composition of grain mantles in interstellar clouds

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  • COMPOSIT ION OF GRAIN MANTLES IN

    INTERSTELLAR CLOUDS

    W. W. DULEY Centre for Research in Experimental Space Science and Physics Dept.,

    York University, Toronto, Canada

    (Received 25 July, 1973)

    Abstract. The question of grain mantle composition in dense clouds is examined in the light of new observational and theoretical results on atomic and molecular concentrations in the gas phase. It is shown that if grain temperatures are less than about 20K these mantles will be primarily solid CO. Methods of identifying CO coated grains are discussed.

    The composition of interstellar grains represents one of the fundamental unsolved problems of astrophysics. Numerous suggestions concerning the nature of interstellar grains have been made (for a review see Wu, 1972). At the present time it appears that interstellar grains are of two principal types; refractory particles perhaps of graphite, silicates or silicon carbide existing in the intercloud medium and core- mantle particles existing within interstellar clouds. It is likely that the refractory particles seen in the intercloud medium and in circumstellar shells are also present in clouds, where they are able to accrete mantles from the gas phase. These mantles represent a condensed sample of the heavier atoms and molecules present in the gas phase within the clouds and are expected to be the source of narrow and broad band spectral features in the infrared, visible and ultraviolet regions of the spectrum.

    Until recently, it was widely thought that mantles would have the chemical com- position proposed by van de Hulst (1949). The failure to observe the 3.07/z absorption band of ice (Knacke et al., 1969) and laboratory experiments on CH 4, NH3 and HzO mixtures at low temperatures (Hunter and Donn, 1971) has shown that H20 is not present on interstellar grains in appreciable quantities. This in turn suggests that the chemical mixture proposed by van de Hulst is unlikely to accurately represent the composition of grain mantles. As recent observations in the radio region of the spectrum have revealed the presence of a wide variety of diatomic and polyatomic molecules and approximate relative abundances are now available, it is of interest to reexamine the question of mantle composition in clouds. It will be shown that grain mantles in these regions are likely to consist primarily of solid CO containing a variety of trapped atoms and molecules.

    In Table I we list the concentrations of some of the simpler atomic and molecular species thought to be present in an interstellar cloud with nil2 =3 x 104 cm -a. This list was compiled from the calculated abundances given by Herbst and Klemperer (1973) and is based on the assumption that cosmic ray ionization produces the primary ions H2 +, He + and H + which then create a wide variety of other species through ion-molecule reactions in the gas phase. As the calculations of Herbst and

    Astrophysics and Space Science 26 (1974) 199-205. All Rights Reserved Copyright 9 1974 by D. Reidel Publishing Company, Dordrecht-Holland

  • 200 W.W. DULEY

    TABLE I

    Gas phase concentrations of simple atoms, molecules and ions in a cloud with density nx, =3 x 104 cm -~ after Herbst and Klemperer

    (1973)

    Atom, molecule or ion Concentration

    H2 3 104 cm -a He 8.45 103 CO 22.5 O 4 N 3 Nz 1.2 Oz 3 x 10 -2 C 1 x 10 -2 H20 5 10 -~ CN 5 10 -4 NHa 2 )< 10 -4 NO 2 10 -4 OH 7 10 .6 HCN 7 10 -a H~CO 1 10 -5 NH 3 10 .6 CEI 6 10 -v C + 2 10 -3 H + 5 10 -4 HCO + 2 10 -4

    Klemperer yield densities which are compatible with measured values in those sources

    for which compar ison of theory with experiment is possible, the densities given in

    Table I can be taken as representative of those expected in a typical dense cloud.

    In this cloud as in clouds of somewhat lower density (Solomon and Klemperer,

    1972) the most abundant molecule with the exception of H 2 is CO. In fact CO is so

    abundant, that it appears that essentially all of the carbon atoms in the gas phase are bound in this molecule. The next most abundant species are O, N, N2 and 02, all. of which have densities which are at least an order of magnitude smaller than that of CO.

    It is now well established that grain temperatures even inside dense clouds are

    unable to drop to H 5 K, the temperature range that would be required for the con- densation of mantles of solid H a (Werner and Salpeter, 1969; Purcell, 1969; Green- berg and de Jong, 1969; Field, 1969). Enhanced binding on the surface of the core

    may result in the covering of this surface with a monomolecular layer of H 2 when the grain temperature is ~< 10K (Hol lenbach and Salpeter, 1971). Wi th a layer of this kind

    on the surface, the condensat ion of subsequent layers is inhibited because of the small

    binding energy of a H2 molecule on a H 2 surface (Lee et al., 1971). Thus, despite the abundance of Hz in the gas phase solid hydrogen mantles are not expected and

    little H2 will be present on grain surfaces. Data on the vapour pressure of solid CO (Meyer, 1970) indicates that CO mantles

    would be stable against thermal evaporat ion on grain surfaces when the grain tem-

  • COMPOSITION OF GRAIN MANTLES IN INTERSTELLAR CLOUDS 201

    perature is less than ~-20K and the gas phase density of CO, nco-~102 cm -3. Even

    for nco=l cm -3, CO mantles would be stable for grain temperatures To~ 10 -7 m) will not be formed on small cores in these clouds as a result of accretion from the gas, sufficient time is available for the growth of CO mantles out to radii of about 10-7 m.

    Much experimental data on the spectroscopic properties of solid CO exists (Buxton, 1972) and this should facilitate identification of CO coated grains in the interstellar medium. In the vacuum ultraviolet solid CO absorbs strongly at the wavelengths given in Table II (Brith and Schnepp, 1965). With high resolution each of the strongest bands of the A'17 ~X'~ + transition exhibits Davydov splitting. With lower re-

    TABLE II Wavelengths of absorption peaks in the AI l I~X1X + transi-

    tion of solid CO (Brith and Schnepp, 1965)

    v' 2(/~) (solid) Width Intensity (solid) (cm -1) (arbitrary units)

    0 1566.6 920 3.3 1555.3

    1 1530.3 1030 3.6 1517.1

    2 1494.3 980 2.2 i483.6

    3 1459.3 780 1.3 1451.2

    4 1426.1 660 0.9 5 1398.8 460 0.45 6 1373.4 400 0.16 7 1350.2 0.07 8 1328.1 0.04 9 1307.0

    10 1287.0 11 1268.1 12 ~ 1250

  • 202 w.w. DULEY

    solution this structure is not observed (Dressier, 1962). In the infrared solid C12016 absorbs at 2138.44-t-0.09 cm -1 (Maki, 1961) while the isotopic species C13016 and C 1201s absorb at 2092 and 2088 cm- ~ respectively (Ewing, 1962). The first overtone of the C12OX6 fundamental absorbs at 4252.74_ 0.40 cm-1 (Maki, 1961). These bands show no structure and have widths which are typically 1.5-2cm -~ (Ewing and Pimentel, 1961) because of the absence of rotational structure in the solid state. Thus high resolution spectra of clouds in the 5 # and 2.5 # regions would enable a distinc- tion to be made between gas phase CO and CO condensed on interstellar grains. It should be noted that where temperature and density conditions are unsuitable for the growth and stabilization of CO mantles, CO can still be chemisorbed on some grain materials. Infreared spectra might then enable the identification of core materials from the shift of the CO fundamental frequency. As examples, CO adsorbed on iron absorbs at 1960 cm -~ while CO adsorbed on Fe203 absorbs at 2127 and 2020 cm -1 (Hair, 1967). Both of these materials have been suggested as possible compo- nents of the interstellar dust. Unfortunately, since a monomolecular layer on the surface of a grain of radius 5 x 10-8m will contain only 105-106 molecules the optical depth at the centre of an absorption band due to adsorbed CO would be small.

    Since solid CO is transparent through most of the infrared region of the spectrum and throughout the visible and ultraviolet to - 1600 A, spectral features due to other species in the gas phase which condence on CO coated grains will appear in extinction measurements. Table lII list some of the products which are expected when the

    TABLE III Products expected when gas phase species impinge on CO coated grains. References refer to experiments which give spectroscopic information on the product molecules in solid CO. Reaction products which likely

    involve overcoming appreciable activation energy are shown with an asterisk

    Incident Product Reference species

    O H20*, CO2 N NH2*, NCO Nz N2 02 02 C CCO, CH2* H20 H20 CN CNCO, H2CN? NH3 NH~ NO NO OH COOH HCN HCN H~CO H2CO NH HNCO, HOCN*, NH3* CH CH~*, HCCO C + C, CO H + H, 02 HCO + CO, H

    Jacox et al. (1965)

    Milligan and Jacox (1971)

    Milligan and Jacox (1967) Milligan and Jacox (1969)

  • COMPOSITION OF GRAIN MANTLES IN INTERSTELLAR CLOUDS 203

    neutral atoms and molecules of Table I become incorporated in CO mantles. Many of these species have been produced in laboratory matrix isolation studies with solid CO (Meyer, 1970) and the wavelengths at which the products absorb have been well characterized. For example HCO trapped in solid CO at 14K gives rise to an extensive series of absorption bands in the 2200-2600 A region and absorbs at 2483, 1863 and 1087 cm -1 in the infrared (Milligan and Jacox, 1969). As the ratio of isolated species to matrix molecules in matrix isolation experiments is typically 10- 2-10-6 by number, it is apparent that condensation of the majority of species given in Table I together with CO would yield mixed solids which can be easily simulated in the laboratory.

    Reactions with H z molecules which collide with a CO coated

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