Faraday Discuss. 1998 109 31»46 What can ISO tell us about gasñgrain chemistry? Ewine F. van Dishoeck L eiden Observatory PO Box 9513 NL »2300 RA L eiden T he Netherlands Recent results of searches for IR absorption lines of gas-phase molecules in the ISO spectra of embedded massive young stars are discussed. Abundant highly excited gas-phase H2O is detected in the ìhot coresœ surrounding the young stellar objects but not in the colder regions. The abundance of gasphase CO is low in all sources in spite of the large observed abundance of solid CO 2 Gas-phase 2. C4 C2H2 H and HCN have been observed as well. The latter two species have high excitation temperatures of the order of 1000 K in some sources. The large abundance of CH suggests that the molecule is formed mainly through grain-surface reactions.CO may be 4 produced by grain-surface chemistry as well athough photochemical pro- 2 duction in ices can also play a role. Comparison of gas-phase with solidstate column densities for the same lines of sight allows accurate determination of gas solid state ratios which can be analyzed as functions of the temperature of the region. The data indicate that signi–cant evaporation of ices likely occurs in the hotter regions but that part of the observed gas-phase H2O may also be produced by high-temperature gasphase reactions. The results are discussed in the context of the physical evolution of the regions. 2 .9 1 Introduction Much of interstellar chemistry in the last 25 years has been concerned with gas-phase molecules which are readily detected at millimeter wavelengths.Detailed models have been developed to reproduce the observed abundances starting with the early work of Bates and Spitzer1 and Herbst and Klemperer.2 The most recent networks contain over 4000 gas-phase reactions between a few hundred species.3,4 Although it was recognized at an early stage that reactions on grain surfaces can also be signi–cant,5h8 they have been largely neglected in models except for the formation of H In the last decade several models have been developed which take account of both the gas-phase and the grain-surface processes.10h12 These have been stimulated by two sets of observational data. First owing to the rapid improvements in the sensitivity of IR detectors ground»based and airborne observations have revealed absorption of solid H CO CH3OH and other species along the lines of sight to bright IR sources.13,14 Second (sub-)millimeter emission data on complex saturated organic molecules such as 2O CH OCH3 HCOOCH3 and CH3CN in so-called ìhot coresœ around massive young 3 stellar objects15,16 have stimulated models in which grain mantles are evaporated into the gas at high temperatures and drive a rapid gas-phase chemistry.17,18 The short wavelength spectrometer (SWS)19 on board the IR space observatory (ISO)20 is particularly well-suited to address several aspects of gas»grain interactions in interstellar chemistry.First the ISO»SWS allows a complete inventory of the solid-state species in a variety of regions through full spectral scans from 2.5 to 45 lm at a resolving power R\j/*jB500 unhindered by the atmosphere.21h23 Secondly ISO can observe the vibration»rotation absorption lines of gas-phase species such as H2O CO2 31 32 ISO and gas»grain chemistry 2 Fig.1 Normalized ISO-SWS spectra in the wavelength region of the CO2 l3 (0,0,1)»(0,0,0) and CO 1»0 vibrational bands toward three embedded young stellar objects. The CO is primarily in solid form as revealed by its broad line shape. Toward NGC 7538 IRS1 CO is also mostly in solid form but toward W 3 IRS5 the ro-vibrational structure of gaseous CO is seen as well. Toward GL 4176 only highly excited gaseous CO is observed. The spectra have been shifted by 0.0 [0.8 and [1.8 for clarity. and CH which are thought to be among the dominant products of gas»grain chem- 4 istry but which cannot be observed from Earth or only with great difficulty.24h27 In addition accurate gas solid ratios can be determined for these species providing important constraints on the evaporation mechanisms and various aspects of ìhot-coreœ chemistry.Finally ISO allows more extensive searches for minor solid-state species especially nitrogen-containing molecules which may be produced by photoprocessing of ices. Together with observations of CO2 they can be used as diagnostics of the radiation –eld and give information on the relative importance of grain-surface reactions compared with photochemical reactions in ices. In this paper an overview of recent results with the ISO»SWS on gas-phase and solid-state species in massive star-forming regions is presented together with new observational data for several lines of sight.2 IR absorption vs. submillimeter emission observations The majority of the nearly 120 interstellar molecules identi–ed to date28,29 have been detected through (sub-)millimeter observations of pure rotational lines in chemically rich sources like TMC»1 Orion-IRc2/KL SgrB2(N) and IRC]10216. This technique has the advantage of very high spectral resolution (RP106) so that the line pro–les are 33 E. F. van Dishoeck resolved. In addition very small abundances down to 10~11 with respect to H2 can be detected and mapped. On the other hand IR observations have many advantages compared with millimeter data.30h32 First they probe the absorption of material along a pencil beam line of sight to the IR source (\1A) whereas the millimeter emission line data often refer to beams of at least 20A.Secondly the population distribution over the rotational energy levels can be directly constrained from a single IR vibration»rotation spectrum whereas often diÜerent receivers or telescopes are needed to determine the rotational excitation from submillimeter data. Also information on much higher energy levels is obtained up to J[20 for species like CO and HCN compared with J\7»9 from submillimeter data. The excitation temperature gives important information on the physical parameters of the gas where the molecule is located. Thirdly molecules without a dipole moment such as CO have strong IR vibration»rotation transitions 2 CH4 and C but negligible millimeter rotational emission.Finally not only gas-phase molecules but 2H2 also solid-state species can be detected at IR wavelengths. A vibrational band of a molecule in the solid-phase can be readily distinguished from that in the gas-phase because the former consists of a single broad spectral feature which lacks the characteristic rovibrational structure of the gas-phase spectrum and is slightly shifted in wavelength (see Fig. 1).33 The main drawback of the ISO data is their limited spectral resolution RB2000 for the grating mode on the SWS. This implies that the lines are unresolved and often optically thick when detected. In practice only abundances down to ca. 10~7 with respect to H can be probed and only lines with widths *V [2 km s~1 can be seen.2 3 Gasñgrain chemistry open issues 3.1 Basic processes The chemistry on the surfaces of interstellar grains has received ample discussion in the literature.7,8,11,12,34h36 Four diÜerent steps can be distinguished in the formation of a molecule (i) accretion ; (ii) diÜusion; (iii) reaction ; and (iv) ejection or evaporation. The timescale for a molecule to collide with a grain and accrete is given by tacc\2 ] y 109/nH yS yr where the sticking probability is thought to lie between 0.1 and 1.0 for S most species.37 Thus at typical densities in dense clouds of 104 cm~3 or more most species (except H and He) are expected to be condensed onto the grains on timescales 2 of less than 105 yr unless efficient desorption occurs.The residence time on the surface depends on the desorption mechanisms. For t is short for H (500 s) but very long ([1015 yr) t thermal evaporation the timescale TD\10 K. Thus all neutral species heavier than He can be con- for C and CH at evap 4 T sidered permanent residents at D\10 K. Once stuck on the grain a species can hop to the next site on a timescale thop which is signi–cantly shorter than the evaporation heavier species are immobile. H and H are so light that they can also tunnel quantum timescale. Light species such as H H2 C N and O can hop from site to site whereas mechanically to the next site. A species can scan the complete surface for a reaction 2 partner on a timescale scanB105thop determined by a random walk process.The efficiencies of the various reactions depend on relative timescales for accretion diÜusion and desorption. Two diÜerent regimes can be distinguished. In the ìaccretion limitedœ regime thop>tacc so that a species can diÜuse on the surface until it –nds a co-reactant. The chemistry is limited by the accretion rate of new species. In the ìreaction limitedœ regime the opposite holds thop?tacc so that a species trapped in a site can only react with migrating species that visit that site. At a typical density of 104 cm~3 a grain accretes only a few atoms or molecules per day. These species thus have a long time namely a day to scan the surface and react before a new species arrives, 34 ISO and gas»grain chemistry indicating that the chemistry is in the ìaccretion limitedœ regime.7 Many of the published gas»grain models have been formulated in the ìreaction limitedœ regime using rate equations for computational convenience.34 The rate equations have recently been reformulated to remedy their shortcomings,38 but the detailed eÜects on published models still remain to be assessed.Molecules can be returned to the gas phase by a variety of mechanisms.35,37,39,40 Most relevant for this paper is the thermal evaporation which is efficient only at higher T temperatures D[20 K and depends on the species involved. The sublimation temperatures of pure CO CO2 CH4 and H2O»ice under interstellar conditions are 20 45 20 and 90 K respectively.41 For traces (\5%) of CH and CO embedded in an H2O- 4 these temperatures increase to ca.90 K. Other desorption mechanisms in cold matrix 2 clouds include cosmic-ray spot heating42 and explosive desorption. The latter can be either cosmic-ray induced or triggered by grain»grain collisions at velocities greater than ca. 0.1 km s~1. In star-forming regions sputtering of icy mantles by shocks in the turbulent boundary layers of out—ows can be important as well. Most mechanisms are eÜective only for ìapolarœ ices containing CO O and N2 H but less for polar ices 2 which contain strong hydrogen bonds. Thus the desorption processes can shape the icy 2O-rich mantles into polar and apolar layers. Only sputtering in strong shocks is likely to remove the mantles completely. 3.2 Model predictions 3 TDB10 K. However in these experiments the atomic O was produced by 2 Because hydrogen is so abundant and mobile all models predict that grain surface chemistry leads primarily to hydrogenated species under hydrogen-rich conditions.Speci –cally hydrogen has enough time to tunnel through activation barriers as large as ca. 2000 K so that H2O NH3 and CH can be formed. Indeed the observed large abun- 4 dances of H2O ice in dark clouds have been argued to be strong evidence of its production by grain surface reactions.43 Hydrogenation of CO can lead to H2CO and CH OH although the efficiencies of these reactions are still subject of discussion.12 At higher densities the amount of atomic hydrogen in the gas phase decreases and reactions with atomic oxygen become important. The long accretion timescale allows reactions with barriers up to ca.400 K to proceed through thermal hopping. A particularly important case is the reaction of CO with O to form CO2 . Early laboratory experiments44 seemed to indicate that this reaction does not proceed at low temperatures photolysis of O2. O Since the was still abundant during the experiments most of the O 2 reacted with O to form O3 . Additional experiments of CO with O in the absence of abundant O are urgently needed. 2 Another process for producing new molecules is through photochemical reactions within the icy mantles. The UV photons can be provided by the external interstellar radiation –eld by the radiation from the young embedded star45 and by the interaction of cosmic rays with H resulting in UV photons.46 Photodissociation of molecules in ices produces radicals (e.g.H2O]OH]H O or ]H2) which can subsequently react 2 to form other molecules. This provides an important alternative route to the formation of solid CO through the CO]OH reaction. Indeed this process is well known from laboratory experiments on H2O»CO ice mixtures just a small amount of radiation 2 results in signi–cant CO production.47 Observations of solid CO in a variety of 2 regions may be able to distinguish between these two possible routes. 2 3.3 Hot core chemistry Once the molecules have been desorbed from the grains at high temperatures they drive a rapid chemistry in the gas phase. This so-called ìhot coreœ chemistry was –rst discussed 35 E. F. van Dishoeck can be reproduced if a mixture of simple ices containing3 H for the Orion hot core from which it derives its name.Recent models have shown that the observed abundances of complex organic molecules such as CH OCH and HCOOCH 3 2O CO 3 CH OH NH and/or HCN is evaporated into the hot gas.17,18,48h50 Reactions with 3 3 CH produce complex organic molecules on timescales of ca. 104 yr. In 3OH H2CO and addition the high temperature in the gas drives atomic oxygen into H2O through the 2 ]OH and OH]H2 ]H2O reactions at temperatures above 230 K.48,51 4 O]H Other molecules such as CO and CH do not participate in an active chemistry and are 2 destroyed on timescales of ca. 105 yr by normal ion»molecule and neutral»neutral reactions. Within 104 yr after evaporation the original composition of the ices is still re—ected in the gas-phase composition of the hot cores.3.4 Scenario for gas-grain interactions during star-formation IR absorption line observations are limited by the availability of a bright source located behind a large column of gas and dust. In practice most of the bright IR sources are deeply embedded massive young stars. The IR continuum radiation at 4»25 lm is due to emission from hot IR observations concern warm star-forming regions rather than cold quiescent clouds. (TDB200 K) dust located close to the young star. Thus most of the The processes discussed above lead to the following scenario for gas»grain inter- (TDB10 K) that most actions at various stages of the star-formation process.12,14,52h56 During the collapse phase the density increases and the temperature stays so low molecules freeze out onto the cold grains.Here the chemistry can be actively modi–ed by surface reactions resulting most likely in hydrogenation and oxidation of O C N and CO leading to and H2O CH4 NH3 CO2 CH3OH H2CO. After the new star has formed its radiation heats the surroundings (TDB20»100 K) and the molecules start to evaporate back into the gas phase probably in a sequence according to their sublimation temperatures. In addition the out—ows create shocks when they interact with the surrounding envelope which can drive high-temperature chemical reactions and return icy mantles and more refractory material containing silicon to the gas phase. These freshly evaporated molecules can then drive a ìhot-coreœ chemistry for a period of ca.105 yr. Finally the chemistry returns to the quiescent phase dominated by ion»molecule reactions. What can ISO contribute to this picture ? First it can con–rm that the major products of grain-surface chemistry under hydrogen-rich conditions are indeed H and NH through direct observation of the ices. Second constraints can be obtained on 2O CH4 the relative importance of grain-surface reactions compared with photochemical pro- 3 cesses in ices. Third information on the temperature structure and history of the region can be derived through determination of the various ice components (polar vs. apolar). Fourth the mechanisms for releasing molecules from the grains back into the gas can be tested by observing gas solid ratios for various species.Finally direct observation of gas-phase molecules in hot cores provides information on species which are evaporating from the ices and those produced in the gas by subsequent high-temperature reactions. This paper will address most of these issues except the diÜerent ice components which are discussed extensively in the paper by Ehrenfreund et al.23 4 ISO observations ISO observations were performed primarily with the SWS,19 which operates from 2.5 to 45 lm. The SWS06 grating mode was used which has a resolving power R ranging from 1350 to 2500. The aperture varies from 14A]20A at 2.4»12 lm up to 20A]33A at the longest wavelengths. Details of the data reduction and procedures for the removal of instrumental fringes are discussed elsewhere.26,57 36 ISO and gas»grain chemistry Searches for H2O lines were also performed with the long wavelength spectrometer, 58 which covers the wavelength range from 43 to 197 lm.Both the low-resolution grating mode LWS01 (RB500) and the high»resolution Fabry»Perot mode LWS04 (RB8000) were used. The LWS aperture is about 80»100A. Details of the reduction can be found in ref. 59. 5 Gas-phase molecules 5.1 Gas-phase CO High-resolution spectra of gas-phase 12CO and 13CO toward a number of massive young stellar objects have been obtained previously from the ground by Mitchell et al.30 Lines originating from levels up to J\24 have been detected indicating the presence of both warm (T \200»1000 K) and cold (T \60 K) gas along the lines of sight.Accurate CO column densities have been derived from the optically thin 13CO data which are important to constrain the total amount of H2 . The fraction of warm gas N the other sources. Thus most sources have already heated a substantial fraction of their warm(H2)/Ntot(H2) ranges from less than 5% for NGC 7538 IRS 9 to typically 50% for envelope. Although hampered by low spectral resolution gas-phase CO can also be detected with the ISO»SWS. Fig. 1 shows spectra in the region of the CO v\1»0 vibrational band at 4.68 lm. ISO is mostly sensitive to the warm gas with broad lines (*V [2 km s~1) which is clearly seen up to J[20 in the P- and R-branches in objects like GL 4176. These data provide information on the fraction of warm gas along the line of sight which is especially useful for sources which have not been observed previously from the ground (e.g.sources in the southern sky). 5.2 Hot abundant gas-phase water Helmich et al.24 and van Dishoeck and Helmich60 presented the –rst detection of IR absorption lines within the bending vibration of water at 6.2 lm toward four massive young stars. The observations are shown in Fig. 2 together with new data on three additional objects using the SWS06 grating mode. The strongest lines absorb 5»15% of the continuum. very diÜerent from that of a linear molecule such as CO and a detailed model is Because of its complex energy level structure the ro-vibrational spectrum of H2O is required to interpret the data. In its simplest form it is assumed that the water level populations can be characterized by a single excitation temperature which should be close to the kinetic temperature if collisions dominate the excitation.Alternatively the rotational level populations of the H2O molecule are readily coupled to the radiation –eld through near- and far-IR pumping in which case the excitation temperature is representative of the color temperature of the radiation –eld. In most regions a mixture of the two processes likely occurs. Under the assumption of LTE simulated spectra can be constructed in which the total H2O column density N the excitation temperature Tex and the Doppler parameter b\2)(ln2)*V are the only input parameters. Details of the method and references to the adopted molecular data can be found in ref.61.61 N(H2O)\2]1018 cm~2 Tex\300 K and Fig. 2 includes a model spectrum for b\5 km s~1. It is seen that this spectrum provides a good –t to the observations of the warm sources GL 2591 GL 2136 and GL 4176. However toward W 3 IRS5 S140 IRS1 and NGC 7538 IRS 1 at most a few H2O lines arising from the lowest energy levels are seen whereas no H2O is detected in the spectrum toward NGC 7538 IRS9. For these sources no constraints on the excitation temperature can be obtained. Gas-phase H2O has also been detected toward GL 7009S but the excitation temperature in this case appears to be low TexB25 K.27 Fig. 2 ISO-SWS normalized spectra of seven massive protostellar sources in the wavelength region of the H2O l2 bending mode showing absorption by hot abundant H2O in at least three sources.Model H N(H2O)\2]1018 cm~2 Tex\300 K and 2O spectra for a column density of Doppler parameter b\5 km s~1 and N(H are shown for comparison. 2O)\1]1018 The derived H2O column densities are summarized in Table 1. The results are accurate to a factor of two and are not sensitive to the adopted values of b and Tex as long as bZ2 km s~1. Since ISO cannot detect very narrow lines with b\1.5 km s~1 large amounts of cold H2O could potentially still be present. However there is no evidence for such narrow lines in submillimeter emission data of various species,62 nor in the 13CO IR absorption lines.30 Because of the low velocity resolution it is also not possible to distinguish whether the absorption occurs at or near the velocities of the cloud core or in the blue-shifted gas.Blue-shifted absorption due to high-velocity gas in the out—ow has been detected in several sources,30 but the fraction of the total column density Table 1 Gas-phase column densities toward massive young stellar objects object NGC 7538 IRS9 W 33A NGC 7538 IRS1 S140 IRS1 W3 IRS5 GL 2136 GL 4176 GL 2591 a From 13CO observations (warm]cold),30 assuming 12CO/13CO\60 and 12CO/H2\2]10~4.63 K. c Derived using b\3»7 km s~1 and Tex\100»300 K. d Results for Tex\100»250 2 e x NGC 7538 IRS9 and W 33A from ref. 26. warm(H2)\Nwarm(H2)/Ntot(H2) is the fraction of hot gas along the line of sight. f Derived from C17O 2»1 SEST spectrum and silicate optical depth.60 E. F. van Dishoeck H COa CO2 b 2Oc \3(17) 3(17) B1(17) 8(15) » 1(16) 2(15) 1(16) 2(16) 9.8(18) 2.6(19) 1.7(19) 7.4(18) 2.6(19) 2.2(19) B1(17) 3(17) 2(18) 2(18) 2(18) 1(16) 2(16) 1.6(19)f 1.9(19) b Derived from the l Q-branch using b\3»7 km s~1 and 37 cm~2 Tex\25 K and b\2 km s~1 CH4 d xwarm e H2 a 0.02 0.53 0.48 0.6 0.48 0.68 4.9(22) 1.3(23) 8.6(22) 3.7(22) 1.3(23) 1.1(23) 5(16) 1(17) 5(16) » » [5(16) [0.5 0.63 8.0(22)f 9.6(22) » [5(16) 38 ISO and gas»grain chemistry 2 l2 bending mode showing absorption by gas-phase CO2 in all sources.The Fig. 3 ISO»SWS normalized spectra of eight massive protostellar sources in the wavelength region of the CO dotted line indicates the position of the Q-branch.The observations have not been corrected for the (small) shifts due to the velocity of the source. contained in this component is only a few percent. If all of the observed H2O would be located in this component it would account for more than the interstellar oxygen abundance in the hottest sources. The H2O H abundances can be determined by comparison with the column den- 2 sities derived from the 13CO data along the same lines of sight assuming 12CO/ 13CO\60 and 12CO/H2\2]10~4.63 Table 1 lists both the total H column density 2 and the fraction of warm gas. For GL 2591 GL 2136 and GL 4176 the resulting H2O abundances range from (2»3)]10~5 if the H2O is homogeneously distributed throughout the source to (3»5)]10~5 if H2O is present only in the warm gas.In the latter case a signi–cant fraction ([5%) of the available oxygen is tied up in H2O. In the other sources the gas-phase H2O abundance integrated over the total line of sight is typically 1]10~6 and at most 5]10~6 in the warm gas. In order to further constrain the H2O abundance and excitation searches for the pure-rotational lines of H2O in emission or absorption have been made with the LWS toward several sources which show strong H2O 6 lm absorption.59 The lack of detected the beam but is present only in the small inner ìhot coreœ region which is typically only emission lines in GL 2591 GL 2136 and W 3 IRS5 suggests that the H2O does not –ll ca. 1016 cm in size (\1000 AU 1A at 1 kpc).CO2 5.3 Gas-phase CO and CH 2 4 l Searches for gas-phase CO l have been made in the asymmetric stretch and 2 3 2 bending modes at 4.3 and 15 lm.25 As Fig. 1 shows the 4.26 lm band is dominated by the strong solid CO feature and no hint of the ro-vibrational P- and R-branch structure of gas-phase N(CO2) \5]1016 is evident. This provides upper limits of 2 39 E. F. van Dishoeck CO2 l2 bending mode is less saturated and a weak sharp superposed 2 2 cm~2 corresponding to surprisingly low CO abundances of less than 5]10~7. The 15.2 lm solid absorption due to the Q-branch of gas-phase CO at 14.98 lm has been detected.25 In Fig. 3 new ISO data toward four additional sources are presented. The inferred column 2 densities are presented in Table 1 and are up to a factor of two higher than derived previously25 due to a better determination of the continuum.In all sources gas-phase CO is detected but at low abundances of (1»2)]10~7. Even toward Orion IRc2 a 2 similarly low value is found.64 No stringent limits on the excitation temperature can be T obtained although the best –ts to the pro–les are obtained for exB200 K or lower. The presence of gas-phase CO had been suggested indirectly from millimeter obser- 2 vations of the HOCO` ion,65 but except for the SgrB2 region these data also suggested low CO abundances of less than 10~6. Gas-phase CH has been identi–ed by Lacy et al.66 but the quality of the groundbased data is severely hindered by the Earthœs atmosphere. Although the ISO-SWS has 4 lower spectral resolution careful reduction of the data in the 7»8 lm region reveals the presence of gas-phase CH toward several sources (Fig.4).26,27,57 The resulting column densities are summarized in Table 1. The abundance of gas-phase CH with respect to 4 4 total H is ca. (0.5»1)]10~6. Comparison with simulated spectra indicates that the 2 CH gas is warm with an excitation temperature of 100^30 K. 4 and HCN and HCN. Lines of these species have been observed from and HCN may even be present,64 consistent with the 5.4 Gas-phase C2H2 The ISO spectra have also been inspected for the presence of a number of less abundant molecules including C2H2 the ground,31,32,67 but only in a few sources. Clear detections of the C2H2 l5 Q-branch at 13.7 l lm and the HCN Q-branch at 14.0 lm have been made in the ISO spectra of 2 half our sources (see Fig.5).57 For GL 2136 and GL 2591 the pro–les are very broad and high excitation temperatures are inferred of the order of 1000»1500 K. Absorption from vibrationally excited C2H2 K. The thick vertical lines above the spectra indicate the optical depths of the R(0) and R(2) Fig. 4 ISO-SWS spectrum toward the massive protostellar objects NGC 7538 IRS9 (top) and W33A (middle) showing the detection of solid CH (broad feature) with gas-phase CH lines CH 4 4 superposed.26 The lower –gure is a model gas-phase 4 100 spectrum for N\1017 cm~2 and Tex\ lines measured in ref. 66. 40 ISO and gas»grain chemistry 5 and the l Q-branch of HCN. The thin dashed line indicates the best –tting single com- N(C2H2)\2]1016 cm~2 Tex(C2H2)\1000 K and ex Fig.5 ISO-SWS normalized spectrum of GL 2136 showing the detection of the l Q-branch of C2H2 ponent model spectra for 2 N(HCN)\6.5]1016 cm~2 K T (HCN)\1500.57 Some of the remaining structure may be due to absorption from vibrationally excited C2H2 and HCN.64 detection of absorption from vibrationally excited CO in the same sources.30 Typical abundances with respect to total H are x(C2H2)B2]10~7 and x(HCN)B6]10~7 for the warm sources. For HCN the abundances are more than an order of magnitude 2 larger than obtained from submillimeter data suggesting again that the hot gas occupies only a small volume. 6 Interstellar ices The availability of complete spectral coverage from 2»45 lm with the ISO»SWS provides a landmark for the study of interstellar ices and allows for the –rst time an unbiased inventory.Overviews of recent results are given in several papers.14,21,22,68,69 Here we summarize only the conclusions for those species for which the gas-phase counterparts have been detected. A striking ISO result is the presence of strong solid CO l stretching mode at 4.27 lm (see Fig. 1) and the l bending absorption in the 2 3 2 mode at 15.2 lm along all lines of sight to embedded young stellar objects.22,70h72 The overall CO abundance in ices is ca. 15% relative to H2O making it one of the most abundant solid-state molecules. This corresponds to an abundance with respect to gas- 2 phase H of ca. 3]10~6»2]10~5 nearly two orders of magnitude larger than that of gas-phase CO 2 2.H2O Information on the amount of ice is available from ground-based observations of the 3 lm band.73 Additional measurements of the less saturated 6.0 lm bending mode allow a more accurate determination in some sources.74 Solid CO has been detected toward the colder sources (see Fig. 1).75 As discussed by Ehrenfreund et al.,23 the solid-state observations provide not only information on abundances but also on the ice environment since the shape and position of the bands are sensitive to the interactions of the molecules with their neighbors. Analysis of the pro–le shapes using laboratory data indicates the presence of grain mantles with distinct polar (H2O-rich) and non-polar (CO O2 and/or N -rich) 2 layers.75,76 CO appears to be present in both phases and may be ì segregatedœ or ìannealedœ in the warmer sources.2 41 E. F. van Dishoeck 2O C2 ice is about 15% similar as that toward young stellar objects andOis pri- 2 . Solid CO has also recently been detected toward Elias 16 a –eld star behind a 2 quiescent part of the Taurus molecular cloud.77 The derived abundance with respect to H marily in the H2O-rich phase. This important result indicates that no embedded energy source is needed for the production of solid CO An absorption feature near 7.67 lm has been detected in the ISO spectra toward 4 4 . is ca. 1% [ca. (1»2)]10~6 with respect to total H is also embedded in the polar H2O-rich ices. three deeply embedded objects (see Fig. 4).26,27,78 Comparison with laboratory spectra shows that it can be identi–ed with the l deformation mode of solid CH Its abundance with respect to H2O-ice 2] and CH Limits on the amount of solid C2H2 of 1»10% with respect to solid H2O have been 4 derived toward NGC 7538 IRS9.79 Because solid C2H2 does not have any strong bands when embedded in H2O-ice the limits are not very stringent.Information on the amount of solid HCN can be derived using recent laboratory data,80 giving typical upper limits of 3% with respect to H2O ice.81 7 ISOœs view on gasñgrain interactions 7.1 Hydrogenated molecules 3. H2O The high abundances of solid in dense clouds certainly support this As discussed in Section 3 one of the main predictions of grain-surface chemistry is the production of large amounts of hydrogenated species in particular H2O CH4 and NH theory.43 The observed abundances of gas]solid CH of ca.2]10~6 with ISO also 4 argue in favor of grain-surface chemistry.26 Time-dependent pure gas-phase chemistry models starting from a diÜuse atomic composition can approach such abundances but only for a very narrow time interval at tB105 yr.3,4 On the other hand conversion of C to CH on the grains is very efficient. The measured abundances suggest that the initial 4 atomic C abundance in the gas must have been low C/COB0.01 indicating that the solid CH was formed during a cold dense cloud phase. The fact that it is embedded in 4 an H2O-rich matrix suggests that the two were formed simultaneously. In contrast with CH4 C2H2 is thought to be produced primarily in the gas phase and then passively accreted onto grains.32 abundances measured in hot cores.14,83,84 The N/N ratio must have been NH 2 No stringent limits on the amount of solid NH are yet available from ISO because its bands are either blended with other strong features or occur in the deep silicate 3 feature where the signal-to-noise ratio of the ISO data is low.Ground»based limits are NH of the order of 5% with respect to solid H2O,82 which is comparable to the gas-phase 3/H2O low \0.1 consistent with the dense collapse phase. Gas-phase has been observed 3 toward GL 2591,85 but the deduced abundance in the 40A beam is low \10~8. Note that the observed abundances of NH in diÜuse clouds have also been cited as evidence of grain surface chemistry.86 Hydrogenation of solid CO may lead to solid and H2CO CH3OH.The latter species is known to be present in substantial abundances of a few percent with respect to H2Oice in these objects.87 Solid formaldehyde has possibly been seen in a few sources at a similar abundance.22,88 These abundances are larger than can be explained by pure gas-phase chemistry and subsequent freeze-out onto the grains suggesting grain-surface formation. 2 7.2 Solid CO2 The observed solid CO abundances of a few ]10~6 to 10~5 are more than an order of magnitude larger than those predicted from pure gas-phase chemistry.3,4 As discussed in 42 ISO and gas»grain chemistry 2 .89 Section 3.2 solid CO may be formed either by surface reactions of CO]O or through 2O»CO ice mixtures where the CO reacts with the O or OH liberated by photolysis of H 2 the photodissociation of other molecules.Can the ISO data distinguish between these two possibilities ? The presence of CO in grain mantles toward the –eld star Elias 16 in Taurus indi- 2 cates that no embedded source of luminosity is required to produce the molecule.77 However other sources of ultraviolet photons are available as well including the external interstellar radiation –eld and the cosmic ray induced photons. Quantitative estimates using laboratory data indicate that the latter route is insufficient to account for the observed abundance of solid CO2 .77 The total visual extinction AV\21 magnitudes of the cloud also appears to preclude the external interstellar radiation –eld.However if the cloud has an inhomogeneous structure and the typical A at any location along the path is about 5 mag this radiation may be sufficient. Alternatively ion bombardment of V ices by cosmic-ray particles may trigger the formation of solid-CO The current data cannot yet fully determine the relative importance of grain surface formation of CO compared with production through photolysis. Additional laboratory 2 experiments on the CO]O surface reaction as well as observations of other molecules which are thought to be produced through energetic processing such as ìXCNœ,90 NO2 and N2O are needed in a variety of sources. Note that XCN has not been detected toward Elias 16.90 7.3 Gas/solid-state abundance ratios In Table 2 the gas/solid state ratios derived from the ISO data for the major species CO CO2 H2O CH4 and are summarized.The temperatures listed in the last column refer to those derived from the gas-phase CO excitation.30 In all eight sources CO is principally in the gas phase although the gas/solid ratio still increases for the warmer sources (see Fig. 1). In contrast CO is primarily in the solid phase even in the warmest 2 regions. The gas/solid H2O ratio varies from less than 5% for NGC 7538 IRS9 and W 33A to more than unity for GL 2591 and GL 4176. The gas/solid CH ratio of 0.5 4 derived for the cold sources is considerably higher than that of O H2O C and but lower than that of CO. The gas/solid ratio of HCN is greater than 0.1 in sources in 2 which gas-phase HCN has been detected while no stringent limits are obtained for gas/solid C2H2 .What do these ratios imply for the source structure and evolution ? It should be recalled that the observed gas/solid ratios are averaged along the line of sight to the star. In reality strong gradients in the temperature and in the abundances of these species Table 2 Gas/solid state abundance ratiosa CO object CO2 H2O CH4 Twarm b/K 0.01 » 0.02 12 43 100 0.4 0.5 [1 180 120 176 390 577 580 \0.04 0.01 0.03 0.04 0.06 0.4 NGC 7538 IRS9 W 33A NGC 7538 IRS1 S140 IRS1 W 3 IRS5 GL 2136 » [ »1 0.01 0.01 0.03 0.08 0.07 121 163 122 [300 380 2.2 1.2 GL 4176 GL 2591 [500 B1000c [ »1 a Using the gas-phase column densities from Table 1 solid CO and H column densities from ref.70 and 75 solid CO from ref. 70 and 72 and 2O 2 solid CH from ref. 26. b From gas-phase CO excitation.30 c Two warm 4 components with T \200 and T \1000 K can be distinguished.30 43 E. F. van Dishoeck could result from outgassing of the icy mantles. exist both in the gas and on the grains.14,51,91 As the young star evolves its radiation heats up an increasing fraction of the surrounding gas and dust leading to enhanced evaporation of the ices and an increasing gas/solid ratio. Are the coldest sources such as NGC 7538 IRS9 therefore just younger versions of the hotter sources like GL 2591? Within a factor of two they have the same luminosity. Comparison of the column densities and abundances of the ices in NGC 7538 IRS9 with the values of the corresponding gas phase species in GL 2591 indicates that the latter are always lower suggesting that this scenario is quantitatively feasible all of the observed gas-phase H2O CO2 and CH4In more detail the trends for the gas/solid H H2O C and with increasing temperature indicate that outgassing of the icy mantles is signi–cant in the hotter regions.4 For NGC 7538 IRS9 and W 33A the CH excitation temperature of ca. 100 K and the 4 large CH gas/solid ratio compared with that of H2O suggest a grain temperature of the order of 80»90 K just below the sublimation temperature of solid H 4 2O. This would imply that the gas and dust temperatures are not fully coupled in spite of the high densities.Sputtering of grain mantles in the powerful out—ows is unlikely to be the dominant grain mantle removal mechanism since this process would have resulted in similar gas/solid ratios for all species. 2 produced by high-temperature gas-phase chemistry in the inner ìhot coreœ region initi- Part of the observed abundant gas-phase H2O in the warmer sources could also be ated by the O]H reaction. This path starts to become signi–cant at temperatures 2 greater than ca. 230 K,48,51 which could be reached by radiative heating close to the star. This possibility can be tested by observations of sources with a range of temperatures of the warm gas. In Fig. 6(a) the observed H2O abundances in the warm gas are shown as functions of the CO excitation temperatures30 assuming that all H2O is in the warm gas N(H O)/N 2 temperature.For three sources W 33A NGC 7438 IRS9 and IRS1 the temperatures are warm(H2). This excitation temperature should be close to the gas below 200 K whereas for the other sources they are signi–cantly higher. It is seen that there is a clear increase in the H2O abundance with increasing temperature. For the lower temperature sources the H2O abundance is around (3^2)]10~6 whereas for the warmer sources it is increased by an order of magnitude to (3^2)]10~5. The errors in the data are too large to determine whether there is a real dichotomy at around ca. 550 K or whether this is just a continuous trend. Does the lower value represent the amount of H2O that has evaporated from the grains and is the higher value representative of the gas-phase reactions ? Or is just the fraction of H2O-ice that has evaporated larger in the warmer sources ? In that case one may have expected that the gas-phase H2O abundances would have been even higher.Fig. 6(b) shows the observed H2O-ice abundances with respect to total H (open symbols) and with respect to cold H2 Ncold(H2)\Ntot(H2)[Nwarm(H2) (–lled symbols). As expected the ice abundance with respect to total H decreases from ca. 2]10~4 in the coldest sources to ca. 1]10~5 in 2 the warmer sources. However although the H2O-ice abundance in the cold gas also shows some decrease it is signi–cantly less and nearly constant at ca. 10~4 within the errors of a factor of two. These results imply that if the warmer sources evolved from a cold phase with H2O-ice abundances of typically 10~4 part of the oxygen must have been converted into another form after evaporation in the gas most likely O or O2 to account for the lower H2O abundances (gas]ice) in the warmer sources.Only at higher temperatures is part of the oxygen driven back into H2O again. Hot core models indeed predict a breakdown of H2O on a timescale of ca. 105 yr provided the temperature is not too high.48 2O However this is unlikely to be the explanation for the Gas-phase production of at least some of the H2O in the warmer sources would be consistent with the lack of abundant gas-phase CO2. H If the fraction of that results from outgassing of grain mantles is only ca. 5% this would be comparable with the amount of evaporated CO2 .44 ISO and gas»grain chemistry abundances in the warm gas Fig. 6 (a) Observed H N(H O)/N 2O warm(H2) H as functions of the gas 2 determined from the CO excitation.30 (b) Observed solid temperature T respect to total warm symbols) as functions of the gas temperature 2O abundances with H2 N(H2O)/Ntot(H2) H (open symbols) and the cold 2 N(H2O)/Ncold(H2) (–lled Twarm determined from the CO excitation.30 warmer sources where the data in Fig. 6 suggest signi–cant evaporation of H2O ice. Another possibility discussed in Section 6 is that most of the solid CO is in a segregated phase located in the outer part of the envelope which has not yet been heated. 2 Once in the gas-phase CO may be destroyed by gas-phase reactions on short time- 2 scales although efficient reactions with abundant species still remain to be identi–ed.25 Altogether the absence of abundant gas-phase CO is still not fully understood.2 8 Concluding remarks In summary the results illustrate that ISO can provide important new insights into the role of gas-grain interactions and the chemical and physical evolution of star-forming regions. Information on the major products of grain surface chemistry can be obtained the mechanisms for evaporation of icy mantles can be tested and the chemistry of hot core regions can be probed. IR absorption spectroscopy remains a unique tool for the study of both the gas-and the solid-state components along the same line of sight. 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