High-precision CTE measurement of aluminum-alloys for cryogenic astronomical instrumentation

We are completing the construction of GIANO, a high resolution near-infrared cryogenic spectrograph for the Telescopio Nazionale Galileo (TNG). Most of the optics are made of aluminium and operate at cryogenic temperature. We evaluated the optical degradation due to mis-matches between the thermal expansion coefficients of the different aluminium parts of the instrument. We performed accurate measurements of the relative thermal expansion coefficients (CTE) of Al-6061 and Al-6082 over the 300–77 K temperatures range. We find that the two alloys have identical thermal expansion coefficient within a maximum (3σ) uncertainty of Δα/α?<?0.28%. Our results show that it is possible to overcome the problem of the alignment of a cryogenic instrument, manufacturing the curved optics, the optics’ holders and the optical bench with different metallic alloys with small CTE mismatch (Al-6061 and Al-6082). This conclusion has also been confirmed by the results of the optical tests with the instrument cooled in the laboratory, showing no significant image quality degradation.     

Distribution and Habitat Preferences of the Introduced MummichogFundulus heteroclitus(Linneaus) in South-western Spain

The distribution and apparent habitat preference of the common mummichog (Fundulus heteroclitus) in south-western Spain were examined during summer–autumn 1996. This introduced species was more or less continuously distributed along the Atlantic coast of Spain, being more abundant in sites near the coastline (usually <10 km inland), mainly in four extensive marshes. The species preferred marsh-related mesohabitats, such as salt lagoons, saltmarsh fish ponds and marsh channels, both natural and man-modified.Fundulus heteroclituswas mostly found at salinities >25. It was the most frequently captured fish species, occurring at 81 of the 272 sites sampled; their frequency of occurrence was almost twice that of the second ranked species (Gambusia holbrooki). However, in over 80% of cases,F. heteroclituswas found alone or with only one sympatric fish species, which usually belonged to a group composed ofGobius niger,mugilids,Anguilla anguilla, Blenniussp.,Lebias ibera, Pomatoschistussp. andDicentrarchus labrax. Finally, the origin and dispersal of mummichog in the Iberian peninsula and the potential effects of this species on native fish populations is discussed.     

A Survey for Hα emitting galaxies AT z ≃ 2.2

The deepest survey for H emitting galaxies at z 2.2has recently been made in narrow-band ( 1%) filters around2.1m using SOFI at the ESO NTT telescope. An effective area of 100 sq. arcmin and a comoving volume of 9000 Mpc³ (forH₀ = 50, q₀ = 0.5) has been covered to a volume weighted 3 line flux limit of 5 × 10^-17 erg s^-1cm^-2. Our survey covered the WFPC2 and STIS fields in the HubbleDeep Field South and an anonymous field about 30 deg. away. Thefaintest limit reached was 3 × 10^-17 erg s^-1 cm^-2 onthe WFPC2 field. In total, 10 convincing candidates with deduced starformation rates in the range 9 - 50 M/yr and an equal number ofmarginal ones have been identified for confirmation and follow-upspectroscopy with ISAAC at the VLT. Based on a very preliminaryanalysis we compare our results with those of earlier surveys andbriefly discuss some possible implications for the form of theevolution out to z 2 and the effects of clustering.     

The role of trace minerals in chemical weathering in a high-elevation granitic watershed (Estibère, France): chemical and mineralogical evidence

We investigated chemical weathering in a high elevation granitic environment in three selected watersheds located in the Pyrenees (France). The sites were located on glacial deposits derived from similar Hercynian (∼300 Ma) granites characterized by the occurrence of zoned plagioclases and trace calcic phases (epidote, prehnite, sphene, apatite). The surface waters at those sites show high Ca/Na molar ratios (>1) which could not be explained by the dissolution of the major plagioclase (oligoclase) present in the rocks. The coupled approach of investigating stream water chemistry and the mineralogy and chemistry of rocks and soils allowed us to explore the role of the weathering of trace calcic minerals in calcium export at the watershed scale. The weathering of the trace calcic minerals which represent ∼ 1% of the total rock volume are responsible for more than 90% of the calcium export at the sites. Annual cationic fluxes (∼ 23.10⁴ eq/km²/yr) calculated for the Estibère watershed are among the highest reported for high elevation systems draining granitic rocks and ∼ 80% of this annual cationic flux can be attributed to the weathering of trace calcic phases. Calculations based on isotopic values (⁸⁷Sr/⁸⁶Sr) go in the same direction. Except apatite, the trace calcic phases appear to be mainly silicates, thus the type of chemical weathering observed in the Estibère watershed may have an influence on atmospheric CO₂ consumption by granite weathering. However, comparison with other watersheds draining granitic environments worldwide, and with the two other sites in the Pyrenees, indicate that the role of trace calcic phases is important in most young environments exposed to chemical weathering (e.g., high elevation catchments on glacial deposits). Other factors such as the date of glacial retreat, the physical denudation rate, the hydrological functioning of the watershed and the nature and structure of the soil cover are also important.     

Evaluation of water resources in a high-mountain basin in <Emphasis Type="Italic">Serra da Estrela</Emphasis>, Central Portugal,using a semi-distributed hydrological model

High-mountain basins provide a source of valuable water resources. This paper presents hydrological models for the evaluation of water resources in the high-mountain Zêzere river basin in Serra da Estrela, Central Portugal. Models are solved with VISUAL BALAN v2.0, a code which performs daily water balances in the root zone, the unsaturated zone and the aquifer and requires a small number of parameters. A lumped hydrological model fails to fit measured stream flows. Its limitations are overcome by considering the dependence of the temperature and precipitation data with elevation and the spatial variability in hydrogeomorphological variables with nine sub-basins of uniform parameters. Model parameters are calibrated by fitting stream flow measurements in the Zêzere river. Computed stream flows are highly sensitive to soil thickness, whereas computed groundwater recharge is most sensitive to the interflow and percolation recession coefficients. Interflow is the main component of total runoff, ranging from 41 to 55% of annual precipitation. High interflows are favored by the steep relief of the basin, by the presence of a high permeability soil overlying the fractured low permeability granitic bedrock and by the extensive subhorizontal fracturing at shallow depths. Mean annual groundwater recharge ranges from 11 to 15% of annual precipitation. It has a significant uncertainty due to uncertainties in soil parameters. This methodology proves to be useful to handle the research difficulties regarding a complex mountain basin in a context of data scarcity.     

Origin of the Coronal Line Emission in NGC1068

We present recent observations of several near infrared emission lines of highly ionized (IP ? 100eV) species in NGC1068 which appear to be emitted predominantly within the NE ionization cone; peak at ? 30pc from the nucleus and are blueshifted by ? 300 km s-1 relative to the systemic velocity. The blueshift is the same as that observed for the better known Fe coronal lines in the visible and the absence of red-shifted components in the less extincted infrared lines suggests that any emission in the counter-cone to the SW is intrinsically faint rather than heavily obscured. Following a review of the possible ionization mechanisms and comparison of the line ratios with recent models we conclude that the coronal emission arises predominantly in outflowing gas photoionized by the EUV continuum of the AGN.     

Hydrogen and helium ionization structure of gaseous nebulae

In this paper we discuss the ionization equilibrium of hydrogen and helium in a nebula with an arbitrary gas density distribution. If we consider the spectral characteristics of hot stars, a power law is found to provide a good approximation to the Lyman continuum spectrum for stars withT _eff≤100 000 K. With this simplification the ionization equilibrium equation is analytically solved first for a pure hydrogen nebula, then for the general case of a nebula containing H, He, and heavy elements. A simple and quite general formula for the determination of the size and the emission of the He⁺ zone is obtained. Finally, the ionization equilibrium He⁺⁺?He⁺ is considered. This problem can be decoupled from that of the ionization of H⁰ and He⁰ if the stellar spectrum is steeper thanv ^?0.9 or, equivalently, if the star effective temperature is lower than 200 000 K. Within this limit, which surely includes all classicalHii regions and the low-medium excitation planetary nebulae, an analytical solution of the problem can be used.     

An ab-initio Hartree-Fock study of α-quartz and stishovite

-quartz and stishovite have been studied using a periodic ab-initio Hartree-Fock method in order to characterize the chemical nature of the Si-O bond and the way in which it changes with the coordination number of Si. Structural properties, including unit cell volume and c/a ratio have been optimized in order to evaluate the reliability of the method and the effect of the basis set. Density of states and electron charge density maps have been taken into account to investigate the electronic properties of the two systems and the rôle played by different orbitals. We also present comparisons with experimental X-ray emission data. The importance of d orbitals is stressed by our calculations, and a possible interpretation of the Si-O bond proposed. Quartz is found to be more covalent than stishovite.