Large Interstellar Grains Research Articles

γ-ray lines from cosmic-ray interactions with interstellar dust grains

As pointed out by Lingenfelter and Ramaty [ApJ 211 (1977) L19], the shapes of some γ-ray lines produced by cosmic-ray interactions with the interstellar medium potentially contain valuable information on the physical properties of dust grains, including their compositions and size distributions. The most promising of such lines are at 847, 1369, 1779 and 6129 keV, from 56Fe*, 24Mg*, 28Si* and 16O*, respectively. We performed detailed calculations of their profiles using, in particular, available laboratory measurements combined with optical model calculations to evaluate the energy distributions of the recoiling excited nuclei. We show that the line shapes are mainly sensitive to relatively large interstellar grains, with radii ≳0.25 μm. Line fluxes from the inner Galaxy are then predicted.

A stream of particles from theβ Pictoris disc: A possible ejection mechanism

Recently, a stream of particles originating from the direction of β Pictoris, a young main sequence star surrounded by a dust disc, has been reported (Baggaley 2000). Standard mechanisms of particle ejection from a disc fail to reproduce the properties of this stream. We find that scattering by a giant proto-planet with properties taken from the literature is consistent with the observations. The fact that a straightforward ejection mechanism reproduces the data supports the identification of the particle stream's source with β Pic. Our work also indicates that protoplanetary dust discs form a potentially rich source of large interstellar grains, as widely detected in the Solar System.

X-Ray Halos and Large Grains in the Diffuse Interstellar Medium

Recent observations with dust detectors on board the interplanetary spacecrafts Ulysses and Galileo have recorded a substantial flux of large interstellar grains with radii between 0.25 and 2.0 μm entering the solar system from the local interstellar cloud. The most commonly used interstellar grain size distribution is characterized by an a-3.5 power law in grain radii a and extends to a maximum grain radius of 0.25 μm. The extension of the interstellar grain size distribution to such large radii will have a major effect on the median grain size and on the amount of mass needed to be tied up in dust for a given visual optical depth. It is therefore important to investigate whether this population of larger dust particles prevails in the general interstellar medium or whether it is merely a local phenomenon. The presence of large interstellar grains can be inferred mainly from their effect on the intensity and radial profile of scattering halos around X-ray sources. In this Letter, we examine the grain size distribution that gives rise to the X-ray halo around Nova Cygni 1992. The results of our study confirm the need to extend the interstellar grain size distribution in the direction of this source to and possibly beyond 2 μm. The model that gives the best fit to the halo data is characterized by (1) a grain size distribution that follows an a-3.5 power law up to 0.50 μm, followed by an a-4.0 extension from 0.50 to 2.0 μm, and (2) silicate and graphite (carbon) dust-to-gas mass ratios of 0.0044 and 0.0022, respectively, consistent with solar abundance constraints. Additional observations of X-ray halos probing other spatial directions are badly needed to test the general validity of this result.

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.

Interstellar grain size. II - Infrared photometry and polarization in Orion

view Abstract Citations (76) References (34) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Interstellar grain size. II. Infrared photometry and polarization in Orion. Breger, M. ; Gehrz, R. D. ; Hackwell, J. A. Abstract Infrared colors, polarization wavelength dependence and criteria for cluster membership are used to examine the interstellar extinction law in the Orion region, in light of 0.36-0.82 micron linear polarimetry and infrared photometry observations of stars within a degree of the Orion Nebula. The stars inside the dust clouds are examined on a star-by-star basis, and the different intracluster grain size indicators are found to be in excellent agreement. Normal radius and maximum wavelength values of about 3.0 and 0.55 microns are observed in stars lying outside the two main regions of nebulosity, indicating normal grain sizes. For a carefully selected group of stars inside the nebulosity that is characterized by extinction by large interstellar grains, the circumstellar emission does not significantly affect the values of R derived from the V-K and V-L colors. Publication: The Astrophysical Journal Pub Date: September 1981 DOI: 10.1086/159225 Bibcode: 1981ApJ...248..963B Keywords: Astronomical Photometry; Cosmic Dust; Infrared Astronomy; Infrared Photometry; Interstellar Extinction; Orion Nebula; Polarimetry; Astronomical Catalogs; Grains; Polarization Characteristics; Size (Dimensions); Astronomy full text sources ADS | data products SIMBAD (114)

Origin of large interstellar grains toward Rho Ophiuchi

The paper considers why the interstellar grains toward Rho Oph are unusually large. Because the visual opacity per gram is lower than average toward Rho Oph, it is concluded that the simplest interpretation of the observations is that grain coagulation has occurred. This result may be important for understanding the evolution of interstellar clouds and may provide insight into the initial stages of the formation of large solid bodies such as planets.