Molecular collisions in circumstellar discs

Inelastic molecular collisions are incorporated into the statistical model for turbulent circumstellar discs (Hämeen-Anttila, 1991; Verronenet al., 1993), and are found to provide a more effective cooling mechanism than molecule-grain impacts. Examination of the viscous evolution of the disc shows that the chemical composition and the coefficient of viscosity are crucial for determining the existence and properties of a state of equilibrium. A detailed study is carried out for carbon monoxide and hydroxyl. Abundances as small as 10^–5 for CO and 10^–8 for OH are sufficient to balance the disc against the viscous production of heat. The disc is usually non-convective. Near the equilibrium state the dust layer becomes thin enough to be gravitationally unstable. Infall of interstellar material is not expected to change the occurrence of instability, since the disc was not found to be convective even in the homogeneous circumstances. The results also remain unaltered even if the abundances of coolants are reduced by several orders of magnitude.     

Interstellar graphite grain temperature and infrared optical constants

The temperature of graphite grains in the interstellar medium has been computed with different assumptions on the extrapolation of its optical constants into infrared. It is found that values computed up to now are generally underestimated by 10–20% in normal interstellar conditions. For extreme conditions (very dark clouds) errors by a factor of two are possible and more attention has to be paid in dealing with problems related to molecule formation.     

Interstellar and circumstellar medium in the direction to IR source IRAS 01005+7910

Using the high-resolution spectra obtained at the 6-meter telescope of the SAO RAS over 2002–2013, we studied the spectral features of the lines of interstellar medium. The radial velocities of the Na I 5890 Å, Na I 5896 Å, Ca II 3934 Å and Ca II 3968 Å absorption lines were analyzed. Seven diffuse interstellar bands 4964, 5780, 5797, 6196, 6203, 6379 Å were identified in the optical spectrum of IRAS01005+7910. Radial velocities V_r and equivalent widths W_λ of these DIBs were measured, for which the values of the interstellar reddening E_B?V and column density of neutral hydrogen log [N(H)] were calculated.     

Interstellar extinction by cometary organic grain clumps

The observed features of the interstellar extinction curve in both the Galaxy and in the external galaxies LMC and SMC are explained on the basis of a model involving clumps of hollow or porous organic particles with an overall volume filling factor of 0.1. The hollow organic particles have a ready explanation in terms of the bacterial grain model. The visual and near infrared observations of extinction are fitted by a size distribution of such aggregates similar to the distribution law discovered for the larger grains in Halley's comet, but with a cutoff at low radii taken at 0.5 m for the Galaxy and 0.3 and 0.4 , respectively, for SMC and LMC. Fine tuning of the theoretical extinction curve to fit observational data in the ultraviolet involves variable contributions from small condensed polymeric units of typical radius 0.012 m and graphite particles of radii 0.02 . These particle species may be regarded as being derived from the primary distribution of clumps. The implication is that cometary sources could provide a major component of the grains in interstellar space.     

Interstellar molecule formation in grain mantles: The laboratory analog experiments,results and implications

Laboratory and theoretical studies have been made of the effects of ultraviolet photolysis of interstellar grain mantles which consist of combinations of hydrogen, oxygen, carbon and nitrogen — dirty ice. It is shown that processes involving photolysis (photoprocessing) of interstellar grains are important during most of their lifetime even including the time they spend in dense clouds. A laboratory designed to simulate the interstellar conditions is described. This is the first time such a laboratory has been able to provide results which may be directly scaled to the astrophysical situations involving interstellar grains and their environment. The evolution of grain analogs is followed by observing the infrared absorption spectra of photolyzed samples of ices deposited at 10 K. The creation and storage of radicals and the production of molecules occur as a result of reactions within the solid. A large number of molecules and radicals observed in the interstellar gas appear in the irradiated ices. Energy released during warm-up is seen from visible luminescence and inferred from vapor pressure enhancement which occurs during warming of photolyzed samples relative to unphotolyzed samples. The evolution of a grain and its role as a source as well as a sink of molecules is pictured as a statistical process within dense clouds. The gradual accretion on and photolysis of an individual grain provides the stored chemical energy the release of which is sporadically triggered by relatively mild events (such as low velocity grain-grain collisions) to produce the impulsive heating needed to eject or evaporate a portion of the grain mantle. An extremely complex and rather refractory substance possessing the infrared signatures of amino groups and carboxylic acid groups and having a maximum mass of 514 amu has been produced at a rate corresponding to a mass conversion rate of interstellar grains of between 2% and 20% in 10⁷ yr. The shape and position of the astronomically observed 3.1 m band is duplicated in the laboratory and is shown to be a natural consequence of the processing of grain mantles.Invited contribution to the Proceedings of a Workshop onThermodynamics and Kinetics of Dust Formation in the Space Medium held at the Lunar and Planetary Institute, Houston, 6–8 September, 1978.     

Lunar soil grain size distribution

A comprehensive review has been made of the currently available data for lunar grain size distributions. It has been concluded that there is little or no statistical difference among the large majority of the soil samples from the Apollo 11, 12, 14, and 15 missions. The grain size distribution for these soils has reached a steady state in which the comminution processes are balanced by the aggregation processes. The median particle size for the steady-state soil is 40 to 130 µm. The predictions of lunar grain size distributions based on the Surveyor television photographs have been found to be quantitatively in error and qualitatively misleading.     

Detection of heavy-metal lines in the spectrum of the circumstellar envelope of a post-AGB star

Splitting of the strongest absorption lines with a lower-level excitation potential χ _low < 1 eV has been detected for the first time in the optical spectra of the post-AGB star V354 Lac obtained with a spectral resolution R = 60 000 at the 6-m BTA telescope. Analysis of the kinematics shows that the short-wavelength component of the split line originates in the star’s thick gas-dust envelope. Disregarding the splitting of strong lines when the chemical composition is calculated leads to overestimated overabundances of s-process elements (Ba, La, Ce, Nd) in the stellar atmosphere. The profiles of strong absorption lines have been found to be variable. The available radial-velocity data suggest the absence of any changes in the velocity field in the atmosphere and circumstellar envelope of V354 Lac over 15 years of its observations.     

Circumstellar dust and circumstellar extinction curves

We have calculated the circumstellar extinction curves produced by dust grains which absorb and scatter the stellar radiation in the shells of pre-main-sequence stars. A Monte Carlo method was used to model the radiative transfer in non-spherical shells. The dependence on the particle size distribution and the dust shell parameters has been examined.The application of the theoretical results to explain the extinction and polarization of the Herbig Be star HD 45677 shows that the dust shell is not disk-like and that very small grains are absent in it.     

Chondrule and metal grain size sorting from jet flows

Abstract— We examine the size sorting of chondrules and metal grains within the context of the jet flow model for chondrule/CAI formation. In this model, chondrules, CAIs, AOAs, metal grains, and related components of meteorites are assumed to have formed in the outflow region of the innermost regions of the solar nebula and then were ejected, via the agency of a bipolar jet flow, to outer regions of the nebula. We wish to see if size sorting of chondrules and metal grains is a natural consequence of this model. To assist in this task, we used a multiprocessor system to undertake Monte Carlo simulations of the early solar nebula. The paths of a statistically significant number of chondrules and metal grains were analyzed as they were ejected from the outflow and travelled over or into the solar nebula. For statistical reasons, only distances ≤3 AU from the Sun were examined. Our results suggest that size sorting can occur provided that the solar nebula jet flow had a relatively constant flow rate as function of time. A constant flow rate outflow produces size sorting, but it also produces a sharp size distribution of particles across the nebula and a metal‐rich Fe/Si ratio. When the other extreme of a fully random flow rate was examined, it was found that size sorting was removed, and the initial material injected into the flow was simply spread over most of the the solar nebula. These results indicate that the outflow can act as a size and density classifier. By simply varying the flow rate, the outflow can produce different types of proto‐meteorites from the same chondrule and metal grain feed stock. As a consequence of these investigations, we observed that the number of particles that impact into the nebula drops off moderately rapidly as a function of distance r from the Sun. We also derive a corrected form of the Epstein stopping time.     

Interstellar water and interstellar ice

There are two ways that water ice can form in the interstellar medium: H₂O molecules can form in the gas phase and then freeze out onto dust grain surfaces, or O and OH can be converted at the surfaces of grains to form H₂O, which is then retained. Bergin et al. (1998) have recently shown that shocks passing through interstellar clouds sufficiently frequently can make the first method effective. However, we present results from a similar chemical model which indicate that this requires significant optical shielding because of the high ionization fraction in regions exposedto a high UV flux. We deduce, therefore, that grain surface reactions probably represent the main source of H₂O ice on lines of sight with visual extinction up to about 6 magnitudes to an embedded source or 12 magnitudes to a background object. This revised version was published online in July 2006 with corrections to the Cover Date.     

Interstellar polyynes and related species

The existence of the polyynes — molecules consisting essentially of long chains of carbon atoms — in the interstellar medium is a discovery that appears to be critical to our understanding of interstellar chemistry. The family relationships of these species to other interstellar species are explored as are their significance to theories of interstellar chemistry. Methods for deducing the numbers of related molecules are presented.     

Molecules and dust in circumstellar shells

Circumstellar shells provide a unique environment for the study of dust formation and the relation of dust composition to specific atomic and molecular components. As a specific example, the formation of carbonaceous dust is discussed in relation to the presence of polycyclic aromatic hydrocarbon molecules and their survival in the interstellar medium. Some conclusions will be drawn concerning the composition of carbonaceous dust in circumstellar sources and that in the diffuse interstellar medium.     

Interstellar Scintillation Studies of Pulsars and Distribution of Scattering Plasma in the Local Interstellar Medium

Pulsar scintillation measurements from the Ooty Radio Telescope (ORT) are used to investigate the distribution of scattering in the Local Interstellar Medium (LISM; region of ≲ 1 kpc of the Sun), specifically the region in and around the Local Bubble. A 3-componentmodel, where the Solar neighborhood is surrounded by a shell of enhanced plasma turbulence, is proposed for the LISM. Further, the Ooty data, along with those from Parkes and other telescopes are used for investigating thedistribution of scattering towards the nearby Loop I Superbubble. This revised version was published online in July 2006 with corrections to the Cover Date.     

Interstellar extinction and diffuse absorption features

The equivalent width of the 2175 Å band,W ₂₁₇₅, well known as the big bump in the interstellar extinction curves, has been found to be closely correlated with the colour excessE _B-V as well as with the extinction differencesE _8–6 andE _9–7 defined to characterize quantitatively the steep slopes of the extinction curves in the far ultraviolet.The equivalent widths of the 5780 and 5797 Å diffuse lines show good correlation withE _B-V. The correlations ofW ₅₇₈₀ andW ₅₇₉₇ withE _8–6 resp.E _9–7 are, however, rather weak. Correlations betweenW ₂₁₇₅ andW ₅₇₈₀ and betweenW ₂₁₇₅ andW ₅₇₉₇ are indicated.The results have been qualitatively interpreted in favour of the dust model consisting of a mixture of small silicate grains and larger silicate grains coated by molecular mantles.Paper presented at the Symposium on Solid State Astrophysics, held at the University College, Cardiff, Wales, between 9–12 July 1974.     

Interstellar depletion anomalies and ionization potentials

Satellite observations indicate that (1) most elements are depleted from the gas phase when compared to cosmic abundances, (2) some elements are several orders of magnitude more depleted than others, and (3) these depletions vary from cloud to cloud. Since the most likely possibility is that the missing atoms are locked into grains, depletions occur either by accretion onto core particles in interstellar clouds or earlier, during the period of primary grain formation. If the latter mechanism is dominant, then the most important depletion parameter is the condensation temperature of the elements and their various compounds. However, this alone is not sufficient to explain all the observed anomalies. It is shown that electrostatic effects—under a wide variety of conditions—can enormously enhance the capture cross-section of the grain. It is suggested that this mechanism can also account for such anomalies as the apparent overabundance of the alkali metals in the gas phase.     

Interstellar gas depletion and dust parameters

It is shown that on theoretical grounds the relative abundances of the elements in the interstellar gas phase should be correlated with the wavelength of maximum interstellar polarization _max. If these correlations can be determined by observations, then there is the possibility to determine the relative abundances of the heavier elements within the mantles as well as within the cores of interstellar dust grains, at least in principle.The observational data available up to now confirm the existence of such correlations between _max and the interstellar gas phase abundances of titanium, iron, magnesium, and carbon. Statements about the chemical composition of the dust particles are not yet possible. For this there are observations of the interstellar gas depletion needed, especially in such lines of sight where _max has extreme values.Paper presented at a Workshop on The Role of Dust in Dense Regions of Interstellar Matter, held at Georgenthal, G.D.R., in March 1986.     

Mid-ir imaging polarimetry and circumstellar magnetic fields

We have obtained diffraction-limited images of the mid-infrared emission and polarization patterns in a number of southern-sky objects. By mapping the polarization produced in absorption or emission by aligned aspherical dust grains, we have been able to trace the detailed spatial structure of magnetic fields in the warm circumstellar material of young molecular-outflow sources and HII regions, in the expanding dust shell of the mass-losing star Eta Carinae, and in the inner parsec of the Galactic Centre.