178,947 research outputs found
Use of airborne vehicles as research platforms
No full textThis is the accepted version of the following chapter: Gratton, G. 2012. Use of Airborne Vehicles as Research Platforms. Encyclopedia of Aerospace Engineering, which has been published in final form at http://onlinelibrary.wiley.com/doi/10.1002/9780470686652.eae604/full. Copyright @ John Wiley & Sons 2012.The use of aircraft is often valuable to position airborne sensors or to conduct experiments in ways not possible purely on the ground. An airframe, typically an older one, must be selected then adapted to the role – likely to include inlets, windows, structural changes, power supply, computing and data recording capacity, and likely the provision of external hardpoints. Once the research vehicle is created, the instruments on board will require calibration, either in isolation or by intercomparison against already calibrated instruments on board another aircraft. This calibration process will continue throughout the life of the airplane. Additionally, an operating organization must be created and obtain any necessary organizational approvals. For some specialist applications, unmanned aerial vehicles (UAVs) may also be used, which carry some special considerations of autonomy and interoperability, but similar concerns of instrument, vehicle, and operational integrity
The origin of R CrA variability
No full textContext. R CrA is the brightest member of the Coronet star-forming region and is the closest Herbig AeBe star with a spectrum dominated by emission lines. Its luminosity has been monitored since the end of the nineteenth century, but the origin of its variability, which shows a stable period of 65.767 ± 0.007 days, is still unknown.
Aims. We studied photometric and spectroscopic data for this star to investigate the nature of the variability of R CrA.
Methods. We exploited the fact that the near-infrared luminosity of the Herbig AeBe stars is roughly proportional to the total luminosity of the stars to derive the absorption, and then mass and age of R CrA. In addition, we modeled the periodic modulation of the light curve as due to partial attenuation of a central binary by a circumbinary disk. This model reproduces the observations very well.
Results. We found that the central object in R CrA is a very young (1.5 ± 1.5 Myr) highly absorbed (AV = 5.47 ± 0.4 mag) binary; we obtain masses of MA = 3.02 ± 0.43 M⊙ and MB = 2.32 ± 0.35 M⊙ for the two components. We propose that the secular decrease of the apparent luminosity of R CrA is due to a progressive increase of the disk absorption. This might be related to precession of a slightly inclined disk caused by the recently discovered M-dwarf companion. This means that R CrA might be a triple system hosting a disk
A search for planets in the metal-enriched binary HD 219542
No full textThe components of the wide binary HD 219542 were recently found to differ in metallicity by about 0.1 dex (Gratton et al. \cite{paper1}). In this paper, we present the results of 2 years of high precision radial velocity monitoring of these stars performed at the Telecopio Nazionale Galileo (TNG) using the high resolution spectrograph SARG. No indication for radial velocity variations above the measurement errors ( ~ 5 m s-1) was found for the metal richer component A. This allows us to place upper mass-limits for planets around this star. HD 219542 B instead shows a low amplitude variation with a 112 day period at a confidence level of ~ 96-97%. This might suggest the presence of a Saturn-mass planet, although it is still possible that these variations are due to moderate activity of the star. Tests based on variations of bisectors, stellar magnitude and line equivalent widths were inconclusive so far.
Based on observations made with the Italian Telescopio Nazionale Galileo (TNG) operated on the island of La Palma by the Centro Galileo Galilei of the INAF (Istituto Nazionale di Astrofisica) at the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofisica de Canarias, and observations collected at the European Southern Observatory, Chile, using FEROS spectrograph at 1.5 m telescope (proposal ID: 69.D-0338)
Accounting for pseudoreplication is not possible when the source of nonindependence is unknown
No full textRepeated observations of the same individuals or other units, which can lead to clustered observations, are common in animal behaviour research, and mixed models are commonly employed to model and account for such clustering in the data and avoid pseudoreplication. However, in some cases, while the data might comprise repeated samples from the same individuals, the precise identity of the individuals from which samples originated is unknown. In a recent paper Garamszegi (2016, Animal Behaviour, 120, 223–234) suggested an approach to account for pseudoreplication which is based on repeatedly assigning random subject identities to the samples and then analysing the data using a mixed model or averaged values for each randomly assigned identity. Here we tested this approach using a simulation study. We found that the approach suggested by Garamszegi leads to clearly inflated type I error rates that were essentially the same as those obtained from a naïve linear model simply ignoring individual identity and that only a model based on the correct subject identities roughly produced the nominal type I error rate. We conclude that, currently, there is no method available that allows pseudoreplication to be controlled when subject identities are unknown
The connection between missing AGB stars and extended horizontal branches
No full textRecent surveys confirm early results about a deficiency or even absence of CN-strong stars on the asymptotic giant branch (AGB) of globular clusters (GCs), although with quite large cluster-to-cluster variations. In general, this is at odds with the distribution of CN band strengths among first ascent red giant branch (RGB) stars. Norris et al. proposed that the lack of CN-strong stars in some clusters is a consequence of a smaller mass of these stars that cannot evolve through the full AGB phase. In this short paper we found that the relative frequency of AGB stars can change by a factor of two between different clusters. We also find a very good correlation between the minimum mass of stars along the horizontal branch (Gratton et al. 2010, A&A, 517, A81) and the relative frequency of AGB stars, with a further dependence on metallicity. We conclude that indeed the stars with the smallest mass on the HB cannot evolve through the full AGB phase, being AGB-manqué. These stars likely had large He and N content, and large O-depletion. We then argue that there should not be AGB stars with extreme O depletion, and few of them with a moderate one
Appropriate Similarity Measures for Author Cocitation Analysis
No full textWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Metal Abundances Of One Hundred Hipparcos Dwarfs
No full textAbundances for Fe, O, and the ff\Gammaelements (Mg, Si, Ca, and Ti) have been derived from high resolution spectra of a sample of about one hundred dwarfs with high precision parallaxes measured by Hipparcos. The stars have metal abundances in the range \Gamma2:5 ![Fe/H]! 0:2. The observational data set consists of high dispersion (20 000 ! R ! 70 000), high S=N (? 200) spectra collected at the Asiago and McDonald Observatories. The abundance analysis followed the same precepts used by Gratton et al. (1997a) for ¸ 300 field stars and for giants in 24 globular clusters (Carretta & Gratton 1997), and includes corrections for departures from LTE in the formation of O lines. Our main results are: (1) that the equilibrium of ionization of Fe is well satisfied in late F -- early K dwarfs; (2) O and ff\Gammaelements are overabundant by ¸ 0:3 dex. This large homogeneous data set was used in the derivation of accurate ages for globular clusters (Gratton et al., this volume). Key words: Stars: c..
The waiting-time phenomena of gravity currents in fluids and non linear diffusion
No full textLa presente investigación de Tesis trata sobre el fenómeno de tiempo de espera en corrientes de gravedad en fluidos y difusión no lineal. Muchos fenómenos se describen mediante la ecuación de difusión no lineal unidimensional (EDNL): h1 = δx(hᵐδxh) (1) en donde los subíndices t y x indican derivadas parciales respecto del tiempo y del espacio, respectivamente. Entre ellos se pueden citar: (a) flujos en acuíferos no confinados en la aproximación de Dupuit-Forchheimer (Polubarinova-Kochina 1962, Eagleson 1970, Peletier l981, m=l), (b) flujos de gases en medios porosos (Muskat 1937, Gilding y Peletier l977a, l977b, Vázquez 1983, m=γ), (c) conducción térmica en plasmas (Zel’dovich y Raizer 1966, m=5/2) (d) conducción del calor por radiación en gases multiplemente ionizados (Zel’dovich y Raizer 1966, Pert 1977, m=4,5-5,5). (e) conducción del calor por radiación en gases completamente ionizados (Marshak 1958, Zel’dovich y Raizer 1966, Larsen y Pomraning 1980, m=l3/2), (f) corrientes viscogravitatorias (CVG, m=3). Las CVG son un tipo particular de corrientes de gravedad en líquidos, e interesan en las ciencias naturales y por sus aplicaciones a la tecnología y al medio ambiente (Simpson, 1982, Huppeit, 1986). Son flujos que se derraman sobre una superficie plana horizontal y rígida en el régimen en que los esfuerzos viscosos balancean la gravedad (bajo número de Reynolds y efectos de capilaridad despreciables) descriptos por la aproximación de lubricación (Buckmaster 1977, Huppert 1982, Gratton y Minotti 1990). Por la facilidad con que se las puede estudiar en el laboratorio, son una herramienta muy útil para el estudio de la difusión no lineal. Una característica de la difusión no lineal es la presencia de frentes que separan dominios donde h>0 de otros en donde h=0 (recordar la velocidad finita de propagación de una onda térmica fuerte); otra característica es la existencia de soluciones con tiempo de espera (STE), cuyo frente queda inmóvil durante un lapso finito tw, mientras ocurren cambios detrás de él (Aronson 1970, Kamin 1980, Knerr 1977, Lacey et al. 1982, Kath y Cohen 1982, Lacey 1983, Aronson et al. 1983, 1985, Vázquez 1984, Thomas et al. 1991, Gratton et al. 1992, Marino et al. 1995). Las soluciones autosemejantes de la ecuación (1) fueron estudiadas extensamente en el caso unidimensional, en el que dependen de una única variable ξ=x/tδ, donde x representa a una coordenada cartesiana (simetría plana) o a una radial (simetría axial). Su interes radica en que se obtienen fácilmente y que representan el comportamiento asintótico intermedio de muchos problemas no autosemejantes (Barenblatt 1952, Barenblatt y Zel’dovich 1957, Pattle 1959, Pen 1977, Grundy 1979, etc.). También se conocen bien las CVG autosemejantes (Buckmaster, 1977, Huppen, 1982, Gratton y Minotti, 1990, Maxworhy 1982, 1983, Huppert 1982, etc.). Reseñas sobre las propiedades de la ecuación (1) y de las STE se encuentran en Gratton (1991a) y Gratton et al. (1992). Una reseña sobre la autosemejanza, sus aplicaciones y las corrientes de gravedad en fluidos, incluyendo las CVG, puede encontrarse en Gratton (1991b). En esta investigación de Tesis Doctoral se investigan STE para CVG planas con condiciones iniciales del tipo h c xp. El proceso comienza en t=—t(...); inicialmente el frente está en x=0 y hǂ0 para 0<x<xo, siendo ho su valor característico: el frente arranca en t=0 (h(xo—tw)ǂ0)), la condición de contorno en xo corresponde a una pared ideal). Las soluciones se obtuvieron numéricamente con gran precisión, para lo cual se desarrolló un código ad hoc. El trabajo de investigación se dirigió a aclarar los siguientes puntos: (a) La relación entre condiciones iniciales y tiempo de espera y otras propiedades de las soluciones. No hay fórmulas teóricas para tw (salvo para un único caso: p=2/m), pero sí cotas superiores e inferiores (Kath y Cohen 1982, Lacey et al. 1982, Vázquez 1984, Aronson et al. 1985). Con condiciones iniciales del tipo h c xp, Kath y Cohen (1982) mostraron que, para m<<l, hay un tiempo de espera no nulo si p≥2/m, y que si p>2/m aparece un corner layer en la solución (un CL, es un pequeño intervalo Δx en el que hx varía fuertemente). Vázquez (1984) extendió este resultado para todo m>0. (b) La asintótica de las STE cerca del frente y para tiempos próximos al momento del arranque (ǀxǀ<<xo, ǀtǀ<<tw ǀhǀ<<ho). En ese dominio ninguno de los parámetros característicos de las condiciones iniciales puede intervenir en la solución, por lo cual se espera que sea autosemejante de II Especie (Barenblatt y Zel’dovich 1972, Barenblatt 1979, Aronson y Vázquez 1994). Se verifica esta conjetura y se encuentra la relación δ=δ(p). Previamente al estudio numérico se investigan las soluciones autosemejantes relevantes, que se agrupan en dos familias de acuerdo con δ (Gratton y Vigo l994a): las soluciones LOT (Lacey et al. 1982) y las soluciones A. Las soluciones LOT existen para todo δ>l y son de tres clases: si l<δ<l3/l0 (clase L) presentan una sucesión infinita de CL cuyo punto de acumulación es el frente; si δ>l3/l0 (clases E y N) no hay CL. La nomenclatura proviene del comportamiento de la singularidad en el plano de fase que les da origen (el ciclo límite L o el punto B, quien de acuerdo al valor de δ, es un punto Espiral o Nodo). Las soluciones A tienen δ≤1, y representan la evolución del corner layer fuerte que está llegando al frente; el caso δ=1 es la solución de onda viajera (0V, ver Gratton y Minotti 1990). Las soluciones numéricas muestran que si p<2/3 el frente arranca de inmediato y si p>2/3 se obtienen soluciones con tiempo de espera con un CL móvil, de acuerdo con la teoría. El CL se refuerza (Δx se reduce y aumenta la variación de hx) mientras avanza hacia el frente que espera, y cuando lo alcanza, éste se pone en movimiento. Se estudia en detalle el movimiento del CL y del frente y otras propiedades de las soluciones. Se determina tw(p) y se compara con las cotas teóricas, viendo que en muchos casos estas no son buenos estimadores de tw. La asintótica de las STE muestra un comportamiento sorprendente. En primer lugar sólo las soluciones clase L y la OV son relevantes. Tan sólo la parte de las soluciones L que está detrás del primer CL de la sucesión representa la asintótica de las soluciones numéricas, y lo hace en un dominio que está detrás del corner layer, y para un intervalo de tiempo que excluye el entorno del arranque (se observa un solo CL, porque cerca del frente la solución verdadera converge demasiado lentamente a la solución L). En cambio la asintótica en el entorno del CL y para tiempos próximos al arranque (e incluyéndolo) está descripta por la 0V (δ=1), de forma tal que el movimiento del CL empalma con continuidad con el del frente, luego del arranque. Los flujos viscogravitatorios son un privilegio para el investigador puesto que constituyen un caso modelo altamente viable para su implementación experimental a escala de laboratorio; en ese sentido la verificación experimental de la teoría fue realizada en colaboración con el grupo del Instituto de Física de Arroyo Seco (IFAS), de la Universidad Nacional del Centro de la Provincia de Buenos Aires (UNCPBA), Tandil, dirigido por el Profesor R. Gratton, en donde se realizaron experimentos de derrames de aceites de siliconas con perfiles iniciales en forma de cuña (correspondientes a p=l), que presentan tiempo de espera (Marino et al. 1996). Se presenta una reseña de estos resultados experimentales y una comparación con algunos de los resultados obtenidos en esta Tesis.The present Thesis Investigation address with the waiting-time viscous gravity currents phenomena and non linear diffusion. A wide range of phenomena are described by the one-dimensional non linear diffusion equation (NLDE): h1 = δx(hᵐδxh) (l) where the t and x subscripts indicate partial time and spatial derivatives, respectively. Among them, we can mention: (a) unconfined groundwater flows, in the Dupuit Forchheimer approximation (Polubarinova-Kochina 1962, Eagleson 1970, Peletier 1981, m=l), (b) gas flow through porous media (Muskat 1937 Gilding y Peletier l977a, l977b, Vázquez 1983, m=γ), (c) thermal conduction in plasmas (Zel’dovich y Raizer 1966, m=5/2) (d) thermal conduction by radiation in multiply ionized gas (Zel’dovich y Raizer 1966, Pert 1977, m=4,5-5,5), (e) thermal conduction by radiation in a fully ionized gas (Marshak 1958, Zel’dovich y Raizer 1966, Larsen y Pomraning 1980, m=l3/2), (f) viscous gravity currents (VGC, m=3). The VGC are a special case of viscous currents in liquids, and are of particular interest in technology applications and natural and environmental sciences, (Simpson, 1982, Huppen, 1986). VGC refers to flows over a horizontal rigid plane surface in the regime where viscous forces balance gravity (low Reynolds number and capillary effects are negligibles), and are described by the lubrication approximation. (Buckmaster 1977, Huppert 1982, Gratton y Minotti 1990). They are a useful tool for the study of non linear diffusion, because they are inexpensive and simple to set up in the laboratory. A characteristic of the non linear diffusion is the presence of fronts that divide domains where h>0 from others where h=0 (remember the finite propagation velocity of the strong thermal wave); another characteristic is the existence of waiting-time solutions (WTS), which have a front that remains motionless during a finite time tw, while some changes occur behind it. (Aronson 1970, Kamin 1980, Knerr 1977, Lacey et al. 1982, Kath y Cohen 1982, Lacey 1983, Aronson et al. 1983, 1985, Vázquez 1984, Thomas et al. 1991, Gratton et al. 1992, Marino er al. 1995). Self similar solutions of equation (l) were studied extensively in the one dimensional case in which they depend on a single variable ξ=x/tδ, where x is a Cartesian coordinate (plane symmetry) or a radial coordinate (axial symmetry). They are interesting because can be easily constructed and represent the intermediate asymptotic behavior of a wide range of non self similar problems. (Barenblatt l952, Barenblatt y Zel’dovich 1957, Pattle 1959, Pert 1977, Grundy 1979, etc.). The self-similar VGC are well known (Buckmaster, 1977, Huppert, 1982, Gratton y Minotti, 1990, Maxworthy 1982, 1983, Huppert 1982, etc.). Reviews about the properties of equation (1) and WTS can be found in Gratton (1991a) and Gratton et al, (1992). Other reviews about self-similarity, its aplications, and the gravity currents in fluids, including VGC, can be found in Gratton (l99lb). In this PHD Thesis Research, we investigate WTS for plane VGC with initial conditions of the type h c xp. The process begins at t=—tw, initially the front is at x=0 y hǂ0 for 0<x<xo, where ho is a characteristic value: the front starts to move at t=0 (h(xo-tw)ǂ0), the boundary condition at xo corresponds to an idealized wall). The solutions are obtained numerically with high precision using a scheme developed ad hoc. The work was focalized on the solution to the following points: (a) the relation between initial conditions and waiting-time and other properties of the solutions. There are no analytical formulae to determine tw (there is a only case, p=2/m), but higher and lower bounds are available (Kath y Cohen 1982, Lacey et al. 1982, Vázquez 1984, Aronson et al. 1985). For initial conditions of the type h c xP. Kath y Cohen (1982) proved that, for m<<l, there exists a non vanishing waiting time if p≥2/m, and if p>2/m, a corner layer (CL) developed in the solution (a CL, is a small interval Δx where hx suffers strong variations). Vazquez (1984) extended this result for all m>0. (b) The asymptotic regime of WTS near the front and close to the startup time (ǀxǀ<<xo, ǀtǀ<<tw ǀhǀ<<ho). In this domain, no characteristic parameters of the initial conditions can enter the solution. It is for this reason that we expect the solution to be self-similar of the Second Kind (Barenblatt y Zel’dovich 1972, Barenblatt 1979, Aronson y Vázquez 1994). We verify this conjecture and found the δ=δ(p) relation. Prior to the numerical study we investigate the relevant WTS, that can be grouped in two families, according to the value of δ (Gratton y Vigo l994a): the LOT solutions (Lacey et al. 1982) and the A solutions. The LOT solutions exist for all δ>l and are of three classes: when 1<δ<l3/10 (L class), they present an infinite series of CL with an accumulation point in the front; if δ>l3/10 (clases E y N) there is no CL at all. The nomenclature comes from the behavior of the singularity in the phase plane, a limit cycle L or a spiral or nodal (according to the value of δ ) point B. The A solutions have δ≤1, and represent the evolution of a strong corner layer that is arriving to the front; the δ=1 case corresponds to the travelling wave (TW, see Gratton y Minotti, 1990). The numerical solutions show that if p<2/3 the front starts to move inmediately and that if p>2/3, we obtain waiting-time solutions with a moving CL, according to the theory. The CL reinforces (Δx decrease and hx increase) while it moves toward the front, and when it reaches it, the front it begins to move. We study in detail the movement of the CL and of the front and another properties of the solutions. We determine tw(p) and compare it with the theoretical bounds, and observe that in many cases, these are not a good estimator of tw. The asymptotic regime of the WTS shows a surprising behavior. First, only the L and TW classes are relevant to the problem. Only the part of the L solutions that is behind the first CL of the series represents the asymptotic of the numerical solutions, and it does so in a domain that is behind the CL, for a time interval that excludes the startup, (only one CL can be observed because near the front, the real solution converges too slowly to the L solution). On the other hand, the asymptotic in a region around the CL for times close to the startup time (including it), is described by the TW (δ=1), in such a way that CL joins smoothly with the front after startup. The viscous gravity flows are a privilege for the researchers, because they constitute a model case of wide experimental implementation in the laboratory; in this sense, the experimental check of the theory was made in conjunction with the group of the Instituto de Física de Arroyo Seco (IFAS), from the Universidad Nacional del Centro de la Provincia de Buenos Aires (UNCPBA), Tandil, directed by Professor R. Gratton. They make experiments on the spreading of silicone oils with initial wedge-like profiles (corresponding to p=l), that presents waiting-time behavior (Marino et al. 1996). We show a review of these experimental results and a comparation with some of the results obtained in this Thesis.Fil:Vigo, Claudio Lionel Martín. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina
Use of global positioning system velocity outputs for determining airspeed measurement error
No full textSeveral methods have been derived since the advent of GPS (Global Positioning System) receivers in aircraft cockpits by which these receivers may be used to calibrate these aircraft’s other instrumentation; in particular the pitot-static system. This paper presents the four most suitable methods, two of which have been developed by the author. These methods are shown with a common symbology, and their strengths, weaknesses, analysis and operational use are compared
The Chemical Evolution of the Galaxy: The Two-Infall Model
No full textWe present a new chemical evolution model for the Galaxy that assumes two main infall episodes, for the formation of the halo-thick disk and thin disk, respectively. We do not try to take into account explicitly the evolution of the halo since our model is better suited for computing the evolution of the disk (thick plus thin), but we implicitly assume that the timescale for the formation of the halo was of the same order as the timescale for the formation of the thick disk. The formation of the thin disk is much longer than that of the thick disk, implying that the infalling gas forming the thin disk comes not only from the thick disk but mainly from the intergalactic medium. The timescale for the formation of the thin disk is assumed to be a function of Galactocentric distance, leading to an inside-out picture for the Galaxy's building. The model takes into account the most up-to-date nucleosynthesis prescriptions and adopts a threshold in the star formation process, which naturally produces a hiatus in the star formation rate at the end of the thick-disk phase, as suggested by recent observations. The model results are compared with an extended set of observational constraints both for the solar neighborhood and for the whole disk. Among these constraints, the tightest is the metallicity distribution of the G-dwarf stars, for which new data are now available. Our model fits these new data very well. The model also predicts the evolution of the gas mass, the star formation rate, the supernova rates, and the abundances of 16 chemical elements as functions of time and Galactocentric distance. We show that, in order to reproduce most of these constraints, a timescale of <=1 Gyr for the (halo) thick disk and of 8 Gyr for the thin disk's formation in the solar vicinity are required. We predict that the radial abundance gradients in the inner regions of the disk (R < 1 Rsun) are steeper than in the outer regions, a result confirmed by recent abundance determinations, and that the inner gradients steepen during the Galactic lifetime. The importance and the advantages of assuming a threshold gas density for the onset of the star formation process are discussed
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