A search for solar-like oscillations in the Am star HD 209625

The goal is to test the structure of hot metallic stars, and in particular the structure of a near-surface convection zone using asteroseismic measurements. Indeed, stellar models including a detailed treatement of the radiative diffusion predict the existence of a near-surface convection zone in order to correctly reproduce the anomalies in surface abundances that are observed in Am stars. The Am star HD 209625 was observed with the Harps spectrograph mounted on the 3.6-m telescope at the ESO La Silla Observatory (Chile) during 9 nights in August 2005. This observing run allowed us to collect 1243 radial velocity (RV) measurements, with a standard deviation of 1.35 m/s. The power spectrum associated with these RV measurements does not present any excess. Therefore, either the structure of the external layers of this star does not allow excitation of solar-like oscillations, or the amplitudes of the oscillations remain below 20-30 cm/s (depending on their frequency range).Comment: 5 pages, 4 figures, A&A accepte

Asteroseismology of the visual binary 70 Ophiuchi

Convection in stars excites resonant acoustic waves. The frequencies of these oscillations depend on the sound speed inside the star, which in turn depends on density, temperature, gas motion, and other properties of the stellar interior. Therefore, analysis of the oscillations provides an unrivaled method to probe the internal structure of a star. Solar-like oscillations in the primary of the visual binary 70 Ophiuchi are investigated. 70 Ophiuchi A was observed with the Harps spectrograph mounted on the 3.6-m telescope at the ESO La Silla Observatory (Chile) during 6 nights in July 2004 allowing us to collect 1758 radial velocity measurements with a standard deviation of about 1.39 m s-1. The power spectrum of the high precision velocity time series clearly presents several identifiable peaks between 3 and 6 mHz showing regularity with a large spacing of Delta_nu = 161.7 +- 0.3 uHz. Fourteen individual modes were identified with amplitudes in the range 11 to 14 cm s-1.Comment: 5 pages, A&A in pres

The Effect of Strain Rate on Diffusion Flames

Several steady state and time-dependent solutions to the compressible conservation laws describing direct one-step near-equilibrium irreversible exothermic burning of initially unmixed gaseous reactants, with Lewis-Semenov number unity, are presented. The quantitative investigation first establishes the Burke-Schumann thin-flame solution using the Shvab-Zeldovich formulation. Real flames do not have the indefinitely thin reaction zone associated with the Burke-Schumann solution. Singular perturbation analysis is used to provide a modification of the thin-flame solution which includes a more realistic reaction zone of small but finite thickness. The particular geometry emphasized is the un bounded counterflow such that there exists a spatially constant rate of strain along the flame. While the solutions for diffusion flames under a finite tangential strain rate may be of interest in and of themselves for laminar flow, the problems are motivated by the authors' belief that they are pertinent to the study of diffusion-flame burning in transitional and turbulent shear flows

Analytic studies on satellite detection of severe, two-cell tornadoes

It is argued that a two-cell structure is likely to be the unique property, and potentially satellite-accessible observable, of the exceptionally severe tornado. Analysis elucidating the dynamic, thermodynamic, and geometric properties of this two-cell structure is described. The analysis ultimately will furnish instrumentation requirements

Asteroseismology and calibration of alpha Cen binary system

Using the oscillation frequencies of alpha Cen A recently discovered by Bouchy & Carrier, the available astrometric, photometric and spectroscopic data, we tried to improve the calibration of the visual binary system alpha Cen. With the revisited masses of Pourbaix et al. (2002) we do not succeed to obtain a solution satisfying all the seismic observational constraints. Relaxing the constraints on the masses, we have found an age t_alpha Cen=4850+-500 Myr, an initial helium mass fraction Y_i = 0.300+-0.008, and an initial metallicity (Z/X)_i=0.0459+-0.0019, with M_A=1.100+-0.006M_o and M_B=0.907+-0.006M_o for alpha Cen A&B.Comment: accepted for publication as a letter in A&

Solar-like oscillations in the metal-poor subgiant nu Indi: II. Acoustic spectrum and mode lifetime

Convection in stars excites resonant acoustic waves which depend on the sound speed inside the star, which in turn depends on properties of the stellar interior. Therefore, asteroseismology is an unrivaled method to probe the internal structure of a star. We made a seismic study of the metal-poor subgiant star nu Indi with the goal of constraining its interior structure. Our study is based on a time series of 1201 radial velocity measurements spread over 14 nights obtained from two sites, Siding Spring Observatory in Australia and ESO La Silla Observatory in Chile. The power spectrum of the high precision velocity time series clearly presents several identifiable peaks between 200 and 500 uHz showing regularity with a large and small spacing of 25.14 +- 0.09 uHz and 2.96 +- 0.22 uHz at 330 uHz. Thirteen individual modes have been identified with amplitudes in the range 53 to 173 cm/s. The mode damping time is estimated to be about 16 days (1-sigma range between 9 and 50 days), substantially longer than in other stars like the Sun, the alpha Cen system or the giant xi Hya.Comment: 5 pages, 7 figures, A&A accepte

Extraction of silymarin compounds from milk thistle (Silybum marianum) seed using hot, liquid water as the solvent

High-value specialty chemicals are usually obtained from natural products by extracting with generally regarded as safe (GRAS) solvents. Because organic solvents are quite often used, high operating and disposal costs occur. When compared to traditional solvents, water is an interesting alternative because of its low operating and disposal costs. Milk thistle contains compounds (taxifolin, silychristin, silydianin, silybinin A, and silybinin B) that display hepatoxic protection properties. This paper examines the batch extraction of silymarin compounds from milk thistle seed meal in 50°C, 70°C, 85°C and 100°C water as a function of time. For taxifolin, silychristin, silybinin A, and silybinin B, extraction with 100°C water resulted in the highest yields. After 210 min of extraction at 100°C, the yield of taxifolin was 1.2 mg/g of seed while the yields of silychristin, silybinin A, and silybinin B were 5.0, 1.8 and 3.3 mg/g of seed, respectively. The ratios of the extracted compounds, and particularly the ratios at long extraction times, showed that the more polar compounds (taxifolin and silychristin) were preferentially extracted at 85°C, while the less polar silybinin was preferentially extracted at 100°C

Binaries among Ap and Am stars

The results of long-term surveys of radial velocities of cool Ap and Am stars are presented. There are two samples, one of about 100 Ap stars and the other of 86 Am stars. Both have been observed with the CORAVEL scanner from Observatoire de Haute-Provence (CNRS), France. The conspicuous lack of short-period binaries among cool Ap stars seems confirmed, although this may be the result of an observational bias; one system has a period as short as 1.6 days. A dozen new orbits could be determined, including that of one SB2 system. Considering the mass functions of 68 binaries from the literature and from our work, we conclude that the distribution of the mass ratios is the same for the Bp-Ap stars than for normal G dwarfs. Among the Am stars, we found 52 binaries, i.e. 60%; an orbit could be computed for 29 of them. Among these 29, there are 7 SB2 systems, one triple and one quadruple system. The 21 stars with an apparently constant radial velocity may show up later as long-period binaries with a high eccentricity. The mass functions of the SB1 systems are compatible with cool main-sequence companions, also suggested by ongoing spectral observations.Comment: 5 pages, 2 figures, to appear in: Proc. of the 26th workshop of the European Working Group on CP stars, Contrib. Astr. Obs. Skalnate Pleso Vol. 27, No

Méthode de dimensionnement du réservoir dans les systèmes de pompage photovoltaïques

L'un des plus urgents problèmes auquel ont à faire face plusieurs pays en voie de développement est l'accès à l'eau potable. La plus grande partie de la population touchée par ce problème se retrouve en milieu rural, éloignée des grands réseaux de distribution d'électricité. Pour tenter de résoudre cette crise, l'introduction de systèmes de pompage photovoltaïques (PV), dans le cadre de programmes de pré-électrification rurale, offre une solution attrayante pour satisfaire les besoins en eau, en terme d'autonomie, de fiabilité et de performance.Un système de pompage photovoltaïque comprend un champ de modules PV qui transforme l'énergie solaire en électricité. Un onduleur (aussi appelé convertisseur) transforme ensuite le courant continu (CC) généré par le champ en courant alternatif (CA), ce dernier permettant d'alimenter un groupe moto-pompe (GMP) qui peut être immergé - dans un puits ou un forage - ou de surface (sur une rivière). II existe aussi des GMP à courant continu. Au lieu de stocker les surplus d'énergie produite dans de dispendieux accumulateurs (batteries), ce sont les surplus d'eau pompée que l'on emmagasine dans un réservoir. Ce système, dit « au fil du soleil », a obtenu d'excellentes performances en conditions réelles d'opération. Cependant, avec les méthodes de conception actuelles, la taille du réservoir est souvent grossièrement estimée.Dans le cas d'un réservoir trop petit, il en résulte une insatisfaction des usagers lorsque des débordements de réservoir se produisent. Ce gaspillaged'eau fait subir à la population des privations en ce qui concerne la consommation personnelle et affecte la rentabilité de l'opération quand les excédents d'eau sont habituellement vendus ou destinés à l'irrigation. Lorsque le réservoir est sur-dimensionné, les villageois ont alors à faire face à des coûts de construction trop élevés.La démarche entreprise dans cet article a pour but d'offrir une méthode de conception du réservoir qui ne se base pas uniquement sur le jugement et/ou les expériences passées.L'analyse du comportement des différentes composantes d'un système de pompage solaire a permis de mettre au point des modèles informatiques qui ont ensuite été validés grâce à des données obtenues sur le terrain au Sénégal. Le logiciel développé dans ce travail a pour but d'offrir une grande flexibilité pour la simulation de différents types d'équipement dans divers milieux d'opération. La qualité des résultats dépend étroitement de la précision des paramètres décrivant l'équipement et des données météorologiques.Grâce au logiciel, on pourra prédire le fonctionnement de nouvelles installations et assurer le suivi des systèmes déjà en place. La personne qui doit concevoir le système PV, de concert avec la communauté villageoise, sera capable de déterminer la taille optimale du réservoir (et peut-être valider les résultats obtenus avec les règles empiriques) permettant ainsi de mieux évaluer les coûts et les risques de pénurie qui sont liés à l'exploitation d'un système de pompage PV.Access to safe water supply is one of the must urgent problems in many developing countries. Most populations affected by this problem are in rural areas, where electricity distribution networks are not yet installed. In order to alleviate this crisis, the introduction of photovoltaic (PV) pumping systems provides an attractive solution to satisfy water needs, in terms of autonomy, reliability and performance.A solar pumping system consists of an array of PV modules that transform solar energy into electricity. An inverter converts the direct current (DC) generated by the array to alternating current (AC), that can then be used by a motor-pump unit, either submerged (in a borehole or a well) or an a river surface. DC motor-pump units can also be used. Instead of charging expansive batteries with the excess energy generated, the water is pumped and stored in a reservoir. This type of system yields excellent performances in real-time operating conditions. The site of the tank is, however, often roughly estimated in conventional design procedures.Consequently, problems such as the heavy financial burden of an over-sized tank or, on the contrary, a waste of water resources due to spillage in the case of a too-small tank are, unfortunately, quite common. These problems lead to dissatislaction among users when they have to deal with periods of personal water shortages and, in situations where the water is sold or used for irrigation, economic losses.The process introduced in this paper provides a design tool for sizing the water tank that goes further than mere « guestimates ».Analysis of the behaviour of the different components of a solar system was incorporated into computerized models, which were then validated with data gathered during field studies carried out in Senegal. The resulting software gives the designer flexibility to simulate different types of equipment under various operating conditions. The quality of the results depends closely an the accuracy of the parameters describing the equipment as well as on the reliability of the meteorological data.Different models are used in the software. The hourly climatic conditions (solar radiation and temperature) are simulated using the normal distribution. The model used to find the current-voltage (I-V) characteristics of the solar array has been developed by the Jet Propulsion Laboratory. The motor and centrifugal pump are modelled with a third order equation often used in practice. Finally, the optimal tank size is found with the volumetric method proposed by Al-Layla.From the climatic data the software enables the designer to study a typical day for a selected period of time, usually during the month with the lowest solar radiation. From the climatic data, the PV array output power for every hour of the day, the pumping flow-rate and finally the amount of water in the tank can be determined. Five scenarios of available sunshine have been calculated, allowing the designer to evaluate low-risk situations (low sunshine expected, thus larger equipment needed) and high-risk situations (higher sunshine, requiring smaller equipment).The software permits the prediction of performance for new systems as well as the diagnosis of existing ones. The system designer, in consultation with users, can then find the optimal size of the water tank (and possibly confirm the results obtained from the rule-of-thumb estimates) and better assess, with the help of a decision-making chart, the cost of a system and the risks of water shortages