ISO Observations of Interstellar Ices and Implications for Comets

ICARUS

130, 1–15 (1997) IS975795

ARTICLE NO.

ISO Observations of Interstellar Ices and Implications for Comets P. Ehrenfreund Leiden Observatory, P.O. Box 9513, 2300 RA Leiden, The Netherlands E-mail: [email protected]

L. d’Hendecourt and E. Dartois Institut d’Astrophysique Spatiale, Bat.121, Campus d’Orsay, 91405 Orsay, France

M. Jourdain de Muizon Observatoire de Paris, DESPA, 92190 Meudon, France, and LAEFF-INTA, ESA Satellite Tracking Station, P.O. Box 50727, 28080 Madrid, Spain

M. Breitfellner ISO Science Operations, ESA/VILSPA Satellite Tracking Station, P.O. Box 50727, 28080 Madrid, Spain

J. L. Puget Institut Astrophysique Spatiale, Bat.121, Campus d’Orsay, 91405 Orsay, France

and H. J. Habing Leiden Observatory, P.O. Box 9513, 2300 RA Leiden, The Netherlands Received October 10, 1996; revised June 20, 1997

molecules such as CH4, H2CO, and OCS which provide important constraints on the origin of cometary ices and for cometary evolution.  1997 Academic Press

We report on first ISO observations of interstellar ices in the direction of the protostellar object RAFGL 7009S. Due to its extreme extinction this source represents a unique target for the detection of interstellar ices. Identified molecules include H2O, CO, and CO2 together with 13CO2, CH4, OCS, and H2CO. Other less firmly identified features are observed that appear in laboratory spectra of interstellar ice analogs. The evolution of interstellar dust grains plays an active role in interstellar chemistry and determines solid state and gas phase abundances. During their lifetime dust grains cycle between dense and diffuse clouds and undergo considerable metamorphism. Comets are likely the least evolved bodies in the Solar System and comet nuclei may be low density aggregates of interstellar dust. An important constraint for the origin and evolution of comets can be derived from the presence of pre-solar ices and organics. To study volatiles and grains in the cometary coma is one of the future goals of the ROSETTA comet rendezvous mission. In comparison with new ISO data we present laboratory studies on interstellar ice analogs which reveal the composition and structure of ices in dense molecular clouds. We discuss the ubiquitous presence and high abundances of interstellar CO2 ice in the cometary context and estimate column densities of

1. INTRODUCTION

The accretion of molecules on interstellar grains and the coagulation of particles are efficient processes in quiescent clouds. During the evolution of molecular clouds into protostellar regions, grains are exposed to considerable metamorphism (Dorschner and Henning 1995). In star formation regions, heating, radiation, and shocks provoke grain processing, which can result in molecule desorption, grain explosion, or total grain destruction. The evolution of interstellar grains plays an active role in interstellar chemistry and determines solid state and gas phase abundances. Interstellar ices such as H2O, CO, and CH3OH cover the grain mantles in dense clouds and have been studied by ground-based infrared spectroscopy in the direction of protostars (Willner et al. 1982, Grim et al. 1991). In IRAS–LRS spectra, CO2 has been detected in the direction 1 0019-1035/97 $25.00 Copyright  1997 by Academic Press All rights of reproduction in any form reserved.

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of a few protostars through its bending mode at 15.2 em (d’Hendecourt and Jourdain de Muizon 1989). Energetic UV processing of icy grain mantles in the natal cloud and in the vicinity of protostars creates new molecules and radicals which are potential targets for observations. Regions with low density are characterized by a high H/CO gas phase ratio and hydrogen reactions will lead to the formation of polar ices (H2O-rich ices). Hydrogenated species are preferentially located in grain mantles accreted at low densities, where atomic H is an important component in the gas phase (Tielens & Whittet 1997). In high density regions with a low H/CO ratio, grains accrete a mantle of apolar ices dominated by CO, CO2, N2, and O2. Temperature differences within the cloud lead to loss or enrichment of certain molecules due to the different volatilities of ices. Most volatile are species such as CO, O2, and N2, which will be the first molecules evaporated around luminous protostars, followed by CO2. Above 90 K all ices including H2O and CH3OH are evaporated under interstellar conditions (Tielens and Whittet 1997). In this context theoretical models and current observations propose the existence of polar and apolar ices arranged in ‘‘onion shells’’ on grain mantles or different grain populations (Tielens et al. 1991, Whittet 1993). Observations of molecular clouds therefore sample the signatures of grains which are processed in the different cloud environments (differing in temperature, density, and UV properties). Only combined information on gaseous species, gas/solid ratios, and profiles of solid state molecules will reveal the conditions and evolution of molecular clouds. Comets are likely the least evolved bodies in the solar system. The origin of comets is extensively discussed by Mumma et al. (1993a). The observed low NH3 and CH4 abundance, the CO/CH4 ratio, and the D-enrichment (relative to solar values) of H2O are key indicators of the interstellar origin of cometary volatiles. Observations of H2CO and CH3OH in comets show higher abundances than expected for solar nebula processes (Mumma et al. 1993a). Comet nuclei might contain low density aggregates of interstellar dust (Greenberg 1982). The interstellar nature of ices in some comets is also supported by observations of Hyakutake. The detection of CH4, C2H6, C2H2, the D/H ratio in water, and the HNC/HCN ratio in Hyakutake was recently reported (Mumma et al. 1996, Brooke et al. 1996, Irvine et al. 1996). The detection of abundant C2H6 and CH4 with respective abundances of 0.4 and 0.7% relative to water ice in Hyakutake showed evidence for an interstellar origin (Mumma et al. 1996). The measured abundance of HNC relative to HCN is very similar to that observed in quiescent interstellar molecular clouds and quite different from the equilibrium ratio expected in the outermost solar nebula, where comets are thought to form (Irvine et al. 1996). C2H2 has been observed with an abundance consistent with observations of interstellar dense

clouds (Brooke et al. 1996). A comprehensive review on this subject is given by Mumma (1996, 1997). An important constraint on the origin and evolution of comets can be obtained from evidence as to what extent interstellar ices have been preserved in comets. In the solar nebula, interstellar dust volatiles may have evaporated and undergone subsequent chemistry before becoming incorporated into comets. The preservation of the nucleus may have been affected by heating from residual radioactive species, thermal evolution resulting from solar radiation, and, to a smaller extent, from cosmic ray penetration and comet collision (Greenberg 1993). Recent observations of important volatiles such as CH4 and CO2 in new cometary targets (Mumma et al. 1996, Crovisier et al. 1996, 1997) can now be directly compared to ISO measurements of interstellar gas phase species and ices. This will strongly guide the interpretation of comet observations concerning cometary origin and evolution. The ISO (Infrared Space Observatory) makes it possible for the first time to study the complete inventory of interstellar ices using high resolution spectroscopy (Whittet et al. 1996). The first ISO results showed the ubiquitous presence of abundant CO2 ice in space. Four young stellar objects (YSOs) embedded in molecular clouds and the Galactic Center source Sgr A* show CO2 abundances between 12–16% relative to H2O ice (de Graauw et al. 1996). The Galactic center source is presumably obscured by foreground clouds (de Graauw et al. 1996). In contrast, a low abundance of solid CH4 has been observed in the direction of the deeply embedded protostars W33A and NGC 7538 IRS9 (Boogert et al. 1996). For several objects, such as NGC 7538 IRS9, AFGL 2136, AFGL 4176, and AFGL 2591, the solid to gas ratio of H2O and CO2 could be assessed, and showed that those objects differ in their evolutionary stage and in the fraction of high temperature gas in the line of sight (van Dishoeck et al. 1996, van Dishoeck and Helmich 1996). Whereas a rather constant abundance of CO2 ice is observed in the directions of the same YSOs, the solid CO abundance is drastically different and indicates the increasing thermal processing of ices in lines of sight which show abundant warm gas (de Graauw et al. 1996, Whittet et al. 1996). Large amounts of solid CO (up to 45% relative to water ice) have been observed by ground-based observations in the Taurus and Serpens clouds (Chiar et al. 1994). Such environments are likely very cold and ‘‘pristine ices’’ may be observed in the Taurus cloud at the end of the ISO mission. Laboratory spectroscopy databases of low temperature ices, performed in preparation for ISO, permit the identification of all these molecules and the determination of accurate abundances. In RAFGL 7009S, we report the detection of the following interstellar ices: H2O, CO, XCN, 12CO2, 13CO2, H2CO, and CH4. The presence of OCS (or CO3) as well as of various hydrocarbons is suggested by the overall identifi-

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cation with laboratory spectra of UV irradiated ices. The exceptionally high column densities of solid state features in the direction of RAFGL 7009S and the large amount of CO2 ice indicate the unique nature of this target. We discuss first observations in the direction of RAFGL 7009S and their implications for past and future cometary observations. 2. RAFGL 7009S

The protostellar source RAFGL 7009S was selected on the basis of its 8–22 em IRAS–LRS spectrum, showing the presence of a very deep, saturated, silicate feature, along with H2O and CO2 ice (d’Hendecourt et al. 1996). The object RAFGL 7009S has been observed on April 17th, 1996 by the ISO (Infrared Space Observatory) with the SWS (Short Wavelength Spectrometer) on board in the 2.5–45 em range in AOT1 mode, speed 3, corresponding to a resolution of p500 and in AOT6 mode with a high resolution (of p1500). Data reduction was performed using the Interactive Analysis software installed at the ISO Science Operations Center (SOC) in Vilspa. The source has such a high extinction that H2O at 3 em, CO2 at 4.27 em, and the prominent silicate absorption at 9.7 em are deeply saturated. Figure 1 shows the ISO SWS spectrum of RAFGL 7009S between 2 and 18 em, displaying ice bands such as CO2, CO, H2CO, H2O, OCS (or CO3), and CH4 in comparison with a laboratory spectrum of a photolyzed ice mixture. The abundance of several molecules may be relevant for comets. The improvement of reduction methods during the ISO mission will help to reveal weak solid state features of interest in the direction of this source. A global comparison with laboratory data shows that most of the solid state features found in the spectrum of RAFGL 7009S are reproduced by solid state features in the laboratory. Exceptions are the two silicate bands at 10 and 18 em and the 6.85 em band. This latter band might originate from hydrocarbons, CH3OH or NH41 (see Whittet 1993 for a review, Schutte et al. 1996b), or carbonates (Hecht et al. 1986). Whereas all these molecules might contribute to this strong band, its real origin is still a mystery. Laboratory data were obtained by condensing pure gas or gas mixtures on the surface of a cooled CsI window (4–12 K) mounted in a high vacuum chamber. Infrared transmission spectra were obtained with a BioRad FTS 40A spectrometer and a Brucker FTS IFS 66v spectrometer at a resolution of 1 or 2 cm21. Part of the samples were photolyzed with a microwave excited hydrogen flow lamp, producing a flux of approximately 1015 photons cm22 s21 in order to simulate the interstellar radiation field (Gerakines et al. 1996). One hour UV irradiation in the laboratory corresponds to a radiation of p103 yr in cloud outskirts (assuming an interstellar radiation field of 8 3 107 photons

FIG. 1. ISO SWS spectrum of the source RAFGL 7009S between 2.5 and 18 em (bottom), compared with a laboratory spectrum of a photolyzed ice mixture H2O : CO : CH4 : NH3 : O2 5 10 : 2 : 1 : 1 : 1 (top). Since the astronomical spectrum can be taken as a transmittance spectrum of a continuum through the cold dense medium, the laboratory spectrum has been multiplied by a 485 K black-body continuum. From left to right, the following absorptions can be noted: the strong band of solid CO2 falls at 4.27 em. Between 4.6 and 4.7 em the XCN feature is present together with the solid CO band. The small band at 4.9 em is attributed to OCS (or CO3). The bending mode of water ice at 6 em shows on the blue wing the presence of solid H2CO (at 5.81 em). Hydrocarbons and CH3OH may contribute to the 6.85 em band; the bulk of the band, however, is still unidentified. The CH4 feature at 7.7 em is clearly seen. The strong (saturated) silicate feature falls at 9.7 em and its reproduction has not been attempted in our laboratory spectrum. The 13 em libration mode of water ice is responsible for the dip between the two silicate features (at 10 and 18 em). Finally, the bending mode of CO2 at 15.2 em shows a three-peak structure (see Fig. 4). The laboratory spectrum has been shifted up by 20 Jy.

cm22 s21, Mathis et al. 1983) and to p108 years in dense clouds with a UV flux of 103 photons cm22 s21 (induced by cosmic rays, Sternberg et al. 1987). In comparison with laboratory measurements we derive abundances of some ice components towards RAFGL 7009S, listed in Table I. The abundances of H2O and 12CO2 have been estimated from their respective bending modes and have a limited accuracy. The H2O bending mode is contaminated by other molecules (such as H2CO) and an incorrect baseline definition can introduce errors, so that the derived column density represents an upper limit. The real H2O abundance may be as much as 15% lower. The uncertainties in the continuum level at 15 em might influence the abundance of 12CO2 (within 610%) as well as the 12 CO2/13CO2 ratio of p60. It has to be stated that these preliminary ISO spectra lead to abundance estimates whose accuracy will be improved due to on-going calibration of the instruments during the ISO mission. The improvement of reduction methods expected in the

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TABLE I Column Densities of Solid State Features in the Direction of RAFGL 7009S from a ISO SWS01 Scan, Using the Band Intensities Am Measured in the Laboratory

a

Gerakines et al. (1995). d’Hendecourt and Allamandola (1986). c Tegler et al. (1995). d Schutte and Greenberg (1997). e Schutte et al. (1996a). f Hudgins et al. (1993). b

near future will also undoubtedly reveal weak solid state features of interest towards this source. In the following section we discuss the spectrum of RAFGL 7009S between 2.5 and 18 em (increasing in wavelength) and explain the properties of interstellar ices in the direction of this target and their implications for comets. 3. INFRARED ACTIVE ICES

3.1. CO2 Solid CO2 has been detected in the direction of a few protostars through its bending mode at 15.2 em and in AFGL 961 the CO2 abundance was estimated to be 3% that of water ice (d’Hendecourt and Jourdain de Muizon 1989). Re-analysis of IRAS spectra indicated that CO2 is a widespread component in interstellar ices (d’Hendecourt and Ehrenfreund 1996). The ubiquitous presence of CO2 has been confirmed by ISO through the detection of the strong stretching mode at 4.27 em in various interstellar environments (de Graauw et al. 1996, d’Hendecourt et al. 1996, Guertler et al. 1996). The CO2 abundance is rather constant towards different objects and of the order of 15% relative to water ice. An even larger amount of solid CO2 (.20%) has been detected in the direction of RAFGL 7009S. The search for gaseous CO2 towards embedded sources showed low abundances of less than 5% of the solid CO2 (van Dishoeck et al. 1996). Gaseous CO2 towards RAFGL 7009S has been measured to be 4% relative to solid CO2 (Dartois et al. 1997). This behavior is in strong contrast to that of gaseous CO, which is 10–100 times

more abundant in the gas than in the ice (van Dishoeck et al. 1996). Recent laboratory results on ‘‘CO2-containing ices’’ shed new light on the behavior of this important molecule in interstellar space as well as in comets (Ehrenfreund et al. 1996a, 1997). CO2 forms particular T-shaped complexes with other molecules present, which lead to ‘‘strong perturbations’’ within the ice, resulting in shifts and broadening of the CO2 band profiles as well as other bands such as H2O and CO (see Ehrenfreund et al. 1996a, 1997). From the profiles of CO2 we can therefore determine the ice composition and derive information on the ice structure. Both parameters may indicate how CO2 will be released in comets. Figure 2 shows the stretching mode of CO2 at 4.27 em and the 13CO2 band at 4.38 em. A laboratory spectrum has been superimposed. The 12CO2 stretch is saturated towards RAFGL 7009S and no exact fit is attempted here. Due to the large column density of solid CO2 a prominent 13 CO2 band is observed. Using the bending mode at 15.2 em we estimate a 12CO2/13CO2 ratio of 60. This value is typical for the interstellar medium (terrestrial value: p90). Figure 3 shows the bending mode of CO2 towards RAFGL 7009S, revealing two subpeaks with equal intensity at 15.1 and 15.23 em and an asymmetric red wing at 15.4 em. The sharp peak on the blue wing at 14.98 em is due to gas phase CO2 (van Dishoeck et al. 1996, Dartois et al. 1997). The double-peak structure of the bending mode of pure CO2 is formed because the vibration is doubly degenerate and the band splits when the axial symmetry of the molecule is broken (Sandford and Allmandola 1990).

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FIG. 2. ISO SWS spectrum of RAFGL 7009S showing the stretching mode of 12CO2 at 4.27 em and 13CO2 at 4.38 em. The 12CO2 band is saturated; the superimposed laboratory mixture of H2O : CO2 5 2 : 1 is used to show a reasonable fit to the isotope band 13CO2.

Within 80 experiments this particular double peak structure is only observed for a phase where CO2 is only interacting with itself, likely by forming a second trapping site in the ice. Figure 4 shows a warm-up sequence for the bending mode of CO2 in a CO/H2O 5 6 : 1 ice mixture and illustrates this important process. At 10 K strong complexes between CO2 and H2O result in a very broad band. With increasing temperature this broad band is converted slowly into a

FIG. 3. ISO SWS spectrum of RAFGL 7009S showing the bending mode of CO2 at 15.2 em. A three-peak structure is observed, showing two narrow peaks with equal intensity at 15.1 and 15.23 em (resembling pure or annealed CO2) and a strong asymmetric wing (centered at 15.4 em), which can be fitted by polar ices (de Graauw et al. 1996). The sharp peak at 14.98 em is due to gas phase CO2. A laboratory spectrum of the CO2 bending mode is shown in Fig. 4 for comparison.

FIG. 4. A warm-up sequence of the bending mode of CO2 in a CO/H2O 5 6 : 1 is shown. Strong aggregates lead to a broad band at 10 K. During the annealing process CO2 and H2O are separated and reside at 75 K as two independent components in the ice. According to a recent database on CO2 ice (containing more than 70 different mixtures) the two narrow peaks (showing equal intensity) are unique for pure or annealed CO2 (Ehrenfreund et al. 1996a, 1997).

double-peak structure, because the water ice and the CO2 ice are segregated into two different ‘‘families,’’ without reacting any more with each other. This process is called ‘‘annealing’’ and has important implications for grain mantles. The two sharp peaks which are observed for pure or annealed CO2 have been detected in ISO spectra and show that CO2 is present in dense clouds in a phase distinct from polar ice. This component of pure or annealed CO2 shows a constant band profile towards RAFGL 7009S (d’Hendecourt et al. 1996), AFGL 2136 and NGC 7538 IRS9 (de Graauw et al. 1966) in addition to a broader component which is attributed to polar CO2 (diluted in water ice). Preliminary estimates of the polar versus apolar CO2 range from 1.1 to 1.8. For NGC 7538 IRS 9 a value of 1.6 is reported (Whittet et al. 1996). However, the amount of polar versus apolar CO2 has to be estimated with caution, because the particle shape of grains can have an effect on the CO2 profile (Ehrenfreund et al. 1997). Therefore these first estimates may be refined. The fraction of polar CO2, however, seems to exceed the apolar fraction in all observed objects. From first ISO spectra we can conclude that a grain mantle layer of pure CO2 is present in the lines-of-sight toward several molecular clouds (representing different

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temperatures and evolution). This has strong implications for interstellar chemistry. The constant CO2 bending mode profile argues more for a separate layer on grains than for a distinct grain population. These CO2 grain mantles could eventually be formed in temperature regions, where volatile molecules such as CO, O2, and N2 are already evaporated and CO2 or CO2/H2O ice layers remain. The laboratory results shown in Fig. 4 illustrate the detailed information which can be obtained from laboratory spectroscopy. From CO2 profiles we can furthermore derive information about irradiation and temperature history of grains in the line-of-sight, which helps to constrain the role of CO2 in interstellar space and for comets (Ehrenfreund et al. 1996a, 1997). CO2 is readily formed in the laboratory by UV irradiation of ices containing H2O, CO, and O2 (d’Hendecourt et al. 1986, Ehrenfreund et al. 1996a, 1997). The formation of solid CO2 in the interstellar medium is not yet elucidated, however. A main route for CO2 formation could be UV photolysis of H2O/CO ices near protostars. Due to the extreme efficiency of this reaction, CO2 may also be formed by this process in embedded regions far from protostars. A source for UV photons in embedded clouds is cosmic ray excitation of H2 molecules, with an expected photon flux of 103 photons per cm2 per second (Sternberg et al. 1987). The ground state of CO2 correlates with CO and O(1D) in the dissociation limit; therefore formation of CO2 from CO and O requires the presence of (excited) O(1D). This can be produced by photodestruction of H2O or of O2, both of which produce singlet oxygen. This is undoubtedly the reason CO2 is successfully produced in ultraviolet processing of astrophysical and cometary ice analogs. The production yield of CO2 by UV photolysis in various ice mixtures is summarized in Ehrenfreund et al. (1997). However, Tielens and Whittet (1997) argue that long accretion time scales in space may allow oxidation reactions, forming CO2 from CO and O on grains. Future observations along lines of sight with low amounts of grain processing, and laboratory experiments of reaction kinetics, will reveal the original conditions forming CO2. Recent ISO results show that CO2 has to be introduced as an important molecule in the chemical network of interstellar gas-phase chemistry. This will open new chemical pathways and force us in the future to consider reaction products of CO2 such as CO3, C3O, O3, possibly carbonates, and many others. 3.2. CO and the ‘‘XCN’’ Band The solid CO and XCN features were first detected along the lines of sight in the directional embedded protostars by Lacy et al. (1984). High abundances of solid CO have been found so far only in the Serpens dark cloud complex, where the abundance of solid CO reaches 45% relative to

FIG. 5. ISO SWS spectrum of RAFGL 7009S between 4.5 and 5 em shows the solid CO band at 4.67 em, accompanied by the XCN band on the blue wing at 4.62 em and a small feature at 4.9 em which is attributed to OCS. The CO and XCN bands at 4.67 and 4.62 em were first detected in the protostellar object W33A (Lacy et al. 1984). RAFGL 7009S shows many similarities with the ground based observations available for the protostellar object W33A and the H2O/XCN ratio is similar in both objects. Also the 4.9 em band is observed toward both targets. The contribution of gaseous CO to the XCN band has been estimated to be negligible.

water ice (Chiar et al. 1994). Laboratory studies of the shape and peak position of the solid CO band in astrophysically relevant ice mixtures show a two-component structure of solid CO at 4.67 em (Sandford et al. 1988). A narrow CO band is observed in apolar ices (containing CO, CO2, O2, and N2) and a broad component originates in H2Orich (or polar) ices. Many lines of sight contain (at least) two independent grain mantle components (Tielens et al. 1991). Figure 5 shows the spectrum of the CO features which peaks at 4.675 em. It is unusually broad and dominated by apolar CO. Solid CO is estimated to be 18% relative to water ice towards the object RAFGL 7009S. The CO band is accompanied by the XCN band on the blue side at 4.62 em. Gas phase CO in the line of sight can contaminate the XCN band. Our estimates, however, indicate only a small contribution of gaseous CO. The XCN band at 4.62 em appears as a strong feature in some interstellar spectra in dense clouds (Tegler et al. 1995) and carriers such as nitriles (Bernstein et al. 1995), OCN2 (Grim and Greenberg 1987, Schutte and Greenberg 1997), or SiH stretching vibrations (Moore et al. 1991) have been suggested. Upper limits of XCN column densities for 10 pre-main-sequence stars have been derived (Tegler et al. 1995). The presence of the XCN band in such stars and the absence of the XCN band in the field star Elias 16 indicate that this feature is created by UV photoprocessing,

INTERSTELLAR ICES AND COMETS

FIG. 6. The high resolution ISO SWS spectrum of RAFGL 7009S between 4.8 and 5.2 em shows the band at 4.9 em which is attributed to OCS. At this high resolution the rovibrational transitions of CO from high J levels can be resolved and are seen throughout the spectrum.

as observed in the laboratory. Tegler et al. (1995) conclude that this band is produced by photoprocessing of apolar ices containing N2. However, only cosmic rays have sufficient energy to dissociate N2, producing free N-atoms. Laboratory results show that the XCN band is not formed by UV photolysis of apolar ices (Ehrenfreund et al. 1997, Elsila et al. 1997). UV photolysis of HMT (hexamethylenetetramine C6H12N4) frozen in H2O readily produces the XCN band as well as other nitriles (Bernstein et al. 1995). The excellent fit of the interstellar XCN band with the n3 mode of OCN2, produced from photolysis of CO and NH3 and confirmed by isotopic shift measurements, makes this ion an interesting candidate for the XCN band (Schutte and Greenberg 1997). The band intensity of the XCN feature has been measured by various authors and varies according to the proposed carrier molecule between 2 and 7 3 10217 cm mol21 (d’Hendecourt and Allamandola 1986, Grim and Greenberg 1987, Schutte and Greenberg 1997). The XCN band towards RAFGL 7009S therefore represents a carrier abundance between 3 and 12% compared to water ice. Despite its debated origin, this band is produced by UV irradiation of interstellar ice analogs in various pathways and could therefore be an important component for defining radiation history towards RAFGL 7009S and other targets. 3.3. OCS versus CO3 The 4.9 em band seen in the direction of RAFGL 7009S, displayed in Figs. 5 and 6, was first detected in the direction of the protostar W33A and assigned to OCS (Geballe et

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al. 1985, Palumbo et al. 1995). This absorption has been recently detected toward several other young stellar objects (Palumbo et al. 1997). Figure 6 shows the high resolution ISO spectrum (AOT6) of the OCS band along with gaseous CO. In the direction of W33A the OCS/H2O ratio was estimated to be p4 3 1024. Solid OCS has been studied by Hudgins et al. (1993) and shows strong profile variations according to the ice mixture. Weaker features such as the CuS stretch and the OUCUS bend fall at 11.62 and 19.23 em, respectively. OCS is a common gas phase molecule in the interstellar medium, present in both quiescent molecular clouds and regions of star formation. In the direction of W33A more OCS exists in the solid phase than in the gas phase (Palumbo et al. 1995). OCS might be created by grain surface reactions of atomic S with CO and atomic O with CS or by UV irradiation of ices containing CO and H2S. Using the measured band intensity (Hudgins et al. 1993) of 1.5 3 10216 cm mol21, we derive an OCS abundance of 0.2% relative to H2O towards RAFGL 7009S. This abundance is rather high and suggests the contribution of an additional component. Taking into account the ubiquitous presence of CO2 in interstellar space, CO3 seems another candidate which could contribute to this band, peaking at 4.892 em. CO3 shows also other weaker bands at 5.31, 9.37, and 10.3 em, respectively (Gerakines et al. 1996). When we attribute the observed band at 4.9 em to CO3 we can estimate an abundance of p5% compared to water ice, which is a rather high value. The band intensity for CO3 (5.4 3 10218 cm mol21) was measured (Gerakines et al. 1996) in a study of the UV radiation of pure CO2 by assuming that all C not tied up in CO and CO2 is contained in CO3. UV irradiation of pure CO2 ice for one hour (see Experimental) can produce a fraction of 15% CO3 in the ice compared to CO2. Our 4.9 em band is observed at 4.892 em with a FWHM of 0.0278 em. Though the CO3 band can not account for the large width of the 4.9 em observed towards RAFGL 7009S, it is possible that CO3 contributes to this feature. Future laboratory experiments and detailed fits of this particular band (including particle shape calculations) and the search for other weak CO3 features will reveal how much CO3 is present towards RAFGL 7009S. 3.4. H2CO Figure 7 shows the spectrum of RAFGL 7009S between 5.5 and 7.5 em, displaying the bands of water ice at 6 em; the blue wing of the spectrum shows the possible contribution of H2CO at 5.81 em. The band at 6.85 em is still unidentified and might be due partly to hydrocarbons and/or CH3OH. Other suggested carriers are carbonates (Hecht et al. 1986). Carbonates are known to show a strong band at 6.8 em, but also display other weaker bands be-

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et al. 1996). The two strong POM bands at 8.61 and 10.67 em will be hard to detect in any interstellar spectra since they are located within the strong 10 em silicate band, which is often saturated. Although it cannot be excluded that the polymerization products of H2CO contribute to the band at 5.81 em, this band has been assigned tentatively to solid H2CO. From the ISO-SWS06 spectrum (resolution 1500) in the direction of RAFGL 7009S an abundance of H2CO of p3% relative to water ice was derived (d’Hendecourt et al. 1996). 3.5. CH3OH

FIG. 7. ISO SWS spectrum of RAFGL 7009S (bottom) between 5.5 and 7.5 em in comparison with a laboratory spectrum of a photolysed ice mixture (top) showing the strong water bending mode at 6 em and the unidentified 6.85 em band, which cannot be attributed to simple ices. The laboratory spectrum has been multiplied by a 485 K black-body continuum and shifted by 20 Jy. On the blue wing the molecule H2CO is detected at 5.81 em. The abundance of H2CO (p3% relative to water ice) has been calculated from the high resolution spectrum of RAFGL 7009S (d’Hendecourt et al. 1996).

tween 10 and 15 em, which are not observed in astronomical spectra. Laboratory results show that those bands at longer wavelengths might be suppressed in specific amorphous configurations (Knacke and Kraetschmer 1980). The large amounts of CO2 found ubiquitously in the interstellar medium suggest that reaction products such as carbonates may be present as well. However, the conditions for carbonate formation are generally not present in the interstellar medium. Pure solid H2CO displays strong sharp bands at 5.81 (n2 band) and 6.69 em (n3 band) in addition to weak bands at 3.47 and 3.54 em (n5 and n1 bands, respectively). The band at 5.81 em (Fig. 7) can be attributed either to pure solid H2CO or to the terminal groups of H2CO polymers such as POM (polyoxymethylene). POM groups show many additional bands. The strongest features occur at 8.61 and 10.67 em (Gerakines et al. 1996). The detection of the weak n5 mode of solid H2CO in the NIR at 3.47 em by ground-based observations with an abundance of 7% H2CO relative to H2O has been reported in the direction of AFGL 2136 (Schutte et al. 1996b), but has to be confirmed by ISO observations of the strong H2CO features between 5 and 7 em. In the direction of RAFGL 7009S the weak n1 and n5 bands of H2CO in the NIR are buried in the saturated water band and cannot be observed. A strong n2 band seems to be present at 5.81 em and indicates the presence of H2CO in this line of sight (d’Hendecourt

Formation of H2CO and CH3OH in the interstellar gas seems to be inefficient (Millar et al. 1991). Both molecules are, however, highly abundant in the interstellar gas. Probably H2CO and CH3OH are formed on the grain surface and thereafter desorbed in the gas phase. H2CO and CH3OH might be formed by grain surface hydrogenation upon CO or by UV photolysis of ices containing H2O and CO. Methanol has a very rich infrared spectrum. The detection of the 3.54 em band, as well as other weaker absorptions at 3.85 and 3.94 em, has been reported in dense clouds (Allamandola et al. 1992, Geballe 1991). The CO stretch and CH3 rock of solid CH3OH have been observed towards AFGL 2136 leading to an estimated abundance of 10% CH3OH relative to water ice (Skinner et al. 1992). The prominent band at 6.8 em has been previously assigned to methanol, but created strong abundance problems (Allamandola et al. 1992). Recent studies conclude that CH3OH would only have a small contribution to the 6.8 em band (Schutte et al. 1996b), consistent with abundances of CH3OH relative to H2O which are below 10% (Whittet et al. 1996). Unfortunately the amount of CH3OH cannot be studied well in the direction of RAFGL 7009S. The saturation of the water ice band at 3 em makes it impossible to study methanol bands at 3.54, 3.85, and 3.94 em. The CUOH stretch at 9.7 em falls in the deeply saturated silicate band. It is to be hoped that the improved reduction will make it possible to determine the abundance of CH3OH at a later stage by deconvolving its contribution to the 6.8 em band. 3.6. CH4 Ground based and airborne (KAO) observations indicated the presence of CH4 in several objects (Lacy et al. 1991, Boogert et al. 1997). In the direction of RAFGL 7009S solid CH4 has been detected at 7.7 em with an abundance of p4% relative to water ice; see Fig. 8. Solid CH4 has also been observed with ISO in the direction of W33A and NGC 7538/IRS9 and estimates for the abundance range from 0.4 to 1.9% compared to water ice (Boogert et al. 1996). Interstellar CH4 is probably formed on

INTERSTELLAR ICES AND COMETS

FIG. 8. The deformation mode of CH4 in the laboratory data (top) as compared to the astronomical feature (bottom). CH4 resides in the polar phase and can be well fitted with a mixture H2O : CH4 5 10 : 1 (or 10 : 0.5). The laboratory spectrum has been multiplied by a 485 K blackbody continuum and shifted by 0.2 Jy.

grain surfaces by hydrogenation of atomic C or by UV photolysis of CH3OH in ice mantles (Boogert et al. 1997). In both cases it will be included in the polar ice phase. For RAFGL 7009S the gas/solid ratio of CH4 was estimated to be 0.28 (Dartois et al. 1997). Other ISO observations also indicate a low gas phase abundance of CH4, namely Ngas/Nice , 1 (Boogert et al. 1996). 4. INFRARED INACTIVE MOLECULES O2 AND N2

Large amounts of O and N are apparently missing in the interstellar gas. Theoretical models support the idea that large amounts of O and N could be depleted on grains in the form of solid O2 and N2 (van Dishoeck et al. 1993). Searches for gas phase O2 were not successful and provided limits of O2/CO , 0.014 (Combes and Wiklind 1995) and 0.1 (Marechal et al. 1997). Recent measurements of the 63 em line in absorption with KAO and ISO indicate, however, that most of the oxygen may be present in atomic form (Poglitsch et al. 1996, Baluteau et al. 1997). Most of the nitrogen is thought to reside in the form of gaseous N2, but only indirect estimates from the N2H1 ion are currently available. Observations indicate that in most clouds, 10% or less of the available nitrogen is locked up in N2 (Womack et al. 1992), although one star forming region (NGC 2264 IRS1) has been found in which all of the available nitrogen must be in N2 (de Boisanger et al. 1996). O2 and N2 are infrared-inactive molecules and also do not have any signature in the radio. In the solid state, however, adjacent molecules can distort the symmetry of these molecules and their fundamental transition becomes

9

weakly infrared active (see Ehrenfreund and van Dishoeck 1997 for a review). The absorption cross section for solid O2 has been determined (Ehrenfreund et al. 1992) and suggests that molecular oxygen can be detected in objects containing large amounts of apolar ices in the line of sight. We have searched for the fundamental transitions of solid O2 and N2 in the spectrum of RAFGL 7009S at 6.45 and 4.28 em, respectively, but could not detect the presence of those molecules. The fundamental transition of N2 falls, unfortunately, at 4.28 em, which is hidden in the strong CO2 band, and the transition of O2 at 6.45 em is located in the strong red wing of water ice (6 em). The presence of a large amount of water ice may prevent the detection of O2. An H2O/O2 ratio of ,4 is needed to detect O2 in the wing of the water bending mode. However, the individual ice composition will also influence the exact band strength of the O2 band. The presence of CO2 is known to enhance the band intensity of O2 by at least a factor of 2 (Ehrenfreund et al. 1992). Another method for inferring infrared active molecules is to determine the band position of isolated water bands between 2.5 and 2.9 em (Ehrenfreund et al. 1996b). These measurements are not possible in the direction of RAFGL 7009S, because the water ice band around 3 em is totally saturated. Irradiation products of O2 and N2, such as O3, N2O, and NO2 can be used to infer the presence of O2 and N2 as well. The search for N2O at 4.47 em as well as for NO2 at 6.19 em was negative. The NO2 band is located in a region of numerous sharp gas phase water lines and only high resolution spectra or improved reduction methods may reveal a weak feature that allows the determination of an upper limit. The strong ozone band at 9.6 em cannot be observed in the direction of RAFGL 7009S because the silicate band extending from 8 to 13 em is totally saturated. The weak overtone and combination mode of O3 at 4.74 em (n1 1 n3 band) could not be detected. A last possibility for tracing the presence of solid O2 and solid N2 is their ‘‘fingerprints’’ in the profiles of the strong CO and CO2 band (Ehrenfreund et al. 1996a, 1997). Figure 9 shows a laboratory spectrum of the behavior of the CO band when N2 and O2 are included in the ice mixture. Whereas the presence of N2 (being rather inert and of the same crystal structure as CO) has little influence on the CO profile, O2 leads to complexes with the CO molecule and broadens and shifts the CO profile when O2 is present in the ice mixture in large amounts (more than p30% relative to CO). Strong broadening of the CO as well as of the CO2 bands is observed when the three molecules CO, CO2, and O2 are mixed in an apolar mixture; see Fig. 9 (Ehrenfreund et al. 1996a, 1997). From the profile of the CO2 bending mode in the direction of RAFGL 7009S we can estimate that CO2 ice resides dominantly in the polar phase; see Section 3.1. The low resolution CO profile, peaking at

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EHRENFREUND ET AL.

N2 will be more accurately inferred from its irradiation products. In any case it will be difficult to determine accurate abundances for solid N2 in the interstellar medium. Gas phase measurements of the protonated ion N2H1 in the mm and sub-mm range are the most promising method to enhance our present knowledge about the missing N (Womack et al. 1992). 5. IMPLICATIONS FOR COMETS

FIG. 9. The CO profile in ice mixtures containing the infrared inactive molecules O2 and N2. Solid N2 does not disturb the CO profile, whereas O2 in larger concentrations (.30% relative to CO) can perturb the matrix and leads to shifts and broadening of the CO profile (Ehrenfreund et al. 1997). CO2 and O2 lead to strong aggregates, which strongly broaden the CO profile as well as the CO2 profile itself.

4.675 em, indicates the dominance of apolar CO in RAFGL 7009S. The large width of the CO band at 0.026 em (indicating strong perturbations in the apolar phase) is unusual and may originate in the presence of abundant O2 and some CO2, which is not locked up in the polar and CO2-rich ice layer. The largest interaction between molecules is generally achieved when the abundances of the interacting molecules are equal. Using the estimated abundance of 18% CO relative to water ice observed in the direction of RAFGL 7009S we derive an upper limit of 15% O2 relative to water ice, which may be included in an apolar layer on grains in the direction of RAFGL 7009S. Detailed analysis of the high resolution profile of the CO band, which is currently being observed by ISO, will give an accurate ratio of polar and apolar CO to quantify more accurately the O2 content as well as the CO gas phase abundance in the direction of RAFGL 7009S. N2 does hardly contribute to the broadening of CO profiles but induces, when present in abundance, blue shifts of the CO band (Ehrenfreund et al. 1996a). This is not observed for the CO profile towards RAFGL 7009S. Comparing the band position of the apolar CO component at 4.675 em with a database of apolar ice analogs (Ehrenfreund et al. 1996a) suggests only a small contribution of N2 (on the order of a few %) compared to water ice. The presence of

Direct observations of volatiles in the cometary coma, such as CO, CO2, and H2CO, show that comets differ in their chemistry. The differences in abundances of volatiles measured in the coma of several comets might be due to differences in the cometary composition or the state of evolution of the comet. Inhomogeneities in the nucleus could also account for such differences. Information on the volatile species is furthermore difficult to interpret because volatiles might be extensively modified in the coma during the sublimation process. The general low abundance of CH4 and the relatively high ratio of CO/CH4 in comets are more consistent with an interstellar origin of cometary volatiles than with a Solar System origin, which requires evaporation of the interstellar ices (Greenberg 1993). The recent advances in high resolution instrument technology and the number of interesting cometary targets in recent years, however, have strongly improved the knowledge of cometary volatiles (see Mumma 1997 for a review). Those observations show that there are differences in the chemical composition of comets. Well established abundances of interstellar ices in the directions of a large number of targets will certainly guide the interpretation of past and future results of cometary observations. ISO provides new insights on the gas-to-solid ratios of many interstellar molecules and first spectra allow measurement of abundances and isotopic ratios of several important molecules which have strong implications for comets. 5.1. H2O Comet Halley investigations show that water is the dominant volatile close to the comet. H2O ice is also the most abundant molecule on interstellar grain mantles and has been observed in the direction of more than 100 sources. ISO allowed for the first time abundance measurements of gas phase H2O, which may result from desorption from icy grains or be formed alternatively by high temperature gas phase reactions. The ratio of gas phase water to water ice was recently measured with ISO in cold regions as Ngas/Nsolid , 0.04 (van Dishoeck and Helmich 1996). A much higher value of Ngas/Nsolid 5 1 was measured in warm objects such as AFGL 2591 (van Dishoeck and Helmich 1996). Dartois et al. (1997) measured a value of

INTERSTELLAR ICES AND COMETS

Ngas/Nsolid 5 0.18 for the object RAFGL 7009S, indicating that the line of sight is characterized by two distinct regions, a young outflow with high gas temperature and a rather cold region (Dartois et al. 1997). 5.2. CO2 ISO observations show that CO2 is one of the major detected volatiles released by comet Hale–Bopp at more than 4 AU from the Sun (Crovisier et al. 1996). Although CO2 can also be produced by photodissociation in the coma, the ubiquitous presence and large amount of CO2 in interstellar space detected with ISO (15 to 20% relative to water ice) will influence future comet observations. Since the abundance of CO2 in the interstellar gas is very low compared to the abundance of CO2 ice, an efficient destruction mechanism could exist, acting in the interstellar gas as well as in the cometary coma. Such a mechanism could explain the low CO2 abundances measured, e.g., for comet Halley (on the 2% level). However, the new observations of comet C/1995 O1 (Hale–Bopp) at 4.6 and 2.9 AU from the Sun, showing strong outgassing of CO2, suggest that this molecule is a major volatile in the cometary coma (Crovisier et al. 1996, 1997). Preliminary estimates towards a few protostellar sources indicate that CO2 is mostly embedded in polar ices, but that grain mantle layers exist which contain pure or segregated CO2, Section 3.1 (de Graauw et al. 1996, Ehrenfreund et al. 1996a, 1997). This component of pure CO2 is certainly evaporated at lower temperatures, which would indicate for comets a sublimation at larger heliocentric distances. However, the gas flow from sublimation of apolar ices will drag polar grains away from the nucleus if they are intimately mixed. Due to their very small size these grains may become superheated and will also liberate polar molecules. Production rates must therefore be approached with caution. 5.3. CH4 ISO made it possible for the first time to derive abundances of interstellar solid and gaseous CH4 (Boogert et al. 1996, d’Hendecourt et al. 1996, Dartois et al. 1997). An abundance of CH4 ice of 1–4% relative to water ice has been confirmed in the direction of several protostars. These values are consistent with measurements in comets. Like CO2, CH4 shows low abundance in the interstellar gas. A CH4 abundance of 2% was derived from ion mass spectrometer data taken on the Giotto fly-by from Halley (Allen et al. 1987). CH4 and C2H6 have recently been detected in comet C/1996 B2 Hyakutake with abundances of 0.7 and 0.4%, respectively, relative to water ice (Mumma et al. 1996). The abundance ratio of successive homologous hydrocarbons can reveal their formation conditions. Mumma et al. (1996) conclude that the presence of such

11

a large amount of C2H6 (compared to CH4) in comet C/1996 B2 Hyakutake is not consistent with production by thermochemical equilibrium processes in the solar nebula. C2H2, which is highly abundant in the interstellar gas (van Dishoeck et al. 1993), may condense onto the grain surface and act as a precursor to create C2H6 efficiently by hydrogenation. C2H6 can also be produced in the ice by photolysis of CH4. During 1 h UV irradiation (corresponding to 1000 yr of UV radiation in the outskirts of a molecular cloud) of pure CH4, about 20% of C2H6 (compared to CH4) is formed in the ice (Gerakines et al. 1996). However, C2H6 ice has not yet been detected in the interstellar medium. 5.4. H2CO H2CO is an abundant molecule in the interstellar gas (van Dishoeck et al. 1993). It is probably synthesized on interstellar grains and thereafter released in the gas, most efficiently in hot cores which show high abundances of gaseous H2CO. In the cometary coma H2CO seems to have an extended source responsible for its production at large nucleocentric distances. Whereas H2CO could be the photodissociation product from H2CO addition polymers (POM), the photodissociation of H2CO could be responsible for a certain amount of CO in the coma. Discrepancies between the H2CO abundances measured in several comets suggest that like CH3OH, H2CO varies among comets (Bockelee-Morvan et al. 1995). The abundance derived for H2CO in comets however depends strongly on whether one assumes that it is released as a ‘‘parent’’ or as a ‘‘daughter’’ fragment. Radio measurements have not been able to answer this question. Infrared measurements show that H2CO as a parent is generally less abundant than 1% (Reuter et al. 1992) except for one vent in Comet Halley, where an abundance of 4% was measured (Mumma and Reuter 1989). To summarize, H2CO is observed in comets usually less than 1%. Measurements in the direction of RAFGL 7009S report 3% H2CO compared to water ice. The detection of solid H2CO in the direction of other targets will allow to study in which ice type H2CO is actually included and how this important molecule is formed in the interstellar medium. 5.5. OCS OCS is a widespread molecule in the interstellar gas in star forming regions as well as in quiescent molecular clouds (Palumbo et al. 1995). Detection of OCS in the solid state before ISO with an abundance relative to water ice of 0.1% and below were reported for several embedded YSO’s (Geballe et al. 1985, Palumbo et al. 1995, 1997). OCS has been detected with an abundance of 0.2% relative to water ice in RAFGL 7009S. However,

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the possible contribution of CO3 to this band has first to be determined with the help of laboratory spectroscopy. The detection of OCS in Hyakutake is weak: Woodney et al. (1996) detected a single line for OCS with low S/N. Earlier searches for OCS in comets were negative, at comparable sensitivities (Mumma et al. 1993a). OCS has been detected in Comet Hale–Bopp (Woodney et al. 1997). 5.6. CH3OH The CH3OH/H2O ratio has been pointed out to allow a dichotomy between comets coming from the Oort cloud and from the Kuiper belt (Mumma et al. 1993b). Methanol abundances relative to water ice were derived for seven comets and range between 0.6 and 5%, indicating no strong evidence for a bimodal distribution of methanol-rich and methanol-poor comets (Bockelee-Morvan et al. 1995). In the interstellar medium the determination of solid CH3OH is still a debated subject, but recent studies suggest abundances between 1 and 10% (relative to water ice). ISO observations will certainly establish consistent abundances of solid CH3OH as well as the gas/solid ratio of CH3OH in the near future. 5.7. O2 and N2 Current studies indicate large amounts of atomic oxygen in the interstellar gas, which could account for most of the missing oxygen. In the contrast, nitrogen is supposed to be abundant in the interstellar gas in molecular form (N2). The amount of solid O2 and N2 incorporated in apolar ice layers on grains is expected to be quantified by future ISO results. Comet observations reveal that N appears to be depleted in Comet Halley. Since Oort cloud comets came from the Jupiter–Neptune region of the nebula, closer to the Sun than the Kuiper belt which supplied the low inclination short period comets, one would expect N2/H2O and CO/H2O to be higher in short period comets than in dynamically new comets (assuming current release rates are cosmogenic). N2/CO should be higher in Kuiper belt comets since the temperatures were colder there than in the Jupiter–Neptune region. It is generally accepted that two distinct reservoirs of N were available in the early solar nebula, a large amount of N2 and to a lesser extent N-compounds (Owen and BarNun 1995). Large amount of interstellar gaseous N2 may be preserved in the outer solar nebula. Abundances in interstellar clouds thus provide a useful guide to abundances in the outer solar nebula and comets. 6. DISCUSSION

Interstellar icy grain mantles formed in dark shielded environments may be processed, heated and be returned

to the gas phase. The gas phase chemical composition in hot molecular cores in star-forming regions is determined by the evaporation of material from icy grain mantles. The high abundance of molecules such as CH3OH, H2CO and CH3CH2OH (ethanol) in the interstellar gas cannot be explained by gas phase chemistry. Therefore ISO results which provide new insights in interstellar gas/grain chemistry are most relevant. During their lifetime dust grains cycle forth and back several time between dense and diffuse clouds. The prolonged exposure of heterogenous icy grains to UV radiation during their evolutionary cycle may result in gradual aromatization or even total carbonization of the grain mantle, forming according to the local environmental conditions, carbonaceous material such as AC (amorphous carbon), HAC (hydrogenated amorphous carbon), or coal and kerogen-like matter. Photolyzed ice mixtures composed of H2O, CH3OH, CO, and NH3 result in the formation of radiation products and radicals such as CO, CO2, HCO, H2CO, CH4, and nitriles (Allamandola et al. 1988, Bernstein et al. 1995). Gradual heating of those mixtures revealed molecules such as ethanol, formamide, acetamide, hexamethylenetetramine (HMT, C6H12N4), and forms of POM ((UCH2UOU)n), as well as ketones, alcohols, and many other organic molecules (Bernstein et al. 1995). A large amount of dust grains may have survived unmodified in the outer solar nebula before incorporation into comets. To trace interstellar ices and heterogenous organic matter (formed during the evolution of interstellar dust grains) in comets is one of the future goals of the ROSETTA comet rendezvous mission. Interstellar ices, released as cometary volatiles, will probe the origin and history of comets. In order to study the formation of polar and apolar ices, to identify the molecules which dominate the grain mantles as well as their evolution into organic matter, laboratory spectroscopy builds the base of any such identification. Infrared spectroscopy of interstellar ice analogs makes it possible to reveal the ice composition, but also the ice structure, which has strong implications for observations of volatiles in the cometary coma. Therefore the abundance and history of ices and organic molecules formed in interstellar space are of vital importance for comets. The dichotomy of comets is not well established (Levasseur-Regourd et al. 1996). Evidence for two classes of comets is found from their whole coma scattering properties at large phase angles (Levasseur-Regourd et al. 1996). Comets with higher maximum polarization Pmax seem to be dust-rich (Bockelee-Morvan et al. 1995). Analysis of the properties of 85 comets (A’Hearn et al. 1995) shows that the dust/gas ratio does not vary with the dynamical age of the comet and a compositional grouping of comets is related to their place of formation. To study the heterogeneity of cometary nuclei will allow to reveal if comets

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have incorporated mainly unaltered interstellar matter or if the original material was chemically processed in the solar nebula. To determine if cometary nuclei contain admixtures of interstellar and nebular material and which fraction dominates in certain types of comets will provide constraints for the origin and evolution of comets and the formation of the Solar System. The segregation of ice layers in the interstellar medium in ‘‘onion shell’’ structures may be of importance for the dichotomy of comets. Whereas it is not clear if polar and apolar ices reside within one grain or on different grain populations, observations of solid CO in the Serpens cloud point strongly towards the ‘‘onion shell’’ model (Chiar et al. 1994). Using laboratory spectroscopy of interstellar ice analogs in comparison with astronomical data allowed to distinguish H2O-rich and CO-rich ices, also called ‘‘polar’’ and ‘‘apolar’’ ices, in the line of sight toward molecular clouds. In the direction of one source (AFGL 2136) the presence of CH3OH-rich ice which could be formed at up to T . 80 K has been suggested (Skinner et al. 1992). Most of the species have already evaporated at this temperature and CH3OH segregates from water ice into a separate phase (Tielens and Whittet 1997). ISO results show the presence of CO2-rich ice mantles in several lines of sight (see Section 4.1). Since sublimation characteristics strongly depend on the ice composition, the characterization of the different ice types is most relevant for comets. To confirm whether these different ice types are present on the same grains or originate from different grain populations is a main issue of interstellar spectroscopy. The first ISO results in the direction of the highly extinguished source RAFGL 7009S already contain an enormous amount of information concerning the ice composition, temperature, and radiation history of grains in dense clouds before Solar System formation. The abundances of interstellar ices towards this source are in good agreement with measurements towards other protostellar targets. The gas/solid ratios of certain ice bands (such as CO2, H2O, CH4) show that this source is representative for a dense and relatively cold interstellar cloud environment. In comparison with laboratory spectra it will be particularly important to define whether the large amounts of CO2, H2CO, and XCN found in the direction of RAFGL 7009S are formed by irradiation conditions in protostellar environments or by grain surface reactions. The abundances of solid H2CO, CO2, and CH4 measured in the direction of RAFGL 7009S are consistent with current cometary observations, indicating that interstellar ices may have been incorporated unaltered into comets. With ongoing improvement of reduction procedures, such extinguished sources as RAFGL 7009S will reveal many weak features which have strong implications for comets, such as N- and S-bearing species. These new and future ISO results on interstellar ices will provide a great challenge for cometary scientists.

ACKNOWLEDGMENTS We are grateful for the numerous helpful comments of the referee M. Mumma and another anonymous referee. These comments have strongly improved the paper. We also thank B. Foing for helpful discussion. The laboratory work was supported by the European Community Grant ERBCHBICT940939. PE is a recipient of an APART fellowship of the Austrian Academy of Sciences.

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