imaging, DWI, siglas en inglés) cerebral obtenidas a partir de resonancia magnética (RM). Las imágenes DWI permiten cuantificar la dirección de la difusión de las moléculas de agua del tejido cerebral al someterlas a un campo magnético constante y a pulsos de radiofrecuencia. Debido a que las moléculas de agua se difunden con mayor facilidad en axones recubiertos de vainas de mielina, la secuencia DWI permite detectar la anisotropía producida por la movilidad de las moléculas de agua en los tractos de fibras nerviosas y, posteriormente, reconstruir las trayectorias más probables de estos haces de fibras a partir de sus mapas de tractografía. No obstante, para realizar una reconstrucción de todos los haces de fibra, tractografía, se necesita de un modelado previo de la imagen cuya finalidad es proveer la orientación de las fibras. En el caso del modelo DTI (diffusion tensor imaging) se obtiene mediante la estimación de tensores. Sin embargo, el cálculo de los tensores se ve afectado por diversos artefactos durante la adquisición de la imagen de difusión que requieren de un procesamiento previo para corregirlos. Nuestro estudio se ha centrado en combinar diferentes técnicas de procesamiento y modelado, y, mediante métodos de evaluación cuantitativos y una estadística posterior, seleccionar los procedimientos que resulten de mejores resultado con el objetivo de obtener una mejor estimación de la reconstrucción de los diferentes tractos, lo cual redundaría en mejores diagnósticos de patologías que cursaran con un daño de estas fibras. Los procedimientos se han aplicado sobre dos adquisiciones diferentes de 15 sujetos cada una. En resumen, nuestro trabajo ha mostrado que un procesamiento óptimo de imágenes de difusión cerebral debería utilizar un método de corrección de artefactos basado en el proceso de Gauss usando la interpolación spline, junto con procedimientos de modelado no lineales (NLLS) o modelar mediante rechazo de valores atípicos (RESTORE).The study of brain connectivity has always been prominent in neuroscientific research because it has allowed investigators to delimit the cerebral circuits that underlie each of the cognitive and motor functions that characterize humans. Since the first studies of neuronal staining performed by Camillo Golgi and Santiago Ramón y Cajal, a great variety of techniques have been designed to study brain connectivity. Maximal structural detail is obtained from electron microscopy analyses, which allow the identification of the synaptic contacts of a local circuit and the number of dendritic spines for each involved neuron. However, this approach does not permit the study of connectivity through in vivo techniques and thus has a very limited capacity for diagnosing diseases of the nervous system. Currently, the most widely used approach to studying brain connectivity is based on properties derived from magnetic resonance imaging (Diffusion weighted imaging, DWI). DWI images allow the direction of the diffusion of water molecules in brain tissue to be quanitified by subjecting them to a constant magnetic field and radiofrequency pulses. Because water molecules diffuse more easily into axons coated with myelin sheaths, the DWI sequence detects the anisotropy produced by the mobility of water molecules in the tracts of nerve fibers and then reconstructs the most likely trajectories of these fiber bundles from their tractography maps. However, tractography (a reconstruction of all fiber bundles) requires prior modeling of the image, the purpose of which is to provide the orientation of the fibers. In the case of DTI (diffusion tensor imaging), the model is obtained by estimating tensors. However, the calculation of the tensors is affected by various artifacts during the acquisition of the diffusion image that require a prior process to correct them. Our study has focused on combining different techniques of processing and modeling. By means of quantitative evaluation methods and a subsequent statistical model, we have strived to identify procedures that lead to improved results. The aim of our work has been to obtain a better estimate of the reconstruction of the different tracts of brain, which would result in better diagnoses of pathologies that could lead to fiber damage. The procedures have been applied on two different acquisitions of 15 subjects each. In summary, our work has shown that optimal processing of brain diffusion images should use an artifact correction method based on the Gauss process using spline interpolation, along with nonlinear modeling (NLLS) or modeling by outliers rejection (RESTORE).Universidad de Sevilla. Máster en Ingeniería de Telecomunicació

Binary evolution along the Red Giant Branch with BINSTAR: The barium star perspective

Barium (Ba), CH, and extrinsic or Tc-poor S-type stars are evolved low- and intermediate-mass stars that show enhancement of slow-neutron-capture-process elements on their surface, an indication of mass accretion from a former asymptotic giant branch (AGB) companion, which is now a white dwarf (WD). Ba and CH stars can be found in the main-sequence (MS), the sub-giant, and the giant phase, while extrinsic S-type stars populate the giant branches only. As these polluted stars evolve, they might be involved in a second phase of interaction with their now white dwarf companion. In this paper, we consider systems composed of a main-sequence Ba star and a WD companion when the former evolves along the Red Giant Branch (RGB). We want to determine if the orbital properties of the known population of Ba, CH, and S giants can be inferred from the evolution of their suspected dwarf progenitors. For this purpose, we use the BINSTAR binary evolution code and model MS+WD binary systems, considering different binary interaction mechanisms, such as a tidally-enhanced wind mass-loss and a reduced circularisation efficiency. To explore their impact on the second RGB ascent, we compare the modelled orbits with the observed period and eccentricity distributions of Ba and related giants. We show that, independently of the considered mechanism, there is a strong period cut off below which core-He burning stars should not be found in binary systems with a WD companion. This limit is shorter for more massive RGB stars and for more metal-poor systems. However, we still find a few low-mass short-period giant systems that are difficult to explain with our models as well as two systems with very high eccentricities.Comment: Accepted for publication in A&

Orbital properties of binary post-AGB stars

Binary post-asymptotic giant branch (post-AGB) stars are thought to be the products of a strong but poorly-understood interaction during the AGB phase. The aim of this contribution is to update the orbital elements of a sample of galactic post-AGB binaries observed in a long-term radial-velocity monitoring campaign. Radial velocities are computed from high signal-to-noise spectra by use of a cross-correlation method. The radial-velocity curves are fitted by using both a least-squares algorithm and a Nelder-Mead simplex algorithm. We use a Monte Carlo method to compute uncertainties on the orbital elements. The resulting mass functions are used to derive a companion mass distribution by optimising the predicted to the observed cumulative mass-function distributions, after correcting for observational bias. As a result, we derive and update orbital elements for 33 galactic post-AGB binaries, among which 3 are new orbits. The orbital periods of the systems range from 100 to about 3000 days. Over 70 percent (23 out of 33) of our binaries have significant non-zero eccentricities ranging over all periods. Their orbits are non-circular despite the fact that the Roche-lobe radii are smaller than the maximum size of a typical AGB star and tidal circularisation should have been strong when the objects were on the AGB. We derive a distribution of companion masses that is peaked around 1.09 $M_\odot$ with a standard deviation of 0.62 $M_\odot$. The large spread in companion masses highlights the diversity of post-AGB binary systems. Furthermore, we find that only post-AGB stars with high effective temperatures (> 5500 K) in wide orbits are depleted in refractory elements, suggesting that re-accretion of material from a circumbinary disc is an ongoing process. It appears, however, that chemical depletion is inefficient for the closest orbits irrespective of the actual surface temperature.Comment: 21 pages total, 3 appendices, 8 figures excluding appendix figures, 3 table

Recipes for bolometric corrections and Gaia luminosities of B-type stars: Application to an asteroseismic sample

We provide three statistical model prescriptions for the bolometric corrections appropriate for B-type stars as a function of: 1) T_eff, 2) T_eff, log g, and 3) T_eff, log g, [M/H]. These statistical models have been calculated for 27 different filters, including those of the Gaia space mission, and were derived based on two different grids of bolometric corrections assuming LTE and LTE+NLTE, respectively. Previous such work has mainly been limited to a single photometric passband without taking into account NLTE effects on the bolometric corrections. Using these statistical models, we calculate the luminosities of 34 slowly pulsating B-type (SPB) stars with available spectroscopic parameters, to place them in the Hertzsprung-Russell diagram and compare their position to the theoretical SPB instability strip. We find that excluding NLTE effects has no significant impact on the derived luminosities for the temperature range 11500-21000 K. We conclude that spectroscopic parameters are needed in order to achieve meaningful luminosities of B-type stars. The three prescriptions for the bolometric corrections are valid for any galactic B-type star with effective temperatures and surface gravities in the ranges 10000-30000 K and 2.5-4.5 dex, respectively, covering regimes below the Eddington limit.Comment: 17 pages, 12 figures, accepted for publication in MNRA

Constraining the overcontact phase in massive binary evolution -- II. Period stability of known O+O overcontact systems

Given that mergers are often invoked to explain many exotic phenomena in massive star evolution, understanding the evolutionary phase directly preceding a merger, the overcontact phase, is of crucial importance. Despite its importance, large uncertainties exist in our understanding of the evolution of massive overcontact binaries. We aim to provide robust observational constraints on the future dynamical evolution of massive overcontact systems by measuring the rate at which the periods change for a sample of six such objects. Furthermore, we aim to investigate whether the periods of unequal mass systems show higher rates of change than their equal mass counterparts as theoretical models predict. Using archival photometric data from various ground- and space-based missions covering up to ~40 years, we measure the periods of each system over several smaller time spans. We then fit a linear regression through the measured periods to determine the rate at which the period is changing over the entire data set. We find that all of the stars in our sample have very small period changes and that there does not seem to be a correlation with the mass ratio. This implies that the orbital periods for these systems are stable on the nuclear timescale, and that the unequal mass systems may not equalize as expected. When comparing our results with population synthesis distributions, we find large discrepancies between the expected mass ratios and period stabilities. We find that these discrepancies can be mitigated to a degree by removing systems with shorter initial periods, suggesting that the observed sample of overcontact systems may originate from binary systems with longer initial orbital periods.Comment: 11 pages, 6 (+5) Figures, accepted for publication in A&

Atmospheric attenuation measurement system for commercial solar plants based on optical spectrum analysis

The measurement of the solar radiation attenuation is one of the main challenges in concentrating solar power technologies. This work presents a new strategy for this measurement, based on the analysis of the VIS and IR spectra, and its related system. The optical design of the system and its implementation in a central receiver solar power plant is described, and the experimental results are detailed. We present, to the best of our knowledge, the first measurements of the atmospheric attenuation spectrum in a solar power plant. This system provides direct measurements of real atmospheric attenuation values, which opens the way to analyze the impact of aerosols and meteorological conditions at surface level in this attenuation

Paleoecología del yacimiento del Neógeno continental de Los Valles de Fuentidueña (Segovia)

Conclusiones: Los datos geológicos nos indican que las capas excavadas no son homogéneas en cuanto a su deposición, un decrecimiento de la energía del medio se observa de muro a techo, esto se refleja claramente en la tafocenosis cuyo grado de aloctonía disminuye de muro a techo. No parece existir una diferencia cualitativa entre la fauna del Nivel Y (alóctono) y del Nivel X (más autóctono), aunque dada la dificultad de extraer los fósiles de este último nivel, los datos siguientes tienen más fiabilidad para el nivel Y. El estudio de la comunidad fósil nos indica el predominio fuerte de algunas especies, pero su análisis de talla parece contradecir la existencia de factores limitantes en esta comunidad, lo que unido a la reconstrucción geológica del medio, puede indicarnos que el medio de transporte ha sido uno de los factores más selectivos en la acumulación de los fósiles de este yacimiento, sin olvidar, que posiblemente la mayor parte de los restos de los animales habían estado sometidos en un largo periodo a la intemperie después de su muerte; la no existencia de ningún resto en conexión anatómica es concluyente a tal respecto. Podría suponerse un tipo de clima estacional, con al menos dos estaciones contrastadas una seca y otra húmeda que ocasionaría arroyadas como la que ha conformado el yacimiento; dentro de este marco climático poseeríamos, en el entorno alrededor del yacimiento, un ecosistema con nichos ecológicos diferentes. Podemos suponer que la zona alrededor del yacimiento ocupa la transición entre relieve moderado y una llanura de tipo sabana; esta zona de tipo transicional, se caracterizaría por la existencia de charcas efímeras. Cercana a esta zona existirían áreas con aguas limpias más o menos permanentes (de escorrentía y surgencias cársticas) en cuyos alrededores se desarrollaría una vegetación densa, que es la que soporta a un gran número de especies poco abundantes en restos (exceptuando Aceratherium sp.). Es de suponer que la zona de llanura sustentaría las formas gregarias muy abundantes en restos, como es el caso de Hipparion.Peer reviewe