Journal articles: 'Spectre de graisse'

1

Greenberg, J. M., and F. Baas. "Infrared spectra from interstellar dust grains." Advances in Space Research 5, no. 3 (January 1985): 19–26. http://dx.doi.org/10.1016/0273-1177(85)90443-0.

Full text

APA, Harvard, Vancouver, ISO, and other styles

2

Maggipinto, G., A. Minafra, and F. Tritto. "Optical spectra of amorophous carbon grains." Astrophysics and Space Science 108, no. 1 (1985): 101–11. http://dx.doi.org/10.1007/bf00650122.

Full text

APA, Harvard, Vancouver, ISO, and other styles

3

Höfner, S., S. Bladh, B. Aringer, and K. Eriksson. "Dynamic atmospheres and winds of cool luminous giants." Astronomy & Astrophysics 657 (January 2022): A109. http://dx.doi.org/10.1051/0004-6361/202141224.

Full text

Abstract:

Context. The winds observed around asymptotic giant branch (AGB) stars are generally attributed to radiation pressure on dust formed in the extended dynamical atmospheres of these long-period variables. The composition of wind-driving grains is affected by a feedback between their optical properties and the resulting heating due to stellar radiation. Aims. We explore the gradual Fe enrichment of wind-driving silicate grains in M-type AGB stars to derive typical values for Fe/Mg and to test the effects on wind properties and synthetic spectra. Methods. We present new radiation-hydrodynamical DARWIN models that allow for the growth of silicate grains with a variable Fe/Mg ratio and predict mass-loss rates, wind velocities, and grain properties. Synthetic spectra and other observables are computed a posteriori with the COMA code. Results. The self-regulating feedback between grain composition and radiative heating, in combination with quickly falling densities in the stellar wind, leads to low values of Fe/Mg, typically a few percent. Nevertheless, the new models show distinct silicate features around 10 and 18 microns. Fe enrichment affects visual and near-IR photometry moderately, and the new DARWIN models agree well with observations in (J − K) versus (V − K) and Spitzer color–color diagrams. The enrichment of the silicate dust with Fe is a secondary process, taking place in the stellar wind on the surface of large Fe-free grains that have initiated the outflow. Therefore, the mass-loss rates are basically unaffected, while the wind velocities tend to be slightly higher than in corresponding models with Fe-free silicate dust. Conclusions. The gradual Fe enrichment of silicate grains in the inner wind region should produce signatures observable in mid-IR spectro-interferometrical measurements. Mass-loss rates derived from existing DARWIN models, based on Fe-free silicates, can be applied to stellar evolution models since the mass-loss rates are not significantly affected by the inclusion of Fe in the silicate grains.

APA, Harvard, Vancouver, ISO, and other styles

4

Dadswell, G., and W. W. Duley. "Simulation of Infrared Spectra of Carbonaceous Grains." Astrophysical Journal 476, no. 1 (February 10, 1997): 184–90. http://dx.doi.org/10.1086/303590.

Full text

APA, Harvard, Vancouver, ISO, and other styles

5

Mariñoso Guiu, Joan, Antoni Macià, and Stefan T. Bromley. "How to accurately model IR spectra of nanosized silicate grains." Proceedings of the International Astronomical Union 15, S350 (April 2019): 431–33. http://dx.doi.org/10.1017/s174392132000006x.

Full text

Abstract:

AbstractWe assess the accuracy of various computational methods for obtaining infrared (IR) spectra of nanosized silicate dust grains directly from their atomistic structure and atomic motions. First, IR spectra for a selection of small nanosilicate clusters with a range of sizes and chemical compositions are obtained within the harmonic oscillator approximation employing density functional theory (DFT) based quantum chemical calculations. To check if anharmonic effects play a significant role in the IR spectra of these nanoclusters, we further obtain their IR spectra from finite temperature DFT-based ab initio molecular dynamics (AIMD). Finally, we also study the effect of temperature on the broadening of the obtained IR spectra peaks in larger nanosilicate grains with a range of crystallinities. In this case, less computationally costly classical molecular dynamics simulations are necessary due to the large number of atoms involved. Generally, we find that although DFT-based methods are more accurate, surprisingly good IR spectra can also be obtained from classical molecular dynamics calculations.

APA, Harvard, Vancouver, ISO, and other styles

6

Imada, Saki, Toshiyuki Isshiki, Nobuyuki Tatemizo, Koji Nishio, Shuichi Mamishin, Yuya Suzuki, Katsuji Ito, et al. "Formation of various-axis-oriented wurtzite nuclei and enlargement of the a-axis-oriented region in AlFeN films deposited on Si(100) substrates." Materials Advances 2, no. 12 (2021): 4075–80. http://dx.doi.org/10.1039/d0ma01026j.

Full text

Abstract:

A-axis-oriented single-crystal AlFeN grains grew from randomly oriented small wurtzite grains with deposition time on Si(100). (a) Deposition time dependence of Al K-edge XANES spectra. (b) Theoretical spectra.

APA, Harvard, Vancouver, ISO, and other styles

7

Matter, A., F. C. Pignatale, and B. Lopez. "Spatially resolving the chemical composition of the planet building blocks." Monthly Notices of the Royal Astronomical Society 497, no. 3 (July 29, 2020): 2540–52. http://dx.doi.org/10.1093/mnras/staa2137.

Full text

Abstract:

ABSTRACT The inner regions of protoplanetary discs (from ∼0.1 to 10 au) are the expected birthplace of planets, especially telluric. In those high-temperature regions, solids can experience cyclical annealing, vapourisation, and recondensation. Hot and warm dusty grains emit mostly in the infrared domain, notably in N-band (8–13 μm). Studying their fine chemistry through mid-infrared spectro-interferometry with the new Very Large Telescope Interferometer (VLTI) instrument Multi AperTure mid-Infrared SpectroScopic Experiment (MATISSE), which can spatially resolve these regions, requires detailed dust chemistry models. Using radiative transfer, we derived infrared spectra of a fiducial static protoplanetary disc model with different inner-disc (<1 au) dust compositions. The latter were derived from condensation sequences computed at local thermodynamic equilibrium (LTE) for three initial C/O ratios: subsolar (C/O = 0.4), solar (C/O = 0.54), and supersolar (C/O = 1). The three scenarios return very different N-band spectra, especially when considering the presence of sub-micron-sized dust grains. MATISSE should be able to detect these differences and trace the associated sub-au-scale radial changes. We propose a first interpretation of N-band ‘inner-disc’ spectra obtained with the former VLTI instrument MID-infrared Interferometric instrument (MIDI) on three Herbig stars (HD 142527, HD 144432, HD 163296) and one T Tauri star (AS 209). Notably, we could associate a supersolar (‘carbon-rich’) composition for HD 142527 and a subsolar (‘oxygen-rich’) one for HD 1444432. We show that the inner-disc mineralogy can be very specific and not related to the dust composition derived from spatially unresolved mid-infrared spectroscopy. We highlight the need for including more complex chemistry when interpreting solid-state spectroscopic observations of the inner regions of discs, and for considering dynamical aspects for future studies.

APA, Harvard, Vancouver, ISO, and other styles

8

Bhowmik, T., A. Boccaletti, P. Thébault, Q. Kral, J. Mazoyer, J. Milli, A. L. Maire, et al. "Spatially resolved spectroscopy of the debris disk HD 32297." Astronomy & Astrophysics 630 (September 24, 2019): A85. http://dx.doi.org/10.1051/0004-6361/201936076.

Full text

Abstract:

Context. Spectro-photometry of debris disks in total intensity and polarimetry can provide new insight into the properties of the dust grains therein (size distribution and optical properties). Aims. We aim to constrain the morphology of the highly inclined debris disk HD 32297. We also intend to obtain spectroscopic and polarimetric measurements to retrieve information on the particle size distribution within the disk for certain grain compositions. Methods. We observed HD 32297 with SPHERE in Y, J, and H bands in total intensity and in J band in polarimetry. The observations are compared to synthetic models of debris disks and we developed methods to extract the photometry in total intensity overcoming the data-reduction artifacts, namely the self-subtraction. The spectro-photometric measurements averaged along the disk mid-plane are then compared to model spectra of various grain compositions. Results. These new images reveal the very inner part of the system as close as 0.15″. The disk image is mostly dominated by the forward scattering making one side (half-ellipse) of the disk more visible, but observations in total intensity are deep enough to also detect the back side for the very first time. The images as well as the surface brightness profiles of the disk rule out the presence of a gap as previously proposed. We do not detect any significant asymmetry between the northeast and southwest sides of the disk. The spectral reflectance features a “gray to blue” color which is interpreted as the presence of grains far below the blowout size. Conclusions. The presence of sub-micron grains in the disk is suspected to be the result of gas drag and/or “avalanche mechanisms”. The blue color of the disk could be further investigated with additional total intensity and polarimetric observations in K and H bands respectively to confirm the spectral slope and the fraction of polarization.

APA, Harvard, Vancouver, ISO, and other styles

9

Bishop, J. L., M. D. Dyar, E. C. Sklute, and A. Drief. "Physical alteration of antigorite: a Mössbauer spectroscopy, reflectance spectroscopy and TEM study with applications to Mars." Clay Minerals 43, no. 1 (March 2008): 55–67. http://dx.doi.org/10.1180/claymin.2008.043.1.04.

Full text

Abstract:

AbstractPhysical alteration of magnetite-bearing antigorite grains is investigated in this study using Mössbauer, visible/near-infrared (VNIR) and mid-IR spectroscopy coupled with scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM) analyses. An expected decrease in grain size with grinding is observed using SEM. The HRTEM images illustrate that the nanophase-sized grains which adhere to larger grains have 7 Å antigorite patterns. Mössbauer spectroscopy shows the presence of antigorite, magnetite and an amorphous phase. Visible/near infrared spectra exhibit features common in serpentine. These spectra also show an increasing continuum slope with grinding, an effect which is characteristic of thin coatings or tiny grains on surfaces. Mid-IR spectra indicate the formation of fine-grained Si-OH and carbonate in these samples with grinding.

APA, Harvard, Vancouver, ISO, and other styles

10

Kalvāns, Juris, and Juris Roberts Kalnin. "Temperature Spectra of Interstellar Dust Grains Heated by Cosmic Rays. III. Mixed-composition Grains." Astrophysical Journal Supplement Series 263, no. 1 (October 21, 2022): 5. http://dx.doi.org/10.3847/1538-4365/ac92e6.

Full text

Abstract:

Abstract Icy grains in the interstellar medium and star formation regions consist of a variety of materials. Such composite grains interact differently with cosmic-ray (CR) particles compared to simple single-material grains. We aim to calculate the spectra of energies and temperatures of mixed-composition grains undergoing whole-grain heating by CRs. The grains were assumed to consist of a mixture of carbon and olivine, covered by ices consisting of carbon oxides and water. The energy and temperature spectra for grains with radii 0.05, 0.1, and 0.2 μm impacted by CRs were calculated for eight values of column density, relevant to molecular clouds and star-forming cores. The approach takes into account changes in ice thickness and composition with increasing column density. These detailed data for CR interaction with interstellar grains are intended for applications in astrochemical models. The main finding is that a more accurate approach on grain heat capacity and other factors prevent frequent heating of 0.1 μm or larger icy grains to high temperatures.

APA, Harvard, Vancouver, ISO, and other styles

11

Capozzi, V., A. Minafra, L. Colangeli, E. Bussoletti, and A. Borghesi. "Amorphous carbon grains: Size effects in ultraviolet spectra." Solid State Communications 65, no. 10 (March 1988): 1193–96. http://dx.doi.org/10.1016/0038-1098(88)90921-0.

Full text

APA, Harvard, Vancouver, ISO, and other styles

12

Osborne, B. G., B. Mertens, M. Thompson, and T. Fearn. "The Authentication of Basmati Rice Using near Infrared Spectroscopy." Journal of Near Infrared Spectroscopy 1, no. 2 (March 1993): 77–83. http://dx.doi.org/10.1255/jnirs.8.

Full text

Abstract:

The feasibility of using near infrared transmission spectroscopy to discriminate between Basmati and other long-grain rice samples was investigated. A discriminant rule was derived using the Fisher linear discriminant function calculated from the first few principal component scores of the NIR spectra. The discriminant rule was assessed by cross-validation. Nine Basmati and 53 other rice samples were classified correctly from NIR spectra measured on 200 g bulk samples but 8% of the Basmatis and 14% of the others were misclassified on the basis of spectra of individual grains. Using average spectra from 23 grains resulted in correct classification of 16 Basmatis but a 20% error rate on 100 other samples.

APA, Harvard, Vancouver, ISO, and other styles

13

Yang, Chenbo, Lifang Song, Kunxi Wei, Chunrui Gao, Danli Wang, Meichen Feng, Meijun Zhang, et al. "Study on Hyperspectral Monitoring Model of Total Flavonoids and Total Phenols in Tartary Buckwheat Grains." Foods 12, no. 7 (March 23, 2023): 1354. http://dx.doi.org/10.3390/foods12071354.

Full text

Abstract:

Tartary buckwheat is a common functional food. Its grains are rich in flavonoids and phenols. The rapid measurement of flavonoids and phenols in buckwheat grains is of great significance in promoting the development of the buckwheat industry. This study, based on multiple scattering correction (MSC), standardized normal variate (SNV), reciprocal logarithm (Lg), first-order derivative (FD), second-order derivative (SD), and fractional-order derivative (FOD) preprocessing spectra, constructed hyperspectral monitoring models of total flavonoids content and total phenols content in tartary buckwheat grains. The results showed that SNV, Lg, FD, SD, and FOD preprocessing had different effects on the original spectral reflectance and that FOD can also reflect the change process from the original spectrum to the integer-order derivative spectrum. Compared with the original spectrum, MSC, SNV, Lg, FD, and SD transformation spectra can improve the correlation between spectral data and total flavonoids and total phenols in varying degrees, while the correlation between FOD spectra of different orders and total flavonoids and total phenols in grains was different. The monitoring models of total flavonoids and total phenols in grains based on MSC, SNV, Lg, FD, and SD transformation spectra achieved the best accuracy under SD and FD transformation, respectively. Therefore, this study further constructed monitoring models of total flavonoids and total phenols content in grains based on the FOD spectrum and achieved the best accuracy under 1.6 and 0.6 order derivative preprocessing, respectively. The R2c, RMSEc, R2v, RMSEv, and RPD were 0.8731, 0.1332, 0.8384, 0.1448, and 2.4475 for the total flavonoids model, and 0.8296, 0.2025, 0.6535, 0.1740, and 1.6713 for the total phenols model. The model can realize the rapid measurement of total flavonoids content and total phenols content in tartary buckwheat grains, respectively.

APA, Harvard, Vancouver, ISO, and other styles

14

Greenberg, J. Mayo. "Basic Laboratory Studies of Grains." Symposium - International Astronomical Union 120 (1987): 501–23. http://dx.doi.org/10.1017/s0074180900154543.

Full text

Abstract:

Laboratory investigations of grain mantles are shown to have a direct application to observations of interstellar grains. the experiments have made it possible to show how infrared spectra may be used as a probe not only of the grains as they are but also as they have evolved both chemically and physically. Growth by accretion, ultraviolet photoprocessing, gradual heating, explosions and general effects of grains on interstellar chemistry provide a consistent basis for understanding evolution of grains in terms of interactions with the gas, and ultraviolet radiation in both diffuse and dense regions of space.

APA, Harvard, Vancouver, ISO, and other styles

15

Avishai, Yshai, Daniel Berend, and Luba Bromberg. "Discrete charging of metallic grains: statistics of addition spectra." Journal of Physics A: Mathematical and General 39, no. 18 (April 19, 2006): 4797–810. http://dx.doi.org/10.1088/0305-4470/39/18/002.

Full text

APA, Harvard, Vancouver, ISO, and other styles

16

Capozzi, Vito, Cristina Flesia, and Antonio Minafra. "Anomalous dips in extinction spectra of disordered carbon grains." Optics Communications 68, no. 3 (October 1988): 166–70. http://dx.doi.org/10.1016/0030-4018(88)90177-0.

Full text

APA, Harvard, Vancouver, ISO, and other styles

17

Fernandes, C. D., D. Johnson, J. C. Bridges, and Monica M. Grady. "UV-Vis spectroscopy of stardust." International Journal of Astrobiology 5, no. 4 (October 2006): 287–93. http://dx.doi.org/10.1017/s1473550406003363.

Full text

Abstract:

NASA's Stardust mission flew through the coma of comet Wild 2 in January 2004, capturing dust grains as it did so. The grains were returned safely to Earth in January 2006, and are in the process of being distributed to investigators. As members of the Spectroscopy Preliminary Examination Team, we are preparing to analyse Stardust grains. Our contribution is to measure the spectrum of the grains between 200 nm (in the near ultraviolet) and 800 nm (near infrared). The purpose of the measurement is to provide an additional technique for characterizing the grains, one that is complementary to other spectroscopic techniques and one that produces results that can be matched directly with spectra acquired remotely (with telescope or spacecraft instrumentation). As part of the preparation for analysis of Stardust materials, we are producing a database of spectra from appropriate minerals, and are honing the technique through analysis of primitive meteorites.

APA, Harvard, Vancouver, ISO, and other styles

18

Xu, Ling Fang, Kang Sun, Xing Feng, Hai Bo Xiao, Rui Long Wang, and Chang Ping Yang. "Abnormal capacitance–voltage behaviors of bismuth-doped CaCu3Ti4O12 ceramics." International Journal of Modern Physics B 31, no. 20 (August 10, 2017): 1750133. http://dx.doi.org/10.1142/s0217979217501338.

Full text

Abstract:

Ca[Formula: see text]Bi[Formula: see text]Cu3Ti4O[Formula: see text] ([Formula: see text] = 0.0, 0.1, 0.2 and 0.3; BCCTO) ceramics were prepared by traditional solid-state sintering method. All samples had pure cubic perovskite-like structure. A drastic grain size reduction was observed with bismuth doping. Dielectric spectra showed two obvious relaxation steps corresponding to two series of peaks in the imaginary part of electric modulus spectra and dielectric loss spectra too. Activation energy fitting by electric modulus spectra reflected different conducting segments in BCCTO ceramics of grains and oxygen vacancies below room temperature. Normalized capacitance dependent of extra bias showed different voltage–capacitance coefficients at special frequencies that suggested multirelaxation mechanisms related with grain and oxygen vacancy. A positive capacitance curvature implies dipolar relaxations in CCTO grains. Whereas a negative curvature suggests oxygen-related relaxations at the interface.

APA, Harvard, Vancouver, ISO, and other styles

19

Kabacińska, Zuzanna, and Alida Timar-Gabor. "Dating Sediments by EPR Using Al-h Centre: A Comparison between the Properties of Fine (4–11 µm) and Coarse (>63 µm) Quartz Grains." Molecules 27, no. 9 (April 21, 2022): 2683. http://dx.doi.org/10.3390/molecules27092683.

Full text

Abstract:

The possibility of EPR dating for sediments using Al-h signals of fine (4–11 μm) grains of quartz has not been previously discussed. Here, the Al-h and peroxy EPR spectra of fine (4–11 μm) and coarse (63–90, 125–180 μm) sedimentary quartz from thoroughly investigated loess sites in Eastern Europe were examined. By comparing experimental spectra with a simulated signal, we evaluated the overestimation observed when using the standard approach established by Toyoda and Falguères to measure Al-h intensity for different doses of radiation, up to 40,000 Gy. This overestimation, caused by the presence of peroxy signals, was much more pronounced for fine grains. Fine grains exhibited some additional dose-dependent signals, which, for some samples, caused a complete distortion of the Al-h spectra at high doses, making it impossible to measure the standard amplitude. We propose a new approach to measuring Al-h signal intensity, focusing on the peak-to-baseline amplitude of the part of the signal at g ≈ 2.0603, which is not affected by the peroxy signals and therefore has the potential of providing more accurate results. The shapes of dose response curves constructed for coarse and fine grains using the new approach show considerable similarity, suggesting that Al-h centre formation in fine and coarse grains upon artificial radiation at room temperature follows the same pattern.

APA, Harvard, Vancouver, ISO, and other styles

20

Sloan, G. C. "Dust in planetary nebulae." Proceedings of the International Astronomical Union 12, S323 (October 2016): 121–27. http://dx.doi.org/10.1017/s1743921317002034.

Full text

Abstract:

AbstractInfrared spectra from the Spitzer Space Telescope trace the evolution of carbon-rich dust from the asymptotic giant branch (AGB) to young planetary nebulae (PNe). On the AGB, amorphous carbon dominates the dust, but SiC and MgS also appear. In more evolved systems with warmer central stars, the spectra reveal the unidentified 21 μm feature, features from aliphatic hydrocarbons, and spectra from polycyclic aromatic hydrocarbons (PAHs), often with shifted feature positions indicative of the presence of aliphatics. More evolved systems with hot central stars show more typical PAH spectra, along with fullerenes and/or an emission feature known as the big-11 feature at ~11 μm. This features arises from a combination of SiC and PAHs, and it is usually accompanied by a shoulder at 18 μm, which while unidentified might be from cool silicate grains. The strong emission from MgS and SiC in young PNe probably arises from coatings on carbonaceous grains.

APA, Harvard, Vancouver, ISO, and other styles

21

Griffin, Ian, C. J. Skinner, and B. R. Whitmore. "Observations and Models for Red Giants with Unusual Dust." International Astronomical Union Colloquium 106 (1989): 367. http://dx.doi.org/10.1017/s0252921100063259.

Full text

Abstract:

We present near IR (H, K and L band) medium resolution (ƛ/Δƛ ∼ 600) spectra for a selection of 9 red giants which have previously been shown to exhibit anomalous dust emission as characterised by their IRAS LRS spectra. The objects observed (during UKIRT and AAT service time) include Carbon stars whose LRS spectra show the 9.7μm silicate feature and also M stars whose LRS spectra display an 11.3μm feature similar to that usually associated with emission from SiC dust grains.

APA, Harvard, Vancouver, ISO, and other styles

22

Nordsiek, Sven, and Andreas Weller. "A new approach to fitting induced-polarization spectra." GEOPHYSICS 73, no. 6 (November 2008): F235—F245. http://dx.doi.org/10.1190/1.2987412.

Full text

Abstract:

Best fitting of induced-polarization (IP) spectra by different models of Cole-Cole type evidences discrepancies in the resulting model parameters. The time constant determined from the same data could vary in magnitude over several decades. This effect, which makes an evaluation of the results of different models nearly impossible, is demonstrated by induced polarization measurements in the frequency range between [Formula: see text] and [Formula: see text] on thirteen mixtures of quartz sand and slag grains. The samples differ in size and the amount of the slag grains. Parameters describing the IP spectra are derived by fitting models of the Cole-Cole type to the measured data. The fitting quality of the generalized Cole-Cole model, the standard Cole-Cole model, and the Cole-Davidson model is investigated. The parameters derived from these models are compared and correlated with mass percentage and grain size of the slag particles. An alternative fittingapproach is introduced, using the decomposition of observed IP spectra into a variety of Debye spectra. Four integrating parameters are derived and correlated with parameters of the slag-sand mixtures and Cole-Cole parameters, respectively. The alternative approach generally enables a better fitting of measured spectra compared with Cole-Cole type models. It proves to be more flexible and stable, even for complicated phase spectra that cannot be fitted by single Cole-Cole type models. The integrating parameters are well correlated with characterizing parameters of the slag-sand mixtures. The total chargeability well indicates the mass percentage of slag grains, and the mean relaxation time is related to the grain size. The relaxation time distribution can be displayed by cumulative normalized chargeability versus relaxation time, similar to granulation curves. Anologous to the latter, a nonuniformity parameter characterizes the width of the relaxation time distribution.

APA, Harvard, Vancouver, ISO, and other styles

23

Kwok, Sun. "From Stardust to Meteorites: The Synthesis of Inorganic and Organic Grains in AGB and Post-AGB Stars." Symposium - International Astronomical Union 213 (2004): 177–80. http://dx.doi.org/10.1017/s0074180900193222.

Full text

Abstract:

Infrared spectroscopic observations of the circumstellar envelopes of AGB and post-AGB stars have identified a variety of inorganic grains including amorphous silicates, crystalline silicates, silicon carbide, carbonates, corundum, spinels and possibly rutiles. Isotopic studies of meteorites have also identified similar species of presolar origin. The existence of aromatic and aliphatic features in the spectra of post-AGB stars suggests that organic compounds in solid-state form are made during the post-AGB phase of stellar evolution. These features show similarity with the IR spectra of kerogen, which is also found in meteorites. These grains therefore represent an important link between stars and the solar system.

APA, Harvard, Vancouver, ISO, and other styles

24

Lamy, P. L., J. Lefevre, J. Millet, and J. P. Lafon. "Electrostatic Charge of Interplanetary Dust Grains: New Results." International Astronomical Union Colloquium 85 (1985): 335–39. http://dx.doi.org/10.1017/s0252921100084876.

Full text

Abstract:

AbstractA continuing program aimed at improving the determination of the charge of interplanetary as well as cosmic grains is presented. Recent data have been combined to generate two high-resolution solar spectra corresponding to the maximum & minimum of activity. The energy distribution of photoelectrons emitted by quartz grains under solar irradiation is calculated using the new laboratory measurements of Quemerais et al. (this volume). Finally, the method of Lafon, Millet and Lamy (Astron. Astrophys., 95,295, 1981) is used to find the electrostatic potential of these grains.

APA, Harvard, Vancouver, ISO, and other styles

25

Riboldi, Alessandro, Laura Borgese, Irene Vassalini, Giovanna Cornacchia, Marcello Gelfi, Marco Virginio Boniardi, Andrea Casaroli, and Laura Eleonora Depero. "Micro-Raman Spectroscopy Investigation of Crystalline Phases in EAF Slag." Applied Sciences 10, no. 12 (June 15, 2020): 4115. http://dx.doi.org/10.3390/app10124115.

Full text

Abstract:

Electric arc furnace (EAF) slags were investigated by micro-Raman spectroscopy. A slag sample characterised by well-developed crystalline phases was obtained. The EDXS elemental composition made it possible to recognize the grains corresponding to the brownmillerite, larnite and magnesioferrite phases, as identified by XRD in the same powdered sample. The grains were collected and analysed by µ-Raman spectra, and the results showed good reproducibility in each grain and good agreement with spectra of the phases reported in the literature. A Raman database devoted to crystalline phases of EAF slag identification was created to be used by portable Raman instruments, allowing the phase characterisation of the slag directly during steel production.

APA, Harvard, Vancouver, ISO, and other styles

26

Koike, C., and H. Shibai. "Mid- and far-infrared absorption spectra of fine fused quartz grains." Monthly Notices of the Royal Astronomical Society 269, no. 4 (August 15, 1994): 1011–18. http://dx.doi.org/10.1093/mnras/269.4.1011.

Full text

APA, Harvard, Vancouver, ISO, and other styles

27

Kurumada, M., C. Koike, and C. Kaito. "Laboratory production of and alumina grains and their characteristic infrared spectra." Monthly Notices of the Royal Astronomical Society 359, no. 2 (May 11, 2005): 643–47. http://dx.doi.org/10.1111/j.1365-2966.2005.08937.x.

Full text

APA, Harvard, Vancouver, ISO, and other styles

28

Zhao, Hong Jie, Ji Zhou, Zhi Lun Gui, and Long Tu Li. "Complex Impedance Spectroscopy of Bi-Substituted Yttrium Iron Garnet (YIG)." Key Engineering Materials 336-338 (April 2007): 709–11. http://dx.doi.org/10.4028/www.scientific.net/kem.336-338.709.

Full text

Abstract:

The effects of Bi-substitution on the complex impedance spectra of yttrium iron garnet (YIG) were studied in this paper. The polycrystalline yttrium iron garnet Y2BiFe5O12 (YIG: Bi) and Y3Fe5O12 (YIG) samples were prepared by solid-reaction method. The complex impedance spectra were measured in the frequency range from 1 KHz to 100MHz at several temperatures between 210oC and 500oC. The complex impedance sample shows that the YIG: Bi can be represented by double Cole-Cole semicircles, and the YIG can be represented by a single Cole semicircle. The physical structure of the specimen was visualized as comprising of small resistance grains separated by large resistance grain boundaries in accordance with the impedance spectra observations. The electrical processes in the sample were modeled in the form of an electrical equivalent circuit made up of a series combination of two parallel RC circuits attributed to grains and grain boundaries. The temperature dependence of bulk resistance indicated an evidence of Arrhenius-type thermally activated process, showing a close to linear variation up to a temperature of 740 K.

APA, Harvard, Vancouver, ISO, and other styles

29

Rouillé, Gaël, Tino Kirchhuebel, Marcel Rink, Marco Gruenewald, Jörg Kröger, Roman Forker, and Torsten Fritz. "Identification of vibrational excitations and optical transitions of the organic electron donor tetraphenyldibenzoperiflanthene (DBP)." Physical Chemistry Chemical Physics 17, no. 45 (2015): 30404–16. http://dx.doi.org/10.1039/c5cp03761a.

Full text

APA, Harvard, Vancouver, ISO, and other styles

30

Blanco, A., V. Orofino, E. Bussoletti, S. Fonti, L. Colangeli, and J. R. Stephens. "Laboratory Spectra of Amorphous and Crystalline Olivine: An Application to Comet Halley IR Spectrum." International Astronomical Union Colloquium 126 (1991): 125–28. http://dx.doi.org/10.1017/s025292110006663x.

Full text

Abstract:

AbstractAmong the various silicates proposed as components of cosmic dust grains, olivine is considered one of the most likely materials. In this work we present the infrared spectra of three different types of olivine grains: crystalline, amorphous and synthetic (also amorphous). While the first and second sample derive from the same natural mineral, the third one has been prepared in the laboratory according to the relative cosmic abundances of the elements. The experimental data are used to fit the emission feature observed in the comet Halley spectrum between 8 and 13μm.Satisfactory results are obtained by using synthetic olivine mixed with a small amount (5%) of crystalline grains.

APA, Harvard, Vancouver, ISO, and other styles

31

Klenner, Fabian, Frank Postberg, Jon Hillier, Nozair Khawaja, René Reviol, Ralf Srama, Bernd Abel, Ferdinand Stolz, and Sascha Kempf. "Analogue spectra for impact ionization mass spectra of water ice grains obtained at different impact speeds in space." Rapid Communications in Mass Spectrometry 33, no. 22 (October 28, 2019): 1751–60. http://dx.doi.org/10.1002/rcm.8518.

Full text

APA, Harvard, Vancouver, ISO, and other styles

32

Greenberg, J. Mayo, and Willem Schutte. "Infrared Spectral Identification of Complex Organic Molecules in Interstellar Grains." Symposium - International Astronomical Union 112 (1985): 145–50. http://dx.doi.org/10.1017/s0074180900146455.

Full text

Abstract:

The chemical evolution of interstellar grains leads ultimately to a composition consisting largely of complex organic molecules. Comparison of infrared absorption spectra of laboratory produced analogue materials with astronomical observations confirm the presence of similar molecules in interstellar space. The abundance of this complex organic matter derived from the strength of the absorption bands is of the order of ten million solar masses and is almost certainly as large or larger than all conceivable planets.

APA, Harvard, Vancouver, ISO, and other styles

33

Wang, Jin, Xuemin Wang, Jian Yu, Tingting Xiao, Liping Peng, Long Fan, Chuanbin Wang, Qiang Shen, and Weidong Wu. "Tailoring the Grain Size of Bi-Layer Graphene by Pulsed Laser Deposition." Nanomaterials 8, no. 11 (November 1, 2018): 885. http://dx.doi.org/10.3390/nano8110885.

Full text

Abstract:

Improving the thermoelectric efficiency of a material requires a suitable ratio between electrical and thermal conductivity. Nanostructured graphene provides a possible route to improving thermoelectric efficiency. Bi-layer graphene was successfully prepared using pulsed laser deposition in this study. The size of graphene grains was controlled by adjusting the number of pulses. Raman spectra indicated that the graphene was bi-layer. Scanning electron microscopy (SEM) images clearly show that graphene changes from nanostructured to continuous films when more pulses are used during fabrication. Those results indicate that the size of the grains can be controlled between 39 and 182 nm. A detailed analysis of X-ray photoelectron spectra reveals that the sp2 hybrid state is the main chemical state in carbon. The mobility is significantly affected by the grain size in graphene, and there exists a relatively stable region between 500 and 800 pulses. The observed phenomena originate from competition between decreasing resistance and increasing carrier concentration. These studies should be valuable for regulating grains sizes for thermoelectric applications of graphene.

APA, Harvard, Vancouver, ISO, and other styles

34

Wei, Chenxi, Yanshuai Hong, Yangchao Tian, Xiqian Yu, Yijin Liu, and Piero Pianetta. "Quantifying redox heterogeneity in single-crystalline LiCoO2 cathode particles." Journal of Synchrotron Radiation 27, no. 3 (March 13, 2020): 713–19. http://dx.doi.org/10.1107/s1600577520002076.

Full text

Abstract:

Active cathode particles are fundamental architectural units for the composite electrode of Li-ion batteries. The microstructure of the particles has a profound impact on their behavior and, consequently, on the cell-level electrochemical performance. LiCoO2 (LCO, a dominant cathode material) is often in the form of well-shaped particles, a few micrometres in size, with good crystallinity. In contrast to secondary particles (an agglomeration of many fine primary grains), which are the other common form of battery particles populated with structural and chemical defects, it is often anticipated that good particle crystallinity leads to superior mechanical robustness and suppressed charge heterogeneity. Yet, sub-particle level charge inhomogeneity in LCO particles has been widely reported in the literature, posing a frontier challenge in this field. Herein, this topic is revisited and it is demonstrated that X-ray absorption spectra on single-crystalline particles with highly anisotropic lattice structures are sensitive to the polarization configuration of the incident X-rays, causing some degree of ambiguity in analyzing the local spectroscopic fingerprint. To tackle this issue, a methodology is developed that extracts the white-line peak energy in the X-ray absorption near-edge structure spectra as a key data attribute for representing the local state of charge in the LCO crystal. This method demonstrates significantly improved accuracy and reveals the mesoscale chemical complexity in LCO particles with better fidelity. In addition to the implications on the importance of particle engineering for LCO cathodes, the method developed herein also has significant impact on spectro-microscopic studies of single-crystalline materials at synchrotron facilities, which is broadly applicable to a wide range of scientific disciplines well beyond battery research.

APA, Harvard, Vancouver, ISO, and other styles

35

Sikora, Magdalena, Marina Valek, Zdenka Šušić, Vera Santo, and Dario Brdarić. "Tree Pollen Spectra and Pollen Allergy Risk in the Osijek-Baranja County." Archives of Industrial Hygiene and Toxicology 64, no. 1 (March 1, 2013): 115–22. http://dx.doi.org/10.2478/10004-1254-64-2013-2235.

Full text

Abstract:

The forests of north-eastern Croatia, as well as various plants and trees in the parks and streets of the Osijek-Baranja County, produce large amounts of pollen during the pollen season, which can cause allergy symptoms in pollen sensitive individuals. The aim of this study was to determine the most frequent types of pollen in this area and estimate possible health risks, especially the risk of allergy. In 2009 and 2010, the staff of the Health Ecology Department of the Osijek Public Health Institute monitored tree pollen concentrations in four cities from the Osijek - Baranja County (Osijek, Našice, Đakovo and Beli Manastir) using a Burkard volumetric instrument. The results were affected by weather conditions. Windy and sunny days facilitated the transfer of pollen, whereas during rainy days, the concentration of pollen grains decreased. High pollen concentrations of Cupressaceae/Taxaceae, Betulaceae, Salicaceae and Aceraceae could be the cause for symptoms of pollen allergy. In 2009, conifers, birch and poplar pollen were dominant at all monitoring stations with 5000 pollen grains (PG), 3188 PG and 3113 PG respectively. The highest number of pollen grains was recorded at measuring site Osijek. The variations in airborne pollen concentration between pollen seasons were recorded at all monitoring stations. The most obvious variations were recorded at measuring site Osijek. The usual pollination period lasts two to three months, which means that most pollen grains remain present from February to early June. However, the Cupressaceae / Taxaceae pollination periods last the longest and their pollen grains remain present until the end of summer. The risk of allergy was determined at four monitored measuring stations and the obtained data confirmed that the largest number of days with a high health risk was at the Đakovo measuring station for a species of birch. The research information aims to help allergologists and individuals allergic to plant pollen develop preventive measures and proper treatment therapies.

APA, Harvard, Vancouver, ISO, and other styles

36

Smith, R. G., G. Robinson, and A. R. Hyland. "Molecular Ices as Temperature Indicators in the ISM." Publications of the Astronomical Society of Australia 10, no. 3 (1993): 241–46. http://dx.doi.org/10.1017/s1323358000025765.

Full text

Abstract:

AbstractMolecular ices are a common component of the dust in many molecular clouds and circumstellar shells. The most abundant molecule, H2O, has several infrared spectral features which have strongly temperature dependent shapes (FWHM) and peak wavelengths. This paper describes how a study of these features, both astronomically and in the laboratory, can lead to constraints on the temperatures of interstellar dust grains. This is demonstrated in part by comparing several astronomical spectra with laboratory spectra of H2O ice.

APA, Harvard, Vancouver, ISO, and other styles

37

Ricci, Claudio, and Stéphane Paltani. "Ray-tracing Simulations and Spectral Models of X-Ray Radiation in Dusty Media." Astrophysical Journal 945, no. 1 (March 1, 2023): 55. http://dx.doi.org/10.3847/1538-4357/acb5a6.

Full text

Abstract:

Abstract Dust can play an important role in shaping the X-ray spectra and images of astrophysical sources. In this work we report on the implementation of dust in the ray-tracing platform RefleX. We illustrate the different effects associated with the interaction between X-ray photons and dust grains, such as dust scattering, near-edge X-ray absorption fine structures, and shielding. We show how the cross sections of the photon–gas interaction change depending on the fraction of metals in dust grains (i.e., the dust depletion factor). We compare RefleX simulations to the most widely used absorption model that includes dust and show how X-ray spectra are affected by the presence of dust in the absorbing/reprocessing medium for different geometries. We also show how RefleX can be used to reproduce the dust scattering halos observed in Galactic sources, and we release the first torus X-ray spectral model that considers dust absorption and scattering (RXTorusD), to reproduce the spectra of active galactic nuclei (AGNs). RXTorusD also considers other physical processes that are not included in the most widely used AGN torus models, such as Rayleigh scattering and scattering on molecular gas, which can lead to remarkable differences in the predicted X-ray spectra for the same set of geometrical and physical parameters.

APA, Harvard, Vancouver, ISO, and other styles

38

V. Pavlenko, Yakiv. "Formation of the Optical Spectra of the Coolest M- and L-dwarfs and Lithium Abundances in their Atmospheres." Symposium - International Astronomical Union 198 (2000): 454–59. http://dx.doi.org/10.1017/s007418090016704x.

Full text

Abstract:

Theoretical aspects of modeling of spectra of late M- and L-dwarfs are discussed. We show, that the processes of formation of spectra of M- and L-dwarfs are basically different. Instead of the case of M-dwarfs, atoms of Ti and VO should be depleted into grains in the atmospheres of L-dwarfs. Overall shape of the L-dwarf spectra is governed by the K I + Na I resonance line wings of the huge strength. To fit lithium lines observed in spectra of the coolest dwarfs we used two additional suggestions: a) there are some extra depletions of molecular species absorbed in the optical spectra of L-dwarfs; b) there may be (a few?) additional (“dusty”?) opacity sources in their atmospheres. Problems of lithium line formation and the “natural” limitation of their use for the ‘lithium test’ for the case of L-dwarfs are discussed.

APA, Harvard, Vancouver, ISO, and other styles

39

Ciesielski, Arkadiusz, Lukasz Skowronski, Ewa Górecka, Jakub Kierdaszuk, and Tomasz Szoplik. "Growth model and structure evolution of Ag layers deposited on Ge films." Beilstein Journal of Nanotechnology 9 (January 8, 2018): 66–76. http://dx.doi.org/10.3762/bjnano.9.9.

Full text

Abstract:

We investigated the crystallinity and optical parameters of silver layers of 10–35 nm thickness as a function 2–10 nm thick Ge wetting films deposited on SiO2 substrates. X-ray reflectometry (XRR) and X-ray diffraction (XRD) measurements proved that segregation of germanium into the surface of the silver film is a result of the gradient growth of silver crystals. The free energy of Ge atoms is reduced by their migration from boundaries of larger grains at the Ag/SiO2 interface to boundaries of smaller grains near the Ag surface. Annealing at different temperatures and various durations allowed for a controlled distribution of crystal dimensions, thus influencing the segregation rate. Furthermore, using ellipsometric and optical transmission measurements we determined the time-dependent evolution of the film structure. If stored under ambient conditions for the first week after deposition, the changes in the transmission spectra are smaller than the measurement accuracy. Over the course of the following three weeks, the segregation-induced effects result in considerably modified transmission spectra. Two months after deposition, the slope of the silver layer density profile derived from the XRR spectra was found to be inverted due to the completed segregation process, and the optical transmission spectra increased uniformly due to the roughened surfaces, corrosion of silver and ongoing recrystallization. The Raman spectra of the Ge wetted Ag films were measured immediately after deposition and ten days later and demonstrated that the Ge atoms at the Ag grain boundaries form clusters of a few atoms where the Ge–Ge bonds are still present.

APA, Harvard, Vancouver, ISO, and other styles

40

Jankovskis, J., V. Yurshevich, and G. Rankis. "Analysis of Grains Distribution Effects on the Magnetic Spectra of Polycrystalline Ferrites." Le Journal de Physique IV 07, no. C1 (March 1997): C1–203—C1–204. http://dx.doi.org/10.1051/jp4:1997176.

Full text

APA, Harvard, Vancouver, ISO, and other styles

41

Hashimoto, Tetsuo, Noriyuki Sugai, Hisanobu Sakaue, Kenya Yasuda, and Nobutoshi Shirai. "Thermoluminescence (TL) spectra from quartz grains using on-line TL-spectrometric system." GEOCHEMICAL JOURNAL 31, no. 4 (1997): 189–201. http://dx.doi.org/10.2343/geochemj.31.189.

Full text

APA, Harvard, Vancouver, ISO, and other styles

42

Koike, Chiyoe, Hiroichi Hasegawa, and Takeshi Hattori. "Grain size dependence of far-infrared extinction spectra of amorphous fine grains." Astrophysics and Space Science 134, no. 1 (June 1987): 95–102. http://dx.doi.org/10.1007/bf00636456.

Full text

APA, Harvard, Vancouver, ISO, and other styles

43

Leary, Ryan J., M. Elliot Smith, and Paul Umhoefer. "Mixed eolian–longshore sediment transport in the late Paleozoic Arizona shelf and Pedregosa basin, U.S.A.: A case study in grain-size analysis of detrital-zircon datasets." Journal of Sedimentary Research 92, no. 8 (August 22, 2022): 676–94. http://dx.doi.org/10.2110/jsr.2021.101.

Full text

Abstract:

ABSTRACT Detrital-zircon U–Pb geochronology has revolutionized sediment provenance studies over the last two decades, and zircon has been successfully analyzed from nearly all sedimentary lithologies, depositional environments, and sediment grain sizes. However, despite the ubiquity of this method and the far-reaching interpretations supported by detrital-zircon data, few studies have investigated the potential role of zircon grain size on age spectra and provenance interpretation. In this study, we investigate the connections between sample grain size, zircon grain size, U–Pb age spectra, and interpreted provenance using 18 detrital-zircon samples (4999 individual grains) collected from Pennsylvanian–Permian strata in central and southern Arizona, USA. In these samples, there is no clear correlation between sample grain size and zircon grain size and no clear correlation between sample grain size and age spectra. However, when all grains are grouped by zircon minimum long-axis dimension, the abundance of some age groups is correlated to zircon grain size. In Pennsylvanian samples, < 400 Ma grains and 2500–3000 Ma zircons are more abundant in the finer fractions, and 1400–1900 Ma zircons are more abundant in coarser fractions of both Pennsylvanian and Permian samples. In Permian samples, 500–800 Ma zircons are most abundant in the finer fractions, and 2500–3000 Ma grains are concentrated in the coarser fractions. Based on changes in abundance and grain-size distribution of 500–800 Ma grains, we interpret a change in zircon provenance across the Pennsylvanian–Permian boundary that reflects regional climate and paleogeographic changes driven in part by the northward drift of Laurentia across the equator. Specifically, we interpret the concentration of 500–800 Ma zircons in Permian samples in central and southern Arizona to indicate that these grains were: 1) sourced from Gondwana, 2) deposited in, and subsequently eroded (recycled) from, Mississippian–Pennsylvanian strata in the Arkoma, Anadarko, and Fort Worth basins at the margins of Laurentia, and 3) finally transported into the Arizona study area as loess by easterly trade winds. This study serves as a case study in the value and interpretive power of basic grain-size characterization of detrital-geochronology datasets.

APA, Harvard, Vancouver, ISO, and other styles

44

Dridi, Wassim, Mohamed Faouzi Zid, and Miroslaw Maczka. "Electrical and Vibrational Studies of Na2K2Cu(MoO4)3." Advances in Materials Science and Engineering 2017 (2017): 1–8. http://dx.doi.org/10.1155/2017/6123628.

Full text

Abstract:

The complex impedance of Na2K2Cu(MoO4)3 material has been investigated in the temperature range of 653–753 K and in the frequency range of 40 Hz–5 MHz. Electrical behavior of the studied material is explained through an equivalent circuit model which takes into account the contributions of grains and grains boundaries. The number of vibrational modes was calculated using group theoretical approach. The infrared and Raman spectra have also been measured and vibrational assignment has been proposed.

APA, Harvard, Vancouver, ISO, and other styles

45

Jäger, Cornelia, Alexey Potapov, Gaël Rouillé, and Thomas Henning. "Laboratory experiments on cosmic dust and ices." Proceedings of the International Astronomical Union 15, S350 (April 2019): 27–34. http://dx.doi.org/10.1017/s1743921319009682.

Full text

Abstract:

AbstractThe existence of cosmic dust is attested by the interstellar extinction and polarization, IR emission and absorption spectra, and elemental depletion patterns. Dust grains are efficiently processed or even destroyed in shocks, molecular clouds, or protoplanetary disks. A considerable amount of dust has to be re-formed in the ISM. In various astrophysical environments, dust grains are covered by molecular ices and therefore contribute or catalytically influence the chemical reactions in these layers. Laboratory experiments are desperately required to understand the evolution of grains and grain/ice mixtures in molecular clouds and early planetary disks. This review considers recent progress in laboratory approaches to dust/ice experiments.

APA, Harvard, Vancouver, ISO, and other styles

46

Gavilan Marin, Lisseth, Salma Bejaoui, Gregory Gate, Michael Haggmark, Nathan Svadlenak, Mattanjah de Vries, Ella Sciamma-O’Brien, and Farid Salama. "Low-temperature condensation of carbonaceous dust grains from PAHs." Proceedings of the International Astronomical Union 15, S350 (April 2019): 465–67. http://dx.doi.org/10.1017/s174392131900646x.

Full text

Abstract:

AbstractInterstellar carbon has been detected in both gas-phase molecules and solid particles. The goal of this study is to identify the link between these two phases of cosmic carbon. Here we report preliminary results on the low temperature formation of carbonaceous dust grains from gas-phase aromatic hydrocarbon precursors. This is done using the supersonic expansion of an argon jet seeded with aromatic molecules and exposed to an electrical discharge. We report experimental evidence of efficient carbon dust condensation from aromatic molecules including benzene and naphthalene. The molecular content of the solid grains is probed with laser desorption mass spectrometry. The mass spectra reveal a rich molecular composition including fragments of the parent molecule but also growth into larger molecular species.

APA, Harvard, Vancouver, ISO, and other styles

47

Krelowski, Jacek, Jacek Papaj, and Walter Wegner. "Residual extinction effects in spectra of newly formed stars." Symposium - International Astronomical Union 137 (1990): 293–97. http://dx.doi.org/10.1017/s0074180900187893.

Full text

Abstract:

Extinction laws observed in spectra of Be stars are shown to be evidently different from the mean extinction curve. They do not contain in many cases the prominent extinction feature - the 2200A bump. The results are derived both from multicolour (ranging from the far-IR until the ultraviolet ANS) photometry and from TD-1 UV spectra. It is strongly suggested that the observed extinction phenomena are originated in circumstellar, disk-shaped shells as the shape of resultant extinction curve suggests both the presence of big, core-mantle grains and continuous infrared emission of circumstellar origin.

APA, Harvard, Vancouver, ISO, and other styles

48

Victoria-Ceballos, César Ivan, Omaira González-Martín, Jacopo Fritz, Cristina Ramos Almeida, Enrique López-Rodríguez, Santiago García-Burillo, Almudena Alonso-Herrero, Mariela Martínez-Paredes, Donaji Esparza-Arredondo, and Natalia Osorio-Clavijo. "The Complex Infrared Dust Continuum Emission of NGC 1068: Ground-based N- and Q-band Spectroscopy and New Radiative Transfer Models." Astrophysical Journal 926, no. 2 (February 1, 2022): 192. http://dx.doi.org/10.3847/1538-4357/ac441a.

Full text

Abstract:

Abstract Thanks to ground-based infrared and submillimeter observations the study of the dusty torus of nearby active galactic nuclei has greatly advanced in the last years. With the aim of further investigating the nuclear mid-infrared emission of the archetypal Seyfert 2 galaxy NGC 1068, here we present a fitting to the N- and Q-band Michelle/Gemini spectra. We initially test several available spectral energy distribution (SED) libraries, including smooth, clumpy and two-phase torus models, and a clumpy disk+wind model. We find that the spectra of NGC 1068 cannot be reproduced with any of these models. Although, the smooth torus models describe the spectra of NGC 1068 if we allow variation of some model parameters among the two spectral bands. Motivated by this result, we produced new SEDs using the radiative transfer code SKIRT. We use two concentric tori that allow us to test a more complex geometry. We test different values for the inner and outer radii, half-opening angle, radial, and polar exponent of the power-law density profile, opacity, and viewing angle. Furthermore, we also test the dust grains’ size and different optical and calorimetric properties of silicate grains. The best-fitting model consists of two concentric components with outer radii of 1.8 and 28 pc, respectively. We find that the size and the optical and calorimetric properties of graphite and silicate grains in the dust structure are key to reproducing the spectra of NGC 1068. A maximum grain size of 1 μm leads to a significant improvement in the fit. We conclude that the dust in NGC 1068 reaches different scales, where the highest contribution to the mid-infrared is given by a central and compact component. A less dense and extended component is present, which can be either part of the same torus (conforming a flared disk) or can represent the emission of a polar dust component, as already suggested from interferometric observations.

APA, Harvard, Vancouver, ISO, and other styles

49

Thebault, P., and Q. Kral. "Is there more than meets the eye? Presence and role of sub-micron grains in debris discs." Astronomy & Astrophysics 626 (June 2019): A24. http://dx.doi.org/10.1051/0004-6361/201935341.

Full text

Abstract:

Context. The presence of sub-micron grains has been inferred in several debris discs, usually because of a blue colour of the spectrum in scattered light or a pronounced silicate band around 10 μm, even though these particles should be blown out by stellar radiation pressure on very short timescales. So far, no fully satisfying explanation has been found for this apparent paradox. Aims. We investigate the possibility that the observed abundances of sub-micron grains could be naturally produced in bright debris discs, where the high collisional activity produces them at a rate high enough to partially compensate for their rapid removal. We also investigate to what extent this potential presence of small grains can affect our understanding of some debris disc characteristics. Methods. We used a numerical collisional code to follow the collisional evolution of a debris disc down to sub-micron grains far below the limiting blow-out size sblow. We considered compact astrosilicates and explored different configurations: A and G stars, cold and warm discs, bright and very bright systems. We then produced synthetic spectra and spectral energy distributions, where we identified and quantified the signature of unbound sub-micron grains. Results. We find that in bright discs (fractional luminosity ≳10−3) around A stars, the number of sub-micron grains is always high enough to leave detectable signatures in scattered light where the disc colour becomes blue, and also in the mid-IR (10 ≲ λ ≲ 20 μm), where they boost the disc luminosity by at least a factor of 2 and induce a pronounced silicate solid-state band around 10 μm. We also show that with this additional contribution of sub-micron grains, the spectral energy distribution can mimic that of two debris belts separated by a factor of ~2 in radial distance. For G stars, the effect of s ≤ sblow grains remains limited in the spectra although they dominate the geometrical cross section of the system. We also find that for all considered cases, the halo of small (bound and unbound) grains that extends far beyond the main disc contributes to ~50% of the flux up to λ ~ 50 μm wavelengths.

APA, Harvard, Vancouver, ISO, and other styles

50

Kalvāns, Juris. "TEMPERATURE SPECTRA OF INTERSTELLAR DUST GRAINS HEATED BY COSMIC RAYS. I. TRANSLUCENT CLOUDS." Astrophysical Journal Supplement Series 224, no. 2 (June 24, 2016): 42. http://dx.doi.org/10.3847/0067-0049/224/2/42.

Full text

APA, Harvard, Vancouver, ISO, and other styles

Journal articles on the topic 'Spectre de graisse'

Create a spot-on reference in APA, MLA, Chicago, Harvard, and other styles