W. A. Schutte

Amino acids from ultraviolet irradiation of interstellar ice analogues

Amino acids are the essential molecular components of living organisms on Earth, but the proposed mechanisms for their spontaneous generation have been unable to account for their presence in Earths early history. The delivery of extraterrestrial organic compounds has been proposed as an alternative to generation on Earth, and some amino acids have been found in several meteorites. Here we report the detection of amino acids in the room-temperature residue of an interstellar ice analogue that was ultraviolet-irradiated in a high vacuum at 12 K. We identified 16 amino acids; the chiral ones showed enantiomeric separation. Some of the identified amino acids are also found in meteorites. Our results demonstrate that the spontaneous generation of amino acids in the interstellar medium is possible, supporting the suggestion that prebiotic molecules could have been delivered to the early Earth by cometary dust, meteorites or interplanetary dust particles.

Infrared Space Observatory with the observations of solid carbon dioxide in molecular clouds

Spectra of interstellar CO2 ice absorption features at a resolving power of lambda/Delta lambda approximate to 1500-2000 are presented for 14 lines of sight. The observations were made with the Short-Wavelength Spectrometer (SWS) of the Infrared Space Observatory (ISO). Spectral coverage includes the primary stretching mode of CO2 near 4.27 mu m in all sources; the bending mode near 15.2 mu m is also detected in 12 of them. The selected sources include massive protostars (Elias 29 [in rho Oph], GL 490, GL 2136, GL 2591, GL 4176, NGC 7538 IRS 1, NCC 7538 IRS 9, S140, W3 IRS 5, and W33 A), sources associated with the Galactic Center (Sgr A*, GCS 3 I, and GCS 4), and a background star behind a quiescent dark cloud in Taurus (Elias 16); they thus probe a diverse range of environments. Column densities of interstellar CO2 ice relative to H2O ice fall in the range 10%-23%: this ratio displays remarkably little variation for such a physically diverse sample. Comparison of the observed profiles with laboratory data for CO2-bearing ice mixtures indicates that CO2 generally exists in at least two phases, one polar (H2O dominant) and one nonpolar (CO2 dominant). The observed CO2 profiles may also be reproduced when the nonpolar components are replaced with thermally annealed ices. Formation and evolutionary scenarios for CO2 and implications for grain mantle chemistry are discussed. Our results support the conclusion that thermal annealing, rather than energetic processing due to UV photons or cosmic rays, dominates the evolution of CO2-bearing ices.

The composition and distribution of dust along the line of sight toward the Galactic center

We discuss the composition of dust and ice along the line of sight to the Galactic center (GC) based on analysis of mid-infrared spectra (2.4-13 μm) from the Short Wavelength Spectrometer on the Infrared Space Observatory (ISO). We have analyzed dust absorption features arising in the molecular cloud material and the diffuse interstellar medium along the lines of sight toward Sgr A* and the Quintuplet sources, GCS 3 and GCS 4. It is evident from the depth of the 3.0 μm H2O and the 4.27 μm CO2 ice features that there is more molecular cloud material along the line of sight toward Sgr A* than toward GCS 3 and GCS 4. In fact, Sgr A* has a rich infrared ice spectrum with evidence for the presence of solid CH4, NH3, and possibly HCOOH. Hydrocarbon dust in the diffuse interstellar medium along the line of sight to the GC is characterized by absorption features centered at 3.4, 6.85, and 7.25 μm. Ground-based studies have identified the 3.4 μm feature with aliphatic hydrocarbons, and ISO has given us the first meaningful observations of the corresponding modes at longer wavelengths. The integrated strengths of these three features suggest that hydrogenated amorphous carbon is their carrier. We attribute an absorption feature centered at 3.28 μm in the GCS 3 spectrum to the C–H stretch in aromatic hydrocarbons. This feature is not detected, and its C–C stretch counterpart appears to be weaker, in the Sgr A* spectrum. A key question now is whether or not aromatics are a widespread component of the diffuse interstellar medium, analogous to aliphatic hydrocarbons.

Theoretical modeling of the infrared fluorescence from interstellar polycyclic aromatic hydrocarbons

We have modeled the family of interstellar IR emission bands at 3.3, 6.2, 7.7, 8.6, 11.3, and 12.7 microns by calculating the fluorescence from a size distribution of interstellar polycyclic aromatic hydrocarbons (PAHs) embedded in the radiation field of a hot star. It is found that the various emission bands are dominated by distinctly different PAHs, from molecules with much less than about 80 C atoms for the 3.3 micron feature, to molecules with 10 exp 2-10 exp 5 C atoms for the emission in the IRAS 12 and 25 micron bands. We quantitatively describe the influence on the emergent spectrum of various PAH properties such as the molecular structure, the amount of dehydrogenation, the intrinsic strength of the IR active modes, and the size distribution. Comparing our model results to the emission spectrum from the Orion Bar region, we conclude that interstellar PAHs are likely fully, or almost fully, hydrogenated. Moreover, it is found that the intrinsic strengths of the 6.2 and 7.7 micron C-C stretching modes, and the 8.6 micron C-H in-plane bending mode are 2-6 times larger than measured for neutral PAHs in the laboratory.

Cosmic ray induced explosive chemical desorption in dense clouds

The desorption due to the energy release of free radicals in the ice mantles of a dust grain is investigated theoretically by calculating the ultraviolet radiation field inside the cloud, the free radical accumulation, the cosmic-ray heating of the grain and then the desorption in this situation starting from the cosmic-ray energy spectra. This model can reproduce the observations of the CO gas abundances and level of depletion in dark clouds such as L977 and IC 5146 with a combination of input parameters which are either constrained by independent observations or have been derived independently from laboratory experiments. We investigate other desorption mechanisms and conclude that they cannot explain the observations. The model also shows that the energy input by the cosmic-ray induced ultraviolet field is almost one order of magnitude larger than the direct energy input by cosmic-ray particles. This strengthens the conclusion that desorption due to the energy release by ultraviolet photon produced radicals dominates over direct cosmic-ray desorption.

Comet Halley as an aggregate of interstellar dust and further evidence for the photochemical formation of organics in the interstellar medium

Photolysis of mixtures of CO:NH3:H2O at 12 K results in the formation of an organic residue which is not volatile in high vacuum at room temperature. Analysis of this fraction by GC-MS resulted in the detection of C2–C3 hydroxy acids and hydroxy amides, glycerol, urea, glycine, hexamethylene tetramine, formamidine and ethanolamine. Use of isotopically labeled gases made it possible to establish that the observed products were not contaminants. The reaction pathways for the formation of these products were determined from the position of the isotopic labels in the mass spectral fragments. The significance of these findings to the composition of comets and the origins of life is discussed.

Ice absorption features in the 5-8 μm region toward embedded protostars

We have obtained 5{8 m spectra towards 10 embedded protostars using the Short Wavelength Spectrometer on board the Infrared Space Observatory (ISO-SWS) with the aim of studying the composition of interstellar ices. The spectra are dominated by absorption bands at 6.0 m and 6.85 m. The observed peak positions, widths and relative intensities of these bands vary dramatically along the dierent lines of sight. On the basis of comparison with laboratory spectra, the bulk of the 6.0 m absorption band is assigned to amorphous H2O ice. Additional absorption, in this band, is seen toward 5 sources on the short wavelength wing, near 5.8 m, and the long wavelength side near 6.2 m. We attribute the short wavelength absorption to a combination of formic acid (HCOOH) and formaldehyde (H2CO), while the long wavelength absorption has been assigned to the C{C stretching mode of aromatic structures. From an analysis of the 6.85 m band, we conclude that this band is composed of two components: a volatile component centered near 6.75 m and a more refractory component at 6.95 m. From a comparison with various temperature tracers of the thermal history of interstellar ices, we conclude that the two 6.85 m components are related through thermal processing. We explore several possible carriers of the 6.85 m absorption band, but no satisfactory identication can be made at present. Finally, we discuss the possible implications for the origin and evolution of interstellar ices that arise from these new results.

Infrared Spectroscopy of Dust in the Diffuse Interstellar Medium toward Cygnus OB2 No. 12

Observations made with the short-wavelength spectrometer of the Infrared Space Observatory are used to investigate the composition of interstellar dust in the line of sight to Cygnus OB2 No. 12, commonly taken as representative of the diffuse (low-density) interstellar medium. Results are compared with data for the Galactic center source Sgr A*. Nondetections of the 3.0 and 4.27 μm features of H2O and CO2 ices in Cyg OB2 No. 12 confirm the absence of dense molecular material in this line of sight, whereas the presence of these features in Sgr A* indicates that molecular clouds may contribute as much as 10 mag of visual extinction toward the Galactic center. The spectrum of Cyg OB2 No. 12 is dominated by the well-known 9.7 μm silicate feature; detection of a shallow feature near 2.75 μm indicates that the silicates are at least partially hydrated, with composition possibly similar to that of terrestrial phyllosilicates such as serpentine or chlorite. However, the 2.75 μm feature is not seen in the Galactic center spectrum, suggesting that silicates in this line of sight are less hydrated or of different composition. The primary spectral signatures of C-rich dust in the diffuse ISM are weak absorptions at 3.4 μm (the aliphatic C=H stretch) and 6.2 μm (the aromatic C=C stretch). We conclude, based on infrared spectroscopy, that the most probable composition of the dust toward Cyg OB2 No. 12 is a mixture of silicates and carbonaceous solids in a volume ratio of approximately 3:2, with the carbonaceous component primarily in an aromatic form such as amorphous carbon.

UV-photoprocessing of interstellar ice analogs: New infrared spectroscopic results

We simulate experimentally the physical conditions present in dense clouds by means of a high vacuum experimental setup at low temperature T ≈ 12 K. The accretion and photoprocessing of ices on grain surfaces is simulated in the following way: an ice layer with composition analogous to that of interstellar ices is deposited on a substrate window, while being irradi- ated by ultraviolet (UV) photons. Subsequently the sample is slowly warmed up to room temperature; a residue remains con- taining the most refractory products of photo- and thermal processing. In this paper we report on the Fourier transform-infrared (FT-IR) spectroscopy of the refractory organic material formed under a wide variety of initial conditions (ice composition, UV spectrum, UV dose and sample temperature). The refractory products obtained in these experiments are identified and the corresponding efficiencies of formation are given. The first evidence for carboxylic acid salts as part of the refractory products is shown. The features in the IR spectrum of the refractory material are attributed to hexamethylenetetramine (HMT, ((CH2)6N4)), ammonium salts of carboxylic acids ((R-COO − )(NH + )), amides (H2NC(=O)-R), esters (R-C(=O)-O-R � ) and species related to polyoxymethylene (POM, ((-CH2O-)n)). Furthermore, evidence is presented for the formation of HMT at room temperature, and the important role of H2O ice as a catalyst for the formation of complex organic molecules. These species might also be present in the interstellar medium (ISM) and form part of comets. Ongoing and future cometary missions, such as Stardust and Rosetta, will allow a comparison with the laboratory results, providing new insight into the physico-chemical conditions present during the formation of our solar system.

Approaching the Interstellar Grain Organic Refractory Component

Infrared spectra have been obtained for laboratory residues of photoprocessed low-temperature ices which have been exposed to long-term solar ultraviolet radiation on the EURECA satellite. This is an analog to the ultraviolet processing of interstellar dust mantles in diffuse clouds after leaving molecular clouds. The 3.4 μm absorption features of these organic materials match those of the diffuse cloud interstellar dust better than any other previously suggested analog to the interstellar organics.