Perry Alexander Gerakines

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.

Interstellar Extinction and Polarization in the Taurus Dark Clouds: The Optical Properties of Dust near the Diffuse/Dense Cloud Interface

Observations of interstellar linear polarization in the spectral range 0.35-2.2??m are presented for several stars reddened by dust in the Taurus region. Combined with a previously published study by Whittet et al., these results represent the most comprehensive data set available on the spectral dependence of interstellar polarization in this nearby dark cloud (a total of 27 sight lines). Extinction data for these and other reddened stars in Taurus are assembled for the same spectral range, combining published photometry and spectral classifications with photometry from the Two Micron All Sky Survey. The polarization and extinction curves are characterized in terms of the parameters ?max (the wavelength of maximum polarization) and RV (the ratio of total to selective extinction), respectively. The data are used to investigate in detail the question of whether the optical properties of the dust change systematically as a function of environment, considering stars observed through progressively more opaque (and thus progressively denser) regions of the cloud. At low visual extinctions (0 3, real changes in grain properties occur, characterized by observed RV values in the range 3.5-4.0. A simple model for the development of RV with AV suggests that RV may approach values of 4.5 or more in the densest regions of the cloud. The transition between normal extinction and dense cloud extinction occurs at AV ~ 3.2, a value coincident with the threshold extinction above which H2O-ice is detected on grains within the cloud. Changes in RV are thus either a direct consequence of mantle growth or occur under closely similar physical conditions. Dust in Taurus appears to be in a different evolutionary state compared with other nearby dark clouds, such as ? Oph, in which coagulation is the dominant physical process.

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.

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.

Detection of abundant CO2 ice in the quiescent dark cloud medium toward Elias 16

We report the first detection of solid carbon dioxide (CO2) in quiescent regions of a dark cloud in the solar neighborhood, a result that has important implications for models of ice formation and evolution in the interstellar medium. The K-type field star Elias 16 was previously known to display solid-state absorption features of H2O and CO ices arising in the Taurus Dark Cloud. Our detection of the CO2 feature at 4.27 μm in this line of sight implies a column density N(CO2)=4.6+ 1.3−0.6×1017 cm-2, equivalent to ~18% and 70% of the H2O and CO column densities, respectively. Comparison with laboratory data indicates that (unlike CO) the CO2 resides primarily in a polar (H2O-rich) component of the ices. CO2 is formed easily in the laboratory by the photolysis of ice mixtures containing CO, but the detection toward Elias 16 indicates that CO2 formation can occur in dark clouds in the absence of a local embedded source of radiation. Possible alternative mechanisms for CO2 production include grain surface reactions and energetic processing driven by the interstellar radiation field or cosmic rays.

The Abundance of Carbon Dioxide Ice in the Quiescent Intracloud Medium

We present new observations with the Infrared Spectrograph on board the Spitzer Space Telescope of the solidCO2 absorption feature near 15 � m in the spectra of eight field stars behind the Taurus complex of dark clouds. Solid CO2isdetectedinsixlinesofsight.Newresultsarecombinedwithpreviousdatatoinvestigatethecorrelationof CO2 columndensitywiththoseofother majoriceconstituents(H2OandCO)andwithextinction.CO2isshowntodisplaya ‘‘thresholdextinction’’ effect, i.e., a minimumextinction(A0 ¼ 4:3 � 1:0 mag)requiredfordetection,behaviorsimilar to that previously reported for H2O and CO. We find a particularly tight correlation through the origin between N(CO2) and N(H2O), confirming that these species form in tandem and coexist in the same (polar) ice layer on the grains. The observedcomposition of themantlesisbroadlyconsistentwiththepredictionsof photochemical modelswithdiffusive surface chemistry proposed by Ruffle & Herbst. Comparison of our results for Taurus with published data for Serpens indicatessignificantdifferencesinicecomposition consistentwithenhancedCO2productioninthe lattercloud.Ourresultsalsoplaceconstraintsonthedistributionofelementaloxygenbetweenicesandotherpotentialreservoirs.Assuming aconstant N(H)toextinctionratio,weshowthat ~65%of thesolarOabundanceisaccountedforbysummingthecontributions of ices (~26%), refractory dust (~30%) and gas-phase CO (~9%). If the Sun is an appropriate standard for the interstellar medium, the ‘‘missing’’ oxygen may reside in atomic O i gas and/or (undetected) O2 within the ices.

Mid- and far-infrared spectroscopic studies of the influence of temperature, ultraviolet photolysis and ion irradiation on cosmic-type ices.

Infrared (IR) studies of laboratory ices can provide information on the evolution of cosmic-type ices as a function of different simulated space environments involving thermal, ultraviolet (UV), or ion processing. Laboratory radiation experiments can lead to the formation of complex organic molecules. However, because of our lack of knowledge about UV photon and ion fluxes, and exposure lifetimes, it is not certain how well our simulations represent space conditions. Appropriate laboratory experiments are also limited by the absence of knowledge about the composition, density, and temperature of ices in different regions of space. Our current understanding of expected doses due to UV photons and cosmic rays is summarized here, along with an inventory of condensed-phase molecules identified on outer solar system surfaces, comets and interstellar grains. Far-IR spectra of thermally cycled H2O are discussed since these results reflect the dramatic difference between the amorphous and crystalline phases of H2O ice, the most dominant condensed-phase molecule in cosmic ices. A comparison of mid-IR spectra of products in proton-irradiated and UV-photolyzed ices shows that few differences are observed for these two forms of processing for the simple binary mixtures studied to date. IR identification of radiation products and experiments to determine production rates of new molecules in ices during processing are discussed. A new technique for measuring intrinsic IR band strengths of several unstable molecules is presented. An example of our laboratory results applied to Europa observations is included.

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.

SOLID CARBON DIOXIDE IN REGIONS OF LOW-MASS STAR FORMATION

We present high-resolution (R D 1500¨2000) spectra of the 4.27 km asymmetric stretching feature of solid in eight lines of sight observed with the Short Wavelength Spectrometer of the Infrared Space CO 2 Observatory. Two of the sources are —eld stars located behind the Taurus molecular cloud; the others are young stellar objects (YSOs) of predominantly low-to-intermediate mass. We —nd a signi—cant source-tosource variation in the solid abundance ratio in our sample: two lines of sight, Elias 18 and CO 2 /H 2 O RAFGL 989, have abundances of D 34%¨37%, considerably higher than in other lines of sight CO 2 studied to date. In agreement with a previous study of Elias 16, we con—rm a substantial (D20%) abundance of solid relative to in the quiescent intracloud medium. We compare the pro—les CO 2 H 2 OC O 2 with laboratory spectra of interstellar ice analogs from the Leiden Observatory Laboratory database. Results show that the 4.27 km pro—les toward —eld stars and embedded low-mass objects are remarkably similar to each other and seem to originate mostly in cold ice. In two higher mass YSOs H 2 O-rich (RAFGL 989 and S255 IRS1), the pro—les are clearly diUerent, and at least the latter source shows signs of thermal processing. . .. . .. . ..