Douglas C. B. Whittet

Observations of the Icy Universe

Freeze-out of the gas-phase elements onto cold grains in dense interstellar and circumstellar media builds up ice mantles consisting of molecules that are mostly formed in situ (H2O, NH3, CO2, CO, CH3OH, and more). This review summarizes the detected infrared spectroscopic ice features and compares the abundances across Galactic, extragalactic, and Solar System environments. A tremendous amount of information is contained in the ice band profiles. Laboratory experiments play a critical role in the analysis of the observations. Strong evidence is found for distinct ice formation stages, separated by CO freeze-out at high densities. The ice bands have proven to be excellent probes of the thermal history of their environment. The evidence for the long-held idea that processing of ices by energetic photons and cosmic rays produces complex molecules is weak. Recent state-of-the-art observations show promise for much progress in this area with planned infrared facilities.

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.

Three Micron Hydrocarbon and Methanol Absorption in Taurus

The 3.3-4.0 μm spectral region contains the fundamental C-H stretching vibrations of alcohols and aliphatic hydrocarbons and provides a powerful method of characterizing the organic component of interstellar ices. Observations of sources in and behind the Taurus dark cloud are presented. We detect the 3.47 μm hydrocarbon feature toward Elias 16 (field star) and HL Tau (T Tauri star). Elias 16 provides the first detection of the 3.47 μm feature in the quiescent cloud medium. Our results are consistent with a location for the 3.47 μm absorber in the grain mantle material rather than the core. The 3.54 μm methanol (CH3OH) feature is not detected in any of the sources to a limit of τ3.54/AV < 0.0009, and CH3OH therefore has an abundance relative to H2O, which is likely to be less than 5%. This result sets limits on models for mantle evolution which involve processing of CH3OH-rich ices to form more complex organic species in regions of active star formation.

Processing of icy mantles in protostellar envelopes.

We have obtained CO absorption profiles of several young stellar objects (YSOs), spanning a range of mass and luminosity, in order to investigate their ice mantle composition. We present the first detection of CO toward the class I YSO L1489 IRS in the Taurus dark cloud. In general, the CO profiles for YSOs show evidence for both processed and pristine ices in the same line of sight, strong indirect evidence for CO, is suggested in R CrA IRS 7, L1489 IRS, Elias 18, and GL 961E. Toward other sources (R CrA IRS 1, IRS 2, W33A, NGC 7538 IRS 9, Mon R2 IRS 2) CO is present in (nearly) pure form. We propose an evolutionary scenario to explain the chemical diversity of the icy mantles toward these objects.

Evidence for Chemical Processing of Precometary Icy Grains In Circumstellar Environments of Pre-Main-Sequence Stars

We report the detection of a broad absorption feature near 2166/cm in the spectrum of the Taurus cloud cource Elias 18. This pre-main-sequence source is the second in Taurus, the third in our survey, and the fifth known in the sky to show the broad 2166/cm absorption feature. Of equal importance, this feature is not seen toward several other embedded sources in our survey, nor is it seen toward the source Elias 16, located behind the Taurus cloud. Laboratory experiments with interstellar ice analogs show that such a feature is associated with a complex C triple bonded to N containing compound (called X(C triple bonded to N)) that results from high-energy processing (ultraviolet irradiation or ion bombardment) of simple ice components into more complex, organic components. We find a nonlinear anticorrelation between the abundance of X(C triple bonded to N) and frozen CO components in nonpolar lattices. We find no correlation between the abundance of X(C triple bonded to N) and frozen CO in polar lattices. Because the abundances of frozen CO and H2O are strongly correlated with each other and with visual extinction toward sources embedded in and located behind the Taurus molecular cloud, these ice components usually are associated with intracloud material. Our results indicate that X(C triple bonded to N) molecules result from chemical processing of dust grains dominated by nonpolar icy mantles in the local environments of pre-main-sequence stars. Such processing of icy grains in the early solar system may be an important source of organic compounds observed in minor solar system bodies. The delivery of these organic compounds to the surface of the primitive Earth through comet impacts may have provided the raw materials for prebiotic chemistry.

Spitzer observations of CO2 ice toward field stars in the taurus molecular cloud

We present the first Spitzer Infrared Spectrograph observations of the 15.2 μm bending mode of CO2 ice toward field stars behind a quiescent dark cloud. CO2 ice is detected toward two field stars (Elias 16 and Elias 3) and a single protostar (HL Tau) with an abundance of ~15%-20% relative to water ice. CO2 ice is not detected toward the source with lowest extinction in our sample, Tamura 17 (AV = 3.9 mag). A comparison of the Elias 16 spectrum with laboratory data demonstrates that the majority of CO2 ice is embedded in a polar, H2O-rich ice component, with ~15% of CO2 residing in an apolar, H2O-poor mantle. This is the first detection of apolar CO2 toward a field star. We find that the CO2 extinction threshold is AV = 4 ± 1 mag, comparable to the threshold for water ice, but significantly less than the threshold for CO ice, the likely precursor of CO2. Our results confirm that CO2 ice forms in tandem with H2O ice along quiescent lines of sight. This argues for CO2 ice formation by means of a mechanism similar to that responsible for H2O ice formation, viz., simple catalytic reactions on grain surfaces.

IS EXTRATERRESTRIAL ORGANIC MATTER RELEVANT TO THE ORIGIN OF LIFE ON EARTH

I review the relative importance of internal and external sources of prebiotic molecules on Earth at the time of lifes origin μ3.7Gyr ago. The efficiency ofsynthesis in the Earths atmosphere was critically dependent on its oxidation state. If the early atmosphere was non-reducing and CO2-dominated, external delivery might havebeen the dominant source. Interplanetary dust grains and micrometeorites currently deliver carbonaceous matter to the Earths surface at a rate of μ 3 × 105kg/yr(equivalent to a biomass in μ2Gyr), but this may havebeen as high as 5 × 107kg/yr (a biomass in onlyμ10Myr) during the epoch of late bombardment. Much ofthe incoming material is in the form of chemically inactive kerogens and amorphous carbon; but if the Earth once had a dense (μ10-bar) atmosphere, small comets rich in avariety of prebiotic molecules may have been sufficiently air-braked to land non-destructively. Lingering uncertainties regarding the impact history of the Earth and the density and composition of its early atmosphere limit our ability to draw firm conclusions.

ultraviolet radiation from F and K stars and implications for planetary habitability

Now that extrasolar planets have been found, it is timely to ask whether some of them might be suitable for life. Climatic constraints on planetary habitability indicate that a reasonably wide habitable zone exists around main sequence stars with spectral types in the early-F to mid-K range. However, it has not been demonstrated that planets orbiting such stars would be habitable when biologically-damaging energetic radiation is also considered. The large amounts of UV radiation emitted by early-type stars have been suggested to pose a problem for evolving life in their vicinity. But one might also argue that the real problem lies with late-type stars, which emit proportionally less radiation at the short wavelengths (λ < 200 nm) required to split O2 and initiate ozone formation. We show here that neither of these concerns is necessarily fatal to the evolution of advanced life: Earth-like planets orbiting F and K stars may well receive less harmful UV radiation at their surfaces than does the Earth itself.

FUSE Measurements of Far-Ultraviolet Extinction. I. Galactic Sight Lines*

We present extinction curves that include data down to far-ultraviolet wavelengths (FUV; 1050-1200 ?) for nine Galactic sight lines. The FUV extinction was measured using data from the Far Ultraviolet Spectroscopic Explorer. The sight lines were chosen for their unusual extinction properties in the infrared through the ultraviolet; that they probe a wide range of dust environments is evidenced by the large spread in their measured ratios of total to selective extinction, RV = 2.43-3.81. We find that extrapolation of the Fitzpatrick & Massa relationship from the ultraviolet appears to be a good predictor of the FUV extinction behavior. We find that predictions of the FUV extinction based on the Cardelli, Clayton, & Mathis (CCM) dependence on RV give mixed results. For the seven extinction curves well represented by CCM in the infrared through ultraviolet (x < 8 ?m-1), the FUV extinction is well predicted in three sight lines, overpredicted in two sight lines, and underpredicted in two sight lines. A maximum entropy method analysis using a simple three-component grain model shows that seven of the nine sight lines in the study require a larger fraction of grain materials to be in dust when FUV extinction is included in the models. Most of the added grain material is in the form of small (radii 200 ?) grains.

Interstellar polarization in the taurus dark clouds, wavelength dependent position angles and cloud structure near tmc-1

Systematic variations with wavelength in the position angle of interstellar linear polarization of starlight may be indicative of multiple cloud structure along the line of sight. We use polarimetric observations of two stars (HD 29647 and HD 283809) in the general direction of TMC-1 in the Taurus dark cloud to investigate grain properties and cloud structure in this region. We show the data to be consistent with a simple two-component model in which general interstellar polarization in the Taurus cloud is produced by a widely distributed cloud component with relatively uniform magnetic field orientation; light from stars close to TMC-1 suffers additional polarization arising in one (or more) subcloud(s) with larger average grain size and magnetic field directions different from the general trend. Toward HD 29647 in particular, we show that the unusually low degree of visual polarization relative to extinction is due to depolarization associated with the presence of distinct cloud components in the line of sight with markedly different magnetic field orientations. Stokes parameter calculations allow us to separate the polarization characteristics of the individual components. Results are fitted with the Serkowski empirical formula to determine the degree and wavelength of maximum polarization. Whereas λmax values in the widely distributed material are similar to the average (0.55 μm) for the diffuse interstellar medium, the subcloud in the line of sight to HD 283809, the most heavily reddened star in our study, has λmax ≈ 0.73 μm, indicating the presence of grains ~30% larger than this average. Our model also predicts detectable levels of circular polarization toward both HD 29647 and HD 283809.