Composition Of Interstellar Grains Research Articles

THE COMPOSITION OF INTERSTELLAR GRAINS TOWARD ζ OPHIUCHI: CONSTRAINING THE ELEMENTAL BUDGET NEAR THE DIFFUSE-DENSE CLOUD TRANSITION

We investigate the composition of interstellar grains along the line of sight toward Zeta Ophiuchi, a well-studied environment near the diffuse-dense cloud transition. A spectral decomposition analysis of the solid-state absorbers is performed using archival spectroscopic observations from the Spitzer Space Telescope and Infrared Space Observatory. We find strong evidence for the presence of sub-micron-sized amorphous silicate grains, principally comprised of olivine-like composition, with no convincing evidence of H2O ice mantles. However, tentative evidence for thick H2O ice mantles on large (a ~ 2.8 microns) grains is presented. Solid-state abundances of elemental Mg, Si, Fe, and O are inferred from our analysis and compared to standard reference abundances. We find that nearly all of elemental Mg and Si along the line of sight are present in amorphous silicate grains, while a substantial fraction of elemental Fe resides in compounds other than silicates. Moreover, we find that the total abundance of elemental O is largely inconsistent with the adopted reference abundances, indicating that as much as ~156 ppm of interstellar O is missing along the line of sight. After taking into account additional limits on the abundance of elemental O in other O-bearing solids, we conclude that any missing reservoir of elemental O must reside on large grains that are nearly opaque to infrared radiation.

A molecular survey of comet C/1995 O1 (Hale-Bopp) at the IRAM telescopes

Spectroscopic observations of comet Hale-Boppwere undertaken near perihelion at millimetre wavelengths with the Institut de Radioastronomie Millimetrique (IRAM) telescopes at Plateau de Bure (France)and Pico Veleta (Spain). They resulted in the first detections of HCOOH,SO2, NH2CHO and HCOOCH3 in a comet. HDO was detected through its312–221 line at 225.897 GHz,complementing the observation of the 101–000 line at 464.925 GHz at the James Clerk Maxwell Telescope (Meier et al., 1998,Science 279, 842).Several unidentified lines are present in the spectra. Observations of HC3N, HNCO, OCS, SO, CN, CO+, HCO+, in addition to more ‘classical’species CO, HCN, HNC, CH3CN, CH3OH, H2CO, CS and H2S (Biveret al., this issue) permit us to make out an extensive inventory of the composition of the coma of comet Hale-Bopp at its perihelion. It presents strong analogies with gas-phase abundances measured in interstellar hot cores and bipolar flows, which are believed to reflect the composition of interstellar grains.

Interplanetary material as a guide to the composition of interstellar grains

Interplanetary material as a guide to the composition of interstellar grains

Measuring interstellar dust grains from the haloes of binary X-ray sources

Overbeck1 first predicted that the images of cosmic X-ray sources might have observable haloes due to coherent forward scattering by interstellar dust grains, and it was recognized that the observations of these haloes would provide a powerful tool for analysing the size distribution and composition of the grains2–5. Observations with the Einstein observatory have provided evidence of faint haloes extending to a few arc minutes beyond the X-ray images of supernova remnants6,7 and several compact galactic sources8–11. The evidence that the haloes are actually due to interstellar grain scattering appears substantial, but it is subject to some uncertainty resulting from the dependence of the point response function (PRF) of the telescope on the source spectra (the observed haloes are brighter than the predicted PRF by factors of approximately 1.5–2). In this letter we show how observations of the time-dependence of the halo X-ray emission may be analysed to determine the location, size distribution, and composition of interstellar grains. Indeed, it should be possible to observe and analyse the halo emission during eclipse even without imaging the source, and we suggest that such haloes probably account for the soft X-rays that have been observed during eclipses or dips in several binary systems.

Organic material and the 2.5?4 ?m spectra of galactic sources

The spectra of galactic infrared sources over the waveband 2.5–4 μm provide clear evidence for an organic composition of interstellar grains.

The evolution of refractory interstellar grains in the solar neighborhood

The abundance of thermally condensed refractory grains and corresponding interstellar metal depletions are investigated using models for the evolution of the solar neighborhood. The calculations include a self-consistent treatment of red giant winds; planetary nebulae; protostellar nebulae and supernovae as sources of grains; star formation and encounters with supernova blast waves as sinks, for a variety of birthrate histories; chemical evolution models; and mass-loss parameters. We find that the maximum possible fraction of an element that can be locked up in thermally condensed cores is about 60% for the highly refractory elements like Ca, Al, and Ti, and about 30% for the more abundant refractory elements like Si, Mg, Mn, and Fe, independent of model parameters. Typically observed interstellar depletions (approx.10--10/sup 3/) and their elemental variations can therefore not be interpreted in terms of condensation efficiencies in the sources, and must instead be attributed to the selective growth of mantles in interstellar clouds. The strong upper limit on the abundance of thermally condensed grains yields constraints on the nature and composition of interstellar grains and the filling factor of the interstellar coronal gas. The relative importance of various condensation sites is also discussed, and we find that any interstellar grainmore » model that requires the depletion of any element in thermally condensed grains to be larger than approx.1.2 must require the formation of grains in supernovae.« less

Organic grains in space

Astronomical data at infrared wavelengths point decisively to an organic composition of interstellar grains.

RECENT REVELATIONS OF INFRARED ASTRONOMY.

ABSTRACT Subjects of recent IR astronomical observations are discussed. The BN point source in the Orion nebula, a likely protostar candidate, is described, and the characteristics of an IR object which may indicate a protostar are outlined. Results obtained on various aspects of the condensation and composition of solid grains in the galaxy are summarized, including condensation of solids in the atmospheres of cool stars, the composition of interstellar grains, and the possibility of grain condensation around hot stars. The physical processes of the radiation mechanism of extragalactic sources are discussed, and it is shown that the continuum energy distribution of some sources, such as NGC 1068 and M 82, consists of radio emission due to synchrotron electron radiation, a poorly understood UV energy source, and thermal radiation by grains within the galactic nuclei. Peculiar sources detected in the 2-micron CIT survey and the AFCRL survey at 4, 11, and 20 microns are described, and include very cool M stars, possible binary components, and possible planetary nebulae. Future developments in IR astronomy are briefly noted.

The Chemical Composition and Distribution of Interstellar Grains

The chemical composition of interstellar grains is derived here on the basis of (1) the cosmic abundance of the elements; (2) the wavelength dependence of extinction and polarization; (3) the average total extinction; (4) the ratio of polarization to extinction; (5) the predominantly dielectric character of grains in the visible spectral region; and (6) infrared spectral characteristics of grains. It is indicated that the major portion of the grains, by mass, consist of core-mantle particles in the 0.1-µm size range, whose cores consist largely of silicates and whose mantles are a solid mixture of O, C, and N with H in a heterogeneous combination of simple and complex molecules with frozen free radicals. A minor constituent of the solid particles exist in the form of very small uncoated particles generally less than 10-6 cm in size whose precise composition is not certain. Inferences of the core-mantle model with respect to spatial distribution are consistent with the proposition that growth of the mantles occurs in the galactic shock region predicted by the density-wave theory. Estimates of the total visual extinction toward the galactic center and the consequent estimates of the total amount of far infrared radiation are shown to depend critically on the grain model. Variations of the ratio of far ultraviolet to visual extinction are correlated with the conditions for growth of mantles on the bare small particles which are generally prevented from accreting mantles primarily because of their extreme temperature fluctuations produced by the ultraviolet photons in the radiation field.