J. Phys. Chem. 1983, 87, 4243-4260
4243
Ices in Space J. Mayo Greenberg,’ C. E. P. M. van de Bult, and L. J. Allamandola Laboratory Astrophyslcs. Huygens Laboratory, University of Leiden, Leiden, The Netherlands (Recelved: November 17, 1982; I n Final Form: April 4, 1983)
The chemical and physical properties of ice grains in interstellar space have been studied in the laboratory and theoretically modeled to compare with astronomical spectra between 2700 and 3700 cm-’. The observed polarization of starlight in this region clearly indicates that elongated particles are involved. Absorption characteristicsfor various shaped grains whose radii vary from -0.1 to 1.0 pm, containing either pure amorphous H 2 0 or amorphous mixtures of H2O with NH3,have been calculated with the aim of narrowing the range of acceptable grain parameters. By comparing the band shapes for spherical, spheroidal, and cylindrical grains with astronomical spectra we show that elongated particles whose radii are -0.15 pm produce an acceptable match and that both spherical and elongated particles whose radii are 20.5 pm are definitely not consistent with observations. Details of the band shape are shown to depend on particle size, shape, and composition. Similar profiies can be produced by using different combinations of particle shape and composition. For example, the NH3signature at 2.97 pm, which is prominent in a spherical grain, is greatly suppressed when in an elongated grain. This is exactly equivalent to reducing the concentration of NH3 in a spherical grain. A morphological grain model is used to explain the large variations in the observed strength of the 3.07-pm ice band from one region of space to another.
1. Introduction
Perhaps the earliest notion that there was a considerable amount of water ice in space arose from the application by van de Hulst’p2 of the idea suggested by Lindblad, that the small particles between the stars (interstellar dust) could have formed by accretion of a t o m in space. Starting with the most abundant condensible species-oxygen, carbon, and nitrogen-and letting them combine with the highly abundant hydrogen after sticking on the already present small solid particles, van de Hulst derived a model for the dust grains consisting of the saturated molecules H20, CHI, and NH, with traces of other constituents. Since the interstellar grains are at temperatures of the order of 10 K, all the molecules are frozen and the mixture came to be called “dirty ice”. About the same time as the dirty ice model was proposed, Whipple4 suggested that the cometary nucleus consisted mostly of H 2 0 ice. Although the origins of comets are somewhat uncertain, one of the current views is that comets have formed directly by coagulation of the interstellar If this is so, then the cometary ices are the same as the interstellar ices which we shall see must be a complex melange which includes many more complicated as well as simple molecules along with the HzO. The prediction of the dirty ice dust model was that about 170of all the material between the stars consisted predominantly of H20. Since this represents a really large amount of H 2 0 when one considers the vast dimensions of space it was reasonable to expect to detect the HzO in space from its strong OH stretching absorption at 3 pm. The initial results of such astronomical observations were surprisingly negative.6 They set upper limits on the amount of HzO present in the grains which were, even by generous estimates, less than 10% of the expectation value. (1) van de Hulst, H. C. N e d . Tijdschr. Nutuurkd. 1943, 10, 25. (2)van de Hulst, H. C. Rech. Abstr. Obs., Utrecht 1949, 11, part 2. (3) Lindblad, B.,Nature (London) 1935, 135, 133. (4) Whipple, F. L. Astrophys. J . 1950, 1 1 1 , 375.
(5) For recent developments and references, see Greenberg, J. M. “Comets”; Wilkening, L. L., Ed.: University of Arizona Press: 1982; pp 131-63. (6)Danielson, E. R.; Woolf, J. N.; Gaustad, J. E. Astrophys. J. 1965, 141, 116.
A subsequent observation was equally unsuccessful.’ The observations were not easy because they required not only a great pathlength through dust but a strong infrared background source as well. These two requirements are, in the normal interstellar medium, quite difficult to satisfy simultaneously. The breakthrough in the discovery of interstellar ice came following the detection of a strong infrared source in the Orion nebula. This, along with the high density of interstellar dust in that region, made it possible to provide the first 3-pm absorption evidence for H 2 0 ice in space.* As a bonus, a strong featureless absorption at 9.7 pm showed up which gave evidence for a new material in space. This has since become attributed to the Si0 stretch in an as yet unidentified amorphous silicate type material. This latter has come to be considered as the most likely nucleus (which was lacking in the accretion model of van de Hulst) on which the condensable atoms and molecules from the gas may stick; Le., it is the seed or core for the interstellar grains. In spite of the first success in the observation of H 2 0 ice, continuing attempts met with highly variable results. There did not appear to be any predictable relationship between the ice band absorption strength and the amount of extinction which is a measure of the totalamount of dust along the line of sight. Most of the observations produced negative results and where the ice band was detected there were distinct discrepancies in shape and peak position when compared with what was then known about the infrared properties of ice.g Although the situation was certainly becoming confusing it was forcing the recognition that not only was the ice in space much more complicated than had been pictured, it also required new laboratory investigations to provide the relevant data for the interstellar conditions. In 1969 Greenberg suggested that the negative 3-pm ice result for such highly reddened stars as CIT 11 and VI Cygni No. 12 may possibly be explained away by effects (7) Knacke, R. F.; Cudaback, D. D.; Gaustad, J. E. Astrophys. J. 1969, 158, 151. (8) Gillett, F. C.; Forrest, W. J. Astrophys. J . 1973, 179, 483. (9) Bertie, J. E.; Labb6, H. E.; Whalley, E. J . Chen. Phys. 1958, 50,
4501.
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of ultraviolet radiation on the dirty ice mixtures.’O The reduction in strength of the OH stretch at 3 fim was attributed to the breakup of the water molecules by photolysis. The use of a laboratory to study the photochemistry of interstellar ices actually arose not only because of the negative ice observations, but it was also motivated by the apparently disconnected astronomical discovery of the then surprisingly complicated molecule formaldehyde (H,CO) in the interstellar gas.” The formation of complicated molecules and the reduction of the strength of the ice band were conjectured to have a common origin in the processing of grains resulting from photolysis of the ices by the ubiquitous ultraviolet radiation in space. Since, not only OH, but other radicals are formed by photolysis as well, it is but one step further to see the concomitant formation of complicated molecules in the grains so that HzCO was probably a simple example of the full complexity of what actually existed in the small solid particles. There was already abundant literature on such problems as the production of “complicated” molecules by the photolysis of low-temperature solids.12 There was also the classic gas-phase experiment of Miller and Urey showing that spark discharges which dissociated the molecules simulating a primitive earth atmosphere led to the formation of complex organic m01ecules.l~ But there had been no experiments on complex, reactive, low-temperature mixtures which could properly simulate the interstellar grains. This was attempted a t the State University of New York at Albany where temperatures approaching the required 10 K level (actually T 2 28 K) were used in the early experiment^.'^ But it was not until the foundation of Laboratory Astrophysics at the University of Leiden in 1975 that it became possible to combine all the principal requirements for a “labratory analogue” of the interstellar grain pr0b1em.l~ It has indeed turned out that without this approach one cannot hope to answer the puzzles about H 2 0 ice and at the same time the connection between grains and the radio observations of complicated gaseous molecules in space. In the subsequent sections we shall attempt to summarize the current status of interstellar ice from the point of view of observations, physical and chemical laboratory studies, optical modeling of interstellar grains, and the underlying astrophysics of grain evolution. In order to maintain a reasonable degree of completeness in this review we shall have to overlap somewhat with another paper in this issue.16a
2. Basic Interstellar Medium The space between the stars which is known as the interstellar medium provides the raw material from which stars and planetary systems are born and is also the burial ground for the remnants of stellar systems which have expired. This extremely tenuous distribution of matter, comprised of gas and dust, accounts for about 10% of the material in the Milky Way. The dust, small particles (10)Greenberg, J. M. ‘Molecules in the Galactic Environment”; Gordon, M. A.; Snyder, L. E., Ed.; Wiley: New York, 1973;p 94. (11) Snyder, L. E.; Buhl, D.; Zuckerman, B.; Palmer, P. Phys. Reo. Lett. 1969, 22, 679. (12)See,for example, “Vibrational Spectrcacopy of Trapped Species”; Hallam, H. E., Ed.; Wiley: London, 1973;and in particular references to the substantial work of Milligan and Jacox, mentioned therein. (13)Miller, S. L. Science 1953, 117, 528. (14)Greenberg, J. M.; Yencha, A. J.; Corbett, J. W.; Frisch, H. L. Men. Soc. R. Sci. LiCge 1972,6th ser, Tome 111, 425. (15)Greenberg, J. M. Ned. Tijdschr. Nutuurkd. 1976,42, 117. (16)(a) Tielens, A. G. G. M.; Hagen., W.; Greenberg, J. M., this issue. (b) Aannestad, P. A.; Greenberg, J. M. Astrophys. J., in press.
Greenberg et ai. approximately 0.1 pm in radius, is extremely cold, being generally at a temperature of about 10 K. The molecules, which have been frozen onto this dust, comprise the ice in space which is discussed in this article. The other major constituents of the interstellar medium in which this ice is found will now be described. A more complete description of the interstellar medium is given in ref 17 and 18. (a) Atoms. Most of the interstellar medium is hydrogen which was created in the earliest stages of our universe. The formation of the heavier elements has been an ongoing process ever since the first stars were born. What we see today, on the grand scale, is a distribution of the elements which have been produced by stars and ejected back into space. Following helium, which is chemically inert, the group of elements comprising oxygen, carbon, and nitrogen constitute about one part in a thousand, by number, relative to hydrogen. The elements of the next most abundant group-magnesium, silicon, iron, and sulfur-are further down by a factor of ten. The mean hydrogen density throughout the galaxy is about 1 ~ m - ~ , (b) Molecules and Dust. Before the late 1960’s, general consensus held that the interstellar radiation field was so harsh that no gaseous polyatomic molecule would survive long enough to permit detection. The spell was broken in 1968 with the detection of gaseous NH, toward the galactic center.lg This discovery was startling for many reasons; important among them is the fact that NH3 is easily photodissociated. Millimeter wave astronomy was now properly launched and it was not long afterward that formaldehyde was discovered, followed quickly by CO. To date, a wide range of molecular species have been detected in the gas, nearly 60 in total, of which carbon monoxide is the most abundant, though still consuming 510% of the available carbon. This is only the tip of the iceberg, since, in addition to the gas, many of these molecules, as well as others which have been formed “in situ” in the grains, are to be found as an ice frozen onto the dust in abundances greater than seen in the gas. Thus, even though it is 10l2 times less abundant by number than hydrogen, each particle can contain as many as lo9 of the smaller molecules such as CO and NH,, so that there are more molecules in the solid than have been found in the gas. Although we can say that the dust and gas have a well-defined general distribution within the galaxy, a brief glance a t the night sky reveals a highly inhomogeneous and patchy structure. This is not due to an uneven distribution of stars, but rather the presence of concentrations of dust, acting like a smoke screen and blocking the light of the background stars. This shows that the interstellar medium actually consists of a chaotic distribution of gas/dust clouds with a variety of densities, always in motion, and passing from one phase to another in their evolution, with the most dramatic phase appearing when new stars are formed. The widest variety of molecules is associated with clouds in intimate association with star formation. In addition to blocking the light of stars,the grains sometimes cause it to be partially linearly polarized. This means that the particles must be nonspherical and, to some extent, aligned. The extinction is quantitatively defined by astronomers in magnitudes as Am(X) E A(X) = -2.5 log [I(A)/I,,(X)], where X is the wavelength and I the intensity of radiation. (17) Spitzer, L., Jr. “Physical Processes in the Interstellar Medium”; Wiley: New York, 1978. (18)Greenberg, J. M. “Cosmic Dust”; McDonnell, J. A. M., Ed.;Wiley: New York, 1978;Chapter 4,p 187. (19)Cheung, A. C.; Rank, D. M.; Townes, C. H.; Thornton, D. C.; Welch, W. J. Phys. Reu. Lett. 1968, 21, 170.
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Since the passage of light through the interstellar medium is attenuated by exp{-r},where 7 is the turbidity or optical depth, the extinction, in terms of the turbidity, is A(X) = 1.0867(X). As was first pointed out some 50 years ago, the interstellar extinction causes a reddening of the light from distant stars which is not to be confused with a Doppler effect.z0 This reddening is commonly called the color excess and is defined as the difference in the extinctions in the “blue” ( B = 435 nm) and the “visual” (V 547 nm); i.e., color excess E(B - V) = A ( B ) - A(V). By measuring the mean amount of starlight extinction per unit distance (which is proportional to the total area of particles per unit area) and from the mean size of the dust grains, which is inferred from the color dependence of this extinction and polarization, a mean spatial density of dust of one particle per 10l2 cm-3 is derived. This is approximately the volume of a cube whose sides are a football field in length. In darker, denser regions, the density may be higher by a factor of lo4 to lo6. The hydrogen number density, nH, is about 10 cm-3 in the socalled diffuse cloud regions and can be as high as 105-106 cm-3 in the denser molecular cloud regions. To provide is equivalent to a point of reference, a density of lo6 a pressure of 3 X mbar! The denser regions are also known as molecular clouds because it is in these that the molecules are detected. ( c ) The Interstellar Radiation Field. The mean ultraviolet radiation coming from the general stellar population photons cm-3 with energy greater consists of about 3 X than 6 eV (A = 200 nm), where 6 eV is chosen as a rough threshold value for photodissociation of molecules (some obvious exceptions being CO and Nz). Thus, in the diffuse medium, the density of photons in the range 912-2000 A is greater than the density of atoms and molecules. In the cores of dense clouds the radiation is attenuated by passage through the dust so that it is reduced enough to reverse this relationship.
3. Ice Observations In this section we shall give some examples of both the positive and negative evidence for HzO in interstellar grains. ( a ) Positive HzO Obseruations. The literature now contains many examples of the 3.07-pm absorption and, in a few cases, the degree of polarization through this band. So far most of the spectra in the 3-pm region have been measured with resolution no better than Av N 50 cm-l, or equivalently, AX N 0.05 pm. Within this limitation the data show a rather high degree of constancy in the value of the peak absorption wavelength. Nevertheless substructure in the band which varies from object to object is clearly evident. Several examples of ice bands are shown in Figure 1 along with the silicate absorptions. We note that although curves 1-4 show a hint of structure near the peaks their deepest absorptions are at X N 3.07 pm whereas the peaks for objects 5-7 occur at wavelengths short of 3.07 pm. The differences are significantly greater than the resolution element. These observations were taken from Willner et ai. and are the infrared spectra of protostars.21a There exist other similar observations but these constitute a fair sample for our purposes. It is well-known that pure H 2 0 also absorbs at 6 and 12 pm. Although the 6-pm feature is evident in the astronomical
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(20) Trumpler, R. J. Lick Obs. Bull. 1930, 24, 154. (21) (a) Willner, S.P.; Gillett, F. C.; Herter, T. L.; Jones, B.; Krasemer, J.; Merrill, K. M.; Pipher, J. L.; Puetter, R. C.; Rudy, R. J.; Russell, R. W.; Soifer, B. T. Astrophys. J. 1982,253, 174. (b) Willner, S.P.; Pipher, J. L. ‘Proceedings of Workshop on the Galactic Center”; California Institute of Technology, 1982.
spectra, it can be distorted or complicated by blending with absorptions due to other molecules, e.g., HzCO and NH3. A good profile of the 6-pm region taken with moderate resolution is not yet available because of atmospheric absorption and whatever measurements exist have been made from an airborne observatory with a relatively small telescope. It is interesting to note that where the 3-pm band is shifted short of 3.07 pm, as in curves 5-7, the 6-pm band is rather obscured. The 12-pm absorption, on the other hand, does not seem to exist at all in the interstellar ices surrounding protostellar objects. The common substructures in the ice band are at X N 2.9 and N 3.4 pm. Another common feature of the interstellar ice band is the absorption wing which extends rather far on the long wavelength side. There are a few exceptions to this; one example is shown by Soifer et al. in their Figure 1F2 Not only are the wing and substructure absent, but the 6-pm band is sharper and the 12-pm band is present, all characteristics of relatively pure, amorphous HzO ice.23 This is notably not a protostellar source and consequently there may be good reasons why the ice band is different.23 We shall not discuss further small features in other regions of the astronomical spectra. We note, for future reference, that the strength of the ice absorption relative to the 9.7-pm silicate absorption is obviously highly variable and, in fact, there may be no ice absorption even with a strong silicate absorption (see, e.g., curve 8 in Figure 1). A very interesting, and suggestive, observation has been made by Joyce and Simon24where it appears that, for compact infrared sources imbedded in dense molecular clouds, the deepest 3.07-wm absorptions are associated with the most highly polarized objects. This may be interpreted to imply that H 2 0 ice becomes a dominant feature only in an extremely dense cloud environment. A limited amount of polarimetry has been done within the ice band. The polarization of the BN source as obtained by two independent observations is shown in Figure 2.25 The HzO absorption is shown for comparison.26 (b)Negatiue HzOObseruations. First of all, it must be stated that no HzO absorptions have ever been detected in the diffuse cloud medium. Starting with the first observations it had been assumed that the detectability of HzO should be greater the greater the extinction by the obscuring dust. Although this is a necessary condition it is far from sufficient. Since some of the brightest infrared sources are at a great distance from us in the center of the Milky Way, they provide an excellent background for observing an ice band if present in the dust along the line of sight. As is obvious in Figure 3, although there is some sort of absorption feature at approximately 3 pm, it is quite different from any of the H 2 0 features shown in Figure 1;it is perhaps three times broader and also its maximum absorbance, where determined, is at a wavelength substantially short of 3 pm. We note also that there is considerably greater variety in this feature than is exhibited by the ice features, and that the ratio of its strength to the total extinction is far less than that of the ice band strengths shown in Figure 1. Indeed this appears to be an absorption by some material quite different from H,O. (22) Soifer, B.T.; Willner, S.P.; Capps, R. W.; Rudy, R. J. Astrophys. J. 1981, 250,631. (23) Hagen, W.; Tielens, A. G. G. M.; Greenberg, J. M. Astron. Astrophys. Suppl. Ser. 1983, 51, 389. (24) Joyce, R.R.; Simon, T. Astrophys. J. 1978, 260,604. (25) (a) Kobayashi, Y.;Kawara, K.; Sato, S.;Okuda, H. Publ. Astron. SOC.Jpn. 1980, 32, 295. (b) Capps, R.W.; Gillett, F. C.; Knacke, R. F. Astrophys. J. 1978, 226,863. (26) Gillett, F.C.; Jones, T. W.; Merrill, K. M.; Stein, W. A. Astron. Astrophys. 1975, 45, 17.
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Some rather higher resolution observations of one of the galactic center sources shows quite clearly that the material contains little if any H20 and, indeed, as inferred from its detailed structure in the 3.4-pm region, must be due to much more complex molecules.27 The attempts to detect solid H,O in molecular clouds-other than those associated with protostellar objects-have met with almost as little success as the diffuse cloud observations. One such example is shown in Figure 4.28 Although the total extinction in the Taurus cloud is not great, one might expect a noticeable ice band because the grains in this cloud are larger than average and may therefore have sufficient H20 per grain to compensate for the lack of large extinction. The observational result obtained is certainly far below what the authors expected. However, we show (see section 6b and ref 45c) that based on an evolutionary picture of grains the level of detection of a 3-pm band is quite consistent with a substantial presence of H,O in the grain mantles. (See Note Added in Proof.) 4. The Astronomical Problem-The Laboratory Solution Grains interact with atoms, molecules, and electromagnetic radiation in space. Let us consider first the photophysical and photochemical interactions with electromagnetic radiation in the vacuum ultraviolet. If we assume, for the moment, that a grain of -0.1-Fm size consists of a dirty ice mixture of HzO, CH,, NH3 and CO, what would happen to it in space? The ultraviolet photons with energies sufficient to photolyze these molecules would penetrate the particle (only -2-3% of the photons are absorbed in the outer few surface layers) and break the molecular bonds.29 This is schematically illustrated in the top frame of Figure 5. Considering the reaction H,O + hv OH + H, we see that, if a photon with energy greater than the photodissociation energy is absorbed, the hydrogen atom could come flying off and even escape from the grain leaving the radical OH, which could either react with a neighboring atom or molecule, or become immobilized before reacting because of the low grain temperature. Similarly for the other molecules. In general, the energies required to photodissociate molecules are of the order of 4 eV or greater. The frozen radicals are chemically very reactive, and should two of them be adjacent to each other they would generally combine since radical-recombination reactions proceed with zero activation energy, and in so doing release energy to the grain. The possibility of recombination is shown in the second frame of Figure 5 where, for example, combining the hydroxyl radical with the methyl radical leads to the new, and more complex, molecule CH30H. The continuation of this type of process leads to a grain with new molecules and frozen radicals as pictured in the last of the sequence shown in Figure 5. The conditions in interstellar space are such that this phenomenon should be important almost everywhere because the time scales for this process are generally very short compared with the interstellar time scales. An extreme case is given by considering placing the grains in the diffuse cloud (DC) medium where the ultraviolet flux given in = lo8 cm-2 s-l. In a cloud, Table I for Ehv> 6 eV is this flux is reduced by an attenuation factor e-w. A lower limit of 6 eV is used as a convenient and perhaps con-
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(27) Allen, D. A.; Wickramasinghe, D. T. Nature (London) 1981,94, 539. (28) Whittet, D. C. B.; Bode, M. F.; Evans, A.; Butchart, I. Mon. Not. R. Astron. SOC.1981,196, 819. (29) Greenberg, J. M. “Infrared Astronomy”; Setti, G.; Fazio, G. G., Ed.; Reidel Dordrecht: Holland, 1978; p 51.
Greenberg et al.
TABLE I : Comparison between Laboratory and Interstellar Conditions laboratory grain mantle initial composition thickness, pm temp, K gas: pressure of condensible species, mbar ultraviolet flux (A < 200 n m ) , c m - ’ s - ’ time scales diffuse clouds molecular clouds
interstellar
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2 10
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l h 1h
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servative critical value for a photodissociation energy. Reducing this to 4.5 eV results in a doubling of the ultraviolet We define the photoprocessing time (rpp) as the time it takes for the number of photons absorbed by the grain to be equal to the number of bonds in the grain. This photoprocessing time is given by 4a - (4/3)na3 ”’- d 3 ~ a 2 @ D C e - 3d3@Dce-Tu~ r~
where a is the grain radius and d is a molecular diameter. For a = 0.1 pm and d = 3 A, the value of T ~ , ,in the diffuse cloud medium where 7uv = 0 is only about 200 years which is indeed very short. The production of complex molecules and radicals in grains leads to physical and chemical processes which play a role, not only in determining the chemical constituents of the grains, but also those of the gas. In order to understand and quantify the relevant phenomena one must study the effect of ultraviolet photons on materials under conditions which prevail in interstellar space. Early attempts to do this at temperatures (2‘ N 28 K) approaching that of the grains and with photon energies -7.5 eV proved that this method would work.l0J4 Similar experiments performed at rather high temperatures (2’= 77 K) and somewhat lower photon energies (E I5 eV) also showed interesting results in terms of the production of large, complex molecules.30 The Astrophysical Laboratory at the University of Leiden which was established in 1975 is the first to succeed in simulating the essential conditions in interstellar space as they affect the evolution of interstellar grains. A schematic of the main elements of the experimental setup is shown in Figure 6. The key components are the cryostat and the ultraviolet sources. The low temperature is achieved by means of a closed cycle helium cryostat with which one reaches temperatures as low as 10 K on a “cold finger” which can variously be an aluminum block or transparent window mounted in a metal ring. Various gases may be controllably allowed to enter the vacuum chamber of the cryostat starting pressure is 5 X mbar) via a small tube. These gases condense as a solid on the cold finger which acts then like the core of the interstellar grains. On one port of the chamber is mounted a vacuum ultraviolet radiation source which, until now, has almost exclusively been a microwave excited hydrogen flow lamp which has a sharp emission peak at 121.6 nm (Lyman a) and a broad component centered at about 160.0 nm, a spectrum which reproduces quite well that in the diffuse interstellar medium.31 The normal flux of vacuum ul(30) Khare, B.; Sagan, C. ‘Molecules in the Galactic Environment”; Gordon, M. A.; Snyder, L. E., Ed.; Wiley: New York, 1973; p 399. (31) Grewing “Diffuse Matter in Galaxies”; NATO Advanced Study Institute; Cargese: France, in press.
The Journal of Physlcal Chemistry, Vol. 87, No. 21, 1983 4247
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traviolet photons from these lamps is -1Ol6 cm-2 s-' at the target. Through another port (or pair of ports) we may direct the beam of an infrared spectrometer which measures the infrared absorption spectrum of the sample on
the cold finger between 2.5 and 25 pm (4000to 400 cm-'). This is the "fingerprint" region for identifying molecules by their stretching, bending, and other normal modes of vibration. Other monitoring measurements are pressure,
The Journal of Physical Chemistry, Vol. 87, No. 21, 1983
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chemiluminescence, mass spectra, and visible absorption. Further details of the equipment may be found A comparison between laboratory and interstellar conditions is made in Table I. The most important, but necessary, difference is in time scales for photolysis. One hour of radiation in the laboratory is equivalent to 1000 years in the diffuse medium and longer in denser clouds. The basic mode of operation consists of the deposition of mixtures of simple volatile molecules-CH4, CO, HzO, COz, NH3, Nz, 02-with and without simultaneous irradiation as they freeze on the cold finger. Sometimes irradiation is continued after deposition is stopped. We simulate in this way the accretion and photoprocessing of grains in molecular clouds. The principal laboratory sequences and operations are the measurement of the following: (1) infrared absorption spectra of pure substances and mixtures at 10 K to study how molecular interactions affect ~~~
(32) Hagen, W.; Allamandola, Space Sci. 1979, 65, 215.
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The Journal of Physical Chemistry, Vol. 87,
No. 21, 1983 4249
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Figure 7. Absorption of amorphous ice, H,O(as), at 10 K and crystalline ice, H20(IC). The dots show the shape of the 3.1-~mabsorption band in the Becklin-Neugebauer (BN) object.
2 Flgure 5. Schematic evolution sequence for a grain mantle at 10 K subjected to ultraviolet photolysis. The processes illustrated are photodissociation, radical-radlcal recombination, and the productlon of new molecules and radlcals.
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Interstellar
Figure 8. A schematic of the laboratory analogue method for studying interstellar grain evolution Is shown in the upper haif. Molecules are deposited onto a 10 K cold finger and Irradiatedby uitraviolet photons. The optical properties of the 3.1-pm ice band are deduced from the infrared abswptlon spectrum. The lower half compares the conflguration of the ice prepared in the laboratory with that in space.
(3) infrared spectra of irradiated material following warmup to follow the disappearance of frozen radicals and formation of new molecules (4) visible and ultraviolet absorption spectra of irradiated and warmed up samples (5) chemiluminescence (visible) and vapor pressure simultaneously during warmup of irradiated and, for com-
parison, unirradiated samples (6) explosions produced in the warmup period (7) infrared and mass spectrometric analyses of complex nonvolatile residues remaining after warmup to room temperature (8) visible absorption spectra of nonvolatile residues We shall indicate briefly in the following some sample results from the laboratory. ( a ) The HzOIce Band. Interpretation of the observations of the 3-pm ice band (0-H.stretch) requires a knowledge of the absorption properties of H 2 0 in various mixtures and at various temperatures relevant to interstellar dust. The first complete measurements of the optical properties of pure solid H209provided an important guide to the early observations. However, because they were made for pure crystalline ice rather than for ice as it occurs naturally in interstellar space they led to some apparent inconsistencies in shape and position of the ice band which even led to suggestions that the ice band may not be due to H,O at all.33 In the Leiden Astrophysics Laboratory it has been possible to study ices under conditions which match those of interstellar space. We have started first with pure HzO ice even though generally H 2 0 must occur in mixtures along with other molecules in interstellar grains. This work has served as a bench mark or standard with which to compare various mixtures prepared under similar conditions. These studies have appeared in detail in a number of p ~ b l i c a t i o n s . We ~ ~ shall ~ ~ ~summarize ~~~ here for completeness a few of the critical results with emphasis on the spectral features around 3 pm. Further details may be found in Tielens et al. (this issue). One of the most important aspects of interstellar ice is that it forms and exists mostly at extremely low temperatures, T N 10 K. The HzO ice deposited very slowly at this temperature is extremely amorphous (there are various degrees of amorphicity) and accounts for the fact that the absorption due to the OH stretch is about twice as broad as that of crystalline ice (Figure 7). Annealing the sample up to 80 K (still amorphous) results in both a shift in peak (33) Mukai, T.; Mukai, S.; Noguchi, K. Astrophys. Space Sci. 1978, 53, 77. (34) Hagen, W.; Allamandola, L. J.; Greenberg, J. M. Astron. Astrophys. 1980, 86, L3. (35) Hagen, W.; Tielens, A. G. G. M.; Greenberg, J. M. Chem. Phys. 1981, 56, 367.
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The Journal of Physical Chemistry, Vol. 87, No. 21, 1983
Greenberg et al. nT
0
1.0
0.5 f
Figure 8. Approximate dependence of the peak value of ”’on for ice mixtures containing a fraction, f, of H,O. TABLE 11: Optical Constants for Various at Peak Absorption (- 3 fim)
dilution
H,O Mixtures ~~
~
halfwidth Av,
fH,Oa
1 1
0.75 0.42 a
form
cm-’
crystalline amorphous 1 0 K amorphous 1 0 K amorphous 1 0 K
150 300 320 350
“Imeas
0.815 0.5 0.29 0.12
“ITh
0.5 0.35 0.16
f ~ , ois the fraction of H,O in the mixture.
absorption and an increase in intensity (at the peak) as well as a decrease in width. Thus the optical properties measured at liquid nitrogen temperature^^^ differ noticeably from those at 10 K. It is important to note that the 6-pm band is narrower at 10 than at 80 K (opposite to the behavior of the 3-pm band) and that the librational absorption at -800 cm-l of unannealed HzO is shifted to lower frequencies, broadened, and reduced in intensity relative to that of annealed forms of solid H20. These effects are enhanced in mixtures. The values of m’and m” (the real and imaginary parts of the index of refraction) at 3280 cm-’ for pure amorphous ice (deposited and maintained at 10 K) are 1.31 and 0.477, respectively. These are to be compared with the values 1.365 and 0.663 obtained by Ldger et alas at 3240 cm-’. In Table I1 are shown some measured and theoretical expectation values of the strength of the 3.07 pm H 2 0 absorption in pure (f = 1)and mixed ( f < 1)samples where f is the fraction of HzO in the mixtures. We see that the measured value of the ratio m ’i/m’rl which is the ratio of the absorption per unit mass of HzO in the mixture relative to that for an equivalent amount of pure HzO is, as expected, less than unity. This is because, as the ice is diluted, larger fractions of the molecules of H20cannot combine to form the oligomers which give the 3.07-pm absorption.37a An extreme case of this is illustrated theoretically by a very dilute system. Bel~ingel.3~~ has shown, for example, that in a simple cubic lattice (and similarly for other structures) the statistical concentrations of monomers, dimers, and trimers as a function o f f are n M = f(1 - f y n D = 3 f ( l - f)” (36) Jkger, A.; Klein, J.; De Chevergne, S.; Guinet, C.; Defourneau, D.; Belin, M. Astron. Astrophys. 1979, 79,256. (37) (a) van Thiel, M.; Becker, E. D.; Pimentel, G. C. J. Chem. Phys. 1957, 27,486. (b) Behringer, R. E. J . Chenz. Phys. 1958, 29, 537.
= 3p(1 - f)13[4
+ (1 - f ) ]
For f = 0.15 only about 10% of all the solute molecules form trimers so that the expected value of the relative absorptivity of HzO (at -3.1 pm) is f e f f = m’i/m’’l < (0.1)(0.15) = 0.015 because even trimers do not absorb significantly at 3.07 pm. This theoretical result leads us to a schematic general representation for the strength of the ice band at various dilutions as the straight line m’)/m’>= (f - 0.15)/0.85 shown in Figure 8. With rare possible exceptions (as might be produced by high abundances of NH3 in the mixture) the relative strength of the expected ice absorption in mixtures should be well below that of equal amounts of pure HzO and this effect becomes much stronger as f gets smaller. ( b ) “In-Situ’’ Production of N e w Molecules and Radicals-Infrared Spectra. In Figure 9 are shown some results of the laboratory analogue studies of dust evolution. The absorption spectra in this figure show first an unirradiated sample and then, for comparison, the spectra after irradiation where the appearance of new molecules and frozen radicals is evident. We see, for example, that molecules such as formaldehyde and formamide are readily created and it may be inferred that much more complicated molecules are also being produced at the low temperatures. Their presence is clearly indicated after warmup as shown in the upper sequence where, as the more volatile molecules are evaporated, the absorption spectrum takes on the very different character shown in the two upper right spectra. Both samples show that the HCO radical is easily created and probably plays an important role in subsequent stages. The sample illustrated in the lower half shows that HzCO is produced in the cold solid in quantities which may even become comparable (in this sample) with the H 2 0 and NH3 as indicated by the comparable absorption intensities in the 1600-cm-’ region. ( c ) Explosions of Irradiated Samples. It was early demonstrated from both chemiluminescence and pressure enhancement in irradiated materials during warmup that energy was released not only as visible light, but also in the form of heat produced by radical-radical or radicalmolecule reactions. Only about of the energy is released as visible light. The fact that these reactions are diffusion controlled is also clear from the observation that the luminescence stops immediately upon recooling and does not resume until a subsequent warmup again reaches the temperature at which the luminescence has stopped. Explosive events can be systematically produced in the laboratory by ensuring that the reaction energy is not conducted away from the sample too rapidly.38 An example of the pressure and luminescence behavior in such a sample is given in Figure 10 showing simultaneous pressure spikes along with light flashes. When such events occur, essentially all the sample is blown off of the cold finger. It has been noted that the explosions appear to occur for a variety of different samples at temperatures of T = 27 K. From infrared measurements of the shape of the NH, absorption in the remaining material we have established that the temperature overshoots to at least 70 K during the explosion thus clearly demonstrating independently the tremendous energy release which occurs. An important criterion for the explosion to take place is that the number of ultraviolet photons absorbed in the sample be at least 1/10 of the number of molecules in the sample. ( d ) Complex Organic Residue. As was illustrated in Figure 9, during the photoprocessing of the grain mantle
-
(38) d’Hendecourt, L.; Allamandola, L. J.; Baas, F.; Greenberg, J. M. Astron. Astrophys. Lett. 1982, 109,L21.
Ices in Space
The Journal of Physical Chemistry, Vol. 87, No. 21, 1983 4251
I
I
bl
’
1 1
I
I 1
1
2 HOUR i n . s i l u PHOTOLYSIS
A =
1400-3000
1800
F R E Q U E N C Y( c m ’ )
1800
1400
1200
1000
F R E Q U E N C Y( c m ’ ) 1
.
1
-
ICWUJIIOM 1e.4 v [WI] Figure B. Infrared absorption spectra of two different ice mixtures showing the effect of photolysis. Left side of upper sequence and the two lower spectra show first the features in unirradiatedsamples and then the spectra of the irradiated samples showing the “in-situ’’ production of new molecules and radicals. The upper right-hand pair of spectra clearly indicate the presence of complex molecules whose spectra become evident as the more volatile species are evaporated away during warmup.
analogue material more and more complex molecules are created. When the volatile components in the sample are evaporated away by warming there always remains a nonvolatile residue material. If one starts with a mixture of CO:H20:NH,:CH4 (10:1:2:6) the ultimate residue appears yellow. We have obtained infrared absorption spectra of nonvolatile residues with various initial compositions. However, we have not followed up on the photoprocessing by examining the results of continued ultraviolet irradiation of the residues themselves. Nevertheless what we have already done provides some important qualitative answers to the grain composition. One of our samples had a molecular weight of 514 and all of our samples do not evaporate at temperatures less than -400 K with at least one pyrolyzing, without evaporating, at 600 K. The infrared absorption spectrum of a residue
is shown in Figure 11 and comparison is made with the spectrum of the original unirradiated mixture. The relative absorption strengths have been normalized by equating the integrated absorptions of the unirradiated sample and the residue between 2000 and 1000 cm-’. This region is chosen to avoid the H 2 0 absorption enhancement which appears in the 3000-cm-l region. We identify the very broad absorption from 3500 to 2000 cm-’ as due to carboxylic acid groups and a number of absorptions between about 1200 and 1500 cm-’ as due to amino groups. The features around 3.4 pm indicate the presence of -CH3 and -CH2- groups. A high-resolution mass spectrometer analysis of the lowest pressure component in the residue whose infrared spectrum is given in Figure 11showed, after warmup and C 0 2release of about 60 C, a mass corresponding to C4H6N2
4252
The Journal of Physical Chemistty, Vol. 87, No. 27, 1983 C 0 : H 2 0 : N H3: C H4 (10:1:1:4) 3x110 MINUTE DEPOSIT
AND
2
HOUR PHOTOLYSIS)
I
PR E 5 5 U R E
26
28
30
TEMPERATURE(’K)
Flgure 10. Correlation of chemiluminescence flashes with pressure bursts during warmup of the mixture CO:H,O:NH,:CH, (10 1: 1:4) which was prepared by three cycles of a 10-min deposition followed by 2 h of photolysis at 10 K.
and traces of urea. The intensity ratios suggest that aminopyroline rings make up a substantial part of this material. Undoubtedly this material will undergo further modification when subjected to continued ultraviolet bombardment. High molecular weight molecules in liquid or solid form have significantly higher real values of the index of refraction than the volatile ices. We have not yet measured this for these residues but a survey of the data for other complex molecules shows that we can expect a value of m’ of at least 1.40 and perhaps greater than 1.5.39 We have adopted a preliminary estimate of m’ = 1.45 in our calculations. 5. Grain Evolution The birth, growth, and death of interstellar grains is a continuing process so that what we see in various regions of the Milky Way is always a passing phase. Whether in the vast, almost empty region between the stars or in the dense active regions of new star formation, the grains are always undergoing change. So, how is it that there are certain uniform observational properties of the dust which one can associate with diffuse clouds and, on the other hand, what are the processes which lead to the differences we observe in molecular clouds? How can we account for the presence or absence of H20 ice; how can we account for the variations in the infrared absorption spectra which indicate not only H 2 0 but also other molecular species? These and many other questions can only be answered by a full theory of the evolution of grains in space. One of the principal new factors which is making it possible to begin to answer these questions is the application of the laboratory results as explicitly shown in the following scheme. We start with the birth of a grain, assumed here to be in the form of an elongated silicate particle of -0.05-pm radius. Such small particles are formed in the atmosphere of cool evolved stars and blown out by radiation pressure into the surrounding space. Ultimately they are swept up into the general gaseous matter and, being coupled to the gas atoms and ions, begin to partake in the evolution of (39) “Handbook of Chemistry and Physics”; The Chemical Rubber Co.: Cleveland, 1966-7; 47th ed.
Greenberg et al.
the clouds. From the fact that the clouds can not be static-either because they are observed to be in motion, or because we infer or see such dramatic energetic events as star formation occurring within them-it is obvious that the physical conditions of density and temperature represent different stages in their evolution (see middle column of Figure 12). The densest part of clouds seems to correspond to times just before, during, or just after star formation. Diffuse clouds become dense by a number of mechanisms, perhaps as a result of collisions or some source of external pressure.40 Within the dense clouds critical densities may be reached which lead to instabilities and further contraction and finally to star f ~ r m a t i o n . ~ ~ After the stars form-if they happen to be large hot stars or if they develop high material ejection speeds by processes other than radiation (accretion disks for example)-the remaining local material from which they have been formed is ejected into the surrounding space.42 Much of this material, being heated and finding itself in a very tenuous low-pressure environment, expands to reappear as diffuse clouds. Should the silicate particles appear first in a diffuse cloud region no gaseous material will accrete (or remain) on them because of the harsh environment. Sticking of atoms or molecules from the gas on the grains only begins within molecular clouds where it proceeds simultaneously with the process of ultraviolet photolysis. We are now at the start shown at the top of the left sequence in Figure 12. After some period of accretion and photoprocessing two grains will collide with each other at suprathermal speeds resulting from turbulent gas m0tions,4~which raises the temperature enough to trigger grain explosions as described in section 4. Following the explosions-which may be complete or partial-the grains now have mantles of various composition and thickness all the way down to the original silicate core. In general, a residue of complex organic material will be gradually built up on the silicate cores, each successive generation within the cloud leading to an additional layer. Since the indication from the laboratory is that the order of 2% to 20% of the condensed material is converted to the nonvolatile residue each lo7 years we may assume that a substantial mantle thickness say a significant fraction of the 0.07 pm required in the mean for diffuse cloud grains, will have been accumulated in the course of time the grain is in the dense cloud, which is of the order of lo8 years. We shall assume that the total cycle time for a grain to pass through the diffuse cloud phase and the subsequent molecular cloud phase is -2 X lo8years with about half of this time spent in each. The maximum age of a grain is limited because ultimately all interstellar material is recycled through the birth of stars in which all particles are fully evaporated. The turnover time, based on star formation rate estimates, is of the order of 5 X lo9years,44 so that a typical grain should go into and out of a molecular (40) (a) Field, G.B.; Saslaw, W. C. Astrophys. J. 1965, 142, 568. (b) Kwan, J. Ibid. 1979, 229, 567. (c) Oort, J. H. Bull. Astron. Inst. Ned. 1954,12,177. (d) Scoville, N. Z.; Hersh, K. Astrophys. J . 1979,29,578. (e) Taff, L.; Savedoff, M. Monthly Notices of the Royal Astronomical Society 1972, 160, 89. (0 Taff, L.; Savedoff, M. Ibid. 1972, 164, 357. (41) (a) Woodward, P.R. Annu. Rev. Astron. Astrophys. 1978,16,555. (b) Bash, F.N.Astrophys. J. 1979, 233, 524. (42) (a) Blitz, L.; Shu, F. H. Astrophys. J. 1980, 238, 148. (b) Lada, C. J.; Harvey, P. M.Ibid. 1981,245,58. (c) Heydari-Malayeri, M.; Testor, G.; Baudry, A.; Lufon, G.; de la Noe, J. Astron. Astrophys. 1982,113,118. (43) (a) Greenberg, J. M. In ’Stars and Star Systems”; Westerlund, B. E., Ed.; Reidel: Dordrecht, 1979, p 173. (b) Volk, H. J.; Jones, F. C.; Morfd, G. E.; and Roser, S. Astron. Astrophys. 1980,85,316. (c) Lichten, S.M. Astrophys. J . 1982, 255, L119. (44) Oort, J. H. In ”Recent Radio Studies of Bright Galaxies”, Shakeshaft, J. R., Ed.;Reidel: Dordrecht, 1974; p 375.
The Journal of Physical Chemistry, Vol. 87, No. 21, 7983 4253
Ices in Space I
I
I
I
I
I
1
I
co
1
1
I
1
2000
3000 FREQUENCY
1
I
1000
cm"
F W e 11. Comparison of the infrared spectrum of a mixture at 10 K, before photolysis, with that of the nonvolatile resMue remaining after photolysis a i d subsequent'warmup to room temperature under vacuum.
cloud-diffuse cloud cycle approximately 20 times during its lifetime. Because of this it becomes possible to maintain a steady state of particle types in diffuse clouds. Furthermore it is statistically more realistic to have started (as shown in the left sequence of Figure 12) with a coremantle particle than with a bare core particle. In the next section we shall look more closely at the individual steps we have proposed above for the chemical and physical processing of grains and what the grains probably look like at various key stages. Whatever processes are expected to take place in each region are limited to those which can occur in the maximum time interval of lo8 years unless otherwise stated. 6. Appearance of Grains at Various Stages What we shall emphasize here is the observational status of HzO in the interstellar grains based on their changing chemical and physical structure from one region of space to another. The following models are derived from the grain evolution scheme described above. They provide the foundation for quantitative calculations some of which have already been done,16and some of which are presented in section 7. ( a )Dust in Diffuse Clouds. Very few volatile molecules may be expected to remain on a grain during and subsequent to ejection from the star formation region. If any do remain they should have been sputtered or evaporated away relatively rapidly in the diffuse medium so that all that can remain is the organic residue. We thus picture the grains in diffuse clouds as consisting of silicate cores with mantles exclusively made up of an organic refractory material. Under normal interstellar conditions this material will continue to be subjected to ultraviolet photolysis so that its composition may undergo still further changes but it is tough enough to remain as a substantial mantle for times longer than the normal lo8 years spent in the diffuse matter.45 The spectrum of a first generation residue is already very different from that of any H,O ice mixture. For one, its broadest structure, centered at about 3 pm, is at least 800 cm-' wide as compared with the -300 cm-' characteristic of the interstellar ice absorption. Secondly it exhibits a higher degree of complexity in the
-
(45) (a) Draine, B. T.; Salpeter, E. E. Astrophys. J. 1979,231,77. (b)
absorptions in the 3.4-pm region. I t is of interest to note that the spectra of residues which are produced from irradiation of dirty ices in which the initial amount C in CHI is comparable with the C in CO, bear a closer resemblance to some of the galactic center featuresn than do the spectra of residues resulting from irradiation of ices in which C is mostly in the form of CO. This leads us to expect that what we see in the interstellar grains which have passed through n cycles ( n < 20) has been so greatly photolyzed during the 2n X los years spent in the diffuse cloud medium that the carboxyl groups which show up as the very broad 3-pm feature in the first generation residue shown in Figure 9 are at least partially destroyed leaving a material which structurally approaches complex hydrocarbons. In any case, the evolutionary picture of diffuse cloud grains having generally emerged violently from protostellar regions leads to the expectation that there is no H 2 0 to be found in their mantles even though the grains probably contain a substantial fraction of oxygen bound up in the residue. A schematic representation of a typical diffuse cloud grain is shown in Figure 13. ( b ) Dust in Molecular Clouds. From the kinds of changes observed in the wavelength dependence of extinction and polarization, one readily deduces that grains in molecular clouds are larger than those in diffuse clouds.18 Therefore the already observed diffuse cloud grains must be the cores on which additional accretion takes place within the molecular clouds. The question is what is the likely chemical composition of the extra accreted material? It turns out that the diffuse cloud grains as modeled in Figure 13 contain a smaller fraction of the cosmically available oxygen than of the carbon and nit r ~ g e n . ~In ? ~fact ~ so little of the oxygen is tied up in diffuse cloud grains that, whereas the cosmic abundance ratio of oxygen to carbon is OCA:CCA = 6.8:3.7 e 2:1, the gas which is in the state of contracting toward the dense molecular cloud probably has 0:C = 5:1! It is therefore to be expected that the mantle material in molecular clouds must be superabundant in oxygen and consequently one might expect a significant portion of this oxygen to have formed as HzO either within the gas or on the grain surface. Why, then, has it been difficult to detect solid
Ibid. 1979, 231, 438. (c) Greenberg, J. M. "Submillimeter Wave Astronomy"; Beckman, J. E.; Phillips, J. P., Ed.; Cambridge University Press: Cambridge, England, 1982, p 261.
(46) De Boer, K. S. Astrophys.
J. 1980, 224, 848.
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The Journal of Physical Chemistry, Vol. 87, No. 21, 1983
.
Greenberg et al.
I/
\
UltmViOW photon I rratiation
\ Accretion of A t o m s t Mobcules.Irradiation
ccf+hoto-
RDcessedm +
Stored Radcals
-
Grain Grain
COlllSlon lgoiocdlisionl Chain reoctmn
by Ehpomtion or Explosion
I Molecule Ejection Farmtion of Nonvolatik Yellow
Residue
\ I \
I
Grains with Nmbdatile Hontles
J Flgure 12. Schematic diagram of grain and cloud evolution. The sequence on the lefl corresponds to the molecular cloud phase and that on the right shows how the grains evolve through the molecular cloud and star formation phase and then back to the diffuse cloud phase.
H20 in molecular clouds? The answer to this question will depend on several factors: ( 1 ) the use of a proper model for grains in a molecular cloud; (2) the relatively small total extinction available in a molecular cloud not surrounding a protostellar object; (3) the effect of molecular dilution on the absorption strength of H 2 0 in a mixture. The schematic representation of a molecular cloud grain is shown in the middle of Figure 13. We shall apply this model to the specific example already mentioned in section 3, namely, the star HD 29647 in the Taurus dark cloud. The color excess (amount of reddening) of this star is E(B-V) = 1.00. For normal sized grains in diffuse clouds this would imply a total extinction in the visual of 3.1 magnitudes. However, what is observed is an extinction of 3.5 magnitudes.28This implies that the grains are larger than the diffuse cloud grains by about 10% so that on top
of the base organic refractory mantle of -0.12-pm radius is an additional layer which is possibly almost pure H 2 0 with thickness about Aa = 0.01 pm; i.e., the total grain has a radius of -0.13 pm. The strength of the ice absorption is defiied as A(3.07). However, in ref 28 the measured ice strength is defined in terms of [A(3.07)- A(V)]/E(B-V) E E(3.07-V)/E(B-V). Theoretically one can relate the value of A(3.07)/A(V)to the quantity E(3.07-V)/E(B-V) by
Since the absorption by small grains is proportional to their volume and their extinction in the visual is proportional to their area one may show that, for the molecular cloud
The Journal of Physical Chemistry, Vol. 87, No. 21, 1983 4255
Ices in Space
u MANTLE
-0.2Lwm GRAINS IN DIFFUSE CLOUDS
-
-
0.3vm
-
GRAINS IN MOLECULAR CLOUDS
4
5 0.Lp-n GRAINS AROUND PROTOSTELLAR OBJECTS
c
Figure 13. Schematic representation of the types of grains expected in different regions of the interstellar medium.
grain model of Figure 13, the ratio of ice absorption to the extinction is5
A(3.07) A(V)
- E - -
16.rr a z 3 - ai3 e2 E (3.07) a; (el + 2)2 + t22 t2 167 -3Aa (6.2) (3.07) (e1 2)2 + t22
+
Aa = a2 - al
> C,,, so that C,,, N Cab For larger particles C,, may become comparable with Cabsand, since the wavelength dependences of cabs and C, are different across an absorption band, we expect to find that the shape of the extinction will depend on particle size as well as on particle optical properties. Since, for simplicity, many calculations have been performed for spherical particles (for extinction only, of course) it will be of some interest to compare the results for spheres with nonspheres. For nonspheres we also introduce a simplification by limiting ourselves to core-mantle, infinite cylinders although, with considerably more calculational complexity, one may obtain equivalent results for spheroidal grains.49 It is customary to express a cross section in terms of an efficiency, Q, which is defined as the cross section C divided by the geometrical cross section. For spheres we shall calculate and present the quantities QZXtand Q”,,, (superscript s denotes sphere) while, for infinite c linders, we shall calculate the basic cross sections Qfxt, Q:,, and QEa where E and H indicate that the particles are aligned with their axis parallel respectively to the electric and magnetic fields of incident plane polarized radiation. We shall present the results for cylinders in the form of extinction, scattering, and polarization by perfectly aligned particles for an unpolarized incident source. These are, respectively
+
d,
(49) Onaka, T. Ann. Tokyo Astron. Obs., Second Ser. 1980,18, No. 1.
The Journal of Physical Chemistry, Vol. 87, No. 27, 1983 4257
Ices in Space
P c = Q% -2 Q%
..........
PU pa
We have calculated the shapes of the 3-pm absorption bands for a variety of particle types. As representative of the molecular cloud and protostellar cloud grains we have considered both spheres and cylinders with ice mixture mantles on cores which are the compound diffuse cloud core residue grains (see Figure 13). The calculational complexity has been reduced for the ice mantle grains by replacing the silicate core-organic refractory (OR) mantle (compound core) by an equivalent homogeneous grain whose size is the same as the organic refractory but whose index is a mean of that of the silicate and the OR material. The latter has some absorption in the 3-pm region which we have not considered here but which will be included in a later paper. Thus we let the mean index of refraction of the compound core be given in terms of the phase shift along the particle diameter by (0.12)m = (0.05)msil+ (0.07)moRwhere 0.05 pm is the core radius and 0.07 pm is the OR mantle thickness. Using msil= 1.6 and mOR= 1.45 (an estimate based on the range of 1.4 to 1.5 given for complex organic molecules in ref 39), we get m N 1.5. As representative of the complex index of refraction of the ice mantle in dense clouds we have used that of a mixture H20:NH3 = 3:l deposited and maintained at 10 K (unannealed) and that of the same mixture warmed to 50 K and recooled to 10 K hereafter called the annealed ice mixture. As representative of H 2 0 as it might appear if accreted directly from the gas we have also used pure H20 unannealed and annealed to 80 K. We have systematically followed the variation in the strength and shape of the absorption band for spheres and cylinders with both pure and complex ice mantles ranging from 0.13- to 1-pm radius and, for cylinders, the shape of the polarization as well. We have also shown under what conditions the scattering makes a significant contribution. It is beyond the scope of this paper to present all of our results in graphical form. However, we have selected a number of examples to illustrate how the principal ice features vary with index of refraction of the pure ice or ice mixture, shape of the particle, and size of the particle. It is obvious that the most important single parameter defining the shape of an absorption band is the optical property of the material. This is certainly true when we limit ourselves to the range of normal interstellar grain sizes which are 50.2 pm, such that, for the range 2.5 pm < X < 4 pm the value of 2?ra/X is
