J. CHEM. SOC. FARADAY TRANS., 1993,89(13), 2261-2268 2261

Observations of Molecules in Diffuse Interstellar Clouds

William B. Somerville and Ian A. Crawford Department of Physics and Astronomy, University College London, Gower Street, London, UK WClE 6BTK

We present results from observations of diffuse interstellar clouds, with discussion of their significance and including a historical survey to put the recent work into perspective. A catalogue has been prepared of molecu- lar data for diffuse clouds, putting observations by different observers onto a common basis. Although most of the spectra were not taken at the highest resolution, the results of comparing column densities for different species, as presented here, has statistical significance for chemical models and in estimating the H, abundance. To consider molecules in individual interstellar clouds, it is essential to observe with the highest available spectroscopic resolution. We have obtained high-resolution (R = A/AA = 3 x lo5) observations of interstellar CHI CH+ and CN. The formation of CH+ in interstellar clouds is a particular problem as relatively high temperatures (ca. 4000 K) are required to drive the most plausible production reaction, which is endothermic, and observa- tions do not confirm the predictions of shock models. The recently installed Ultra-High-Resolution-Facility on the Anglo-Australian Telescope gives resolving powers R of order lo6, which is expected to result in fundamental advances in dealing with this and other outstanding problems in interstellar chemistry. Finally, we present the results of observational searches for the C,, molecule, buckminsterfullerene.

1. Introduction The essential raw material of interstellar chemistry is the information on molecular abundances in individual inter- stellar clouds that comes from observational astronomy. We are concerned here with diffuse interstellar clouds ; these have much lower densities and somewhat higher temperatures than the dense molecular clouds that are studied by observ- ing radiowave, microwave and infrared spectral lines in emis- sion and where the greatest variety of molecular species has been found. The diffuse clouds are thin enough that stars can be seen through them, and the atoms and molecules in the clouds are studied at optical and ultraviolet wavelengths by observing their absorption lines in the spectra of the back- ground stars. The chemistry in diffuse clouds is significantly different from that in dense clouds, because the destruction of molecules by stellar ultraviolet photons plays a major role. A typical diffuse molecular cloud’ has density of the order of 102-103 molecules cm-3 and kinetic temperature of the order of 10-80 K. They have radii of a few parsec? and masses hundreds to thousands of solar masses.

The discovery of interstellar absorption lines in starlight was made in 1904 by Hartmann,’ who found that the Ca 11 K line (393.36 nm) in the spectrum of 6 Orionis did not show the variable Doppler shifts found for the other lines in this spectroscopic binary star. The stationary nature of this line clearly indicated that it did not arise in the atmosphere of the star, and Hartmann concluded that ‘at some point in space in the line of sight between the sun and 6 Orionis there is a cloud which produces that absorption’. In 1909, Slipher3 observed stationary Ca 11 K lines towards several stars in the Milky Way constellations Scorpius, Orion and Perseus, and concluded that ‘the calcium absorption has its origin in an interposing cloud covering at least certain extensive regions of the sky’. Slipher also suggested that observations should be made of the sodium D lines at 589.00 and 589.59 nm to see if they showed the same effect, although it was another decade before stationary D lines were discovered by Heger4

1 pc = 3.09 x 10’’ km. In the region near the Sun, stars are a few parsec apart.

towards /3 Scorpii and 6 Orionis. The realisation that a number of interstellar clouds, each with its own heliocentric velocity, exist towards some stars came in 1936 with the observation by Beak5 that the K line seen towards E Orionis, ( Orionis and p Leonis was in each case split into multiple components corresponding to different line-of-sight Doppler shifts. These were interpreted as produced in distinct inter- stellar clouds at different positions along the line between the star and the sun. This conclusion was supported by a study of the interstellar Na I D lines by Wilson and Merrill,6 who found that although the lines were not resolved by their spec- trograph, their relative strengths were not consistent with there being just a single absorption component.

In 1934 Merrill’ confirmed the interstellar nature of four unidentified absorption lines with wavelengths 578.0, 579.7, 628.4 and 661.4 nm, two of which had been first observed at least as early as 1921.*i9 These lines were all significantly broader than the sodium and calcium interstellar lines. These are among the strongest of what are now called the diffuse interstellar bands (DIBs). The strongest one of all, at 443.0 nm, was first discussed by Merrill” in 1936, and many more have since been observed. Despite much debate,’ ’ their origin is still unclear. It should be noted that the words ‘diffuse’ and ‘bands’ both simply refer to the fact that the lines are broad; no allusion is intended to a molecular band structure.

The introduction of improved technology, in the form of a Schmidt camera and diffraction gratings (replacing glass lenses and prisms, respectively) in the coudk spectrograph of the Mt. Wilson 100 inch telescope in the mid 193Os, greatly increased the ability of that instrument to detect weak absorption lines’’ and soon led to the discovery of new inter- stellar lines. Dunham and Adams’ reported observations of the 330.2 nm doublet of Na I and four lines of Ti 11, and Dunham14 reported the discovery of the 422.67 and 769.90 nm lines of Ca I and K I, respectively, as well as unidentified lines at 395.77, 423.26 and 430.03 nm. Almost immediately Swings and Rosenfeld,” in a discussion on the possible exis- tence of interstellar molecules, suggested that the 430.03 nm line might be due to CH, a suggestion confirmed by McKel- lar.16 Douglas and Herzberg,” on the basis of laboratory spectroscopy, determined that the 395.77 and 423.26 nm lines

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2262 J. CHEM. SOC. FARADAY TRANS., 1993, VOL. 89

were due to the molecular ion CH'. An additional line at 387.46 nm, which seems first to have been reported by Adams and Seares,18 was identified as being due to CN by McKel- lar. These molecular observations clearly indicated that chemical processes must be taking place in the diffuse inter- stellar medium. After these discoveries in the late 1930s, no other interstellar molecule was found until the detection of OH emission at 18 cm wavelength in 1963, followed by ammonia in 1968 and water and formaldehyde in 1969.19

The low temperatures of interstellar space mean that essen- tially all atoms and molecules are in their ground state when they absorb radiation, and therefore only transitions arising from the ground state can be observed by absorption spec- troscopy. For molecules, this means the lowest vibrational state of the ground electronic state. In some cases, such as CN, several of the lowest rotational levels are populated, but for CH and CH' only single lines from the ground rotational state can be observed, an unfamiliar situation for laboratory molecular spectroscopists. Several single lines are seen, corre- sponding to different vibrational levels of the upper electronic state. For most atoms and ions the resonance line transitions occur in the ultraviolet part of the spectrum and, since the Earth's atmosphere is opaque to wavelengths shorter than about 300 nm, they cannot be observed from the ground. By 1943, when Adams2' successfully detected interstellar Fe I in the optical region, virtually all the common atoms, ions and molecules with suitable transitions in the optical region had been observed, and further progress had to await the opening up of the ultraviolet spectral region to astronomical spectros- copy. In 1959 Spitzer and Zabriskie21 published a paper emphasising the importance of the ultraviolet region for the study of the diffuse interstellar medium. This paper (published less than two years after the launch of Sputnik I) argued strongly for the construction of a satellite dedicated to ultraviolet spectroscopy. The first ultraviolet interstellar observations were obtained in 1966 when Morton and Spitzer,, detected several interstellar lines in the wavelength range 126-200 nm towards 6 Scorpii and 'II Scorpii with a spectrograph carried on an Aerobee rocket. This was fol- lowed by several other rocket flights before the first successful launch of a dedicated astronomical satellite (OAO-A2) in 1968. Since then, the use of other satellites, particularly Copernicus (OAO-C, launched in 1972) and the International Ultraviolet Explorer (IUE) (launched in 1978 and still oper- ating more than 14 years later) has revolutionised our know- ledge of the diffuse interstellar medium through the study of several hundred UV atomic and molecular lines. In principle the greater spectroscopic resolution obtainable with the Goddard High Resolution Spectrograph on the Hubble Space Telescope (HST), launched in 1990, is expected to give enhanced information. Some results have been published' 3-2 and despite the well publicised technical problems (including a failure of the highest-resolution system for short wavelengths), data of excellent quality have been obtained. Results published to date are concerned mainly with observations of interstellar atoms but do include2' CO.

The most abundant interstellar molecule by far is of course the hydrogen molecule, H, . Detected first in a rocket flight in 1970,26 it was studied from Copernicus in the spectra of just over 100 stars. Unfortunately, IUE cannot reach the wave- lengths at which H, is observed; because it is absorption from the lowest vibrational state, the longest wavelength is around 110.8 nm. The HST does cover this range, but with poor sensitivity and not now at its highest spectral resolution. Further study of H, in cold interstellar regions will await the planned Lyman-FUSE mission.27 Infrared emission lines28 in the rotation-vibration spectrum within the ground electronic state of H,, the first of which were observed,' in 1976, are

seen only from hot regions of the interstellar gas where there is sufficient energy to excite the upper levels. In practice, these are very localised regions and, the transitions being highly forbidden, the lines are weak. The second most important interstellar molecule is CO, which has been the object of an enormous amount of work in the microwave and far-infrared spectral regions, using its pure rotational spectrum. It was detected in the ultraviolet spectra of stars in 197l3' and has been very well studied with IUE. The OH radical was detected in 1976, both in ground-based observations in the near-ultraviolet at 307.8 nm3 and in Copernicus observa- tions in the far-ultraviolet at 122.2 nm.32 Both of these being rather inaccessible wavelengths, neither of these transitions has been greatly studied since then. The carbon molecule C, was first detected in the absorption of starlight at red wave- lengths in 1977.33 This molecule has been an important object of study in recent years. Like H, , C, is a homonuclear molecule and does not have a microwave spectrum, so the absorption-line measurements provide the only means of studying these important molecules in cold interstellar regions. Recently, interstellar NH has been detected34 in ground-based observations at 335.4 and 335.8 nm.

2. Catalogue of Molecular Observations In the 1940s, using the coudk spectrograph on the 100 inch telescope on Mount Wilson, Adams3' carried out an inten- sive study of optical interstellar absorption lines. Since then, a large volume of data has been obtained by a variety of observers. Smith and S ~ m e r v i l l e ~ ~ . ~ ~ have gathered these observations together in a catalogue of molecular column densities in the directions of reddened stars. The different observers over the years have used a variety of values for the molecular oscillator strengths, so we have scaled their results to put them onto a common basis which enables direct com- parisons to be made between different sight-lines. The cata- logue contains data for almost 300 stars, for 30 molecules (including isotopic variants such as HD and 13C0, and also including several undetected molecules for which there are upper limits). Optical, ultraviolet and radio observations are included. The radio data must, however, be treated with caution when considered in relation to optical or ultraviolet data. The optical and ultraviolet observations take the form of looking at absorption lines in the spectrum of the star; the interstellar material being sampled necessarily lies between the star and the observer and the beam angle, being defined by the disk of the star itself, is very small. By contrast, the radio observations are of emission lines in the general direc- tion of the star; the beam angle is much larger and may include interstellar clouds not sampled in the absorption observations, and the signal may include emission from material lying beyond the star. This question was examined by Dickman et who compared radio emission and ultraviolet absorption measurements of CO in the same directions. The results agree well, showing that the patchy nature of the clouds is not a major problem; where there is serious contamination by material beyond the star the differ- ence is great and in practice such cases can be identified and isolated. In CO observations from IUE, there is the further problem that the molecular band structure is not fully re- solved; this has been discussed in detail, for example, by Black.39 The generally good agreement found between ultra- violet and microwave column densities by Dickman et al. suggests that the various workers studying CO with IUE have dealt with this problem reasonably successfully.

One major purpose of such a catalogue is to study the correlation between abundances of the different species. The

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abundance is usually expressed as a column density, the number of molecules in a column of cross-section 1 cm2 extending from the star to the observer. A limitation in this case is that many of the observations were made at relatively low spectral resolution and do not resolve the separate inter- stellar clouds along the line of sight; we are dealing with the total abundance for each species. Such a comparison can still be valid, so long as its limitations are recognised. If it is found that the column densities of two molecules are closely related, this suggests that everywhere, in diffuse clouds of dif- ferent density and temperature, they co-exist and their abun- dances are related; if a poor correlation is found then the two molecules may not co-exist. If the spectral resolution is ade- quate such conclusions can be supported, in the case where a single cloud is being studied, by comparing the radial velo- cities of the two species. In the earlier publications, however, radial velocity values are generally not given.

Sample results are given in Fig. 1 and 2, showing the corre- lation of H, with CO and with CH. Comparing these dia- grams, two differences are apparent. For CH, the data are fitted well by a line of slope 1.0, but for CO the slope is 0.5. These being log-log relations, the CH column density N(CH) is thus linearly proportional to N(H,), but N(C0) varies as the square of N(H,); there is relatively more CO in denser clouds. These relations were noticed previously by Federman and c o - ~ o r k e r s ~ ' * ~ ~ and by Mattila,42 with smaller datasets than here, and are consistent with cloud models, which indi- cate that CH is formed directly from H, and thus is a first- stage molecule in the photochemical processes, whereas CO

22

h N I

20

19

A 0 0

18 11 12 13 14 15 16

log N ( C 0 )

Fig. 1 Correlation of the column densities of CO and H,. The line has slope 0.5. Reproduced with permission from ref. 44.

22

21

14

N

5 20 m -

19

18 10 11 12 13 14 15

log N(CH)

Fig. 2 Correlation of the column densities of CH and H, . The line has slope 1.0. Reproduced with permission from ref. 44.

is formed from OH and so is a second-stage molecule.43 The second difference between Fig. 1 and 2 is in the amount of scatter about the line, which is substantially greater for CO. The CH data are from optical observations of single lines; the CO data come mainly from IUE observations of only partially resolved molecular bands, so can be expected to be less accurate. Apart from observational error, the scatter in the data must come from differences in physical conditions between different sight-lines, such as cloud density, tem- perature and optical thickness.

These results have been used by Somerville and Smith44 to examine the best methods to estimate the amount of molecu- lar hydrogen in diffuse clouds. In thin clouds of low density, the H, abundance is much less than that of atomic hydrogen, but in the denser diffuse clouds the H, and H abundances are comparable. From Copernicus observation^,^^?^^ H , has been studied in the spectra of just over 100 stars; neither HST nor Lyman is likely to be used for general survey obser- vations of interstellar H, towards large numbers of relatively bright galactic stars. Therefore it is useful to consider what are the most accurate methods for estimating the H, abun- dance, for comparison with other molecules and to give a better measure of the cloud density and total mass. Atomic hydrogen is readily studied using IUE short-wavelength spectra by measuring the wings of the Lyman-a absorption line, and data are available for large numbers of stars. The Copernicus relation gives

N(H,) = 3C5.8 x 1021E(B - V) - N(H)] (1)

where the colour excess E(B - V), obtained from stellar pho- tometry, measures the amount of dust in the line of sight. It is recommended that this relation (1) be used except for sight- lines where the amount of molecular hydrogen is less than about 10% of the amount of atomic hydrogen, in which case the relation

N(H,) = 5.3 x 102'E(B - V ) (2)

gives greater accuracy. From Fig. 1 and 2, it is clearly better to use CH rather than CO as an alternative calibration; in fact, with the linear relation for CH and the quadratic rela- tion for CO (which fit the data well) we have

N(H,) = 1.6 x 1013[N(CO)]1'2 (3)

with r.m.s. scatter 1.3 x 10,' ern-,, and

N(H,) = 2.6 x 107N(CH) (4)

with r.m.s. scatter 9.0 x lo1' ern-,. The optical CH lines have the advantage over CO that they can be measured in ground-based observations, and at high spectral resolution. For diffuse clouds, the atomic hydrogen relation (1) has r.m.s. scatter 2.3 x 10,' cm-, so the CH relation (4) is more accu- rate. In cases where both N(H) and N(CH) are available, it is suggested that the results be combined, weighted according to the respective r.m.s. scatters.

3. High-resolution Observations of Interstellar Molecules

The spectral resolution of interstellar line observations is usually expressed in terms of the resolving power, R = A/AA, where A is the wavelength of the line observed and AA is the FWHM of the instrumental response function at that wave- length. The term ' high-resolution' as applied to astronomical spectroscopy is generally taken to refer to resolving powers

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R 2 10'. A major reason for performing high-resolution observations of interstellar clouds is the determination of the dynamical state of the interstellar medium and it is often con- venient to give the resolution in terms of the instrumental FWHM expressed in velocity units, Av = c /R, where c is the speed of light; R = lo5 corresponds to a velocity resolution Av = 3 km s-'.

In recent years, several excellent astronomical spectro- graphs with R x lo5 have become available and these have led to renewed interest in optical spectroscopy as a probe of the physical, chemical and dynamical state of the interstellar gas. High resolution is important in this work for the follow- ing purposes : (a) The accurate determination of equivalent widths of the absorption lines (the equivalent width is a measure of the strength of the line and thus of the abundance of the atom or molecule giving rise to it). (b) Measurement of the intrinsic line profiles, which provides information on the temperature and dynamical state of the absorbing cloud. (c) Accurate measurement of the radial velocity of an absorbing cloud (which often provides the vital information needed to locate the cloud in its galactic context). ( d ) Determination of the number of discrete clouds present in the line of sight towards a given star (the lines from which are typically separated by several km s- in velocity units owing to differ- ential Doppler shifts).

Key instruments involved in this work include the high- resolution coudk spectrographs attached to telescopes at the following observatories : Mt. Stromlo Observatory (1.9 m telescope; Canberra, Australia), Anglo-Australian Telescope (3.9 m; New South Wales, Australia), European Southern Observatory (3.6 m, 1.4 m coudk auxiliary telescope; La Silla, Chile), McDonald Observatory (2.7 m; Texas, USA), Lick Observatory (3 m ; California, USA) and Cerro-Tololo Inter- American Observatory (4 m; Cerro-Tololo, Chile), and a similar instrument at the Nasmyth focus of the William Her- schel Telescope (4.2 m; La Palma, Canary Islands). Recent work relevant to the chemistry of interstellar clouds, per- formed with one or more of these instruments, includes the following.

Molecular Abundance

Measurement of the interstellar abundances of various mol- ecules, in particular the carbon-bearing diatomic molecules CH, CH', CN and C,, have been made.41*47-55 A s an example, Fig. 3 and 4 show R = lo5 observation^,^^.^^ per- formed with the coudk echelle spectrograph at the Mt. Stromlo Observatory, of the Na I D, (588.995 nm), Ca 11 K (393.366 nm), CH' R(0) (423.255 nm) and CH R,(1) (430.031 nm) lines towards the sixth-magnitude star HD 152235. This star is a member of a group of hot, 'early-type' stars in the southern constellation of Scorpius (the Sco OB1 association), which is situated some 1900 pc from the solar system. The atomic lines (Na I, Ca 11) are seen to exhibit very complex velocity structure, formed as the line of sight passes through discrete interstellar clouds moving at their own particular velocities with respect to the sun (the entire velocity range being from about -60 to + 10 km s-I). In contrast, the molecular spectra are much simpler, with only two lines of CH and CH' present towards the star, both of which occur within the velocity range occupied by the low-velocity, fully satur- ated, Na I D line. This tells us that the latter is caused by at least two molecular clouds along the line of sight (separated by about 6 km s-l in velocity space), whereas the higher velocity clouds, seen in the atomic lines, are not associated with interstellar molecules.

In both Fig. 3 and 4, the observed data points (dots and histograms, respectively) are overlain by theoretical line pro-

I l l 1 1 1 ' I 1 I l l I l l I " ]

1 2 3 4 5 6

> 0.0 .- CI l l l 1 l l , l i , l l l 1 , , I , l l 2 -80 -60 -40 -20 0 20 40

- -

al CI

-80 -60 -40 -20 0 20 40 heliocentric velocity/km s-l

Fig. 3 Interstellar Na I (a) and Ca II (b) lines towards HD 152235. 0, Observed data; smooth curve, theoretical line profile fitted to data. See ref. 56 for full details. Reproduced with permission from ref. 56.

files (smooth curves) that have been generated in an attempt to determine the velocities, column densities and internal velocity dispersions of the absorbing clouds. Full results of this analysis have been given by Crawford and co- w o r k e r ~ . ~ ~ - ~ ~ Here we note that the two CH' components towards this star have column densities of (48 f 8) x 10l2 and (11 & 4) x 10I2 cm-2, and that the corresponding CH column densities are (31 & 7) x 1 O I 2 and (13 & 2) x lo', ern-,. However, we note that at R = lo5 these lines have barely been resolved by the spectrograph, and even higher resolution observations are therefore desirable for more accu- rate column density determinations (see the discussion in Section 4).

> v)

al

c. .-

c. .-

1 .o

0.8

0.6

I I 1 .o

0.8

1 .o

0.8

4 0 -20 0 20 40 heliocentric velocity/km s-

Fig. 4 Interstellar CH (a) and CH+ (b) lines towards HD 152235. Histogram, observed data; smooth curve, theoretical line profile fitted to data. See ref. 49 for full details. Reproduced with permission from ref. 49.

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Chemical Models and the CH' Problem

The accurate measurements of interstellar molecular abun- dances that high-resolution spectroscopy have made avail- able has stimulated the development of elaborate chemical models of the interstellar gas.57-60 These models attempt to predict the relative abundances of molecular species through either equilibrium or time-dependent calculations involving a large number (often hundreds or thousands) of separate chemical reactions. The results are then compared with the observed column densities of the relatively small number of observable species, in the hope that a good match can be used further to constrain the physical conditions existing within an interstellar cloud. A comprehensive and up-to-date list of reaction rates used in these calculations has been given by Millar et aL61

With the exception of CH', these models generally yield molecular abundances in good agreement with the observa- tions. The case of CH+ is a well known exception, as its interstellar abundance [N(CH+) x 10'2-10'3 cm-,] is typi- cally two orders of magnitude higher than predicted by the chemical models (see Black6' for a review). This circumstance has led to the suggestion that interstellar CH+ is produced by the endothermic reaction

( 5 )

However, as this reaction requires a kinetic temperature of several thousand Kelvin before significant CH+ can be pro- duced, it cannot take place in quiescent interstellar clouds, and it has been suggested63 that the ion is formed in the hot gas behind interstellar shock fronts. Although shock models incorporating magnetic fields appear to be able to reproduce the observed CH+ a b ~ n d a n c e s , ~ ~ . ~ ~ they also predict a sig- nificant velocity difference (up to several km s-') between lines due to CH+ and CH (the latter cannot be formed in the immediate post-shock region). Recent high-resolution obser- vations have not confirmed this p r e d i ~ t i o n ~ ' * ~ ~ - ~ * (cf. Fig. 4, where the two molecules have essentially the same radial velocities in both clouds), and the origin of interstellar CH+ is still far from understood.

C+ + H, + 0.4 eV + CH+ + H

Distances of the Interstellar Clouds

There are currently only two ways in which the distances of the clouds responsible for interstellar molecular absorption lines may be estimated: (1) the cloud must be closer to the sun than the background star, which puts an upper limit on its distance; (2) the observed radial velocity of the absorption lines may be used to estimate the distance using the known rotation curve of the Galaxy, or to test a hypothesised associ- ation between the diffuse cloud and a denser (radio-emitting and/or optically opaque) cloud that lies in a nearby direction in the sky.

Recent work by Cra~fo rd ,~ ' -~ ' which has compared the velocities of strong interstellar atomic and molecular lines with those of nearby dark clouds (as mapped in CO emission lines by Dame et aL7,), has strongly indicated that the diffuse molecular clouds are often actually the outlying, less dense regions of the dense clouds. In particular, the velocities of the molecular lines observed towards the star HD 152235 (Fig. 4), and other stars in the Sco OB1 association, are indicative of an origin in the outlying region of the Lupus dark-cloud complex.70 Federman and W i l l ~ o n ~ ~ came to essentially the same conclusion for stars in the northern hemisphere. If this view is correct, it follows that the diffuse molecular clouds are not randomly distributed in space but are intimately associ- ated with the spiral structure of the Galaxy.

Physical Conditions in Clouds

The interstellar temperature and density may be determined through an analysis of the rotational excitation of diatomic molecules. The degree of rotational excitation in an inter- stellar molecule is determined by measuring the relative strengths of the absorption lines arising from the different rotational levels. As radiative transitions between the rota- tional levels of homonuclear molecules are forbidden, and the populations are therefore primarily controlled by collisions, the C, molecule is a particularly useful probe of physical con- ditions in interstellar clouds [see ref. 74 for a detailed discussion]. These analyses generally yield kinetic tem- peratures of about 20-50 K and densities of several hundred H, molecules ~ m - ~ , for diffuse molecular ~ l o u d s . ' ~ * ~ ~ In addition to C,, the collisional excitation of low-lying rota- tional levels of CN may be used to provide information on the electron densities of diffuse clouds, from which the H, density may also be estimated.' In regions of lower density, the rotational excitation of heteronuclear molecules comes into equilibrium with the background radiation, a method used to estimate the black-body temperature of the micro- wave b a c k g r o ~ n d . ~ ~

Isotopic Ratios

Interstellar molecules may be used to determine the isotopic abundances of common elements. For example, the R(0) line of the isotopic variant 13CH+ is off-set by -19 km s-' (i.e. 19 km s-' to the blue) from the R(0) line of I2CH+ at 423.2548 nm. These lines are easily resolvable with modern high-resolution spectrographs, and provide a convenient means of determining the interstellar ',C: 13C ratio, which is an important parameter in models of galactic stellar nucleo- synthesis (see the discussion by Hawkins and M e ~ e r ~ ~ and references therein).

4. The Ultra-high Resolution Facility The ultra-high resolution facility (UHRF) is a major new addition to British astronomical instrumentation. It was designed and built at University College London under the supervision of Dr. F. Diego, and was commissioned at the coude focus of the 3.9 m Anglo-Australian Telescope in July 1992. It is a quasi-littrow Cchelle spectrograph, and is designed such that the diffraction-limited resolving power of the echelle grating is not significantly degraded by the other optical or detector elements. The first results from the com- missioning run7 have confirmed that the intended resolving power (R z lo6, corresponding to a velocity resolution of 0.3 km s-' FWHM) has been successfully achieved. This means that the UHRF is currently the highest-resolution optical astronomical spectrograph in the world, and it will enable a number of outstanding problems of the interstellar medium to be tackled satisfactorily for the first time [see ref. 78 for a detailed discussion of the need for very-high-resolution spec- troscopy in this field].

In particular, it will now be possible to resolve the ther- mally broadened line profiles of interstellar lines formed in gas with temperatures as low as 100 K, and to place strong limits on the degree to which interstellar clouds are turbulent. Among other applications, this should make it possible to determine once and for all whether or not interstellar CH+ is formed in hot gas in the wake of interstellar shock fronts. The higher resolution will also enable us to detect much weaker absorption lines, as an increase in resolving power by an

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5000

4000

3000

2000

1000

0

-50 -40 -30 -20 -10 0 10 heliocentric velocity/km s- '

Fig. 5 Interstellar Na I D, line (588.995 nm) observed towards O p h i ~ c h i . ~ ~ The known separation of the hyperfine components (1.05 km s- ') is indicated for each unsaturated velocity component by vertical tick-marks. The feature at -40 km s- ' is an atmospheric water line. Reproduced from ref. 77.

order of ten decreases the minimum detectable equivalent width by about the same factor. This will, inter alia, greatly increase the number of clouds for which the temperature and density can be determined from measurements of the intrinsi- cally weak c, absorption lines.

As an example of the performance of the UHRF, Fig. 5 shows one of the early observation^,^^ the interstellar sodium D, line observed in the direction of the star c Ophiuchi. The total exposure time was 3600 s and the slit width was 10 pm, giving a spectral resolution (in velocity units) of 0.3 km s-'. The spectrum shows that at least five different velocity com- ponents are present in the direction of c Oph, and in some of these, especially the component with a radial velocity of - 9 km s- ', the hyperfine structure (indicated by vertical markers separated by 1.05 km s-') has clearly been resolved. The observation that the hyperfine structure is much better resolved in the -9 km s - l component than in the others immediately indicates that this cloud is colder and/or less turbulent than the others. The previously highest-resolution spectrum of this line in [ Oph, which was at a resolution of 2 km s-l , did not show the hyperfine structure.79

5. The Quest for C,o

The most remarkable development in chemistry of recent times has been first the discovery" and then the growing understanding of the properties of the c60 molecule, buck- minsterfullerene. From early on in its history it has been con- sidered as a possible constituent of the interstellar medium, because it could be expected to form readily in the extended cool atmospheres of carbon stars and proto-planetary nebulae, and because it is very stable.81 If c60 molecules could form in suitable astronomical locations, they might well be present with detectable abundance in some inter- stellar clouds or circumstellar shells, perhaps in the ionised form C:o or with added atoms. Further, it has been suggested that the c60 molecule or ion could be a carrier of the unidentified diffuse interstellar bandsa2 This suggestion is based on the general property that large molecules tend to have broad absorption features and to survive the absorption process; there is no experimental evidence for any actual wavelength coincidences with the diffuse bands. In theoretical model calculations, Ballester et aLa3 have found that Mg and Si atoms trapped in c60 cages have absorptions in the wave- length region of the diffuse bands. Recently, Kroto and

J ~ r a ~ ~ have discussed the possible circumstellar and inter- stellar existence of fullerenes and their complexed analogues. Further, Websteras has suggested that the carriers of the diffuse bands could be hydrogenated forms of the fullerenes, for which he has coined the name fulleranes.

In laboratory spectroscopic studies made in 1987, Heath et a1.86 reported an absorption feature near 386 nm which they attributed to c60, tentatively assigning the absorption to the 0-0 band of the first allowed 'Tlu-lAB transition, which was predicted in the calculations of Larsson et al.87 to have energy about 3.6 eV (corresponding to a wavelength of 344 nm). The measured oscillator strength was 0.004. Their experiment used two different modified forms of the molecule, with attached groups C6H6 and CH,Cl,; the central wave- length for c60 itself was expected to be within a few tenths of a nm of 386 nm. The feature is about 2 nm wide. Snow and Seabaa examined spectra of suitable reddened stars, and found no evidence for the presence of this absorption in the general interstellar medium. In observations with the Isaac Newton Telescope, Bellis and Somervillea9 have reached the same conclusion.

Somerville and Bellisgo also observed the 386 nm region in the Egg Nebula, the reflection nebula CRL 2688, which is considered to be a proto-planetary nebula (an object interme- diate in its evolutionary state between the red-giant carbon stars and the planetary nebulae). It is found to contain many molecules, and Kroto'l and Jura and Kroto9, had suggested that it is very likely to contain considerable amounts of c60. In the observation of CRL 2688 the signal-to-noise ratio was very poor, but there was a suggestion of a possible absorp- tion feature centred near 385.4 nm and of the width expected for c60. Very recently, a further observation of CRL 2688 has been obtained, using the service observation scheme on the Isaac Newton Telescope. The signal-to-noise ratio is good, but the spectral resolution is lower than in the earlier work. This observation confirms the presence of a broad feature near 385 nm. Other explanations for this feature cannot be ruled out; there are many ordinary atomic and molecular lines in this spectral region, that could be present in the nebula, and it could well be that the apparent feature is produced by blending rather than by a single broad absorp- tion corresponding to the laboratory process studied by Heath et a1.86

In recent laboratory spectroscopy of c60 by Leach et al.93 the 386 nm feature is not seen. The question remains as to what it was that Heath et al. did study in their work, presum- ably some species related to c60. It is desirable that the absorber be identified and the precise laboratory wavelength established. Leach et al. found absorptions from the ground electronic state at about 408 and 307 nm. In the new CRL 2688 spectrum, there is no trace of either of these absorp- tions. Further, a spectrum of HD 44179, the central star of the Red Rectangle (a carbon-rich nebular object with peculiar radiative properties at all wavelength^,^^ centred on a post- asymptotic-giant-branch starg5 and related to the proto- planetary nebulae), has at our request been obtained recently by I. D. Howarth in the commissioning run of the new Utrecht Echelle Spectrograph on the William Herschel Tele- scope. It shows no evidence for these lines or for the 386 nm feature either.

It is in the nebular spectrum of the Red Rectangle that Scarrott et have recently found evidence supporting the view that some of the diffuse interstellar bands are molecular in origin. These absorptions are distributed throughout the interstellar medium with strengths that relate closely to the dust-grain distribution, much more closely than the relation of any known simple molecule to the dust." A molecular carrier must therefore be widely distributed, existing in inter-

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stellar clouds of very different density and in the presence of the ultraviolet radiation field, and so must be very stable. Further, it is found that the relative strengths of different diffuse bands show considerable variations between different s i g h t - l i n e ~ , ~ ~ . ~ ~ while Somerville'OO has found that the strength of the 443.0 nm diffuse band is independent of cloud density whereas Fosseyg has shown that the strengths of certain other bands, including that at 578.0 nm, do vary with density. These results suggest that there is not a single simple carrier that produces all the absorptions. It may be noted that the laboratory spectra of C,, obtained by Leach et ~ 2 1 . ~ ~

do not show any absorptions at the wavelengths of known diffuse interstellar bands.

Further evidence for the absence of C,o from interstellar and circumstellar environments comes in the infrared. Spectra have been taken by Barlow and colleagues,"' with the CGS3 detector on UKIRT and covering the 10 and 20 pm regions with good spectral resolution. They have observed a variety of proto-planetary nebulae, including CRL 2688, and carbon stars. None of these shows any evidence for the vibrational features expected for C,o from the work of Frum et ~ 1 . " ~

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Paper 21055955; Received 16th October, 1992

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