C O M P O S I T I O N O F G R A I N M A N T L E S IN

I N T E R S T E L L A R C L O U D S

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 H 2 0 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

2 0 0 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

K lempere r yield densities which are compat ib le wi th measured values in those sources

for which compar i son of theory with exper iment is possible, the densities given in

Table I can be taken as representat ive o f those expected in a typical dense cloud.

In this c loud as in clouds of somewhat lower densi ty ( S o l o m o n and Klempere r ,

1972) the mos t a b u n d a n t molecule with the except ion o f H 2 is CO. In fact CO is so

abundan t , tha t it appears tha t essentially all o f the ca rbon a toms in the gas phase are

b o u n d in this molecule. The next mos t a b u n d a n t species are O, N, N2 and 0 2 , all.

o f which have densit ies which are at least an order of magn i tude smal ler t han tha t of CO.

I t is now well es tabl ished tha t gra in t empera tures even inside dense clouds are

unable to d rop to H 5 K, the t empera tu re range that would be requi red for the con-

densa t ion of mant les of solid H a (Werner and Salpeter, 1969; Purcell , 1969; Green-

berg and de Jong, 1969; Field, 1969). Enhanced b ind ing on the surface of the core

m a y result in the covering of this surface with a monomolecu l a r layer of H 2 when the

gra in t empera tu re is ~< 10K (Hol lenbach and Salpeter, 1971). W i t h a layer of this k ind

on the surface, the condensa t ion of subsequent layers is inhibi ted because of the small

b ind ing energy of a H2 molecule on a H 2 surface (Lee et al., 1971). Thus, despite

the abundance of H z in the gas phase solid hydrogen mant les are no t expected and

litt le H2 will be present on grain surfaces. D a t a on the v a p o u r pressure of solid CO (Meyer , 1970) indicates tha t CO mant les

would be stable against thermal evapora t ion on gra in 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 n c o = l cm -3, CO mantles would be stable for grain temperatures To~<17K.

Thus cores would be able to accrete CO mantles in most clouds if the grain tempera-

ture is ~ 1 5 K . Temperature fluctuations (Duley, 1973) might reduce the efficiency with which mantles are accreted but there seems little doubt that grains in dense

clouds would be of the core-solid CO type. The rate at which a grain of radius a accretes atoms or molecules of mass m at kinetic temperature T is

"C a ~ 7"ca2 Fla - - S - 1 ,

where n a is the density of gas phase atoms or molecules. When n a =nco =10 cm -3,

~,=2.1 x 10 -5 s -1 for a = 5 x 10 -8 m and T=100K. Assuming that each impacting

molecule freezes out a grain with this radius would accrete a mantle of ---108 mole-

cules or thickness 5 x 10 -8 m in a time -~ 108/~, ~ - 1013 s. This time is about equal to

gravitational collapse time for a cloud with nn=104 cm -3. Thus although large

grains (a> 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 ' 1 7 ~ X ' ~ + transition exhibits Davydov splitting. With lower re-

TABLE II

Wavelengths of absorption peaks in the A I l I ~ X 1 X + 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 1 0 - 8 m 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 grain while some of the radicals given in Table I are still on the surface will complicate predictions of resulting reaction products. For example, while the impact of a N atom on a CO surface would normally be expected to yield the radical NCO an, encounter between the N atom and an Ha molecule which is migrating over the surface before evapora- ting may result in the creation of the radical N H 2. This radical could be ejected into the gas or react further to yield NH2CO. Some possible reaction products of this sort involving H 2 molecules are also given in Table III. It should be noted that although a reaction such as O + H2 ~ H 2 0 is exothermic, it can proceed at low temperatures only if quantum-mechanical tunnelling can overcome the activation energy associated with the bond in H2 (Watson and Salpeter, 1972). We expect, however, that such reactions may be infrequent when a simple reaction with the CO mantle is energetical- ly favoured. For this reason most O atoms incident on CO coated grains are expected to result in the formation of CO2. Similarly NCO will be the primary product when N atoms impinge on grains of this sort.

Positive molecular ions in the gas phase can also collide efficiently with grains provided the grains have a zero or slightly negative charge. Some of the processes that can occur when a positive ion collides with a negatively charged grain have been discussed by Watson and Salpeter (1972). Ions may either recombine with an elec- tron from the grain forming a neutral which then can return to the gas or become

TABLE IV

Abundance of major spe- cies trapped on grains inside the cloud of Table

I. Percentage is by number

CO 67.5 CO2 15.9 NCO 12.8 N2 3.6 O~ -~0.1 H20 ~0.01 Others -~ 0.09

2 0 4 W.W. DULEY

bound to the surface where it can form a new molecular ion. Since the two most abundant ions C + and H + (Herbst and Klemperer, 1973) will not react efficiently with either H2 or CO (at least in the gas phase) but will react with 0 2 to give the products CO § + CO and O2 + H respectively, it is likely that if these ions are not immediately neutralized by gaining an electron from the grain, they will reside on the surface until the grain traps an Oa molecule t tom the gas at which point the above reactions will occur. The abundant ion HCO + is stable against reaction in the gas phase and thus is expected only to undergo recombination with elections on grain surfaces. The reaction would be HCO + + e -o CO + H.

Table IV summarizes the grain composition derived from the abundances calculated by Herbst and Klemperer (1973). The only assumptions that have been made in deri- ving this composition are that (i) the grain temperature is less than about 20K so that CO condensation is favoured and (ii) that every impinging atom or molecule has a sticking efficiency of unity. We see that apart from an abundance of ~" 10% for NCO, all other molecules predicted to reside on grains are unreactive species.

The molecules listed in Table IV can be removed from grains by a variety of mechanisms. If the radiation field within these clouds in the ultraviolet and visible is such that temperature fluctuations are significant (Duley, 1973) then direct thermal evaporation can occur. Watson and Salpeter (1972) have shown that if low energy cosmic rays penetrate these clouds then molecules physically adsorbed on small particles can be desorbed by cosmic ray heating. Photodesorption may also be important with a quantum yield of ~-10-6 being reported (Greenberg, 1972) for CO and near UV photons. All of these mechanisms are expected to be important in low density clouds. In dense clouds, both thermal and photodesorption processes are unlikely to be significant and cosmic ray evaporation should dominate. However using the estimates of Watson and Salpeter (1972) for cosmic-ray desorption rates from a graphite particle of radius 5 x 10- s m and the CO abundances of Herbst and Klem- perer (1972) it appears that CO mantles could grow in clouds with nH>~ 104 cm -3.

As the wavelengths at which solid CO absorbs in the infrared and ultraviolet are well known, a search should be made for CO coated grains in the interstellar medium. If such searches yield negative results then this would indicate that most interstellar grains do not accrete mantles anywhere in the interstellar medium. This could arise only if grain temperatures never drop below 20 K or if other desorption processes are more efficient than condensation. If CO grains are identified in the interstellar medium then this would represent the first spectral observation on any molecule known to reside on grains. By studying the spectral properties of various atoms and molecules in solid CO further information could be obtained on reactions occurring in the interstellar gas and on grain surfaces.

Acknowledgement

This research was supported by a grant from the National Research Council of Cana- da. I thank Prof. T. Carrington for useful discussions.

COMPOSITION OF GRAIN MANTLES IN INTERSTELLAR CLOUDS 205

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