8,650 research outputs found

    Continuous-flow IRMS technique for determining the 17O excess of CO2 using complete oxygen isotope exchange with cerium oxide

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    This paper presents an analytical system for analysis of all single substituted isotopologues (<sup>12</sup>C<sup>16</sup>O<sup>17</sup>O, <sup>12</sup>C<sup>16</sup>O<sup>18</sup>O, <sup>13</sup>C<sup>16</sup>O<sup>16</sup>O) in nanomolar quantities of CO<sub>2</sub> extracted from stratospheric air samples. CO<sub>2</sub> is separated from bulk air by gas chromatography and CO<sub>2</sub> isotope ratio measurements (ion masses 45 / 44 and 46 / 44) are performed using isotope ratio mass spectrometry (IRMS). The <sup>17</sup>O excess (Δ<sup>17</sup>O) is derived from isotope measurements on two different CO<sub>2</sub> aliquots: unmodified CO<sub>2</sub> and CO<sub>2</sub> after complete oxygen isotope exchange with cerium oxide (CeO<sub>2</sub>) at 700 °C. Thus, a single measurement of Δ<sup>17</sup>O requires two injections of 1 mL of air with a CO<sub>2</sub> mole fraction of 390 ÎŒmol mol<sup>−1</sup> at 293 K and 1 bar pressure (corresponding to 16 nmol CO<sub>2</sub> each). The required sample size (including flushing) is 2.7 mL of air. A single analysis (one pair of injections) takes 15 minutes. The analytical system is fully automated for unattended measurements over several days. The standard deviation of the <sup>17</sup>O excess analysis is 1.7&permil;. Multiple measurements on an air sample reduce the measurement uncertainty, as expected for the statistical standard error. Thus, the uncertainty for a group of 10 measurements is 0.58&permil; for &Delta; <sup>17</sup>O in 2.5 h of analysis. 100 repeat analyses of one air sample decrease the standard error to 0.20&permil;. The instrument performance was demonstrated by measuring CO<sub>2</sub> on stratospheric air samples obtained during the EU project RECONCILE with the high-altitude aircraft Geophysica. The precision for RECONCILE data is 0.03&permil; (1&sigma;) for ÎŽ<sup>13</sup>C, 0.07&permil; (1&sigma;) for ÎŽ<sup>18</sup>O and 0.55&permil; (1&sigma;) for &delta;<sup>17</sup>O for a sample of 10 measurements. This is sufficient to examine stratospheric enrichments, which at altitude 33 km go up to 12&permil; for &delta;<sup>17</sup>O and up to 8&permil; for ÎŽ<sup>18</sup>O with respect to tropospheric CO<sub>2</sub> : &delta;<sup>17</sup>O ~ 21&permil; Vienna Standard Mean Ocean Water (VSMOW), ÎŽ<sup>18</sup>O ~ 41&permil; VSMOW (LĂ€mmerzahl et al., 2002). The samples measured with our analytical technique agree with available data for stratospheric CO<sub>2</sub>

    OH-selected AGB and post-AGB objects I.Infrared and maser properties

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    Using 766 compact objects from a survey of the galactic Plane in the 1612-MHz OH line, new light is cast on the infrared properties of evolved stars on the TP-AGB and beyond. The usual mid-infrared selection criteria, based on IRAS colours, largely fail to distinguish early post-AGB stages. A two-colour diagram from narrower-band MSX flux densities, with bimodal distributions, provides a better tool to do the latter. Four mutually consistent selection criteria for OH-masing red PPNe are given, as well as two for early post-AGB masers and one for all post--AGB masers, including the earliest ones. All these criteria miss a group of blue, high-outflow post-AGB sources with 60-mum excess; these will be discussed in detail in Paper II. The majority of post-AGB sources show regular double-peaked spectra in the OH 1612-MHz line, with fairly low outflow velocities, although the fractions of single peaks and irregular spectra may vary with age and mass. The OH flux density shows a fairly regular relation with the stellar flux and the envelope optical depth, with the maser efficiency increasing with IRAS colour R21. The OH flux density is linearly correlated with the 60-mum flux density.Comment: 16 pages, LaTex, 22 figures, AJ (accepted

    Surface defreezing of glasses

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    A glass surface may still flow below the bulk glass transition temperature, where the underlying bulk is frozen. Assuming the existence at T=T* of a bulk thermodynamical glass transition, we show that the glass-vapor interface is generally wetted by a liquid layer of thickness ~ -ln(T*-T) when T--> T*. Contrary to standard surface melting of crystals however, the integrated value of the diffusivity across the interface remains finite for T-->T*. Difference in shape induced by bulk and by surface flow is discussed as a possible means of experimental detection of surface defreezing.Comment: five pages, three figure

    Mass-luminosity relation and pulsational properties of Wolf-Rayet stars

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    Evolution of Population I stars with initial masses from 70M_\odot to 130M_\odot is considered under various assumptions on the mass loss rate \dot M. The mass-luminosity relation of W-R stars is shown to be most sensitive to the mass loss rate during the helium burning phase \dot M_{3\alpha}. Together with the mass-luminosity relation obtained for all evolutionary sequences several more exact relations are determined for the constant ratio f_{3\alpha}=\dot M/\dot M_{3\alpha} with 0.5 \le f_{3\alpha} \le 3. Evolutionary models of W-R stars were used as initial conditions in hydrodynamic computations of radial nonlinear stellar oscillations. The oscillation amplitude is larger in W-R stars with smaller initial mass or with lower mass loss rate due to higher surface abundances of carbon and oxygen. In the evolving W-R star the oscillation amplitude decreases with decreasing stellar mass M and for M < 10M_\odot the sufficiently small nonlinear effects allow us to calculate the integral of the mechanical work W done over the pulsation cycle in each mass zone of the hydrodynamical model. The only positive maximum on the radial dependence of W is in the layers with temperature of T\sim 2e5K where oscillations are excited by the iron Z--bump kappa-mechanism. Radial oscillations of W-R stars with mass of M > 10M_\odot are shown to be also excited by the kappa-mechanism but the instability driving zone is at the bottom of the envelope and pulsation motions exist in the form of nonlinear running waves propagating outward from the inner layers of the envelope.Comment: 15 pages, 10 figures, submitted to Astronomy Letter

    Spectral Characteristics of Greenland Lichens

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    Spectral reflectance measurements conducted during two field campaigns in west Greenland, and in the laboratory using samples collected during those campaigns, are discussed to evaluate the spectral signature of lichens. Given the diversity in lichen species, colors, and appearance — ranging from crust-like (crustose) to almost like mini shrubs (fructicose) — it is not surprising that no single signature was found. Some of the brighter fructicose lichens have reflectance characteristics very similar to those of green vegetation, with a pronounced rise in reflectivity around 750 nm. However, the most abundant lichen species covering rocks in the ice-marginal zone of west Greenland are dark grey to black crustose and foliose ephilithic (rock-growing) lichens and the shape of the reflectance spectrum for these lichens is generally very different from that of other surface types and landcovers, with near-zero reflectance at visible wavelengths, and a maximum around 1 600 nm. This characteristic allows rock-covered lichen to be identified on multispectral satellite imagery.L’évaluation de la signature spectrale des lichens est effectuĂ©e Ă  partir de mesures prises en laboratoire et sur le terrain, au Groenland occidental. Aucune signature spĂ©cifique ne peut ĂȘtre identifiĂ©e, en raison de la diversitĂ© des espĂšces quant Ă  leur couleur et Ă  leur port, allant de la croĂ»te Ă  l’arbuste nain. Les lichens arbustifs les plus brillants montrent une signature spectrale semblable Ă  celle des plantes vertes, avec un pic trĂšs prononcĂ© autour de 750 nm. Toutefois, les lichens les plus abondants sur les roches Ă  proximitĂ© des glaces sont gris foncĂ©s Ă  noirs et du type crustacĂ© ou foliacĂ©; leur spectre de rĂ©flectance montre une allure trĂšs diffĂ©rente de celle des autres types de surface et de couverture, et se rapproche de zĂ©ro dans le spectre visible avec un pic autour de 1 600 nm. Cette caractĂ©ristique permet l’identification des roches recouvertes de lichens par l’imagerie satellitaire multispectrale

    The COBE DIRBE Point Source Catalog

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    We present the COBE DIRBE Point Source Catalog, an all-sky catalog containing infrared photometry in 10 bands from 1.25 microns to 240 microns for 11,788 of the brightest near and mid-infrared point sources in the sky. Since DIRBE had excellent temporal coverage (100 - 1900 independent measurements per object during the 10 month cryogenic mission), the Catalog also contains information about variability at each wavelength, including amplitudes of variation observed during the mission. Since the DIRBE spatial resolution is relatively poor (0.7 degrees), we have carefully investigated the question of confusion, and have flagged sources with infrared-bright companions within the DIRBE beam. In addition, we filtered the DIRBE light curves for data points affected by companions outside of the main DIRBE beam but within the `sky' portion of the scan. At high Galactic latitudes (|b| > 5 degrees), the Catalog contains essentially all of the unconfused sources with flux densities greater than 90, 60, 60, 50, 90, and 165 Jy at 1.25, 2.2, 3.5, 4.9, 12, and 25 microns, respectively, corresponding to magnitude limits of approximately 3.1, 2.6, 1.7, 1.3, -1.3, and -3.5. At longer wavelengths and in the Galactic Plane, the completeness is less certain because of the large DIRBE beam and possible contributions from extended emission. The Catalog also contains the names of the sources in other catalogs, their spectral types, variability types, and whether or not the sources are known OH/IR stars. We discuss a few remarkable objects in the Catalog. [abridged]Comment: Accepted for publication in the Astrophysical Journal Supplement. The full tables are available at http://www.etsu.edu/physics/bsmith/dirbe

    Patterns of calculated basal drag on ice streams B and C, Antarctica

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    This is the published version.Patterns of strain rate and slope on the ice streams are unusual. They cannot be accounted for in the usual way as due to standing waves in ice flow over a basal obstruction to flow (such as a sticky spot) . The features are studied using the force-budget technique. The conventional flow law is used, together with measurements of surface strain rate and shape of the glacier, to compute basal drag. The results for Ice Stream C are as expected, in that the drag varies from site to site but is directed inland, restraining the flow. The calculated drag at the base of Ice Stream B, on the other hand, is in places such that it acts to propel the glacier forward. This result is untenable. Either the conventional flow law is not applicable to Ice Stream B or there are large spatial variations in ice stiffness, perhaps associated with foliation, or both