Large Wavelength Coverage Research Articles

Sub-MHz ultrahigh-resolution optical spectrometry based on Brillouin dynamic gratings.

We propose and demonstrate an ultrahigh-resolution optical spectrometry based on Brillouin dynamic gratings (BDGs). Taking advantage of creating a long grating in an optical fiber, an ultra-narrow bandwidth optical filter is realized by operating a BDG in a long single-mode fiber (SMF), and the optical spectrometry is performed by sweeping the center wavelength of the BDG-based filter through a swept-tuned laser. The BDG-based optical spectrometry features ultrahigh resolution, large wavelength coverage, and a simple direction-detection scheme. In the experiment, a 4fm (0.5MHz) spectral resolution is achieved by operating a BDG in a 400m SMF, and the wavelength coverage can be readily extended to C+L bands with a commercial tunable laser.

GRB 100219A with X-shooter – abundances in a galaxy at z =4.7

Abundances of galaxies at redshifts z > 4 are difficult to obtain from damped Ly {\alpha} (DLA) systems in the sightlines of quasars (QSOs) due to the Ly {\alpha} forest blanketing and the low number of high-redshift quasars detected. Gamma-ray bursts (GRBs) with their higher luminosity are well suited to study galaxies out to the formation of the first stars at z > 10. Its large wavelength coverage makes the X-shooter spectrograph an excellent tool to study the interstellar medium (ISM) of high redshift galaxies, in particular if the redshift is not known beforehand. Here we determine the properties of a GRB host at z = 4.66723 from a number of resonant low- and high ionization and fine-structure absorption lines. This is one of the highest redshifts where a detailed analysis with medium-resolution data has been possible. We detect one intervening system at z = 2.18. The velocity components of the absorption lines are fitted with Voigt-profiles and we determine a metallicity of [M/H] = -1.0 \pm 0.1 using S. The absorption lines show a complicated kinematic structure which could point to a merger in progress. Si II* together with the restframe UV energy release determined from GROND data gives us the distance of 0.3 to 1 kpc of the absorbing material from the GRB. We measure a low extinction of AV = 0.24 \pm 0.06 mag using X-ray spectral information and the flux calibrated X-shooter spectrum. GRB-DLAs have a shallower evolution of metallicity with redshift than QSO absorbers and no evolution in HI column density or ionization fraction. GRB hosts at high redshift might continue the trend towards lower metallicities in the LZ-relation with redshift, but the sample is still too small to draw a definite conclusion. While the detection of GRBs at z > 4 with current satellites is still difficult, they are very important for our understanding of the early epochs of star- and galaxy-formation.

The 1.5 meter solar telescope GREGOR

AbstractThe 1.5 m telescope GREGOR opens a new window to the understanding of solar small‐scale magnetism. The first light instrumentation includes the Gregor Fabry Pérot Interferometer (GFPI), a filter spectro‐polarimeter for the visible wavelength range, the GRating Infrared Spectro‐polarimeter (GRIS) and the Broad‐Band Imager (BBI). The excellent performance of the first two instruments has already been demonstrated at the Vacuum Tower Telescope. GREGOR is Europe’s largest solar telescope and number 3 in the world. Its all‐reflective Gregory design provides a large wavelength coverage from the near UV up to at least 5 microns. The field of view has a diameter of 150″. GREGOR is equipped with a high‐order adaptive optics system, with a subaperture size of 10 cm, and a deformable mirror with 256 actuators. The science goals are focused on, but not limited to, solar magnetism. GREGOR allows us to measure the emergence and disappearance of magnetic flux at the solar surface at spatial scales well below 100 km. Thanks to its spectro‐polarimetric capabilities, GREGOR will measure the interaction between the plasma flows, different kinds of waves, and the magnetic field. This will foster our understanding of the processes that heat the chromosphere and the outer layers of the solar atmosphere. Observations of the surface magnetic field at very small spatial scales will shed light on the variability of the solar brightness (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

AKARI-SDSS-GALEX SURVEYS: SPECTRAL ENERGY DISTRIBUTIONS OF NEARBY GALAXIES

ABSTRACT A sample of nearby galaxies was built from the AKARI/FIS all sky survey cross-correlated withthe SDSS and GALEX surveys. The spectral energy distributions from 0.15 to 160 microns of thesegalaxies are analysed to study dust attenuation and star formation properties. The calibrationsof the amount of dust attenuation as a function of the IR-to-UV flux ratio and the FUV-NUVcolour are re-investigated: the former one is confirmed to be robust and accurate whereas the useof the FUV-NUV colour to measure dust attenuation is found highly uncertain. The current starformation rate given by the SED fitting process is compared to that directly obtained from theUV and total IR luminosities. It leads to an accurate estimate of dust heating by old stars. Weemphasize the importance of such a sample as a reference for IR selected star forming galaxies inthe nearby universe Key words: infrared: galaxies; ultraviolet: galaxies; dust: extinction1. INTRODUCTIONThe nearby universe remains the best laboratory tounderstand the physical properties of galaxies and isa reference for any comparison with high redshift ob-servations. The all sky (or very large) surveys thathave been performed from the ultraviolet (UV) to thefar-infrared (far-IR) provide us with large datasets andvery large wavelength coverage to perform a referencestudy. The broad-band spectral energy distribution(SED) of a galaxy represents the combination of theemission from both stars of all ages and interstellardust that interact in a complex way by means of theabsorption and scattering of the stellar light by dustgrains. The stars emit from the UV to the near-IRwhereas the mid and far-IR emission comes from inter-stellar dust heated by the stellar emission. By compar-ing data with models, one can attempt to derive somephysical parameters related to the star formation his-tory and dust attenuation in a homogeneous way andsimultaneously for all galaxies of a given sample. Withthe availability of mid and far-IR data for large samplesof galaxies, we can combine stellar and dust emissionto analyse SEDs and deduce accurate star formationrates (SFRs).We investigate the properties of nearby galaxies by se-lecting a sample of objects that is as large as possible,observed as part of the

A MAGELLAN-IMACS-IFU SEARCH FOR DYNAMICAL DRIVERS OF NUCLEAR ACTIVITY. I. REDUCTION PIPELINE AND GALAXY CATALOG

Using the Inamori Magellan Areal Camera and Spectrograph (IMACS) integral-field unit (IFU) on the 6.5m Magellan telescope, we have designed the first statistically significant investigation of the two-dimensional distribution and kinematics of ionized gas and stars in the central kiloparsec regions of a well-matched sample of Seyfert and inactive control galaxies selected from the Sloan Digital Sky Survey. The goals of the project are to use the fine spatial sampling (0.2 arcsec/pixel) and large wavelength coverage (4000-7000A) of the IMACS-IFU to search for dynamical triggers of nuclear activity in the central region where active galactic nucleus (AGN) activity and dynamical timescales become comparable, to identify and assess the impact of AGN-driven outflows on the host galaxy and to provide a definitive sample of local galaxy kinematics for comparison with future three-dimensional kinematic studies of high-redshift systems. In this paper, we provide the first detailed description of the procedure to reduce and calibrate data from the IMACS-IFU in `long mode' to obtain two-dimensional maps of the distribution and kinematics of ionized gas and stars. The sample selection criteria are presented, observing strategy described and resulting maps of the sample galaxies presented along with a description of the observed properties of each galaxy and the overall observed properties of the sample.

Spectral energy distributions of an AKARI-SDSS-GALEX sample of galaxies

The nearby universe remains the best laboratory to understand physical properties of galaxies and is a reference for any comparison with high redshift observations. The all sky (or very large) surveys performed from the ultraviolet (UV) to the far-infrared (far-IR) gives us large datasets with a very large wavelength coverage to perform a reference study. We want to investigate dust attenuation characteristics as well as star formation rate (SFR) calibrations on a sample of nearby galaxies observed over 13 bands from 0.15 to 160 microns. A sample of 363 galaxies is built from the AKARI /FIS all sky survey cross-correlated with SDSS and GALEX surveys. Broad band spectral energy distributions are fitted with the CIGALE code optimized to analyze variations in dust attenuation curves and SFR measurements and based on an energetic budget between the stellar and dust emission. Our galaxy sample is primarily selected in far-IR and mostly constituted of massive, actively star forming galaxies. There is some evidence for a dust attenuation law slightly steeper than that used for starburst galaxies but we are unable to constrain the presence or not of a bump at 220 nm. We confirm that a time dependent dust attenuation is necessary to perform the best fits. Various calibrations of the dust attenuation in the UV as a function of UV-optical colours are discussed. A calibration of the current star formation rate combining UV and total IR emissions is proposed with an accurate estimation of dust heating by old stars: for the whole sample 17 % of the total dust luminosity is not related to the recent star formation

X‐shooting Herbig Ae/Be stars: Accretion probed by near‐infrared He I emission

AbstractThe Herbig Ae/Be stars are intermediate mass pre‐main sequence stars that bridge the gap between the low mass T Tauri stars and the Massive Young Stellar Objects. In this mass range, the acting star forming mechanism switches from magnetically controlled accretion to an as yet unknown mechanism, but which is likely to be direct disk accretion onto the star. We observed a large sample of Herbig Ae/Be stars with X‐shooter to address this issue from a multi‐wavelength perspective. It is the largest such study to date, not only because of the number of objects involved, but also because of the large wavelength coverage from the blue to the near‐infrared. This allows many accretion diagnostics to be studied simultaneously. By correlating the various properties with mass, temperature and age, we aim to determine where and whether the magnetically controlled mass accretion mechanism halts and the proposed direct disk accretion takes over. Here, we will give an overview of the background, present some observations and discuss our initial results. We will introduce a new accretion diagnostic for the research of Herbig Ae/Be stars, the HeI 1.083 μm line (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

The impact of the warm outflow in the young (GPS) radio source and ULIRG PKS 1345+12 (4C 12.50)

(Abridged) We present new deep VLT/FORS optical spectra with intermediate resolution and large wavelength coverage of the GPS radio source and ULIRG PKS1345+12 (4C12.50; z=0.122), taken with the aim of investigating the impact of the nuclear activity on the circumnuclear ISM. PKS1345+12 is a powerful quasar and is also the best studied case of an emission line outflow in a ULIRG. Using the density sensitive transauroral emission lines [S II]4068,4076 and [O II]7318,7319,7330,7331, we pilot a new technique to accurately model the electron density for cases in which it is not possible to use the traditional diagnostic [S II]6716/6731, namely sources with highly broadened complex emission line profiles and/or high (Ne > 10^4 cm^-3) electron densities. We measure electron densities of Ne=2.94x10^3 cm^-3, Ne=1.47x10^4 cm^-3 and Ne=3.16x10^5 cm^-3 for the regions emitting the narrow, broad and very broad components respectively. We calculate a total mass outflow rate of 8 M_sun yr^-1. We estimate the total mass in the warm gas outflow is 8x10^5 M_sun. The total kinetic power in the warm outflow is 3.4x10^42 erg s^-1. We find that only a small fraction (0.13% of Lbol) of the available accretion power is driving the warm outflow, significantly less than currently required by the majority of quasar feedback models (~5-10\% of Lbol), but similar to recent findings by Hopkins et al. (2010) for a two-stage feedback model. The models also predict that AGN outflows will eventually remove the gas from the bulge of the host galaxy. The visible warm outflow in PKS1345+12 is not currently capable of doing so. However, it is entirely possible that much of the outflow is either obscured by a dense and dusty natal cocoon and/or in cooler or hotter phases of the ISM. This result is important not just for studies of young (GPS/CSS) radio sources, but for AGN in general.

The thermal emission of the young and massive planet CoRoT-2b at 4.5 and 8 μm

We report measurements of the thermal emission of the young and massive planet CoRoT-2b at 4.5 and 8 <i>μ<i/>m with the <i>Spitzer<i/> Infrared Array Camera (IRAC). Our measured occultation depths are 0.510<i>±<i/>0.042% at 4.5 and 0.41<i>±<i/>0.11% at 8 <i>μ<i/>m. In addition to the CoRoT optical measurements, these planet/star flux ratios indicate a poor heat distribution on the night side of the planet and agree better with an atmosphere free of temperature inversion layer. Still, such an inversion is not definitely ruled out by the observations and a larger wavelength coverage is required to remove the current ambiguity. Our global analysis of CoRoT, <i>Spitzer<i/>, and ground-based data confirms the high mass and large size of the planet with slightly revised values (<i>M<i/><sub>p<sub/> = 3.47<i>±<i/>0.22 <i>M<sub>J<sub/><i/>, <i>R<i/><sub>p<sub/> = 1.466<i>±<i/>0.044 <i>R<sub>J<sub/><i/>). We find a small but significant offset in the timing of the occultation when compared to a purely circular orbital solution, leading to = -0.00291<i>±<i/>0.00063 where <i>e<i/> is the orbital eccentricity and <i>ω<i/> is the argument of periastron. Constraining the age of the system to at most a few hundred Myr and assuming that the non-zero orbital eccentricity does not come from a third undetected body, we modeled the coupled orbital-tidal evolution of the system with various tidal <i>Q<i/> values, core sizes, and initial orbital parameters. For = 10<sup>5<sup/>-10<sup>6<sup/>, our modeling is able to explain the large radius of CoRoT-2b if <i>≤<i/> 10 through a transient tidal circularization and corresponding planet tidal heating event. Under this model, the planet will reach its Roche limit within 20 Myr at most.

Comparing Spectroscopic and Photometric Stellar Mass Estimates

The purpose of this letter is to compare the quality of different methods for estimating stellar masses of galaxies. We compare the results of (a) fitting stellar population synthesis models to broad band colors from SDSS and 2MASS, (b) the analysis of spectroscopic features of SDSS galaxies (Kauffmann et al. 2003), and, (c) a simple dynamical mass estimate based on SDSS velocity dispersions and effective radii. Knowing that all three methods can have significant biases, a comparison can help to establish their (relative) reliability. In this way, one can also probe the quality of the observationally cheap broadband color mass estimators for galaxies at higher redshift. Generally, masses based on broad-band colors and spectroscopic features agree reasonably well, with a rms scatter of only ~ 0.25 dex over almost 4 decades in mass. However, as may be expected, systematic differences do exist and have an amplitude of ~ 0.15 dex, corrleting with Halpha emission strength. Interestingly, masses from broad-band color fitting are in better agreement with dynamical masses than masses based on the analysis of spectroscopic features. In addition, the differences between the latter and the dynamical masses correlate with Halpha equivalent width, while this much less the case for the broad-band masses. We conclude that broad band color mass estimators, provided they are based on a large enough wavelength coverage and use an appropriate range of ages, metallicities and dust extinctions, can yield fairly reliable stellar masses for galaxies. This is a very encouraging result as such mass estimates are very likely the only ones available at significant redshifts for some time to come.

A comprehensive set of elemental abundances in damped Lyαsystems: Revealing the nature of these high-redshift galaxies

By combining our UVES-VLT spectra of a sample of four damped Lyα systems (DLAs) toward the quasars Q0100+13, Q1331+17, Q2231−00 and Q2343+12 with the existing HIRES-Keck spectra, we covered the total optical spectral range from 3150 to 10 000 A for the four quasars. This large wavelength coverage and the high quality of the spectra allowed us to measure the column densities of up to 21 ions, namely of 15 elements − N, O, Mg, Al, Si, P, S, Cl, Ar, Ti, Cr, Mn, Fe, Ni, Zn. This comprehensive set of ionic column densities and elemental abundances severely contrasts with the majority of DLAs for which only a handful of ions and elements is typically observed. Such a large amount of information is necessary to constrain the photoionization and dust depletion effects, two important steps in order to derive the intrinsic chemical abundance patterns of DLAs. We evaluated the photoionization effects with the help of the Al + /Al ++ ,F e + /Fe ++ ,N 0 /N + and Ar/Si, S ratios, and com- puted dust corrections. Our analysis revealed that the DLA toward Q2343+12 requires important ionization corrections. This makes the abundance determinations in this object uncertain. The access to the complete series of relatively robust intrinsic ele- mental abundances in the other three DLAs allowed us to constrain their star formation history, their age and their star formation rate by a detailed comparison with a grid of chemical evolution models for spiral and dwarf irregular galaxies. Our results show that the galaxies associated with these three DLAs in the redshift interval zabs = 1.7-2.5 are either outer regions of spiral disks (radius ≥ 8 kpc) or dwarf irregular galaxies (showing a bursting or continuous star formation history) with ages varying from some 50 Myr only to 3.5 Gyr and with moderate star formation rates per unit area of −2.1 < log ψ< −1.5 Myr −1 kpc −2 .

Highly extinguished emission line outflows in the young radio source PKS 1345+12

We present intermediate resolution, large wavelength coverage spectra for the compact radio source PKS 1345+12 (4C 12.50, z = 0.122) that were taken with the aim of investigating the impact of the nuclear activity on thecircumnuclear interstellar medium (ISM). Our spectra show extended line emission up to ∼20 kpc from the nucleus, consistent with the asymmetric halo of diffuse emission observed in optical and infrared images. At the position of the nucleus we observe complex emission line profiles and Gaussian fits to the [O III] emission lines require three components (narrow, intermediate and broad), the broadest of which has width ∼2000 km s - 1 (FWHM) and is blueshifted by ∼2000 km s - 1 with respect to the halo of the galaxy and HI absorption. We interpret this latter component as material in outflow. We also find evidence for large reddening [0.92 4200 cm - 3 ) for the most kinematically disturbed component. The corresponding total mass of line-emitting gas in the kinematically disturbed components is M g a s < 10 6 M O .. Not all emission line profiles can be reproduced by the same model: [OI]λλ6300, 6363 and [SII]λλ6716, 6731 require separate, unique models. We argue that PKS 1345+12 is a young radio source with nuclear regions that are enshrouded in a dense cocoon of gas and dust. The radio jets are expanding through this cocoon, sweeping material out of the nuclear regions. Emission originates from three kinematically distinct regions, though gradients (e.g. in density, ionization potential, velocity) must exist across the regions responsible for the emission of the intermediate and broad components.

Quantitative Spectroscopy of Supergiants

Blue supergiants of spectral types B and A are the visually brightest stars in spiral and irregular galaxies, with their most luminous members (at ) outshining entire dwarf galM 10 V axies. This characteristic allows us to use them as probes to study the local universe in great detail. In principle, already existing large telescopes and instrumentation facilitate quantitative spectroscopy of these objects as far as the Virgo and Fornax clusters of galaxies. Beyond their challenging stellar atmospheres and opportunities for testing sophisticated nonLTE physics, they offer numerous applications to modern astrophysics. Quantitative spectroscopy of supergiants will contribute to improving our understanding of massive-star evolution. Galactic abundance gradients and abundance patterns, as will be obtained from studies of large ensembles of supergiants in our own and other galaxies, will foster the understanding of galactochemical evolution. Finally, they are promising independent indicators for calibrating the extragalactic distance scale, by application of the wind momentum–luminosity and the flux-weighted gravity–luminosity relationships (R. P. Kudritzki et al. 1999, AA R. P. Kudritzki, F. Bresolin, & N. Przybilla 2003, ApJ, 582, L83). In view of this large potential, the objective of this thesis is to improve the status of quantitative spectroscopy of BA-type supergiants and to provide first applications on a sample of Galactic and extragalactic targets. It is shown that among the model atmospheres available at present, the best suited for analyses of supergiants are line-blanketed classical LTE models. An investigation of the impact of various parameters such as helium abundance and line blanketing on the atmospheric structure shows that for the most luminous objects, an accurate treatment of these parameters is essential for a quantitative analysis, whereas the less luminous supergiants react less sensitively. Spectrum synthesis is used to model the line spectra. It is the only technique capable of providing analyses of spectra of different qualities from low to high resolution and able to cope with heavy line blending at a broad range of signal-tonoise ratios. The stellar parameters are determined from purely spectroscopic indicators, from temperatureand gravity-sensitive ionization equilibria and the Balmer line wings. Elemental abundances are derived by modeling individual spectral features. Several tens of thousands spectral lines from 28 chemical species are included in the line formation, allowing almost the entire observed spectra to be reproduced. Non-LTE effects become important in blue supergiants, where a strong radiation field at low particle densities favors deviations from LTE. Comprehensive model atoms are therefore constructed for C i/ii, N i/ii, O i, and Mg i/ii in order to determine non-LTE level populations (N. Przybilla, K. Butler, & R. P. Kudritzki 2001, AA N. Przybilla & K. Butler 2001, AA N. Przybilla et al. 2000, AA N. Przybilla et al. 2001, A&A, 369, 1009). Highly accurate radiative and collisional atomic data recently determined for astrophysical and fusion research using the R-matrix method in the closecoupling approximation are incorporated. In addition, model atoms for H, He i, O ii, S ii/iii, Ti ii, and Fe ii are adopted from the literature, the atomic data being updated to more modern values in some cases. Thus, an improved treatment for the main elements of astrophysical interest is achieved. Extensive testing of the atomic data is performed for the nearby bright main-sequence standard Vega (A0 V), at welldetermined stellar parameters and atmospheric structure. A high-resolution and low-noise spectrum with large wavelength coverage from the visual to the near-IR is used for this purpose. Further tests are performed for the Galactic supergiants h Leo, HD 111613, HD 92207, and b Ori, with similar high-quality spectra in order to study the non-LTE effects across the parameter space. Accurate and consistent stellar parameters are derived for these. Non-LTE ionization equilibria of several elements—typically N i/ii, O i/ii, Mg i/ii, and S ii/iii—agree simultaneously, provided that a realistic treatment of line blocking is used. These parameters also constitute important input data for further studies of the stellar winds of these objects. Accounting for non-LTE reduces the random errors and removes systematic trends in the analyses. In particular, the improved treatment of electron collisions largely removes longstanding discrepancies in analyses of lines from different spin systems of a given ion. The computed non-LTE line profiles fit the observations well for the different species at a given elemental abundance. In the parameter range covered, all lines from He i, C i/ii, N i/ii, O i/ii, and S ii/iii are significantly

Spectroscopy of T Tauri stars with UVES

We present the first results of our observations of classical T Tauri Stars with UVES/VLT. The data consists of high signal-to-noise (150) and high spectral resolution (R 60 000) spectra. A large simultaneous wavelength coverage throughout most of the visible spectrum and comparatively short integration times allow us to study variability on short time-scales, using a number of diagnostics reflecting a wide range of physical processes. In particular we concentrate on the properties and geometry of the accretion process in the strongly accreting and highly variable CTTS RU Lup. We use the evolution of the level of veiling, the shapes of absorption and emission lines, and correlations between these diagnostics, to make new measurements of the fundamental stellar parameters as well as constraints on the accretion process and its geometry. We also derive the shortest time-scale of incoherent changes, which has implications for the nature of the accretion process in RU Lup. Since their discovery one of the most intriguing properties of the classical T Tauri stars (CTTS) has been their apparently chaotic variability visible with nearly all observational tech- niques, from broad-band photometry to high-resolution spec- troscopy. The spectra of these low-mass pre-main sequence objects are densely populated with emission lines including hydrogen, permitted and forbidden metallic lines and lines of helium (for a complete compilation of observational proper- ties see e.g. Bertout 1989). The CTTS phase in the evolution of low-mass stars is rather short - a few million years. In the same age interval one finds, apart from CTTS, a group of weak line T Tauri stars (WTTS), with very weak or absent emission spectra. The characteristic properties of CTTS are attributed to the presence of an accretion disk while WTTS apparently lack such a disk. Whether there exists an evolutionary connection between the two groups is still unclear. CTTS represent a real challenge for both modelling and ob- servations. In this paper we will concentrate on the observa- tional signatures of CTTS, on their variability and the plausible connection to the accretion processes. The observational picture of CTTS is very complex. Photometry reveals a high level of variability on timescales from minutes to years with changes in brightness by a factor of two or more. CTTS show an excess of radiation in the IR

Galileo NIMS Near-Infrared Observations of Jupiter's Ring System

We present the first observations of the jovian ring system viewed at near-infrared wavelengths in forward-scattered light. The eclipse of the Sun by Jupiter during the “C3” orbit of the Galileo spacecraft afforded favorable conditions for the Near Infrared Mapping Spectrometer (NIMS) to observe the ring system between 0.7 and 5.2 μm at a spectral resolution of ≈0.04 μm. NIMS images show the nearly edge-on main ring from just inside the ansa at 1.80 R J down to 1.05 R J (where 1 R J=71398 km), at a spatial resolution of 450 km along the ring and 1130 km out of the ring plane. At these spatial resolutions, no fine structure is apparent in the ring. The large wavelength coverage of the NIMS observations permits an analysis of the scattering properties of the ring system even though the ring subtends only a small range of scattering angles. Our retrieved particle size distribution shows a decrease in the number of particles at larger radii although there is a reversal of this trend between 0.6 and 18 μm. This distribution can be modeled as the combination of a power law with index p=3.9±0.2 and a log-normal distribution of mean radius 4.5 μm. Analysis shows that Voyager data lacked the required wavelength coverage and viewing geometry to determine the particle distribution uniquely. We conclude that the NIMS data set is a better determinant of the particle size distribution, especially for particles larger than a micrometer. We suggest that the power law distribution is the result of collisional processes that form grains less than 1 micrometer, in agreement with previous theory. We suggest that the log-normal distribution is characteristic of another, yet undetermined, process that dominates grain formation and evolution for particles several micrometers in size.

An ISO View on Interstellar and Cometary Ice Chemistry

The ISO-SWS instrument offering a large wavelength coverage and a resolution well adapted to the solid phase has changed our knowledge of the physical-chemical properties of ices in space. The discovery of many new ice features was reported and the comparison with dedicated laboratory experiments allowed the determination of more accurate abundances of major ice components. The presence of CO2 ice has recently been confirmed with the SWS (Short Wavelength Spectrometer) as a dominant ice component of interstellar grain mantles. The bending mode of CO2 ice shows a particular triple-peak structure which provides first evidence for extensive ice segregation in the line-of-sight toward massive protostars. A comparison of interstellar and cometary ices using recent ISO data and ground-based measurements has revealed important similarities but also indicated that comets contain, beside pristine interstellar material, admixtures of processed material. The investigation of molecules in interstellar clouds is essential to reveal the link between dust in the interstellar medium and in the Solar System.

Analysis of Keck high-resolution spectra of VB 10

We use a preliminary version of our ``NextGen'' grid of cool star model atmospheres to compute synthetic line profiles which fit high resolution Keck spectra of the cool M~dwarf VB10 satisfactorily well. We show that the parameters derived from the Keck data are consistent with the parameters derived from lower resolution spectra with larger wavelength coverage. We discuss the treatment of van der Waals broadening in cool, molecular (mostly ${\rm H_2}$) dominated stellar atmospheres. The line profiles are dominated by van der Waals pressure broadening and are a sensitive indicator for the gravity and metallicity. Therefore, the high-resolution Keck spectra are useful for determining the parameters of M dwarfs. There is some ambiguity between the metallicity and gravity. For VB10, we find from the high-resolution spectra that $ 5.0 < \log(g) < 5.5$ and $0 < \left[\case{{\rm M}}{{\rm H}}\right] < +0.5$ for an adopted fixed effective temperature of 2700~K (Schweitzer, 1995), which is consistent with recent interior calculations (Baraffe etal, 1995).

Integral field spectroscopy with optical fibres

AbstractAn efficient technique for obtaining complete spectral informations over moderately extended objects is to transform the geometry of their telescopic images to match a classical long ”slit” aperture. This anamorphosis is conveniently made with bundles of optical fibres. Such devices are especially useful when high spatial resolution and large wavelength coverage are required at the same time. We review the existing instruments based on this technique and present some typical results. We discuss also the future developments of integral field spectroscopy with optical fibres (visible or IR domains) for the new generation of very large telescopes.