The Experts below are selected from a list of 282 Experts worldwide ranked by ideXlab platform

A. Jones - One of the best experts on this subject based on the ideXlab platform.

  • A hidden reservoir of Fe/FeS in interstellar Silicates?
    Astronomy and Astrophysics - A&A, 2014
    Co-Authors: M. Köhler, A. Jones, N. Ysard
    Abstract:

    Context. The depletion of iron and sulphur into dust in the interstellar medium and the exact nature of interstellar amorphous Silicate Grains is still an open question. Aims: We study the incorporation of iron and sulphur into amorphous Silicates of olivine- and pyroxene-types and their effects on the dust spectroscopy and thermal emission. Methods: We used the Maxwell-Garnett effective-medium theory to construct the optical constants for a mixture of Silicates, metallic iron, and iron sulphide. We also studied the effects of iron and iron sulphide in aggregate Grains. Results: Iron sulphide inclusions within amorphous Silicates that contain iron metal inclusions show no strong differences in the optical properties of the Grains. A mix of amorphous olivine- and pyroxene-type Silicate broadens the Silicate features. An amorphous carbon mantle with a thickness of 10 nm on the Silicate Grains leads to an increase in absorption on the short-wavelength side of the 10 μm Silicate band. Conclusions: The assumption of amorphous olivine-type and pyroxene-type Silicates and a 10 nm thick amorphous carbon mantle better matches the interstellar Silicate band profiles. Including iron nano-particles leads to an increase in the mid-IR extinction, while up to 5 ppm of sulphur can be incorporated as Fe/FeS nano inclusions into Silicate Grains without leaving a significant trace of its presence.

  • The role of laboratory experiments in the characterisation of silicon-based cosmic material
    Astronomy and Astrophysics Review, 2003
    Co-Authors: L. Colangeli, Th Henning, John Robert Brucato, D. Fabian, Olivier Guillois, Friedrich Huisken, Elmar K. Jessberger, D. Cl�ment, C. J�ger, A. Jones
    Abstract:

    International audienceSilicate Grains in space have attracted recently a wide interest of astrophysicists due to the increasing amount and quality of observational data, especially thanks to the results obtained by the Infrared Space Observatory. The observations have shown that the presence of Silicates is ubiquitous in space and that their properties vary with environmental characteristics. Silicates, together with carbon, are the principal components of solid matter in space. Since their formation, Silicate Grains cross many environments characterised by different physical and chemical conditions which can induce changes to their nature. Moreover, the transformations experienced in the interplay of Silicate Grains and the medium where they are dipped, are part of a series of processes which are the subject of possible changes in the nature of the space environment itself. Then, chemical and physical changes of Silicate Grains during their life play a key role in the chemical evolution of the entire Galaxy. The knowledge of Silicate properties related to the conditions where they are found in space is strictly related to the study in the laboratory of the possible formation and transformation mechanisms they experience. The application of production and processing methods, capable to reproduce actual space conditions, together with the use of analytical techniques to investigate the nature of the material samples, form a subject of a complex laboratory experimental approach directed to the understanding of cosmic matter. The goal of the present paper is to review the experimental methods applied in various laboratories to the simulation and characterisation of cosmic Silicate analogues. The paper describes also laboratory studies of the chemical reactions undergone and induced by Silicate Grains. The comparison of available laboratory results with observational data shows the essential constraints imposed by astronomical observations and, at the same time, indicates the most puzzling problems that deserve particular attention for the future. The outstanding open problems are reported and discussed. The final purpose of this paper is to provide an overview of the present stage of knowledge about Silicates in space and to provide to the reader some indication of the future developments in the field

  • The role of laboratory experiments in the characterisation of silicon-based cosmic material
    Astronomy and Astrophysics Review, 2003
    Co-Authors: L. Colangeli, Th Henning, John Robert Brucato, D. Clément, D. Fabian, Olivier Guillois, Friedrich Huisken, Elmar K. Jessberger, C. Jäger, A. Jones
    Abstract:

    Silicate Grains in space have attracted recently a wide interest of astrophysicists due to the increasing amount and quality of observational data, especially thanks to the results obtained by the Infrared Space Observatory. The observations have shown that the presence of Silicates is ubiquitous in space and that their properties vary with environmental characteristics. Silicates, together with carbon, are the principal components of solid matter in space. Since their formation, Silicate Grains cross many environments characterised by different physical and chemical conditions which can induce changes to their nature. Moreover, the transformations experienced in the interplay of Silicate Grains and the medium where they are dipped, are part of a series of processes which are the subject of possible changes in the nature of the space environment itself. Then, chemical and physical changes of Silicate Grains during their life play a key role in the chemical evolution of the entire Galaxy. The knowledge of Silicate properties related to the conditions where they are found in space is strictly related to the study in the laboratory of the possible formation and transformation mechanisms they experience. The application of production and processing methods, capable to reproduce actual space conditions, together with the use of analytical techniques to investigate the nature of the material samples, form a subject of a complex laboratory experimental approach directed to the understanding of cosmic matter. The goal of the present paper is to review the experimental methods applied in various laboratories to the simulation and characterisation of cosmic Silicate analogues. The paper describes also laboratory studies of the chemical reactions undergone and induced by Silicate Grains. The comparison of available laboratory results with observational data shows the essential constraints imposed by astronomical observations and, at the same time, indicates the most puzzling problems that deserve particular attention for the future. The outstanding open problems are reported and discussed. The final purpose of this paper is to provide an overview of the present stage of knowledge about Silicates in space and to provide to the reader some indication of the future developments in the field.

L. Colangeli - One of the best experts on this subject based on the ideXlab platform.

  • The role of laboratory experiments in the characterisation of silicon-based cosmic material
    Astronomy and Astrophysics Review, 2003
    Co-Authors: L. Colangeli, Th Henning, John Robert Brucato, D. Fabian, Olivier Guillois, Friedrich Huisken, Elmar K. Jessberger, D. Cl�ment, C. J�ger, A. Jones
    Abstract:

    International audienceSilicate Grains in space have attracted recently a wide interest of astrophysicists due to the increasing amount and quality of observational data, especially thanks to the results obtained by the Infrared Space Observatory. The observations have shown that the presence of Silicates is ubiquitous in space and that their properties vary with environmental characteristics. Silicates, together with carbon, are the principal components of solid matter in space. Since their formation, Silicate Grains cross many environments characterised by different physical and chemical conditions which can induce changes to their nature. Moreover, the transformations experienced in the interplay of Silicate Grains and the medium where they are dipped, are part of a series of processes which are the subject of possible changes in the nature of the space environment itself. Then, chemical and physical changes of Silicate Grains during their life play a key role in the chemical evolution of the entire Galaxy. The knowledge of Silicate properties related to the conditions where they are found in space is strictly related to the study in the laboratory of the possible formation and transformation mechanisms they experience. The application of production and processing methods, capable to reproduce actual space conditions, together with the use of analytical techniques to investigate the nature of the material samples, form a subject of a complex laboratory experimental approach directed to the understanding of cosmic matter. The goal of the present paper is to review the experimental methods applied in various laboratories to the simulation and characterisation of cosmic Silicate analogues. The paper describes also laboratory studies of the chemical reactions undergone and induced by Silicate Grains. The comparison of available laboratory results with observational data shows the essential constraints imposed by astronomical observations and, at the same time, indicates the most puzzling problems that deserve particular attention for the future. The outstanding open problems are reported and discussed. The final purpose of this paper is to provide an overview of the present stage of knowledge about Silicates in space and to provide to the reader some indication of the future developments in the field

  • The role of laboratory experiments in the characterisation of silicon-based cosmic material
    Astronomy and Astrophysics Review, 2003
    Co-Authors: L. Colangeli, Th Henning, John Robert Brucato, D. Clément, D. Fabian, Olivier Guillois, Friedrich Huisken, Elmar K. Jessberger, C. Jäger, A. Jones
    Abstract:

    Silicate Grains in space have attracted recently a wide interest of astrophysicists due to the increasing amount and quality of observational data, especially thanks to the results obtained by the Infrared Space Observatory. The observations have shown that the presence of Silicates is ubiquitous in space and that their properties vary with environmental characteristics. Silicates, together with carbon, are the principal components of solid matter in space. Since their formation, Silicate Grains cross many environments characterised by different physical and chemical conditions which can induce changes to their nature. Moreover, the transformations experienced in the interplay of Silicate Grains and the medium where they are dipped, are part of a series of processes which are the subject of possible changes in the nature of the space environment itself. Then, chemical and physical changes of Silicate Grains during their life play a key role in the chemical evolution of the entire Galaxy. The knowledge of Silicate properties related to the conditions where they are found in space is strictly related to the study in the laboratory of the possible formation and transformation mechanisms they experience. The application of production and processing methods, capable to reproduce actual space conditions, together with the use of analytical techniques to investigate the nature of the material samples, form a subject of a complex laboratory experimental approach directed to the understanding of cosmic matter. The goal of the present paper is to review the experimental methods applied in various laboratories to the simulation and characterisation of cosmic Silicate analogues. The paper describes also laboratory studies of the chemical reactions undergone and induced by Silicate Grains. The comparison of available laboratory results with observational data shows the essential constraints imposed by astronomical observations and, at the same time, indicates the most puzzling problems that deserve particular attention for the future. The outstanding open problems are reported and discussed. The final purpose of this paper is to provide an overview of the present stage of knowledge about Silicates in space and to provide to the reader some indication of the future developments in the field.

  • Carbon and Silicate Grains in the laboratory as analogues of cosmic dust.
    Spectrochimica acta. Part A Molecular and biomolecular spectroscopy, 2001
    Co-Authors: Vito Mennella, John Robert Brucato, L. Colangeli
    Abstract:

    Carbon and Silicate Grains are the two main components of cosmic dust. There is increasing spectroscopic evidence that their composition varies according to the cosmic environment and the experienced processing. Irradiation from ultraviolet photons and cosmic rays, as well as chemical interactions with the interstellar gas play a crucial role for grain transformation. The study of 'laboratory analogues' represents a powerful tool to better understand the nature and evolution of cosmic materials. In particular, simulations of grain processing are fundamental to outline an evolutionary pathway for interstellar particles. In the present work, we discuss the ultraviolet and infrared spectral changes induced by thermal annealing, ultraviolet irradiation, ion irradiation and hydrogen atom bombardment in carbon and Silicate analogue materials. The laboratory results give the opportunity to shed light on the long-standing problems of the attribution of ultraviolet and infrared interstellar spectral features.

  • Temperature dependence of the FIR absorption coefficient for carbon and Silicate Grains
    Formation and Evolution of Solids in Space, 1999
    Co-Authors: John Robert Brucato, L. Colangeli, V. Mennella, E Bussoletti
    Abstract:

    In this paper we present some preliminary results concerning the measure of the mass absorption coefficient of carbonaceous and Silicates Grains at various temperatures in the far-infrared spectral region. We have performed transmittance measurements in the 20–200 µm spectral range at temperature between 24 and 300 K. Data show significant spectral index variations with the temperature. In particular when the temperature decreases, β increases from 0.8 to 1.0 for the amorphous carbon, from 1.4 to 1.6 for amorphous fayalite and from 1.8 to 2.3 for crystalline forsterite. These studies are relevant for astrophysical applications since in the far-infrared, information on the total mass and, in some cases, discrimination among dust models, can be derived from opacity measurements of interstellar Grains.

  • Infrared laboratory spectra and composition changes of synthetic amorphous Silicate Grains
    1992
    Co-Authors: Armando Blanco, L. Colangeli, E Bussoletti, Sergio Fonti, Maurizio Martino, V. Mennella, John R. Stephens
    Abstract:

    We present here infrared laboratory spectra of some amorphous Silicates of olivine composition. By changing the metal to silicon ratio as well as the content of iron relative to magnesium, we investigate the spectroscopic effects of chemical fractionation during formation and processing of Grains materials in astronomical environments.

John Robert Brucato - One of the best experts on this subject based on the ideXlab platform.

  • Formation of molecular hydrogen on amorphous Silicate surfaces
    arXiv: Astrophysics, 2007
    Co-Authors: G. Manicò, John Robert Brucato, E. Congiu, Joe Roser, Sol Swords, Hagai B. Perets, A. Lederhendler, Ofer Biham, Valerio Pirronello
    Abstract:

    Experimental results on the formation of molecular hydrogen on amorphous Silicate surfaces are presented and analyzed using a rate equation model. The energy barriers for the relevant diffusion and desorption processes are obtained. They turn out to be significantly higher than those obtained for polycrystalline Silicates, demonstrating the importance of grain morphology. Using these barriers we evaluate the efficiency of molecular hydrogen formation on amorphous Silicate Grains under interstellar conditions. It is found that unlike polycrystalline Silicates, amorphous Silicate Grains are efficient catalysts of H_2 formation in diffuse interstellar clouds.

  • Molecular Hydrogen Formation on Amorphous Silicates under Interstellar Conditions
    The Astrophysical Journal, 2007
    Co-Authors: Hagai B. Perets, G. Manicò, E. Congiu, Joe Roser, Sol Swords, A. Lederhendler, Ofer Biham, Gianfranco Vidali, John Robert Brucato
    Abstract:

    Experimental results on the formation of molecular hydrogen on amorphous Silicate surfaces are presented for the first time and analyzed using a rate equation model. The energy barriers for the relevant diffusion and desorption processes are obtained. They turn out to be significantly higher than those obtained earlier for polycrystalline Silicates, demonstrating the importance of grain morphology. Using these barriers we evaluate the efficiency of molecular hydrogen formation on amorphous Silicate Grains under interstellar conditions. It is found that unlike polycrystalline Silicates, amorphous Silicate Grains are efficient catalysts of H$_{2}$ formation within a temperature range which is relevant to diffuse interstellar clouds. The results also indicate that the hydrogen molecules are thermalized with the surface and desorb with low kinetic energy. Thus, they are unlikely to occupy highly excited states.

  • The role of laboratory experiments in the characterisation of silicon-based cosmic material
    Astronomy and Astrophysics Review, 2003
    Co-Authors: L. Colangeli, Th Henning, John Robert Brucato, D. Fabian, Olivier Guillois, Friedrich Huisken, Elmar K. Jessberger, D. Cl�ment, C. J�ger, A. Jones
    Abstract:

    International audienceSilicate Grains in space have attracted recently a wide interest of astrophysicists due to the increasing amount and quality of observational data, especially thanks to the results obtained by the Infrared Space Observatory. The observations have shown that the presence of Silicates is ubiquitous in space and that their properties vary with environmental characteristics. Silicates, together with carbon, are the principal components of solid matter in space. Since their formation, Silicate Grains cross many environments characterised by different physical and chemical conditions which can induce changes to their nature. Moreover, the transformations experienced in the interplay of Silicate Grains and the medium where they are dipped, are part of a series of processes which are the subject of possible changes in the nature of the space environment itself. Then, chemical and physical changes of Silicate Grains during their life play a key role in the chemical evolution of the entire Galaxy. The knowledge of Silicate properties related to the conditions where they are found in space is strictly related to the study in the laboratory of the possible formation and transformation mechanisms they experience. The application of production and processing methods, capable to reproduce actual space conditions, together with the use of analytical techniques to investigate the nature of the material samples, form a subject of a complex laboratory experimental approach directed to the understanding of cosmic matter. The goal of the present paper is to review the experimental methods applied in various laboratories to the simulation and characterisation of cosmic Silicate analogues. The paper describes also laboratory studies of the chemical reactions undergone and induced by Silicate Grains. The comparison of available laboratory results with observational data shows the essential constraints imposed by astronomical observations and, at the same time, indicates the most puzzling problems that deserve particular attention for the future. The outstanding open problems are reported and discussed. The final purpose of this paper is to provide an overview of the present stage of knowledge about Silicates in space and to provide to the reader some indication of the future developments in the field

  • The role of laboratory experiments in the characterisation of silicon-based cosmic material
    Astronomy and Astrophysics Review, 2003
    Co-Authors: L. Colangeli, Th Henning, John Robert Brucato, D. Clément, D. Fabian, Olivier Guillois, Friedrich Huisken, Elmar K. Jessberger, C. Jäger, A. Jones
    Abstract:

    Silicate Grains in space have attracted recently a wide interest of astrophysicists due to the increasing amount and quality of observational data, especially thanks to the results obtained by the Infrared Space Observatory. The observations have shown that the presence of Silicates is ubiquitous in space and that their properties vary with environmental characteristics. Silicates, together with carbon, are the principal components of solid matter in space. Since their formation, Silicate Grains cross many environments characterised by different physical and chemical conditions which can induce changes to their nature. Moreover, the transformations experienced in the interplay of Silicate Grains and the medium where they are dipped, are part of a series of processes which are the subject of possible changes in the nature of the space environment itself. Then, chemical and physical changes of Silicate Grains during their life play a key role in the chemical evolution of the entire Galaxy. The knowledge of Silicate properties related to the conditions where they are found in space is strictly related to the study in the laboratory of the possible formation and transformation mechanisms they experience. The application of production and processing methods, capable to reproduce actual space conditions, together with the use of analytical techniques to investigate the nature of the material samples, form a subject of a complex laboratory experimental approach directed to the understanding of cosmic matter. The goal of the present paper is to review the experimental methods applied in various laboratories to the simulation and characterisation of cosmic Silicate analogues. The paper describes also laboratory studies of the chemical reactions undergone and induced by Silicate Grains. The comparison of available laboratory results with observational data shows the essential constraints imposed by astronomical observations and, at the same time, indicates the most puzzling problems that deserve particular attention for the future. The outstanding open problems are reported and discussed. The final purpose of this paper is to provide an overview of the present stage of knowledge about Silicates in space and to provide to the reader some indication of the future developments in the field.

  • Carbon and Silicate Grains in the laboratory as analogues of cosmic dust.
    Spectrochimica acta. Part A Molecular and biomolecular spectroscopy, 2001
    Co-Authors: Vito Mennella, John Robert Brucato, L. Colangeli
    Abstract:

    Carbon and Silicate Grains are the two main components of cosmic dust. There is increasing spectroscopic evidence that their composition varies according to the cosmic environment and the experienced processing. Irradiation from ultraviolet photons and cosmic rays, as well as chemical interactions with the interstellar gas play a crucial role for grain transformation. The study of 'laboratory analogues' represents a powerful tool to better understand the nature and evolution of cosmic materials. In particular, simulations of grain processing are fundamental to outline an evolutionary pathway for interstellar particles. In the present work, we discuss the ultraviolet and infrared spectral changes induced by thermal annealing, ultraviolet irradiation, ion irradiation and hydrogen atom bombardment in carbon and Silicate analogue materials. The laboratory results give the opportunity to shed light on the long-standing problems of the attribution of ultraviolet and infrared interstellar spectral features.

Frank J. Stadermann - One of the best experts on this subject based on the ideXlab platform.

  • An Investigation into the Origin of Fe-Rich Presolar Silicates in Acfer 094
    The Astrophysical Journal, 2010
    Co-Authors: Maitrayee Bose, Christine Floss, Frank J. Stadermann
    Abstract:

    Presolar Silicate and oxide Grains from primitive meteorites are recognized as “stardust” on the basis of their extremely anomalous O isotopic compositions. We report data on 48 O-anomalous Grains that were identified in grain size separates of the ungrouped carbonaceous chondrite Acfer 094. A majority of these Grains exhibit high 17 O/ 16 O isotopic ratios along with solar to sub-solar 18 O/ 16 O ratios and may have originated in low-mass stars with close-to-solar metallicity. Four Silicate Grains that contain 18 O enrichments were also measured for their Si isotopes. A comparison of their O and Si isotopic compositions with model predictions indicates that these 18 O-rich Grains may have formed in supernova ejecta. Four of the O-anomalous Grains are oxides while the remaining 44 are Silicates, based on elemental compositions determined by Auger spectroscopy. The presolar oxides include a TiO2 grain and a grain with spinel stoichiometry. The Silicate Grains largely exhibit ferromagnesian compositions, although a few Grains also contain small amounts of Ca and/or Al. Stoichiometric Silicates were further classified as either olivine-like or pyroxene-like, and in this study pyroxene-like Grains are more abundant than olivinelike ones. The majority of Silicates contain more Fe than Mg, including a few Grains with Fe-rich end-member compositions. Spectroscopic observations indicate the presence of Mg-rich Silicates in the atmospheres of stars and the interstellar medium. Mg-rich minerals such as forsterite and enstatite form by equilibrium condensation in stellar environments. However, non-equilibrium condensation can result in higher Fe contents and the occurrence of such processes in the outflows of stars may account for the Fe-rich Grains. Alternatively, secondary processes may play a role in producing the Fe enrichments observed in the presolar Silicate Grains identified in the matrix of Acfer 094.

  • The Use of Auger Spectroscopy for the in situ Elemental Characterization of Sub-micrometer Presolar Grains
    Meteoritics & Planetary Science, 2009
    Co-Authors: Frank J. Stadermann, Christine Floss, Maitrayee Bose, Alan S. Lea
    Abstract:

    Presolar Grains are small samples of stardust that can be found at low abundances in some of the most unaltered types of extraterrestrial materials. While earlier laboratory studies of stardust mainly focused on grain types that can be extracted from bulk meteorites by acid dissolution techniques, such as silicon carbide and graphite, recent analyses of presolar Silicates rely on isotope imaging searches for locating these Grains in situ. Since presolar Silicates are generally less than a micrometer in diameter and represent at best only a few hundred ppm of their host materials (e.g., primitive meteorites or interplanetary dust particles), locating and studying these particles can be analytically challenging. Recently, we began using scanning Auger spectroscopy for the in situ elemental characterization of presolar Silicate Grains as a complement to NanoSIMS isotopic studies for obtaining spatially matched compositional data. Auger spectroscopy is a well-established analytical technique for elemental characterizations in the material sciences, but has not been widely used in geological applications. We discuss the application of this technique to sub-micrometer sized Silicate Grains and address practical issues such as sample preparation, measurement settings, spatial resolution, data processing, and elemental quantification.

  • Identification of isotopically primitive interplanetary dust particles: A NanoSIMS isotopic imaging study
    Geochimica et Cosmochimica Acta, 2006
    Co-Authors: Christine Floss, Frank J. Stadermann, John P. Bradley, Zu Rong Dai, Saša Bajt, Giles A. Graham, A. Scott Lea
    Abstract:

    We have carried out a comprehensive survey of the isotopic compositions (H, B, C, N, O, and S) of a suite of interplanetary dust particles (IDPs), including both cluster and individual particles. Isotopic imaging with the NanoSIMS shows the presence of numerous discrete hotspots that are strongly enriched in 15 N, up to 1300&. A number of the IDPs also contain larger regions with more modest enrichments in 15 N, leading to average bulk N isotopic compositions that are 15 N-enriched in these IDPs. Although C isotopic compositions are normal in most of the IDPs, two 15 N-rich hotspots have correlated 13 C anomalies. CN/C ratios suggest that most of the 15 N-rich hotspots are associated with relatively N-poor carbonaceous matter, although specific carriers have not been determined. H isotopic distributions are similar to those of N: D anomalies are present both as distinct D-rich hotspots and as larger regions with more modest enrichments. Nevertheless, H and N isotopic anomalies are not directly correlated, consistent with results from previous studies. Oxygen isotopic imaging shows the presence of abundant presolar Silicate Grains in some of the IDPs. The O isotopic compositions of the Grains are similar to those of presolar oxide and Silicate Grains from primitive meteorites. Most of the Silicate Grains in the IDPs have isotopic ratios consistent with meteoritic Group 1 oxide Grains, indicating origins in oxygen-rich red giant and asymptotic giant branch stars, but several presolar Silicates exhibit the 17 O and 18 O enrichments of Group 4 oxide Grains, whose origin is less well understood. Based on their N isotopic compositions, the IDPs studied here can be divided into two groups. One group is characterized as being ‘‘isotopically primitive’’ and consists of those IDPs that have anomalous bulk N isotopic compositions. These particles typically also contain numerous 15 N-rich hotspots, occasional C isotopic anomalies, and abundant presolar Silicate Grains. In contrast, the other ‘‘isotopically normal’’ IDPs have normal bulk N isotopic compositions and, although some contain 15 N-rich hotspots, none exhibit C isotopic anomalies and none contain presolar Silicate or oxide Grains. Thus, isotopically interesting IDPs can be identified and selected on the basis of their bulk N isotopic compositions for further study. However, this distinction does not appear to extend to H isotopic compositions. Although both H and N anomalies are frequently attributed to the survival of molecular cloud material in IDPs and, thus, should be more common in IDPs with anomalous bulk N compositions, D anomalies are as common in normal IDPs as they are in those characterized as isotopically primitive, based on their N isotopes.

  • Samples of stars beyond the solar system: Silicate Grains in interplanetary dust.
    Science (New York N.Y.), 2003
    Co-Authors: Scott Messenger, Frank J. Stadermann, Lindsay P. Keller, Robert M. Walker, Ernst Zinner
    Abstract:

    We have identified six circumstellar Silicate Grains within interplanetary dust particles (IDPs). Their extrasolar origins are demonstrated by their extremely anomalous oxygen isotopic compositions. Three 17O-rich Grains appear to originate from red giant or asymptotic giant branch stars. One 16O-rich grain may be from a metal-poor star. Two 16O-poor Grains have unknown stellar sources. One of the Grains is forsterite, and two are amorphous Silicate “GEMS” (glass with embedded metal and sulfides), which is consistent with astronomical identifications of crystalline and amorphous Silicates in the outflows of evolved stars. These observations suggest cometary origins of these IDPs and underscore the perplexing absence of Silicates among circumstellar dust Grains from meteorites.

Th Henning - One of the best experts on this subject based on the ideXlab platform.

  • Temperature dependence of the submillimeter absorption coefficient of amorphous Silicate Grains
    The Astrophysical Journal, 2005
    Co-Authors: N. Boudet, Th Henning, Cornelia Jäger, Harald Mutschke, C. Nayral, J.-p. Bernard, C. Meny
    Abstract:

    We have measured mass absorption coefficients of amorphous Silicate materials for wavelengths between 100 μm and 2 mm (5-100 cm-1) and at temperatures between 300 and 10 K. For both interstellar analog MgSiO3 and simple silica SiO2, we find evidence for a strong temperature and frequency dependence. We define two distinct wavelength regimes, 500 μm-1 mm and 100-250 μm, for which the absorption coefficient presents different trends with frequency. To evaluate this frequency dependence, we fit our absorption coefficient using two power laws with spectral index β that varies with temperature. We do not find a significant variation of β with temperature between 100 and 250 μm, whereas between 500 μm and 1 mm a pronounced anticorrelation between T and β exists. Globally, β-values decrease from 2.5 to 1.5 between 10 and 300 K. This anticorrelation for interstellar analog Grains has the same trend as the one observed using the balloon-borne experiment PRONAOS. We show that physisorbed water is not responsible for the observed temperature and frequency dependence and that OH groups could be at the origin of the submillimeter properties of the materials. As discussed in the literature, OH groups are often related to tunneling processes in two-level systems (TLS). In the case of the more complex MgSiO3 Silicates, TLS could also be produced by the Mg+2 ions, which act as network modifiers, similar to how they act with OH groups.

  • The role of laboratory experiments in the characterisation of silicon-based cosmic material
    Astronomy and Astrophysics Review, 2003
    Co-Authors: L. Colangeli, Th Henning, John Robert Brucato, D. Fabian, Olivier Guillois, Friedrich Huisken, Elmar K. Jessberger, D. Cl�ment, C. J�ger, A. Jones
    Abstract:

    International audienceSilicate Grains in space have attracted recently a wide interest of astrophysicists due to the increasing amount and quality of observational data, especially thanks to the results obtained by the Infrared Space Observatory. The observations have shown that the presence of Silicates is ubiquitous in space and that their properties vary with environmental characteristics. Silicates, together with carbon, are the principal components of solid matter in space. Since their formation, Silicate Grains cross many environments characterised by different physical and chemical conditions which can induce changes to their nature. Moreover, the transformations experienced in the interplay of Silicate Grains and the medium where they are dipped, are part of a series of processes which are the subject of possible changes in the nature of the space environment itself. Then, chemical and physical changes of Silicate Grains during their life play a key role in the chemical evolution of the entire Galaxy. The knowledge of Silicate properties related to the conditions where they are found in space is strictly related to the study in the laboratory of the possible formation and transformation mechanisms they experience. The application of production and processing methods, capable to reproduce actual space conditions, together with the use of analytical techniques to investigate the nature of the material samples, form a subject of a complex laboratory experimental approach directed to the understanding of cosmic matter. The goal of the present paper is to review the experimental methods applied in various laboratories to the simulation and characterisation of cosmic Silicate analogues. The paper describes also laboratory studies of the chemical reactions undergone and induced by Silicate Grains. The comparison of available laboratory results with observational data shows the essential constraints imposed by astronomical observations and, at the same time, indicates the most puzzling problems that deserve particular attention for the future. The outstanding open problems are reported and discussed. The final purpose of this paper is to provide an overview of the present stage of knowledge about Silicates in space and to provide to the reader some indication of the future developments in the field

  • The role of laboratory experiments in the characterisation of silicon-based cosmic material
    Astronomy and Astrophysics Review, 2003
    Co-Authors: L. Colangeli, Th Henning, John Robert Brucato, D. Clément, D. Fabian, Olivier Guillois, Friedrich Huisken, Elmar K. Jessberger, C. Jäger, A. Jones
    Abstract:

    Silicate Grains in space have attracted recently a wide interest of astrophysicists due to the increasing amount and quality of observational data, especially thanks to the results obtained by the Infrared Space Observatory. The observations have shown that the presence of Silicates is ubiquitous in space and that their properties vary with environmental characteristics. Silicates, together with carbon, are the principal components of solid matter in space. Since their formation, Silicate Grains cross many environments characterised by different physical and chemical conditions which can induce changes to their nature. Moreover, the transformations experienced in the interplay of Silicate Grains and the medium where they are dipped, are part of a series of processes which are the subject of possible changes in the nature of the space environment itself. Then, chemical and physical changes of Silicate Grains during their life play a key role in the chemical evolution of the entire Galaxy. The knowledge of Silicate properties related to the conditions where they are found in space is strictly related to the study in the laboratory of the possible formation and transformation mechanisms they experience. The application of production and processing methods, capable to reproduce actual space conditions, together with the use of analytical techniques to investigate the nature of the material samples, form a subject of a complex laboratory experimental approach directed to the understanding of cosmic matter. The goal of the present paper is to review the experimental methods applied in various laboratories to the simulation and characterisation of cosmic Silicate analogues. The paper describes also laboratory studies of the chemical reactions undergone and induced by Silicate Grains. The comparison of available laboratory results with observational data shows the essential constraints imposed by astronomical observations and, at the same time, indicates the most puzzling problems that deserve particular attention for the future. The outstanding open problems are reported and discussed. The final purpose of this paper is to provide an overview of the present stage of knowledge about Silicates in space and to provide to the reader some indication of the future developments in the field.

  • The influence of ice-coated Grains on protostellar spectra.
    Astronomy and Astrophysics, 1993
    Co-Authors: Th. Preibisch, Volker Ossenkopf, Harold W. Yorke, Th Henning
    Abstract:

    Information about protostellar sources obtained by fit calculations contain a number of uncertainties due to our poor knowledge of the properties of dust Grains in dense clouds. From the results of recent observational and theoretical studies of dust in dense regions we construct a new dust model that consists of amorphous carbon Grains and ice-coated Silicate Grains. The model is based on new refractive indices of amorphous carbon and ice-coated Silicate Grains. Our model of the dirty ice assumes a mixture of H 2 O and NH 3 ice with inclusions of amorphous carbon. The optical constants are calculated using effective-medium theory. We determine the opacities of coremantle-particles with varying mantle thickness and pollution by spherical Mie calculations and investigate the effects on the spectrum of a embedded source