Interstellar organics and possible connections with the carbonaceous components of meteorites and IDPs

Studying the chemical and isotopic composition of interstellar ice and dust provides insight into the composition and chemical history of the solid bodies in the solar nebula and the nature of the materials subsequently brought into the inner part of the solar system by comets and meteorites. It is now possible to probe the composition of these microscopic interstellar particles. High quality IR spectra of many different astronomical sources (some associated with dark molecular clouds, and others in the diffuse interstellar medium) are now available. Comparisons of these spectra with laboratory spectra and complex organic components of these materials are discussed

The origin, composition and history of cometary ices from spectroscopic studies

The spectroscopic analysis of pristine cometary material provides a very important probe of the chemical identity of the material as well as of the physical and chemical conditions which prevailed during the comet's history. Concerning classical spectroscopy, the spectral regions which will most likely prove most useful are the infrared, the visible and ultraviolet. Newer spectroscopic techniques which have the potential to provide equally important information include nuclear magnetic resonance (NMR) and electron spin resonance (ESR). Each technique is summarized with emphasis placed on the kind of information which can be obtained

Interstellar grain chemistry and organic molecules

The detection of prominant infrared absorption bands at 3250, 2170, 2138, 1670 and 1470 cm(-1) (3.08, 4.61, 4.677, 5.99 and 6.80 micron m) associated with molecular clouds show that mixed molecular (icy) grain mantles are an important component of the interstellar dust in the dense interstellar medium. These ices, which contain many organic molecules, may also be the production site of the more complex organic grain mantles detected in the diffuse interstellar medium. Theoretical calculations employing gas phase as well as grain surface reactions predict that the ices should be dominated only by the simple molecules H2O, H2CO, N2, CO, O2, NH3, CH4, possibly CH3OH, and their deuterated counterparts. However, spectroscopic observations in the 2500 to 1250 cm(-1)(4 to 8 micron m) range show substantial variation from source reactions alone. By comparing these astronomical spectra with the spectra of laboratory-produced analogs of interstellar ices, one can determine the composition and abundance of the materials frozen on the grains in dense clouds. Experiments are described in which the chemical evolution of an interstellar ice analog is determined during irradiation and subsequent warm-up. Particular attention is paid to the types of moderately complex organic materials produced during these experiments which are likely to be present in interstellar grains and cometary ices

Composition, structure and chemistry of interstellar dust

The observational constraints on the composition of the interstellar dust are analyzed. The dust in the diffuse interstellar medium consists of a mixture of stardust (amorphous silicates, amorphous carbon, polycyclic aromatic hydrocarbons, and graphite) and interstellar medium dust (organic refractory material). Stardust seems to dominate in the local diffuse interstellar medium. Inside molecular clouds, however, icy grain mantles are also important. The structural differences between crystalline and amorphous materials, which lead to differences in the optical properties, are discussed. The astrophysical consequences are briefly examined. The physical principles of grain surface chemistry are discussed and applied to the formation of molecular hydrogen and icy grain mantles inside dense molecular clouds. Transformation of these icy grain mantles into the organic refractory dust component observed in the diffuse interstellar medium requires ultraviolet sources inside molecular clouds as well as radical diffusion promoted by transient heating of the mantle. The latter process also returns a considerable fraction of the molecules in the grain mantle to the gas phase

Aromatic components in cometary materials

The Raman spectra of interplanetary dust particles (IDPs) collected in the stratosphere show that two bands at about 1350 and 1600 delta/cm and a broader feature between 2200 and 3300 delta/cm that are characteristic of aromatic molecular units with ordered domains smaller than 25 A in diameter. This suggests that the carbonaceous material in IDPs may be similar to the polymeric component seen in meteorites, where this material is thought to consist of aromatic molecular units that are randomly interlinked by short aliphatic bridges. The features in the Raman spectra of IDPs are similar in position, and relative strength to interstellar infrared emission features that have been attributed to vibrational transitions in free molecular polycyclic aromatic hydrocarbons. Taken together, these observations suggest that some fraction of the carbonaceous materials in IDPs may have been produced in circumstellar dust shells and only slightly modified in interstellar space

The infrared spectra of very large, compact, highly symmetric, polycyclic aromatic hydrocarbons (PAHs)

The mid-infrared spectra of large PAHs ranging from C54H18 to C130H28 are determined computationally using Density Functional Theory. Trends in the band positions and intensities as a function of PAH size, charge and geometry are discussed. Regarding the 3.3, 6.3 and 11.2 micron bands similar conclusions hold as with small PAHs. This does not hold for the other features. The larger PAH cations and anions produce bands at 7.8 micron and, as PAH sizes increases, a band near 8.5 micron becomes prominent and shifts slightly to the red. In addition, the average anion peak falls slightly to the red of the average cation peak. The similarity in behavior of the 7.8 and 8.6 micron bands with the astronomical observations suggests that they arise from large, cationic and anionic PAHs, with the specific peak position and profile reflecting the PAH cation to anion concentration ratio and relative intensities of PAH size. Hence, the broad astronomical 7.7 micron band is produced by a mixture of small and large PAH cations and anions, with small and large PAHs contributing more to the 7.6 and 7.8 micron component respectively. For the CH out-of-plane vibrations, the duo hydrogens couple with the solo vibrations and produce bands that fall at wavelengths slightly different than their counterparts in smaller PAHs. As a consequence, previously deduced PAH structures are altered in favor of more compact and symmetric forms. In addition, the overlap between the duo and trio bands may reproduce the blue-shaded 12.8 micron profile.Comment: ApJ, 36 pages, 9 fig

Spatial variations of the 3 micron emission features within nebulae

The 3 micron spectra is presented for the Orion bar region and the Red Rectangle. In both objects spectra were obtained at more than one location, corresponding to different distances from the excitation source. The well known 3.3 and 3.4 micron emission bands are seen in both objects as well as the recently discovered features at 3.46, 3.51, and 3.57 microns in the Orion bar spectra. The spectra show that the relative strengths of the 3 micron emission features vary within the Orion bar. As distance from the exciting star increases, the 3.4 and 3.51 micron features increase, and the 3.46 micron feature decreases in strength, relative to the strong 3.3 micron feature. These are two possible interpretations which are postulated, each of which involves the breaking of bonds by UV radiation, which removes the modes responsible for the 3.4 micron emission near the star. The two possible bond ruptures are the CH bond in small polycyclic aromatic hydrocarbons (PAHs), or the bond to an aliphatic subgroup. It has to be pointed out that neither interpretation appears entirely satisfactory. The vibrational overtone interpretation cannot explain the presence or behavior of the 3.46 micron feature, whereas the laboratory spectra of aliphatic sidegroups contain many more features in the 3 micron region than are observed in the astronomical sources

Spatial distribution of emission in Unidentified Infrared Bands from Midcourse Space Experiment Survey

Recently the Midcourse Space Experiment (MSX) has surveyed the Galactic plane in mainly four infrared bands between 6 and 25 micron. Two of these bands cover several Unidentified Infrared emission Bands (UIBs). With the aim of extracting the spatial distribution of the UIB emission on a large scale, a scheme has been developed to model the MSX data with emission in the UIBs alongwith the underlying thermal continuum from the interstellar dust. In order to test this scheme, a sample of five Galactic compact H II regions (Sh-61, Sh-138, Sh-152, Sh-156, Sh-186; Zavagno & Ducci 2001) for which imaging study in some individual UIBs is available from ISOCAM measurements, has been studied. The results of this comparative study on small angular scale are as follows : (i) the morphological details extracted from our scheme agree very well with those from the superior ISOCAM measurements; (ii) the integrated strength of UIBs extracted from the MSX database correlates extremely well with the sum of the strengths of individual UIBs measured from ISOCAM. This tight correlation is very encouraging and promises the potential of MSX database for study of large scale spatial distribution of UIB emission (and the carriers of UIBs) in the entire Galactic plane.Comment: to be published in Astronomy & Astrophysics; (9 pages including 2 tables & 11 figures

Interstellar grain chemistry and the composition of comets

During the past 15 years considerable progress in observational techniques has been achieved in the middle infrared, the spectral region most diagnostic of molecular vibrations. Spectra of many different astronomical infrared sources are now available. By comparing these astronomical spectra with the spectra of lab ices, one can determine the composition and abundance of the icy materials frozen on the cold dust grains present in the interior of molecular clouds. In the experiments described, the assumption is made that cometary ices are similar to interstellar ices. As an illustration of the processes which can take place as an ice is irradiated and subsequently warmed, the infrared spectra is presented of the mixture H2O:CH3OH:CO:NH3:C6H14 (100:50:10:10:10). Apart from the last species, the ratio of these compounds is representative of the simplest ices found in interstellar clouds

Life and the Universe: From Astrochemistry to Astrobiology

Great strides have been made in our understanding of interstellar material thanks to advances in infrared astronomy and laboratory astrophysics. Ionized polycyclic aromatic hydrocarbons (PAHs), shockingly large molecules by earlier astrochemical standards, are widespread and very abundant throughout much of the cosmos. In cold molecular clouds, the birthplace of planets and stars, interstellar atoms and molecules freeze onto extremely cold dust and ice particles forming mixed molecular ices dominated by simple species such as water, methanol, ammonia, and carbon monoxide. Within these clouds, and especially in the vicinity of star and planet forming regions, these ices and PAHs are processed by ultraviolet light and cosmic rays forming hundreds of far more complex species, some of biogenic interest. Eventually, these are delivered to primordial planets by comets and meteorites. As these materials are the building blocks of comets and related to carbonaceous micrometeorites, they are likely to be important sources of complex organic materials delivered to habitable planets (including the primordial Earth) and their composition may be related to the origin of life. This talk will focus on the chemical evolution of these cosmic materials and their relevance to astrobiology