Diffuse interstellar bands in low E (B – V) objects

We analyse spectra of slightly reddened, early-type stars in which the major diffuse interstellar bands at 5870 and 5797 Å are either strong or weak in relation to E(B – V) . It is demonstrated that among the low E(B – V) objects one can find stars obscured by 'sigma' as well as 'zeta' type clouds. The profiles of the diffuse bands seem not to be dependent on the total opacities of the interstellar clouds. We also discuss the physical conditions leading to the formation of the diffuse band carriers in low-opacity interstellar clouds; the carriers are apparently formed in media in which one can observe complex velocity fields.     

Kinetic Monte Carlo method for simulating astrochemical kinetics: Hydrogen chemistry in diffuse clouds

We perform numerical simulations of the molecular hydrogen production on the surface of interstellar dust grains and its dissociation by the ultraviolet background in conditions typical for the interstellar medium. The kinetic version of the Monte Carlo method is used for the modeling of the catalytic chemical reactions on the surface of the dust fraction and in the surrounding medium. Our simulations show the importance of the interstellar dust particles for hydrogen chemistry in diffuse molecular clouds.     

Ultraviolet observations of interstellar molecules and grains fromSpacelab

Observational results obtained to date on interstellar grains and molecules are briefly reviewed, and several promising areas for further research withSpacelab are suggested. Regarding grains, useful data can be expected on the shape of the ultraviolet extinction curve for new interstellar regions; the nature of UV extinction at short wavelengths, down to the Lyman limit; the presence or absence of structure in the UV extinction curve comparable to the visible-wavelength diffuse bands; the scattering properties of grains in new kinds of clouds and nebulae; and the polarization properties of grains in UV wavelengths. The principal advances which may be expected in observations of molecules will include the ability to probe more heavily-obscured regions, where molecular species are more abundant than in the diffuse clouds observed to date; coverage of wavelength regions (such as 1400–3200) not well-studied with previous instruments such asCopernicus; and the capability of observing in optical absorption species detected in the same line of sight in radio emission, which provides unique information on cloud geometry and physical conditions.     

Primitive Grain Clumps and Organic Compounds in Carbonaceous Chondrites

Refractory interstellar grains acquire tarry polymeric coatings in dense protostellar molecular clouds. Collisions between polymer-coated grains lead to the formation of micron sized grain clumps that are subsequently expelled into the diffuse interstellar medium. Such grains could contain the building blocks of life such as amino acids in their interiors protected from dissociative ultraviolet radiation. This revised version was published online in July 2006 with corrections to the Cover Date.     

Correlation patterns between 11 diffuse interstellar bands and ultraviolet extinction

We relate the equivalent widths of 11 diffuse interstellar bands, measured in the spectra of 49 stars, to different colour excesses in the ultraviolet. We find that most of the observed bands correlate positively with the extinction in the neighbourhood of the 2175-Å  bump. Correlation with colour excesses in other parts of the extinction curve is more variable from one diffuse interstellar band to another; we find that some diffuse bands (5797, 5850 and 6376 Å) correlate positively with the overall slope of the extinction curve, while others (5780 and 6284 Å) exhibit negative correlation. We discuss the implications of these results on the links between the diffuse interstellar band carriers and the properties of the interstellar grains.     

Dust metamorphosis in the galaxy

Summary Cosmic dust grains play an important role for the thermal, dynamical, and chemical structure of the interstellar medium. This is especially true for the star formation process and the late stages of stellar evolution. Dust grains determine the spectral appearance of protostars, very young stellar objects with disk-like structures as well as of evolved stars with circumstellar envelopes.In this review, we will demonstrate that solid particles in interstellar space are both agent and subject of galactic evolution. We will especially discuss the different dust populations in circumstellar envelopes, the diffuse interstellar medium, and the molecular clouds with strong emphasis on the evolutionary aspects and the metamorphosis of these populations.     

On a vibronic origin for the diffuse band spectrum

Duley (1982) has proposed that many of the diffuse interstellar bands in the wavelength interval 677 nm>λ>542 nm arise from vibronic transitions of Cr³⁺ ions in MgO grains. No explanation has been offered for the fact that as many as 85 of the possible 108 transitions of this system have not been observed in the interstellar medium. Moreover, the relative intensities of the diffuse bands which are observed appear to be inconsistent with their assignment. We therefore conclude that this model is not consistent with the observations.     

ISO observations of 3–200 μm emission by three dust populations in an isolated local translucent cloud

We present isophot spectrophotometry of three positions within the isolated high-latitude cirrus cloud G 300.2−16.8, spanning from the near- to far-infrared (NIR to FIR). The positions exhibit contrasting emission spectrum contributions from the unidentified infrared bands (UIBs), very small grains (VSGs) and large classical grains, and both semi-empirical and numerical models are presented. At all three positions, the UIB spectrum shapes are found to be similar and the large grain emission may be fitted by an equilibrium temperature of  ∼17.5 K  . The energy requirements of both the observed emission spectrum and optical scattered light are shown to be satisfied by the incident local interstellar radiation field (ISRF). The FIR emissivity of dust in G 300.2−16.8 is found to be lower than in globules or dense clouds and is even lower than model predictions for dust in the diffuse interstellar medium (ISM). The results suggest physical differences in the ISM mixtures between positions within the cloud, possibly arising from grain coagulation processes.     

An Observer’s Overview of the Galactic Interstellar Medium

A number of problems related to understanding diffuse interstellar clouds are the focus of this contribution. Observations are discussed relating to the high velocity clouds, to the morphology of diffuse interstellar clouds and to the time variability of features in the diffuse clouds. Abundances and ionization issues are dealt with as needed. In some of these areas, progress may be substantial over the next few years, through observation and modeling, but in others, it may be that a new generation of space satellites will be required to make substantial progress.     

The identification of diffuse bands

Our present knowledge of the diffuse interstellar bands (two of which were first noted by Wilson in 1958) is briefly summarized. Other broad and very broad interstellar features (the 220nm extinction bump, the very broad structure in the extinction curve, the extended red emission in reflection and other nebulae, and the unidentified-sometimes called the “overidentified”-infrared bands) are also briefly described. The origins of all these features remain unknown, in spite of intensive study. Possible relations between these various features and families is briefly discussed. Recent observations are shown to support the hypothesis that the carriers of the diffuse bands are free-flying molecules, whereas the alternative hypothesis of dust grains now seems to be untenable. Candidate types of molecular carriers are mentioned, and the possible source of such molecules in the diffuse interstellar medium is briefly discussed.     

Emission lines due to interstellar dust in the visible spectra of nebulae

It is suggested that certain sharp lines in the visible and near ultraviolet spectra of nebule may be due to fluorescence from grains. Evidence is presented that some of these lines could arise from MgO or CaO grains. Some sharp diffuse interstellar bands may also appear in the spectra of nebulae.     

Polyatomic molecules in diffuse clouds

The consequences on the chemistry in diffuse clouds of the injection of small hydrocarbon molecules is explored. It is assumed that the injection arises from the erosion of carbonaceous grains at cloud boundaries, and that the injected species take part in conventional interstellar chemistry. The models indicate that for plausible injection rates the daughter species of the injected parent molecules should appear in significant amounts, at least during the injection period, and that daughter species can appear in gas that is otherwise almost entirely atomic. Therefore, injection of hydrocarbons is a mechanism that is capable of providing polyatomic molecules in detectable amounts in diffuse atomic regions of the interstellar medium. The implications of this result for the carriers of the unidentified diffuse interstellar bands are briefly described.     

Zur zeitlichen Variation des Metallgehaltes in der Galaxis

The two oldest known open clusters, NGC 188 and M67, are observed to have a higher heavy-element abundance than the sun and the stars in the Hyades. This observation might be explained by assuming that these clusters were formed from unusually dusty and hence metal-rich interstellar clouds. Alternatively it may be supposed that the radiation pressure produced by stars in the spiral arms of the Galaxy ejected dust from high-latitude clouds. The calculations presented in this paper show that the loss of dust from such clouds might just be sufficient to produce a significant decrease in the mean heavy-element abundance of the interstellar gas. According to this picture, the first burst of star formation in the Galaxy led to a rapid increase in the interstellar heavy-element abundance. Subsequently, the metal abundance of the interstellar gas decreased due to the radiation pressure by young stars. The present rate of change of the heavy-element abundance in the Galaxy depends on the ratio of heavy-element production by stars to ejection of these elements by radiation pressure on dust grains. Since noble gases do not condense on grains, the neon abundance in the interstellar gas should be a monotonously increasing function of time. The observation that the neon abundance in the sun is much lower than that in young stars and nebulae lends some support to the suggestion that ejection of grains from the Galaxy effects the heavy-element abundance in the interstellar gas.     

Growth of interstellar grains by coagulation

A discussion is given of the spontaneous coagulation of interstellar grains due to their natural Brownian motion. It is shown that the predicted present day size of such grains, assuming such a mechanism being operative, is in reasonable agreement with observations for three different environments that have been considered: namely, normal interstellar gas clouds, molecular clouds and the atmospheres of cool stars.     

The inventory of interstellar materials available for the formation of the solar system

Tremendous progress has been made in the field of interstellar dust in recent years through the use of telescopic observations, theoretical studies, laboratory studies of analogs, and the study of actual interstellar samples found in meteorites. It is increasingly clear that the interstellar medium (ISM) contains an enormous diversity of materials created by a wide range of chemical and physical processes. This understanding is a far cry from the picture of interstellar materials held as recently as two decades ago, a picture which incorporated only a few generic types of grains and few molecules. In this paper, I attempt to review some of our current knowledge of the more abundant materials thought to exist in the ISM. The review concentrates on matter in interstellar dense molecular clouds since it is the materials in these environments from which new stars and planetary systems are formed. However, some discussion is reserved for materials in circumstellar environments and in the diffuse ISM. The paper also focuses largely on solid materials as opposed to gases since solids contain a major fraction of the heavier elements in clouds and because solids are most likely to survive incorporation into new planetary systems in identifiable form. The paper concludes with a discussion of some of the implications resulting from the recent growth of our knowledge about interstellar materials and also considers a number of areas in which future work might be expected to yield important results.     

Cosmic Ray-Induced Polycondensate Hydrocarbons in Giant Molecular Clouds

Astrophysical and cosmochemical data show that many kinds of hydrocarbons are widespread in space, including giant molecular clouds, diffuse interstellar medium, comets, interplanetary dust particles, and carbonaceous meteorites. Here an effort is made to show the close relation between high-molecular weight hydrocarbons observed in space and existing on Earth. Results of astrochemical modelling of dust grains in dense collapsing cores of giant molecular clouds are also presented. They show that about 10% of the total abundance of dust grains may be the result of aliphatic hydrocarbons. This dust serves as initial material for comets, formed in protosolar nebula. The problem of survival of cometary organics during impact onto the Earth is discussed, and it is shown that the so-called soft-landing comet hypothesis may explain the accumulation of complex hydrocarbons on the Earth's surface. We conclude that a significant fraction of terrestrial prebiotic petroleum was delivered by extraterrestrial matter.     

Large interstellar molecules

We propose that photocycloaddition reactions in molecular complexes in normal interstellar clouds will create unusually large molecules. These may be sufficiently radiation stable to be circulated with the interstellar gas, and so provide convenient nucleation centres for growth of loosely bound grains in dark regions.     

Interstellar Dust in the Solar System

We deduce the mass distribution and total mass density of interstellar dust streaming into the solar system and compare the results to the conditions of the very local interstellar medium (VLISM). The mass distribution derived from in situ measurements shows a gentler slope and includes larger grains, compared to a model distribution proposed for the wavelength dependence of the interstellar extinction. The mass density of grains in the solar system is consistent with that expected from measurements of the visible interstellar extinction and the abundance constraints of elements in the diffuse interstellar medium (ISM), instead of those in the VLISM. This may imply that interstellar dust grains are not associated with the VLISM and that the conditions of the grains are better represented by the ones expected in the diffuse ISM. If this is the case, then the flatter slope in the mass distribution and the detection of larger interstellar grains in the solar system may even indicate that coagulation growth of dust in the diffuse ISM is more effective than previously inferred. This revised version was published online in July 2006 with corrections to the Cover Date.     

Aromatic Hydrocarbons in Very Small Interstellar Grains

We show that the recently observed 3.3 μm emission feature in the diffuse radiation from the galactic disk might be due to an ensemble of aromatic molecules distributed within very small interstellar grains. The same particles also provide an explanation of the λ2200 ? interstellar absorption feature. This revised version was published online in July 2006 with corrections to the Cover Date.     

Three components of 3–4 μm absorption bands

We present 2–4 μm spectra of six infrared sources for which the extinction is not purely interstellar, but is dominated by circumstellar or molecular cloud dust. In all cases the absorption bands differ from the interstellar case, though a component of the interstellar absorption is often present. We also present an improved absorption spectrum for the galactic centre source IRS 7, correcting spurious features in our previous spectrum. Three independent components can be identified: (i) The interstellar component, probably of organic origin, and itself not necessarily invariant; (ii) The symmetrical water ice feature at 3.06 μm, found most commonly in molecular clouds; (iii) A component at 3.53 μm possibly identified with solid formaldehyde grains, and seen only in molecular clouds because of its weakness. Other absorption components appear to be unrelated to those in the 3–4 μm region, most notably the 10 μm absorption found in oxygen-rich giants and the interstellar medium, and presumable inorganic in nature. Our observations include the first detection of water ice absorption in a source in the ρ Oph dark cloud. Biological materials provide the best fit to the interstellar case, but do not presently account for the distinct 3.53 μm component. We stress the need for further laboratory experiments using simpler organic materials.