Observations and theories of interstellar dust

Abstract

Interstellar dust has a major effect on the Universe and our view of it by strongly affecting the physical and chemical processes within the interstellar medium. Its properties are best determined by the wavelength dependence of its extinction (scattering plus absorption), reflection from nearby stars, emissions (especially in certain near-infrared bands, along with continuum), and polarization of starlight by aligned grains. There are other clues as well, such as the depletion of certain elements from the gas phase onto the grains.The extinction varies strongly with direction, typically with the outer parts of molecular clouds having less extinction in the ultraviolet (relative to the visual) than the diffuse interstellar medium. There is a rather well defined one-parameter mean extinction law extending over the whole range of wavelengths for which observations are available.There are several rival theories regarding the nature of the grains and of their evolution. Most have a common base: that amorphoos silicate materials and hydrogenated carbon are almost surely important constituents of grains. Various ideas regarding the arrangements are reviewed.

Study of putative microfossils in space dust from the stratosphere

“…However, Hoyle & Wickramasinghe (1977) (a) (d) later proposed the alternative molecular explanation of the 217.5 nm absorption; they argued that the requirement of graphite, which has a planar lattice structure to be in the form of small spheres in interstellar medium (ISM), appears scarcely relevant, and instead suggested the involvement of aromatic organic molecules. The feature has since been found along every galactic line of sight (Bradley et al 1992;Draine & Malhotra 1993;Mathis 1993;Sandford 1996;Bradley et al 1999). Many astronomers attribute this feature to very small grains of carbon, possibly of aromatic and/or impure graphite (Draine & Malhotra 1993;Mathis 1993 ;Sandford 1996).…”

Early Solar Nebula Grains – Interplanetary Dust Particles

“…The size range of the grains inferred from extinction is between 0.1 and 0.5 mm (Kim et al, 1994). These grains are believed to have originally formed in the atmospheres and outflows of oxygenrich post-main-sequence AGB stars (Henning, 1999;Mathis, 1993). With the exception of carbon, most of the condensed rock-forming elements in ISM are associated with these silicate grains (Snow and Witt, 1996).…”

“…Alternatively, most GEMS may be isotopically normal (solar) because they formed in the solar system, although the chemical (elemental), mineralogical, and petrographic properties of anomalous GEMS are identical to those of the isotopically normal GEMS. Furthermore, if GEMS truly are interstellar amorphous silicates, it is not entirely clear that they should have nonsolar isotopic compositions, because grains undergo considerable processing during their 0.1-1 Ga lifetimes in the ISM such that the chemical and isotopic compositions of most grains are likely homogenized (Jones and Nuth, 2011;Mathis, 1993;Seab et al, 1987). This dilemma was best articulated by the late R.M.…”

“…The grains originally formed in the circumstellar outflows of other stars and in the interstellar medium (ISM) itself. They were modified to various degrees, ranging from negligible modification to complete destruction and reformation during their $100 Ma lifetimes in the ISM (Jones and Nuth, 2011;Mathis, 1993;Seab et al, 1987). Finally, they were incorporated into the solar system.…”

“…Finally, they were incorporated into the solar system. Submicrometer-sized silicates and carbonaceous material are the most common grains in the ISM (Mathis, 1993;Sandford, 1996), but it is not known to what extent these presolar grains survive in the solar system and whether they can be unambiguously distinguished from solar-system grains. Spatially resolved observations of (the surfaces of) accretion disks around some young stars indicate significant variation in the distribution and properties of grains as a function of heliocentric distance.…”