Dust Spectrum Research Articles

Near-IR spectroscopic monitoring of WR 140 (WC7pd+O4-5) during the 2001 periastron passage

The near-IR spectrum of the periodic dust making WCpd+O4-5 binary WR 140 was monitored to cover the 2001 periastron passage and maximum colliding-wind activity. The He i λ1.083μm emission-line profile showed the appearence of a subpeak on epochs close to periastron passage. The evolution of the subpeak was consistent with the motion of the stars and the colliding wind region. The appearance and evolution of the emission subpeak suggests that the theoretical 1/r dependence of X-ray flux does not hold, so that there is no need to change the values of eccentricity and epoch of periastron passage derived from the RV orbit. JHK spectra show variations of the continuum and and dilution of the emission lines, in agreement with the production and cooling of dust.

The Zodiacal Emission Spectrum as Determined byCOBEand Its Implications

We combine observations from the DIRBE and FIRAS instruments on the COBE satellite to derive an annually averaged spectrum of the zodiacal cloud in the 10-1000 μm wavelength region. The spectrum exhibits a break at ~150 μm that indicates a sharp break in the dust size distribution at a radius of about 30 μm. The spectrum can be fitted with a single blackbody with a λ-2 emissivity law beyond 150 μm and a temperature of 240 K. We also used a more realistic characterization of the cloud to fit the spectrum, including a distribution of dust temperatures representing different dust compositions and distances from the Sun, as well as a realistic representation of the spatial distribution of the dust. We show that amorphous carbon and silicate dust with respective temperatures of 280 and 274 K at 1 AU, and size distributions with a break at grain radii of 14 and 32 μm, can provide a good fit to the average zodiacal dust spectrum. The total mass of the zodiacal cloud is 2-11 Eg (Eg = 1018 g), depending on the grain composition. The lifetime of the cloud, against particle loss by Poynting-Robertson drag and the effects of solar wind, is about 105 yr. The required replenishment rate is ~1014 g yr-1. If this is provided by the asteroid belt alone, the asteroids lifetime would be ~3 × 1010 yr. But comets and Kuiper belt objects may also contribute to the zodiacal cloud.

C–H Bond Formation in Carbon Grains by Exposure to Atomic Hydrogen: The Evolution of the Carrier of the Interstellar 3.4 Micron Band

We present the results of a systematic study on the interaction of nano-sized carbon grains with atomic hydrogen. The effects of H processing have been analyzed by infrared spectroscopy. The samples were irradiated with fluences ranging between 9.2 × 1016 and 1.3 × 1020 H atoms cm-2. Hydrogen atoms lead to the activation of the aliphatic C–H stretching and bending modes, whose intensity increases with exposure until saturation of hydrogenation. The plateau value of the absorption coefficient per unit mass of material at 3.4 μm is 1.6 × 103 cm2 g-1. The estimated cross section of C–H bond formation by H atoms for carbon particles is σf = (1.9 ± 0.5) × 10-18 cm2 per H atom, as derived from the behavior of the 3.4 μm band intensity as a function of the H atom fluence. We have found that the C–H bond formation depends on the structure of the carbon material that is exposed to atomic hydrogen. In view of the basic role of the hydrogenation of carbon particles by H atoms in interpreting the presence of the 3.4 μm band in the diffuse interstellar medium, the behavior of carbon materials under H processing becomes a fundamental criterion for constraining their applicability as analogs of the interstellar aliphatic component. The 3.4 μm band and the doublet at 6.86 and 7.26 μm of carbon particles processed by H atoms reproduce the peak positions and the relative intensities of those observed in the spectrum of interstellar dust toward the Galactic center. The estimation of the formation cross section of C–H bonds by H atoms, together with the previously determined destruction cross section by UV photons, allows a complete description of the evolution of the interstellar aliphatic carbon component due to UV and H processing. The conclusion of our analysis is that the interstellar component (i.e., the C–H bonds in the CH2 and CH3 groups) responsible for the 3.4 μm stretching band and the associated bending features at 6.85 and 7.25 μm is formed in the diffuse medium, since the carrier readily loses memory of its birthsite, wherever it is, because of interstellar processing, which determines a new equilibrium value for its degree of hydrogenation.

Dust and the spectral energy distribution of the OH/IR star OH 127.8+0.0: Evidence for circumstellar metallic iron

We present a t to the spectral energy distribution of OH 127.8+0.0, a typical asymptotic giant branch star with an optically thick circumstellar dust shell. The t to the dust spectrum is achieved using non-spherical grains consisting of metallic iron, amorphous and crystalline silicates and water ice. Previous similar attempts have not resulted in a satisfactory t to the observed spectral energy distributions, mainly because of an apparent lack of opacity in the 3{8 m region of the spectrum. Non-spherical metallic iron grains provide an identication for the missing source of opacity in the near-infrared. Using the derived dust composition, we have calculated spectra for a range of mass-loss rates in order to perform a consistency check by comparison with other evolved stars. The L (12 m) colours of these models correctly predict the mass-loss rate of a sample of AGB stars, strengthening our conclusion that the metallic iron grains dominate the near-infrared flux. We discuss a formation mechanism for non-spherical metallic iron grains.

Remote sounding of the Martian dust from ISO spectroscopy in the [formula omitted] bands

This paper presents a study of Martian aerosols based on the spectral observations near 2.7 μm . Simulations by Titov et al. [Planet. Space Sci. 48 (2000) 67] showed that the depth and the shape of saturated 2.7 μm band of atmospheric CO 2 are sensitive to the presence of aerosols. This allows one to determine the aerosol optical depth, the scale height, and optical properties from the spectral measurements in the band. For the first time this method is applied to two disk-averaged spectra of Mars recorded by the Infrared Space Observatory (ISO). The analysis implies existence of sharp absorption band in the dust spectrum between 2.7 and 2.8 μm that probably indicates the presence of phyllosilicates (for instance, montmorillonite) in the dust. The best fit to the ISO spectra is obtained in the model with column optical depth of 0.33±0.13. This value is in good agreement with simultaneous Mars Pathfinder and Hubble Space Telescope observations.

Geochemical and mineralogical analyses of palagonitic tuffs and altered rinds of pillow basalts in Iceland and applications to Mars

Abstract Samples of altered pillow basalts and hyalotuffs were collected from a volcanic tuya and hyaloclastite ridge in western Iceland. Altered basaltic material from regions such as Hlöðufell tuya and Thórólfsfell ridge may be similar to the altered basaltic surface fines on Mars. Geochemical and mineralogical analyses have been performed on the Icelandic samples in order to characterize the properties that distinguish palagonitization from other forms of low temperature alteration in this environment. Major elements were measured using an electron microprobe and mienralogy was determined through X-ray diffraction and visible-infrared reflectance spectroscopy. The primary focus in this study was on the <2 μm size fractions of the Hlöðufell altered pillow basalt and Thórólfsfell palagonitic tuff samples. Both volcanic alteration products contain at least some smectite and serpentine clay minerals, as well as poorly crystalline layer silicates. The palagonitic tuff contains more crystalline clay minerals, fewer nanophase iron oxides/oxyhydroxides, and has a higher A1/Fe ratio compared to the altered pillow basalt. Spectra of the <2 μm fractions of both Icelandic samples share similarities with the extended visible region spectra of the bright martian soils measured by Pathfinder and the infrared spectra of the martian dust measured by spectrometers on the Mariner missions.

Hydrogenation of carbon grains by exposure to hydrogen atoms: Implications for the 3.4 μm interstellar absorption band

The 3.4 μm absorption band provides direct evidence for the presence of organic compounds in the diffuse interstellar medium. There is, however, a distinct absence of the band in the spectra of molecular cloud dust. The difference between diffuse and dense environments represents a strong constraint for any description of the formation and evolution of the band carrier material. Laboratory research under simulated interstellar medium conditions is of crucial interest for improving our understanding of the nature of this interstellar dust component. In fact, the composition of carbonaceous materials in the interstellar medium can vary in response to several evolutionary factors. Irradiation from UV photons and cosmic rays, as well as chemical interactions with the interstellar gas, can drive grain transformation. Here we discuss the results of experiments aimed at studying the interaction of nano-sized carbon grains with atomic hydrogen. H atoms induce the formation of CH bonds in carbon grains. After hydrogen exposure, the infrared spectrum of carbon grains is similar to that of the aliphatic component observed toward the Galactic center. The efficiency of the hydrogenation process and the amount of carbon which is necessary to reproduce the intensity of the interstellar feature are reported. Finally, implications for the formation and evolution of the organic materials responsible for the 3.4 μm band are discussed.

Light Curves and Spectra of Dust Echoes from Gamma‐Ray Bursts and Their Afterglows: Continued Evidence That GRB 970228 Is Associated with a Supernova

The relative brightening and reddening of the optical afterglows of GRB 970228 and GRB 980326 around 20-30 days after these bursts have been attributed to supernovae, which are red (blueward of their spectral peak) and which peak in time after ≈20(1 + z) days. However, this direct evidence for a gamma-ray burst/supernova (GRB/SN) connection has recently been challenged. It has been suggested that the late afterglows of these bursts can be explained by dust echoes, of which we consider two cases: (1) the scattering of light from the afterglow (the forward shock), the optical flash (the reverse shock), and/or the optical light from the burst itself by a shell of dust at a radius R from the progenitor and (2) the absorption and thermal reemission of this light by this shell of dust. In this paper, we model and compute dust echo light curves and spectra for both cases. Although the late-time afterglow of GRB 980326 was not sufficiently well sampled to rule out a dust echo description, we find that the late-time afterglow of GRB 970228 cannot be explained by a dust echo.

Calculations of "blue-body" spectra for cosmic dust emission

We derive useful relations for "blue-body" thermal dust spectra that are analogous to the Wien displacement laws for blackbody radiation. PACS No.: 94.10Nh

Light curves and spectra of dust echoes from gamma-ray bursts and their afterglows: Continued evidence that GRB 970228 is associated with a supernova

Light curves and spectra of dust echoes from gamma-ray bursts and their afterglows: Continued evidence that GRB 970228 is associated with a supernova

Analysis of the magnetic properties experiment data on Mars: results from Mars Pathfinder

The paper presents the results of a new analysis of the data from the Magnetic Properties Experiment onboard the Mars Pathfinder Spacecraft. The spectral data from the extended visible region of material adhering to the magnets are presented as well as the time dependence of dust accumulation to the magnets. A new model is developed to estimate the magnetic properties of the accumulated dust. It is found that the optical reflection spectra of the magnetic dust differ slightly from the reflection of ground soil/dust in the vicinity of the lander. Several possible hypotheses are given to account for this difference. Magnetic dust has been removed from the magnets at least twice during the 80 sol mission by wind gusts, and it is suggested that this happens in a manner similar to sand blasting. A lower limit for the saturation magnetisation of the most magnetic particles is found to be ∼5 A m 2/ kg .

The DIRTY Model. II. Self‐consistent Treatment of Dust Heating and Emission in a Three‐dimensional Radiative Transfer Code

In this paper and a companion paper we present the DIRTY model, a Monte Carlo radiative transfer code, self-consistently including dust heating and emission and accounting for the effects of the transient heating of small grains. The code is completely general: the density structure of the dust, the number and type of heating sources, and their geometric configurations can be specified arbitrarily within the model space. Source photons are tracked through the scattering and absorbing medium using Monte Carlo techniques, and the effects of multiple scattering are included. The dust-scattering, -absorbing, and -emitting properties are calculated from realistic dust models derived by fitting observed extinction curves in Local Group galaxies, including the Magellanic Clouds and the Milky Way. The dust temperature and the emitted dust spectrum are calculated self-consistently from the absorbed energy, including the effects of temperature fluctuations in small grains. Dust self-absorption is also accounted for, allowing the treatment of high optical depths, by treating photons emitted by the dust as an additional heating source and adopting an iterative radiative transfer scheme. As an illustrative case, we apply the DIRTY radiative transfer code to starburst galaxies wherein the heating sources are derived from stellar evolutionary synthesis models. Within the context of the starburst model, we examine the dependence of the ultraviolet to far-infrared spectral energy distribution, dust temperatures, and dust masses predicted by DIRTY on variations of the input parameters.

Gas and Dust in NGC 7469: Submillimeter Imaging and COJ = 3–2

We present sensitive submillimeter imaging of the Seyfert 1 galaxy NGC 7469 at 850 and 450 μm with the Submillimeter Common User Bolometer Array on the James Clerk Maxwell Telescope and 12CO J = 3-2 line observations of its central starbursting region. The global dust spectrum, as constrained by the new set of submillimeter data and available 1.30 mm and IRAS 100 and 60 μm data, reveals a dominant warm dust component with a temperature of Td ~ 35 K and a global molecular gas-to-dust ratio M(H2)/Md ~ 600. Including the atomic gas component yields a total gas-to-dust ratio of ~830. Such high values are typical for IR-bright spiral galaxies, and in order to reconcile them with the significantly lower ratio of ~100 obtained for the Milky Way, a cold dust reservoir, inconspicuous at far-infrared wavelengths, is usually postulated. However, while there is good evidence for the presence of cold gas/dust in NGC 7469 beyond its central region, our 450 μm map and available interferometric 12CO J = 1-0 maps show the bright submillimeter/CO emission confined in the inner ~2.5 kpc, where a high 12CO (J = 3-2)/(J = 1-0) ratio (~0.85-1.0) is measured. This is consistent with molecular gas at Tkin 30 K, suggesting that the bulk of the interstellar medium in the starburst center of NGC 7469 is warm. Nevertheless, the corresponding total gas-to-dust ratio there remains high, of the order of ~500. We argue that, rather than unaccounted cold dust mass, this high ratio suggests an overestimate of M(H2) from its associated 12CO J = 1-0 line luminosity by a factor of ~5 when a Milky Way value for this conversion is used. Finally, the diffuse cold gas and dust that is the likely source of the observed faint extended 450 μm and 12CO J = 1-0 emission has an estimated total gas-to-dust ratio of ~50-160, closer to the Galactic value.

Evolving Optical Properties of Annealing Silicate Grains: From Amorphous Condensate to Crystalline Mineral

Laboratory studies of the evolution of a magnesium silicate smoke from an amorphous condensate to a crystalline mineral by annealing in vacuum provide a foundation for the development of a silicate evolution index (SEI). The SEI can be used to predict the emergent IR spectrum of silicate dust in a circumstellar shell based on the time-temperature history of the silicate grains in the out—ow. Optical constants for the magnesium silicate smoke samples compatible with those of Draine & Lee are derived over the range 0.001¨1000 km. Subject headings: circumstellar matterdust, extinctionISM: moleculesmethods: laboratory ¨ molecular processes

VLA Imaging of the Disk Surrounding the Nearby Young Star TW Hydrae.

The TW Hydrae system is perhaps the closest analog to the early solar nebula. We have used the Very Large Array to image TW Hya at wavelengths of 7 mm and 3.6 cm with resolutions of 0&farcs;1 ( approximately 5 AU) and 1&farcs;0 ( approximately 50 AU), respectively. The 7 mm emission is extended and appears dominated by a dusty disk of radius greater than 50 AU surrounding the star. The 3.6 cm emission is unresolved and likely arises from an ionized wind or gyrosynchrotron activity. The dust spectrum and spatially resolved 7 mm images of the TW Hya disk are fitted by a simple model with temperature and surface density described by radial power laws, T&parl0;r&parr0;~r-0.5 and Sigma&parl0;r&parr0;~r-1. These properties are consistent with an irradiated gaseous accretion disk of mass approximately 0.03 M middle dot in circle with an accretion rate approximately 10-8 M middle dot in circle yr-1 and viscosity parameter alpha=0.01. The estimates of mass and mass accretion rates are uncertain since the gas-to-dust ratio in the TW Hya disk may have evolved from the standard interstellar value.

ISO observations of interstellar ices: Implications for the pristinity of comets

A significant part of the ISO-SWS (Short Wavelength Spectrometer) guaranteed time is devoted to measure infrared spectra toward protostars, in order to probe interstellar ices and molecules. ISO (Infrared Space Observatory) allows to study for the first time the complete inventory of interstellar ices over the entire spectrum between 2.5–45 μm. First ISO data showed the ubiquitous presence of ices in dense clouds which were never seen before, such as CO 2 and CH 4 ice and provided new insights into the gas to solid ratio of many interstellar molecules. ISO spectra of interstellar gas and dust in comparison with laboratory results and theoretical models constrain gas/grain interactions, temperature and irradiation conditions towards star-forming regions, and consequently their evolution. Well established abundances of interstellar molecules also guide the interpretation of past and future cometary observations. Current ISO results indicate that cometary ices are a mixture of original interstellar material and material that has been processed to some extent in the solar nebula.

A Spectroscopic Study of Dust around 18 Oxygen‐rich Mira Variables in theNBand. I. Dust Profiles

We have obtained N-band spectra for 18 M Mira variables with the Mid-Infrared Camera and Spectrometer (MICS) to investigate the dust materials formed around M Mira variables. The observed spectra show a wide variety; 15 of them show a strong feature at 10 μm, and eight and seven stars show 11 and 13 μm bumps, respectively. We have made an analysis of oxygen-rich dust spectra by using two representative spectra empirically derived. One is the 10 μm feature, and the other is the 11 μm feature. Linear combinations of the two dust spectra together with the stellar continuum can describe all the observed spectra quite successfully, indicating the presence of two kinds of dust materials around M Mira variables. The 10 μm feature is ascribed to silicate grains, while we propose to attribute the 11 μm feature to amorphous alumina grains. We found that crystalline silicates do not appear necessary to fit any component of the spectrum. We have compared the strengths of the dust emissions with the parameters of stellar variability. We found that the strength of the silicate emission increases as the light curve becomes asymmetric, while that of the alumina emission stays constant against the change of the asymmetry factor. This can be interpreted in terms of the differences in the dust-forming region.

Dust in the Local Interstellar Wind

The gas-to-dust mass ratios found for interstellar dust within the solar system, versus values determined astronomically for the cloud around the solar system, suggest that large and small interstellar grains have separate histories and that large interstellar grains preferentially detected by spacecraft are not formed exclusively by mass exchange with nearby interstellar gas. Observations by the Ulysses and Galileo satellites of the mass spectrum and flux rate of interstellar dust within the heliosphere are combined with information about the density, composition, and relative flow speed and direction of interstellar gas in the cloud surrounding the solar system to derive an in situ value for the gas-to-dust mass ratio, Rg/d = 94. This ratio is dominated by the larger near-micron-sized grains. Including an estimate for the mass of smaller grains, which do not penetrate the heliosphere owing to charged grain interactions with heliosheath and solar wind plasmas, and including estimates for the mass of the larger population of interstellar micrometeorites, the total gas-to-dust mass ratio in the cloud surrounding the solar system is half this value. Based on in situ data, interstellar dust grains in the 10-14 to 10-13 g mass range are underabundant in the solar system, compared to a Mathis, Rumple, & Nordsiek mass distribution scaled to the local interstellar gas density, because such small grains do not penetrate the heliosphere. The gas-to-dust mass ratios are also derived by combining spectroscopic observations of the gas-phase abundances in the nearest interstellar clouds. Measurements of interstellar absorption lines formed in the cloud around the solar system, as seen in the direction of CMa, give Rg/d = 427 for assumed solar reference abundances and Rg/d = 551 for assumed B star reference abundances. These values exceed the in situ value suggesting either that grain mixing or grain histories are not correctly understood or that sweptup stardust is present. Such high values for diffuse interstellar clouds are strongly supported by diffuse cloud data seen toward λ Sco and 23 Ori, provided B star reference abundances apply. If solar reference abundances prevail, however, the surrounding cloud is seen to have greater than normal dust destruction compared to higher column density diffuse clouds. The cloud surrounding the solar system exhibits enhanced gas-phase abundances of refractory elements such as Fe+ and Mg+, indicating the destruction of dust grains by shock fronts. The good correlation locally between Fe+ and Mg+ indicates that the gas-phase abundances of these elements are dominated by grain destruction, while the poor correlation between Fe+ and H0 indicates either variable gas ionization or the decoupling of neutral gas and dust over parsec scale lengths. These abundances, combined with grain destruction models, indicate that the nearest interstellar material has been shocked with shocks of velocity ~150 km s-1. If solar reference abundances are correct, the low Rg/d value toward λ Sco may indicate that at least one cloud component in this direction contains dust grains that have retained their silicate mantles and are responsible for the polarization of the light from nearby stars seen in this general region. Weak frictional coupling between gas and dust in nearby low density gas permit inhomogeneities to be present.

The Radio-to-Submillimeter Spectral Index as a Redshift Indicator

We present models of the 1.4 to 350 GHz spectral index α for starburst galaxies as a function of redshift. The models include a semianalytic formulation, based on the well-quantified radio-to-far-infrared correlation for low-redshift star-forming galaxies, and an empirical formulation, based on the observed spectrum of the starburst galaxies M82 and Arp 220. We compare the models to the observed values of α for starburst galaxies at low and high redshift. We find reasonable agreement between the models and the observations and, in particular, that an observed spectral index of α≥+0.5 indicates that the target source is likely to be at high redshift, z≥1. The evolution of α with redshift is mainly due to the very steep rise in the Rayleigh-Jeans portion of the thermal dust spectrum shifting into the 350 GHz band with increasing redshift. We also discuss situations in which this relationship could be violated. We then apply our models to examine the putative identifications of submillimeter sources in the Hubble Deep Field and conclude that the submillimeter sources reported by Hughes et al. are likely to be at high redshifts, z≥1.5.

Diatoms on Earth, Comets, Europa and in Interstellar Space

There exists a close correspondence between the measured infrared properties of diatoms and the infrared spectrum of interstellar dust as observed in the Trapezium nebula and toward the galactic center source GC-IRS 7. Diatoms and bacteria also exhibit an absorbance peak near 2200 A, which is found to agree with the observed ultraviolet absorbance properties of interstellar grains. We review the observational data and consider the known properties of diatoms and bacteria. It is suggested that these characteristics are consistent with the concept of acosmic microbiological system in which these or similar microorganisms might exist on comets, Europa and in interstellar space.