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The interstellar Li I 6707-A absorption line is detected toward Rho Oph A at an equivalent width of 3.1 + or - 1.9 (3 sigma)mA. Upper limits near 1 mA also are found toward seven other stars, including 55 Cyg, toward which relatively strong Li I absorption was previously reported. The four positive detections and the eight upper limits now available for the 6707-A line indicate that Li is not significantly depleted from the interstellar gas.
A survey of 15 lines of sight for the CN B2Sigma(+) X2Sigma(+) interstellar absorption lines shows that the CN column density in diffuse interstellar clouds follows the relation log N(CN) is proportional to m log N(H2), where m is approximately equal to 3. This result is reproduced by a reaction network in which CN is produced primarily from C2 by the neutral-neutral reaction C2 + N yields CN + C, and photodissociation is the main destruction pathway for the neutral molecules CH, C2, and CN. The CN radical is the first molecular species observed in diffuse clouds that requires a neutral-neutral reaction for its formation in the gas phase. The network also reproduces the observed ratio N(CN)/N(H2).
A search for absorption from neutral rubidium at 7800 A was conducted. No evidence for absorption to a 3 sigma limit of less than 1.5 mA was seen in the diffuse interstellar gas toward the stars omicron Persei, zeta Persei, and zeta Ophiuchi. Present results do not confirm the detection by Jura and Smith (1981) toward zeta Oph. A possible reason for the discrepancy is presented. In light of the present measurements, the abundance of interstellar rubidium in reconsidered.
An interstellar line has been detected in emission at the expected submillimeter wavelength of the 1(10)-1(11) transition of H2D(+), the deuterated verison of the primary ion (H3/+/) in the favored ion-molecule reaction scheme for interstellar gas phase chemistry. The strength of the line is in approximate agreement with the theoretically anticipated H2D(+) abundance.
A survey is initiated of interstellar densities, abundances, and cloud structure in the Galaxy, using the International Ultraviolet Explorer. From high-resolution (0.1-A) spectra, interstellar column densities are derived toward 244 early-type stars by fitting the damping wings of the Ly-alpha absorption line. Published B-V photometry is used to derive E(B-V) color excesses, stellar distances r, and the mean hydrogen densities, n-bar = N(H I)/r. The data set includes stars out to r = 8.5 kpc and E(B-V) = 0.86, with 68 stars in the galactic halo (absolute b greater than 20 deg). The statistical sample consists of 205 stars of type O3-B2.5 within 5 kpc (53 stars in the halo). The column densities, log N(H I), range from less than 19.44 per square centimeter for alpha Vir to 21.81 for HD 37061. A statistical analysis of the E(B-V) distribution suggests variations in the line-of-sight cloud density and the mean reddening per cloud.
The International Ultraviolet Explorer (IUE) Satellite was used to collect interstellar extinction curves for 58 moderately reddened lines of sight. Spectral types of the background stars ranged from O7 to A0. All luminosity classes are represented, but early B main-sequence stars predominate. E(B-V) reddening values ranged from 0.14 to 1.13 mag, with an average of 0.63 mag. These extinction curves, less some 10 curves judged of lower quality, were co-added in order to reduce the random and fixed-pattern noise of the instrument. The resulting averaged curve was examined for evidence of small-scale structure, with negative results. The 2-sigma limits were found to be 0.09 mag for wavelengths between 1150 A and 2000 A, and 0.16 mag for wavelengths between 2000 A and 2300 A. Because of mismatch errors in weak stellar lines, it is unlikely that much improvement in these results is possible. The implications of these negative results are discussed with particular reference to the MgO theory of interstellar grains and diffuse bands.
Large Polycyclic Aromatic Hydrocarbons (PAH) are the likely origin of the Unidentified Infrared Emission Feature. The molecules or their ions are also attractive candidates for the carriers of the Diffuse Interstellar Bands (DIBs) in the visible range. The PAHs have optically active transitions in the visible spectrum; have the ability to survive the ultraviolet photons in the Diffuse Interstellar Medium; and are the most abundant among the detected molecular species after H2 and CO. In particular, PAHs are better candidates than the long carbon chains that were proposed previously.
There exists a discrepancy between calculated destruction rates of grains in the interstellar medium and postulated sources of new grains. This problem was examined by modelling the global life cycle of grains in the galaxy. The model includes: grain destruction due to supernovae shock waves; grain injection from cool stars, planetary nebulae, star formation, novae, and supernovae; grain growth by accretion in dark clouds; and a mixing scheme between phases of the interstellar medium. Grain growth in molecular clouds is considered as a mechanism or increasing the formation rate. To decrease the shock destruction rate, several new physical processes, such as partial vaporization effects in grain-grain collisions, breakdown of the small Larmor radius approximation for betatron acceleration, and relaxation of the steady-state shock assumption are included.
An interstellar origin has been attributed to certain organic and apparently elemental forms of C which occurs in carbonaceous meteorites. Evidence for such an origin comes from anomalous isotopic composition either of the C itself or of elements, such as N, H, or the noble gasses, which are combined with or trapped within the carbonaceous material. Suggested sources for these anomalous compositions include atmospheres of red giant stars, novae and super-novae, and interstellar molecular clouds.
Four new transitions of the interstellar formyl radical, HCO have been detected. Five transitions are now known for interstellar HCO, and thus its identification is secure. The column density found by assuming NGC 2024 is an extended source is N subT(HCO) = (8.5 + or - 4.0) x 10 to the 12th/sq cm. This gives a fractional abundance (abundance relative to hydrogen) for NGC 2024 which agrees quite well with some theoretical predictions. Several unidentified lines were detected and are reported here. Tentative identification for some of the unidentified lines are suggested.
Carbonaceous material from several distinct astrophysical environments that has been identified in primitive meteorites is discussed. Most of this material is not well crystallized graphite, but ranges from kerogenlike macromolecular organic matter to poorly graphitized carbon. Of the small fraction of the carbonaceous material which is graphite, most may have resulted from the graphitization of macromolecular precursors in the solar nebula. Since graphite is more stable than those precursors, preexisting interstellar graphite should have survived in meteorites to at least the same extent as did other forms of carbon. It is concluded from this dearth of graphite that graphitic carbon was not a major component of the interstellar dust at the time of the formation of the solar system.
Laboratory experiments and theoretical modeling show that the chemistry of chlorine is fairly simple in dense interstellar clouds, with Cl and HCl as the only species whose fractional abundances are significant. The estimated fraction of gas-phase chlorine present as HCl lies between 25-65 percent, in good agreement with the recent observations of the ground state HCl transition by Blake, Keene, and Philips (1985). These results, combined with the observational limits on HCl, indicate that chlorine is not severely depleted in dense interstellar clouds.
A method for determining the location, size distribution, and composition of interstellar dust grains from the haloes of binary X-ray sources is described. The method is based on the time dependence of the halo X-ray emission during an eclipse of the point source. The distance of the dust cloud is inferred directly from observations of the growth of the dark circle around the eclipsed point source. Grain size distribution in the interstellar dust clouds is derived from the halo brightness profile at a given photon energy. The composition of the grains is obtained by observing the spectral features in the total optical depth due to X-ray scattering near the photoionization thresholds of the elements in the grains. The haloes become dim just below the ionization threshold, and sharply increase in function brightness as the photon energy increases beyond the absorption edge of the relevant atom. The method makes it possible to observe and analyze the halo emission during eclipse even without imaging the source. It is suggested that haloes probably account for the soft X-rays observed during eclipses and dips observed in sources including 4U1915-05 and X1755-338.
The results of an Einstein Observatory imaging proportional counter investigation of the halos produced by the scattering of X-rays from interstellar grains of four compact Galactic (low-latitude) and two extragalactic (high-latitude) X-ray sources are reported. It is found that the intensity of these halos correlates well with the amount of visual extinction and the distance through the Galaxy's dust layer: quantities which measure the column density of grains to a given source. From this result, and from the size and shape of the halos from the Galactic sources, a number of grain parameters are derived in the context of two alternate grain size distributions: the Mathis-Rumpl-Nordsieck (MRN) and the Oort-van de Hulst distributions, either of which is capable of producing the observed halos. Though no single grain size is capable of producing the shapes observed for these halos, a mean size of about 0.1 micron and a number density of about 10 to the -12th grains/cu cm produce the correct halo intensities. From the additional parameters determined from the size and shape of the halos, results concerning the amount of material in grains which are in general agreement with the observed depletion of the elements from the gas phase of the interstellar medium, as well as with the detailed predictions of the MRN size distribution, are found.
The chemical evolution of interstellar grains leads ultimately to a comparison consisting largely of complex organic molecules. Comparison of infrared absorption spectra of laboratory produced analogue materials with astronomical observations confirm the the presence of similar molecules in interstellar space. The abundance of this complex organic matter derived from the strength of the absorption bands is of the order of ten million solar masses and is almost certainly as large or larger than all conceivable planets.
It is shown that the combined equivalent widths for a large population of Gaussian-like interstellar line components, each with different central optical depths tau(0) and velocity dispersions b, exhibit a curve of growth (COG) which closely mimics that of a single, pure Gaussian distribution in velocity. Two parametric distributions functions for the line populations are considered: a bivariate Gaussian for tau(0) and b and a power law distribution for tau(0) combined with a Gaussian dispersion for b. First, COGs for populations having an extremely large number of nonoverlapping components are derived, and the implications are shown by focusing on the doublet-ratio analysis for a pair of lines whose f-values differ by a factor of two. The consequences of having, instead of an almost infinite number of lines, a relatively small collection of components added together for each member of a doublet are examined. The theory of how the equivalent widths grow for populations of overlapping Gaussian profiles is developed. Examples of the composite COG analysis applied to existing collections of high-resolution interstellar line data are presented.
Measurements of the equivalent widths of the diffuse interstellar bands at 6284 A and 6613 A in the spectra of 23 stars with E(B-V) less than 0.1 mag are reported. The band strengths do not correlate with interstellar H2 column density. They are probably produced by molecular mantles on grains or very stable molecules such as the polycyclic aromatic hydrocarbons.
IRAS fluxes of early-type galaxies have been examined. From studying a magnitude-limited sample, it seems that although the statistics are poor, perhaps a third of these objects and possibly even more have an appreciable amount of dust. In general, the infrared emission is strongest at 100 microns, and quite often the far-infrared luminosity of an early-type galaxy can be well in excess of 10 to the 8th solar luminosities. The data are most easily understood if the infrared results from dust reprocessing of starlight. Within 3 kpc of the center of an early-type galaxy, there may be more cold matter than hot gas; recent models that the interstellar media of these objects are composed primarily of hot gas appear to be oversimplifications. The X-ray data do indicate that the thermal pressure in the early-type galaxies often is sufficiently high that in view of the expected low temperatures of their interstellar clouds, it can be speculated that the cold material that is present may be forming low-mass stars.