1. Introduction

INDIGENOUS POLYCYCLIC AROMATIC HYDROCARBONS IN CIRCUMSTELLAR GRAPHITE GRAINS FROM PRIMITIVE METEORITES S.MESSENGER,1 S.AMARI,W X. GAO, AND R. M. ALKER McDonnell Center for the Space Sciences and Physics Department, Washington University, One Brookings Drive, St. Louis, MO 63130; srm=nist.gov, sa=howdy.wustl.edu, xg=howdy.wustl.edu, rmw=howdy.wustl.edu S. J.CLEMETT,C X. D. F. HILLIER,2 AND R. N. ZARE Department of Chemistry, Stanford University, Stanford, CA 94305; simon=d31rz3.stanford.edu, chillier=ssa1.arc.nasa.gov, rnz=d31rz3.stanford.edu AND R. S. LEWIS Enrico Fermi Institute, University of Chicago, 5630 Ellis Avenue, Chicago, IL 60637; royl=rainbow.uchicago.edu Received 1997 May 21; accepted 1998 March 4

ABSTRACT We report the measurement of polycyclic aromatic hydrocarbons (PAHs) in individual circumstellar graphite grains extracted from two primitive meteorites, Murchison and Acfer 094. The 12C/13C isotope ratios of the grains in this study range from 2.4 to 1700. Roughly 70% of the grains have an appreciable concentration of PAHs (500È5000 parts per million [ppm]). Independent molecule-speciÐc isotopic analyses show that most of the PAHs appear isotopically normal, but in several cases correlated isotopic anomalies are observed between one or more molecules and their parent grains. These correlations are most evident for 13C-depleted grains. Possible origins of the PAHs in the graphite grains are discussed. Subject headings: circumstellar matter È dust, extinction È ISM: molecules È molecular processes È stars: carbon

1. INTRODUCTION The molecules measured in this study, polycyclic aromatic hydrocarbons (PAHs), represent an important class Several kinds of refractory circumstellar grains have been of organic molecules that are believed to be abundant and found preserved in primitive meteorites(Anders & Zinner widely distributed throughout the cosmos. PAHs have been 1993). These grains are known to have an extrasolar origin identiÐed in carbonaceous and ordinary chondrites (Hahn on the basis of their extremely anomalous isotopic composi- et al.1988; Zenobi et al. 1989; Clemett et al. 1992; Zenobi et tions. Circumstellar grains isolated to date include silicon al.1992), interplanetary dust particles (Clemett et al. 1993; carbide(Bernatowicz et al. 1987; Tang & Anders 1988), Thomaset al. 1995), and the reducing atmospheres of outer silicon nitride(Hoppe et al. 1994; Nittler et al. 1995), graph- solar system bodies(Sagan et al. 1993). ite (Amari et al.1990; 1993a), corundum (Huss et al. 1994; PAHs are among the few molecular species capable of Nittleret al. 1994), diamonds (Lewis et al. 1987), and Ti, surviving the harsh ultraviolet (UV) radiation environment Ti-Zr-Mo, and Fe carbides(Bernatowicz et al. 1991). With of the di†use interstellar medium. They are considered to be the exception of diamonds, whose mean size is D16 A the best candidates responsible for the unidentiÐed infrared (Careyet al. 1987), and the refractory carbides, which are (UIR) emission features(Leger & Puget 1984; Allamandola, found only as small D20 nm inclusions in graphite grains, Tielens, & Barker1985) observed in a variety of astro- individual grains can be measured for their isotopic com- physical environments including planetary nebulae (Cohen position. et al.1986), reÑection nebulae (Geballe et al. 1985), H II These circumstellar grains come from a variety of isotopi- regions(Bregman et al. 1989), and galactic nuclei (Willner et cally distinct stellar sources. Isotopic analysis has been used al.1977). Interstellar PAHs have been estimated to make up to determine their probable stellar origins, which include C- 3%È15% of the cosmic C, based on the strength of the and O-rich asymptotic giant branch (AGB) stars, super- infrared (IR) emission features(Allamandola et al. 1989). novae, and possibly novae and Wolf-Rayet stars. It is often Neutral, radical, and/or ionized PAHs may also be car- possible to perform isotopic analyses of both major and riers of the di†use interstellar bands (DIBs)(Salma et al. minor elements, providing detailed constraints on stellar 1996 and references therein). DIBs are a family of broad nucleosynthesis models. Transmission electron microscopy absorption features superposed on the interstellar extinc- (TEM) studies of circumstellar graphite grains have recently tion curve. Well over 100 such bands have been identiÐed, been used to provide information on the temperature, pres- varying considerably in strength and shape. Positive identi- sure, and chemical composition of the stellar atmospheres Ðcation of PAHs as carriers of some of the DIBs is likely to from which the grains condensed(Bernatowicz et al. 1996). be extremely difficult owing to the complex variability in Here we extend the range of measurements to include the the shapes, positions, and relative strengths of these features identiÐcation of isotopically correlated, and hence indige- along di†erent lines of sight. nous, organic molecules in circumstellar graphite grains. While there appears to be abundant evidence for aromatic compounds in the interstellar medium, spectroscopic 1 Present address: National Institute of Standards and Technology, Building 222, Room A113, Gaithersburg, MD 20899. methods are generally limited to detecting gas phase mol- 2 Present address: NASA Ames Research Center, MS 245-6, Mo†ett ecules in UV-rich environments and provide little informa- Field, CA 94035-1000. tion on what speciÐc compounds are present. Because of 284 POLYCYCLIC AROMATIC HYDROCARBONS 285 these limitations, PAH formation mechanisms and chem- grains were crushed into the Au substrate with a spectro- istry in interstellar or circumstellar regions remain uncer- scopic grade, clean quartz disk in order to increase their tain. In this work we report the identiÐcation of speciÐc surface area and improve their contact with the mount PAH molecules likely formed in the circumstellar environ- surface. ments that also produced the circumstellar graphite grains Grains were measured for the presence of PAHs by two- found in primitive meteorites. step laser desorption laser ionization mass spectrometry (kL2MS) as described below. Following the organic 2. EXPERIMENTAL TECHNIQUES analysis, 89 of the grains were measured for their bulk C- isotopic composition using the Washington University 2.1. Sample Preparation modiÐed Cameca IMS-3f ion microprobe mass spectro- Circumstellar graphite grains represent an extremely meter. This measurement sequence was chosen to avoid the small fraction([1 part per million [ppm]; Amari, Lewis, & possibility that the ion probe measurements would a†ect Anders1994) of the meteorites that contain them. Isolation the PAHs present. of the graphite is achieved through a series of chemical The techniques for making C- and N- isotopic measure- digestion steps and physical separations that successively ments by ion microprobe have been described by Zinner, destroy speciÐc mineral phases while enriching graphite in Tang, & Anders (1989). the resulting meteoritic residue(Amari et al. 1994). Most of the mass of the parent meteorite is in the form of silicates 2.2. kL2MS Technique (D96%), which are destroyed by HF/HCl acid treatments. The kL2MS technique(Kovalenko et al. 1992; Clemett & Sulfur-bearing phases (D1% of the meteorite) are removed Zare1997) combines the advantages of laser desorption to by treatment with organic solvents and KOH. Kerogen volatilize intact neutral molecules from a solid sample with (D2% of the meteorite) is removed by a combination of the molecular selectivity and sensitivity of resonance- Cr O~2 andH O . Diamonds (D600 ppm) are separated enhanced multiphoton ionization (REMPI) and the high from2 the7 residue2 by2 forming a colloid at pH D 11 by adding ion transmission, unlimited mass range, and multichannel 0.1MNH ÉH O. Graphite grains are extracted from the detection of a reÑectron time of Ñight (RTOF) mass spectro- remaining3 residue2 (now representing D0.2% of the original meter. meteorite) through a density separation followed by a size In the Ðrst step, constituent molecules of a circumstellar separation ([1 km). grain are desorbed with a pulsed IR laser beam focused on a Although greatly enriched in abundance, circumstellar 35 km spot using a Cassegrainian microscope objective. The graphite still represents a small part (0.1%È30%) of the Ðnal laser power is kept low to minimize decomposition and density separate from which the grains are extracted. Never- assure that only neutral species are desorbed. In the second theless, previous work has shown that circumstellar graph- step, the desorbed molecules are selectively ionized with a ite grains have distinct morphologies (spherical with a platy UV laser beam and extracted into an RTOF mass spectro- ““ onion ÏÏ or knobby ““ cauliÑower ÏÏ appearance) that allow meter. them to be identiÐed among numerous other phases (Amari, One of the main advantages of using a laser photoioniza- Zinner, & Lewis1993b). Small aliquots of the graphite tion process is that one can choose conditions that give rise separate were dispersed on Au foil for examination by scan- to species-selective ionization. In this study a (1 ] 1) ning electron microscopy/energy dispersive X-ray analysis REMPI scheme was used, where absorption of one UV (SEM/EDX). Candidate grains were selected on the basis of photon causes a PAH molecule to make a transition to its high C content (as determined by EDX) and morphological Ðrst excited electronic singlet state(S ] S ) and absorption features characteristic of Murchison (CM2) circumstellar of a second UV photon causes ionization.0 1 Selectivity in graphite. Although the distinctive appearance of circumstel- ionization results from the UV photon energy being reso- lar graphite grains serves as a useful tool for their identiÐca- nant with the transition to an intermediate excited state, tion, isotopic analysis is required to distinguish grains that because the ionization efficiency for resonant ionization is are demonstrably of circumstellar origin from those that are greater by many orders of magnitude than that of nonreso- isotopically normal and may have a solar system origin. nant multiphoton processes(Pappas et al. 1989). Hence A total of 124 spherical graphite grains were chosen from only those molecules in resonance with the UV laser wave- acid residues of four primitive chondrites. The grains length are appreciably ionized. Because nearly all PAHs ranged in size from 3 to 6 km in diameter. Most of the have a high photon absorption cross section at a wave- grains (105) came from two residues of the Murchison CM2 length of 266 nm associated with electronic excitation of the chondrite; 15 of which were selected from the KFC1 density aromatic ring (n ] n*), this procedure provides a highly separate (2.15È2.20 g cm~3) prepared at the University of selective ionization window for PAHs. Furthermore, this Chicago(Amari et al. 1994). All other residues were pre- ionization process causes negligible fragmentation of the pared at Washington University, including 90 grains from a PAH species because there is very little rotational and Murchison separate (MKE2: density range 2.0È2.3gcm~3), vibrational excitation(Winograd, Baxter, & Kimock 1982; Ðve grains from the unique meteorite Acfer 094(Newton et Shibanov 1985) acquired during the ionization event. Con- al.1995), six grains from the enstatite chondrite Indarch sequently, the mass spectra obtained by the kL2MS instru- (E4), and eight grains from the unequilibrated ordinary ment are easily interpreted since observed mass peaks chondrite Tieschitz (H3). overwhelmingly reÑect parent ion species. All candidate grains were individually transferred to a sputter-cleaned Au substrate. While observing with a 100 ] 2.3. kL2MS Analysis Procedure binocular optical microscope, each grain was picked from The Au substrate on which several individual circumstel- the residue by employing a dry W needle with a 1 km tip lar grains are crushed is placed on a brass sample mount attached to a micromanipulator. Once transferred, the and introduced into the kL2MS instrument through a 286 MESSENGER ET AL. Vol. 502 vacuum interlock. After introduction into the main vacuum a fast preampliÐer (LeCroy VV100BTB: 500 Mhz chamber, samples are left to outgas for about 15 minutes or bandwidth) and a timing Ðlter (Ortec 474 timing/Ðlter amp) until the pressure in the main chamber returns to the orig- and is displayed on a 350 MHz digital oscilloscope (LeCroy inal base pressure of less than 5 ] 10~8 torr. 9450). As an internal reference, a metered leak valve is used The sample platter is positioned just below the extraction to introduce a constant background of perdeuterotoluene region of the RTOF mass spectrometer and brought into (12C2H ) as a reference gas. This enables periodic opti- the focus of the Cassegrainian microscope objective using mization7 8 of the ion collection optics and aids in mass cali- an x-y-z manipulator. For sample viewing, a macro zoom bration. lens is used in a telescope mode so that an image of each grain to be analyzed can be projected onto a CCD video 2.4. ConÐdence in PAH Mass IdentiÐcations camera. To couple the video camera to the microscope Several independent lines of evidence show that peaks in objective,a2mmthick Ge Ñat is used, which reÑects visible the kL2MS spectra of circumstellar grains originate pre- light used for sample viewing but transmits the IR light of dominantly or exclusively from PAHs. First, the nature of the desorption laser. The sample is illuminated from the the ionization process used for kL2MS, (1 ] 1) REMPI, is exterior to the vacuum chamber through a side viewport highly selective toward PAHs(Clemett 1996 and references with a focused quartz halogen lamp, which is incident at a therein). The photoionization cross sections of PAHs at the glancing angle. wavelength of 266 nm are many orders of magnitude higher IR light from a pulsedCO laser (Alltec AL 853: 10.6 km, than other organic or inorganic species(Zenobi & Zare 120 ns FWHM with a 4 ks2 tail) is attenuated and beam 1991 and references therein), with only a few notable excep- shaped using a variable high-power (Venetian blind/spatial tions such as the alkali metals, e.g., K and Na, which are Ðlter) attenuator similar to that described by Bialkowski efficiently ionized by a one-photon process. Second, while in (Bialkowski 1987). The beam is then collimated and clipped principle it is possible that there are unknown and pre- before entering the vacuum chamber with two collimating viously unobserved inorganic or nonaromatic organic irises opened to a diameter equal to that of the microscope species in sufficient abundance to appear in the kL2MS spider mirror (5 mm) and then focused with the reÑecting spectra, this possibility is considered extremely unlikely in microscope objective (Ealing ]36) into a 35 km diameter the results presented here. The circumstellar graphite grains spot on the sample. The laser pulse energy is chosen to measured in this study are D97.5% C, D2.4% N, and maximize the desorption yield without producing appre- D0.1% H by weight, and hence inorganic (nonÈC-bearing) ciable fragmentation or ionization. The typical pulse energy species can be present at only trace abundance. Thirdly, the is 22 kJ, with D 5.2 kJ contained in the initial spike and grains have been extracted from their parent meteorites by D16.8 kJ in the tail. The pulse shape depends on the CO harsh chemical digestion processes, involving both strong laser gas mixture. The gas mixture of 68.6% He, 18% N 2, acids and oxidizing conditions, which less chemically 9%CO ,H 4% CO, and 0.4% used in these experiments2 resilient organic species are unlikely to survive. Finally, the provides2 the most stable operation;2 shot-to-shot Ñuctua- measurement of the exact mass of a peak can be used as an tions in the peak intensity of the laser proÐle are less than independent method for verifying its assignment as an aro- 10%. To Ðnd the laser power providing the optimal PAH matic species. The molecular weights of all PAHs have desorption yields from grains, a standard sample platter small excesses relative to integer atomic numbers. For with fragments of Allende (CV3) matrix (D100 km average example, the exact mass of phenanthrene(12C 1H ) is diameter) pressed onto KBr was used. The IR laser power 178.0780 atomic mass units (amu) compared14 with10 that density is estimated to be D2.5 ] 106 Wcm~2, and this based on integer atomic weights of 178 amu. The reason value is representative of power densities for all grains dis- arises from variations in nuclear binding energies; the exact cussed in this study. mass of 1HisD1.0078 amu relative to the mass of 12C, After an appropriate time delay (25 ks), the fourth harmo- which is by deÐnition 12.0000 amu. All other elements have nic of a pulsed Nd:YAG laser (Spectra Physics DCR11: 266 mass deÐcits relative to 12C, with the exception of He, Li, nm, 2.5 ns FWHM fast pulse mode with a Gaussian mode Be, B, N, Th, and U. As a result, with a few exceptions, only output coupler) is attenuated with a half waveplateÈcalcite hydrocarbon species show mass excesses. Assuming that the polarizer combination, apertured to a 1 mm diameter with mass excess in a peak occurs principally from H, which has an iris, and directed between the center electrodes of the the largest mass excess per unit mass of any element, quanti- modiÐed Wiley-McLaren(Wiley & McLaren 1955) ion Ðcation of this excess provides a means to estimate the extraction region. The ionization laser pulse energy is number of H atoms in that molecular species. chosen to maximize parent ion signal intensities produced Consider the molecule phenanthrene. This species has by (1 ] 1) REMPI with minimal fragmentation; a typical been independently detected in the Murchison meteorite pulse energy is D2 mJ, giving a UV laser power density of (Pering& Ponnamperuma 1971; Levy, Grayson, & Wolf D1.25 ] 106 Wcm~2. The ions produced by the UV laser 1973), and thus a peak of mass 178 amu in the kL2MS are extracted from the source and injected into an RTOF analysis of bulk Murchison can be reasonably assumed to mass spectrometer. The reÑectron geometry greatly arise from phenanthrene.Figure 1 shows the accurate mass- improves the mass resolution (t/2 *t D 1500) essential for calibrated kL2MS spectrum in the region about 178 amu molecular isotopic analysis. To enhance ion transmission to for a small particle of Murchison matrix. The exact mass of the detector, the ion trajectories are controlled with an the 178 amu peak is found to be 178.078 amu, a value in Einzel lens and two sets of deÑection plates before injection precise agreement with the calculated exact mass of phen- into the reÑectron. The total Ñight path length is 3 m. A 20 anthrene. The agreement in the observed mass excess cm2 active area dual microchannel plate detector with an excludes the possibility of inorganic species.Figure 2 rep- oversized anode (Galileo TOF-4000) is used in a Chevron resents the result of applying this mass di†erence analysis to conÐguration to detect the ions; the output passes through other known parent PAHs and their alkylated homologs in No. 1, 1998 POLYCYCLIC AROMATIC HYDROCARBONS 287

laser ionization they are not correctly mass calibrated and 0.078 amu thus lead to signal artifacts. Hence, the PAHs observed from circumstellar grains are not artifacts of the measure- 1H = 1.0078 amu ment process and truly reÑect species present in the grains 12C = 12.0000 amu at the time of measurement.

Phenanthrene (C14H10) 2.5. Molecular Isotopic Measurements Mass = 178.0780 amu Any hydrocarbon measured by kL2MS has n ] 1 mass peaks in which n is the number of C atoms in the molecule.

Signal Intensity The substitution of 2H for 1H can be neglected owing to the extremely low cosmic abundance of 2H(2H/1H D 1.5 ] 10~5). Each mass peak (isotopomer) represents a di†erent number of 12C and 13C atoms in the molecule. The relative abundance of the various isotopomers follows a simple binomial distribution 176.5 177.0 177.5 178.0 178.5 179.0 179.5 Mass (amu) n(n [ 1)an~2b2 (a ] b)n \ an ] nan~1b ] ]ÉÉÉ , (1) 2! FIG. 1.ÈMass spectrum of Murchison matrix obtained by the kL2MS technique about the mass region 178 amu. The large peak at mass 178.078 where a is the fractional abundance of 12C, b is the abun- amu is phenanthrene(12C 1H ). The small mass excess of 0.078 amu dance of 13C, and n is the number of C atoms in the mol- from 178.000 amu is caused14 by the10 slight mass excess of 1H (1.0078 amu). ecule. For solar isotopic composition a \ 0.989 and b \ 0.011. In this expansion, the Ðrst term represents the Murchison. Excellent agreement is observed in all cases relative abundance of the ““ parent ÏÏ peak (12C atoms only), between calculated mass excesses and those experimentally the second term gives the relative abundance of the Ðrst measured. isotopomer peak (one 13C substitution), and so forth. Extensive kL2MS measurements of carbonaceous stan- In solar system materials (12C/13C D 89) forn [ 89 the dards, including pyrolytic graphite and graphite square most abundant molecules observed will contain only 12C weave fabric, have demonstrated that PAHs are not formed atoms, with heavier isotopomers occurring in decreasing in situ from laser desorption in the kL MS instrument. abundance. For instance, the PAH pyrene(C H ) has 17 2 16 10 While PAHs may be produced at high desorption power isotopomers. For an isotopically normal sample, the most densities, at and beyond the plasma ignition threshold, abundant molecule is12C H , while a smaller fraction is 16 10 these are conditions that are never used in kL MS measure- 12C13C H and a still smaller fraction is12C13C H . In 2 15 1 10 14 2 10 ments. The reasons for this are threefold: First, selectivity in isotopically normal samples heavier isotopomers are ionization is lost since the desorption process indiscrimi- usually below detection limits. nately generates ions. Second, only low mass resolution Because the 12Cto13C ratio R is given by a/b, the spectra can be obtained since the high density of charged R-value of a given molecule can be determined from the species leads to Coulombic repulsion in the desorption ratio of the intensity of any two isotopomer peaks, provided plume allowing both ions and neutral species (through that the number of C atoms in the molecule is also known. collisions) to acquire high kinetic energies. This leads to Thus, given sufficient signal strength and minimal spectral poor mass resolution since the Wiley-McLaren/reÑectron crowding it is possible to determine the C-isotopic composi- geometry of the kL2MS instrument provides only space tion of individual molecules in a PAH mass spectrum. The focusing and not energy focusing. Thirdly, since ions gener- technique of molecule-speciÐc isotopic analysis by kL2MS ated in the desorption plume are formed prior to the UV is described in detail byMaechling, Clemett, & Zare (1995). Molecule-speciÐc isotopic analysis begins with assigning a given mass peak to a known PAH species, thereby deter-

Exact Excess mining the number of C atoms in the molecule. The number 0.18 Murchison Matrix of C atoms in the molecule and the intensity of the parent peak are used to calculate the ““ expected ÏÏ intensity of the 0.13 Ðrst isotopomer peak, assuming isotopically normal C. The measured intensity of the isotopomer peak is then com- 0.08 pared with the expected value. In practice, however, PAH spectra contain a complex

Mass Excess (amu) mixture of aromatic species, many of which overlap each 0.03 Zero Excess Line other at the mass resolution of the kL2MS instrument. For instance, the parent peak of phenanthridine (12C H N: -0.02 179.073 amu) cannot be resolved from the Ðrst isotopomer13 9 peak of phenanthrene(12C13C H : 179.082 amu). Such Pyrene chemical interferences are indistinguishable13 1 10 from excesses in Chrysene Naphthalene

-Naphthalene -Naphthalene the Ðrst isotopomer peak, compromising the identiÐcation 2 4 -Phenanthrene -Phenanthrene -Phenanthrene 1 3 5 Anthanthracene C C C C C of 13C-rich molecules. However, chemical interferences only cause 13C-poor molecules to appear more isotopically FIG. 2.ÈComparison of the exact and measured mass excesses for the dominant PAH species observed in Murchison matrix. The close agree- normal. Stated in another way, we can be more conÐdent ment between measured and calculated mass excess justiÐes mass assign- about the measurement of 13C depletion than about 13C ment as PAH species. enrichment. 288 MESSENGER ET AL. Vol. 502

As a self-consistency check,Figure 3 shows the molecule- depleted PAHs exists in the spectra of any of the bulk acid speciÐc isotope ratio measurements for the Ðve most abun- residues beyond the statistical uncertainties associated with dant parent PAH species (naphthalene: C H ; measurement e†ects at the weak signal limit. phenanthrene:C H ;C pyrene:H ; chrysene:10 8 C H ;Cbenzopyrene:14 10 H ) and their16 alkylated10 homo- 2.6. ConÐdence in Molecular Isotope Measurements logs18 from12 11 meteorite acid20 residues12 (CI, CM, L, LL, and H Concurrent with the PAH analysis of circumstellar classes). All the residues have normal C isotope ratios and grains, extensive checks were performed to ascertain the contain complex mixtures of aromatic species whose conÐdence with which experimental molecule-speciÐc iso- kL2MS spectra have intensities and distributions compara- topic measurements could be made. C isotope ratios for the ble to those observed from the individual grains in this three deuterated PAHs, toluene-1H (C1H ), toluene- 8 7 8 study. The integrated peak intensity of the 12C isotopomer 2H (C2 H ), andmetaÈxylene-2H (C2H ) were measured 8 7 8 10 8 10 peak is plotted against that of the Ðrst 13C isotopomer peak. over a wide range of signal intensities, bracketing the range In such a plot, molecules of solar isotope composition lie observed from the grains in this study. The role of mass along a diagonal line whose slope isR . From Figure 3 discrimination e†ects was accurately quantiÐed in isotope this behavior is seen to be a limiting casesolar with a spread in determination. These PAHs were speciÐcally chosen the data that appears to indicate 13C enrichment relative to because they could be conveniently introduced into the solar. The cause of this e†ect has been discussed above, kL2MS instrument through a calibrated leak valve and from heteroaromatic species and fragmentation, both of have masses that are atypical of those observed in the which can contribute to the intensity of the Ðrst 13C iso- spectra of circumstellar grains. Hence, residual traces of topomer peak. However, apparent 13C depletions cannot be these PAHs, which can typically reside in the chamber for produced by such interferences, and no indication of 13C several days after introduction, did not provide any chemical interferences in the analysis of grain spectra. Addi- tionally, each of the isotope references was independently analyzed with a commercial high-precision isotope mass 102 9 spectrometer. The results are summarized byMaechling et 8 7 13 al.(1995) and indicate that although the kL2MS instrument 6 C Poor 5 is currently unable to distinguish between subtle isotope 4 variations produced by chemical isotopic fractionation C = 500 13 (R 3), it is able to resolve clearly the anticipated large 3 ^ C/ solar 12 nucleosynthetic C isotope variations (R \ 2È6000) of any 2 PAHs indigenous to circumstellar graphite grains.

Isotopically Normal 3. RESULTS 1 10 9 Of the 89 graphite grains measured by ion microprobe 8 7 mass spectrometry, 38 were found to be 13C poor, and 20 6 were found to be 13C rich relative to the solar system value; 5 seeFigure 4. The C-isotopic ratios R of these grains range 4 from 2.4 to 1700. These values lie well outside the range of 3 solar system materials (R \ 89È93), demonstrating the 2 extrasolar origin of these grains. The remaining 31 grains, including all grains from the Tieschitz and Indarch meteor-

Parent Peak Intensity (V.ns) 0 10 9 30 8 Solar 7 6 25 5 4 13 C Rich 20 13C Rich 13C Poor 15 10-3 2345610-2 2345610-1 234567100 234 13 10

First C Isotopomer Peak Intensity (V.ns) Number of Grains

FIG. 3.ÈMolecular isotope analysis of PAH molecules from 11 bulk 5 acid residues (CI, CM, L, LL, and H petrologic classes). All the residues have bulk C isotope ratios that are normal, i.e., R \ 89. The integrated 0 signal intensity of the 12C parent peak is plotted against the integrated 234567 234567 234567 23 100 101 102 103 signal intensity of the Ðrst 13C isotopomer peak divided by the number, n, 12 13 of carbon atoms in the molecular species (e.g., n \ 14 for phenanthrene). C/ C On such a plot the data should fall on a line whose slope is 89. From the plot it is clear that R-values fall on the line and o† to the 13C-rich region of FIG. 4.ÈBulk carbon-isotopic measurements of the 89 grains measured the graph. The latter spread is an inevitable result of chemical interferences by ion microprobe mass spectrometry. The range of 12C/13C observed and instrumental measurement e†ects. However, none of the molecules among the grains in this study (2.4È1720) agrees well with previous measured in this study appear signiÐcantly depleted in 13C, demonstrating analyses of circumstellar graphite in meteorites. The strong peak centered that 13C depletions are not observed by the kL2MS technique on isotopi- about the solar value includes the 14 normal grains measured from the cally normal materials. Tieschitz and Indarch meteorites. No. 1, 1998 POLYCYCLIC AROMATIC HYDROCARBONS 289 ites, have C-isotopic compositions indistinguishable from the solar value. The fact that all candidate grains from Ties- chitz and Indarch are isotopically normal is consistent with previous work that has shown circumstellar graphite to be in very low abundance in Tieschitz and as yet unknown in Indarch. Of the 124 grains studied by kL2MS, 86 grains had PAH concentrations that were equal to or greater than Murchi- son bulk (HF-HCl) acid residue, i.e., D1000 ppm. The estimated concentration of PAHs in Murchison bulk acid residue (composed primarily of PAHs and macromolecular material) is based on their abundance in bulk Murchison of 15È28 ppm(Cronin et al. 1988) and the relative concentration factor in the residue of D37 (Clemett 1996). In general, these grains were found to have extensive suites of PAHs, spanning the mass range from 100 to 600 amu, containing both parents and alkylated homologs. The spectra show a variety of aromatic species, with large varia- FIG. 6.ÈkL2MS spectra of the PAHs in bulk Murchison and its acid tions in both signal strength and mass distribution among residue (EKG 11.11.86). Each spectrum is independently scaled and com- the grains (seeFig. 5). Large variations persist when the parison of signal intensities are valid only within each spectrum. The grains are subdivided into those with light, heavy, and increase in low-mass PAHs in the acid residue is attributed to the acid residue procedure, however, no new peaks are observed and the residue normal C. Although the dominant PAH species observed in preparation is considered to have only a minimal impact on the native the bulk Murchison meteorite (naphthalene, pyrene, phen- PAH distribution. anthrene, and benzopyrene;Clemett 1996) are also present in most grains, substantial grain-to-grain di†erences exist in the relative abundance of these molecules. In comparison to C (720 amu) and related fullerenes and fulleranes have bulk Murchison or its acid residue, the mass distributions of been60 postulated to be potential major constituents of car- PAHs observed among the grains show higher complexity bonaceous material produced in circumstellar outÑows and include a larger proportion of high molecular weight (Krotoet al. 1985; Kroto 1988). The kL2MS instrument is species ([250 amu). The extreme variability in the PAHs capable of detecting the presence of such species, albeit at observed among the grains strongly argues against a per- lower detection sensitivities than that for PAHs (Clemett vasive laboratory contaminant or production of the PAH 1996). No evidence exists in any of the grains analyzed in species during the chemical digestion process used to this study for the presence ofC or related species in cir- prepare the residues from which the grains were isolated. cumstellar graphite. This Ðnding60 does not, however, pre- Furthermore, PAHs present in meteoritic acid residues clude their presence at concentrations below current di†er only slightly from those in untreated bulk meteorite detection limits (D500 ppm) and/or their destruction samples, as illustrated inFigure 6. The primary e†ect of the during the chemical and physical treatments used to extract acid treatments appears to be partial destruction of more the grains from their parent meteorite. labile species, not PAH production. We conclude that the All molecular peaks with sufficient signal intensity and observed molecules resided with their parent graphite minimal spectral crowding were subjected to isotopic grains in their meteorite parent bodies. analysis. Many grain spectra had peak densities too high to permit isotopic analysis, with heteroaromatic species and/or fragmentation peaks overlapping with isotopomer peaks of interest. Somewhat less than one-third of the peaks with sufficient signal strength were also free of obvious µ Grain 7.26 (5 m) interferences, limiting our isotopic measurements to 85 R = 12.7 Ionprobe peaks from 13 13C-poor grains and 30 peaks from 6 normal x 4.2 grains. See ° 2.5 for a discussion of molecular isotopic measurements. Grain 7.16 (5.8 µm) The limited number of total counts (n ] n \ N) R = 400 Ionprobe making up the parent(n ) and Ðrst isotopomer1 2 peaks x 3.1 (n )Ètypically 50È200 countsÈmakes1 it necessary to deter- mine2 whether an apparent isotopic anomaly is due simply Signal Intensity µ Grain 8.23 (4 m) to a statistical Ñuctuation. We choose to approach this issue R = 89 Ionprobe by assessing the likelihood that a given molecule represents x 1 a sample of isotopically normal material. However, because the sample size (N) is very small, the relative probabilities of 100 200 300 400 500 observing di†erent proportions of N ions in the parent and Mass (amu) Ðrst isotopomer peaks follows a binomial (asymmetric) rather than a Gaussian probability distribution. This makes FIG. 5.ÈMass spectra of the PAH distributions observed in heavy, it necessary to calculate the probability distribution for normal, and light C grains illustrating the molecular diversity of PAHs each molecular isotopic measurement. observed with the kL2MS technique. The vertical scales of the spectra from grains 7.16 and 7.26 are expanded relative to that of grain 8.23, by factors The relative probability of observingn ions in the parent of 3.1 and 4.2, respectively. peak andn ions in the Ðrst isotopomer1 peak, given a 2 290 MESSENGER ET AL. Vol. 502 sample ofn ] n \ N ions from an isotopically normal meteorite acid residues. However, a limited number of such reservoir, is given1 2 by the binomial theorem low-probability events are expected to be observed from an isotopically normal reservoir, given the number of obser- N! vations performed. Hence, we Ðnd no evidence of 13C P (n ) \ pn1pn2 , (2) N 1 n ! n ! 1 2 depleted molecules occurring in isotopically normal grains 1 2 beyond expected levels of statistical Ñuctuations. wherep andp are the relative probabilities of observing Most peaks from graphite grains with light C also appear an ion in1 the Ðrst2 and second peak, respectively. The relative isotopically normal. However, approximately Ðve peaks probabilities of observing a parent ion and a Ðrst iso- from four grains appear signiÐcantly depleted in 13C, topomer peak ion are given by the Ðrst two terms of the beyond the variations expected from isotopically normal binomial expansion, as discussed in ° 2.5. We determine the materials given the number of measurements performed. In probability distribution for a given molecule by calculating the most extreme case, illustrated inFigure 7b, the molecule the probabilities of observing di†erent proportions of the C -phenanthrene in grain M7-20 (R \ 648) has 136 ions in (total observed) N ions in the Ðrst two isotopomer peaks, the2 parent peak (206 amu) and 7 ions in its Ðrst isotopomer fromn \ Nn and\0 ton\0 andn\N. An example peak (207 amu), where 24 are expected for a solar 12C/13C of a probability1 distribution2 derived1 in this2 way is shown in ratio. The probability of observing this distribution for an Figure 7a. The resultant distribution is asymmetric, falling isotopically normal sample is approximately three chances o† more gradually for apparent 13C excesses than for in 10,000 (0.0003). Clearly such an observation is very apparent 13C depletions. unlikely to be the result of statistical Ñuctuations, given the A given isotopomer distribution generally falls outside of limited number (85) of measurements performed. For two some conÐdence interval centered about the most likely additional cases, the probabilities of observation [PN(n ) \ isotopomer distribution for an isotopically normal sample. 0.0035, 0.0018] are again well below what one would expect2 By integrating the probability distribution over this con- to observe in a normal sample given the number of obser- Ðdence interval, we can assess the likelihood of observing a vations. In one notable case, two low-probability distribu- particular isotopomer distribution. As an example, the mol- tions are observed within a single grain, M11.15 (R \ 1727). ecule phenanthrene in the isotopically normal grain M8.23 It is interesting to note that these two molecules, phen- has 195 ions in the parent peak (178 amu) and 19 ions in the anthrene[PN(n ) \ 0.038] and C -phenanthrene [PN(n ) \ Ðrst isotopomer peak (179 amu). The most likely distribu- 0.012], may share2 a genetic relationship,1 both representing2 tion for N \ 214 (195 ] 19) ions from an isotopically part of an alkylation series of phenanthrene. The likelihood normal reservoir is found to ben \ 185 andn \ 29. Thus, of observing two such low-probability distributions within in this case, the conÐdence interval1 extends from2 n \ 20 to the same grain is extremely low. Considered together, these 2 n \ 38, wherePN (n \ 20) [ PN (n \ 38). We observations provide a compelling case for the presence of Ðnd2 that 94% of all/214 observations2 fall within/214 this2 conÐdence an isotopically anomalous PAH component in some cir- interval, and thus the probability of observing this particu- cumstellar graphite grains. lar isotopomer distribution from an isotopically normal The isotopic analyses of all molecules measured from reservoir is less than 6%. This example represents the lowest normal and light C graphite grains are summarized in probability isotopomer distribution we observed among all Figure 8. Most molecules measured in both isotopically molecules analyzed from isotopically normal grains and normal grains and those with 13C depletions are consistent

0.08 Probability Distribution for Grain 8.23 Probability Distribution for Grain 7.20 0.08

0.06

0.04

Number 0.04 Observed

0.02 Number Confidence Interval 0.02 Observed Confidence Interval

Relative Probability of Observation for Solar C 0.00 0 1020304050 Relative Probability of Observation for Solar C 0.00 0 1020304050 Number of First Isotopomer Peak Ions Number of First Isotopomer Peak Ions FIG.7a FIG.7b FIG. 7.È(a) Probability distribution calculated for the molecule phenanthrene in grain M8.23. The distribution represents the probability of observing a range of relative numbers of ions in the parent(n )( and Ðrst isotopomer peakn) for this molecule from an isotopically normal reservoir given the total number of ions actually measured. In this case, the1 probability that this molecule2 has a solar 12C/13C ratio is estimated by integrating the probability distribution fromn \ 20 ton \ 38, which have equivalent relative probabilities of observation. This integration takes into account the asymmetricity of the distribution, which2 becomes pronounced2 as the total number of ions observed is reduced. This conÐdence interval includes roughly 94% of all observations, which indicates that the observed isotopomer distribution occurs less than 6% of the time in isotopically normal materials. Since D30 peaks were measured in normal grains, this distribution can be explained by expected statistical Ñuctuations. (b) Probability distribution for the moleculeC -phenanthrene in grain M7-20. Unlike the example given inFig. 7a, the conÐdence interval here is 99.97%, and thus it is very unlikely (.0003) that this molecule2 has a solar C-isotopic composition. No. 1, 1998 POLYCYCLIC AROMATIC HYDROCARBONS 291

20 Molecules From Light-C Grains Molecules From Normal Grains 18 Predicted Histogram For a Normal Sample

16 Apparent 13C Depletion Apparent 13C Excess 14

Number of Cases 6

0 0.0001 0.001 0.01 0.1 1 0.1 0.01 Probability of Observation for Solar C

FIG. 8.ÈSummary of the isotopic analysis of all molecules measured in isotopically normal and 13C-poor grains. The histograms illustrate the relative probabilities that the analyzed molecules are isotopically normal, as described in the text. The histogram drawn with a solid line shows the predicted frequency of observation of these events in the given probability intervals. The probability histogram for molecules from isotopically normal grains is strongly shifted to the 13C-rich side of the predicted histogram. This apparent bias toward 13C-rich molecules has resulted from isobaric interferences. This shift is not apparent in the probability histogram for molecules from 13C-poor grains, possibly indicating that many molecules from these grains are 13C-poor but are masked by minor isobaric interferences. Several molecules from 13C-poor grains appear strongly depleted in 13C, well beyond expected statistical Ñuctuations. with being isotopically normal, within expected statistical forms a normal distribution about the predicted values. variations. Chemical interferences lead to apparent 13C Figure 9 also shows mass excesses determined for molecules excesses in some cases and likely cause both distributions to identiÐed as depleted in 13C. Again all these molecules be somewhat more 13C rich than their intrinsic values. exhibit mass excesses, strongly indicating that the species Indeed, the distribution observed for molecules from are hydrocarbons. normal C grains and meteoritic acid residues is strongly The identiÐcation of 13C-rich molecules on grains is com- shifted toward higher 13C abundances than expected. It is plicated, as discussed earlier, by interferences perturbing the interesting to note that the distribution observed for mol- actual isotopomer peak distribution. Isotopic analysis does ecules from 13C-poor grains is not biased in this way. One become feasible, however for very 13C-rich molecules that explanation for this di†erence is that the molecules appear in uncongested spectral regions. This is because the observed from 13C-poor graphite grains are free from any intensity distribution of the isotopomer peaks is now no molecular interferences. However, given that the spectral longer concentrated in the Ðrst and second peaks but rather complexity observed among these grains is at least as rich is dispersed over multiple isotopomer peaks, all with appre- as that observed among normal C grains and meteoritic ciable intensity. By way of illustration,Figure 10 demon- acid residues, it is more likely that a substantial portion of strates the calculated isotopomer distribution for the the molecules measured are 13C poor but are masked by molecule phenanthrene at three values of R, chosen to span isobaric interferences and an isotopically normal com- the range of those observed among the grains in this study. ponent. Indeed, in most cases only a few ions arising from Thus in a spectral region where there is little evidence for chemical interferences would be sufficient to obscure a 13C interference peaks, an extended isotopomer distribution depletion. associated with a particular molecule is unlikely to be As discussed in ° 2.4, the validity of the molecular identiÐ- caused by chemical interference because it would likely cation, and therefore the isotopic analysis, can be tested by have an impact on only one or two isotopomer peaks. measuring the precise mass of a given peak. Because precise Hence, it is statistically improbable that a chemical inter- mass measurements rely on the peaks in question being well ference would a†ect the entire isotopomer distribution or formed, it was not possible to apply this additional criterion that it would correctly mimic the expected isotopomer dis- to many of the grain spectra owing to the relatively low tribution for that grain. signal strengths. Nevertheless, for most of the molecules In the mass spectrum of grain M7.25 (R \ 18), a strong subject to isotopic analysis, the peaks were sufficiently mass peak is observed at 154 amu and is assigned to the strong to resolve the predicted mass excesses. Figure 9 PAH species acenaphthalene(C H ). Figure 11 shows the shows the deviations from integer atomic mass for mol- expanded region of the mass spectrum12 12 about 154 amu with ecules from normal C grains whose PAH signals were the readily apparent high-mass trailing distribution of iso- strong enough to permit precise mass determinations. In all topomer peaks. The relative intensities of the peaks follow- cases mass excesses are observed, and the scatter in the data ing 154 amu is identical to the isotopomer distribution omlganM.4 n ucio cdrsde Both residue. acid Murchison and M8.54, grain normal this for distribution mass the not is acenapthalene and PAHs the there in low-mass that abundant for in interferences simple for chemical unique potential is case such few a M7.25 the only same grain generally respect is are the this to as In subject analysis grains. be statistical cannot and rigorous tentative only showing present additional molecules of IdentiÐcation grains. candidate other some on observed 502 Vol. AL. ET MESSENGER 292 rsn rmnptaeewihis which naphthalene amu 156 caution at some interferences from with molecular interpreted arising potential result be of The com- grain. to because graphite isotopic parent nevertheless C its as its needs same assuming the is molecule position this for predicted ag fteiooial omlseis uprigterasgmnsa PAHs. as assignments their supporting species, normal isotopically the of range as hsrsl rusprusvl o h asasgmnso hs pce sPH.As hw r h esrdms xessfrfu molecular four for excesses mass measured the are that shown of Also PAH PAHs. relevant the as for species calculated for these value evidence exact of show the that assignments about grains mass distributed C normally the light are for in grains observed persuasively C species argues normal from result molecules This of mass. excesses mass measured the cases niheti on nganK0( K10 grain in found is enrichment dilu- isotopic of e†ects tion. the mitigating residue insoluble acid F togcrusata vdneo rmtcisotopomer dramatic of evidence circumstantial Strong IG .Cmaio fteeatadmaue asecse o h oiatmlclrseisosre nbt omlCadlgtCgan.I all In grains. C light and C normal both in observed species molecular dominant the for excesses mass measured and exact the of 9.ÈComparison . C 2 k akl(C -alkyl L 2 Ssetao ihrMrhsno its or Murchison either of spectra MS ae nteioe hthstehgetpoooiaincosscina 6 m ec,ms 178 mass Hence, nm. 266 anthracene. at than section rather cross phenanthrene photoionization as highest assigned the is has amu that isomer the on based netit,addsrpniswt r o unexpected. not are large a has number this here), listed molecules the in 14 with discrepancies to and 2 uncertainty, (from observed ions peak isotopomer M11-7 K25...... M7-20 M11-15 M11-15 Mass Excess (amu) -0.02 a b 0.03 0.08 0.13 0.18 Grain ic h measured the Since ntestainweeoeo oeioei Aseita ie as asasgmnsare assignments mass mass, given a at exist PAHs isomeric more or one where situation the In 13 ...... 4 206 648 ...... -ihgan h isotopically the grain, C-rich

13 Naphthalene R nihet r at are enrichments C Zero ExcessLine \ I R C -Naphthalene 2720003 7 C 576 0.0035 270 1237 5828001 1 hyeeC Chrysene 216 0.0018 228 1508 771200235C 355 0.012 192 1727 771800825Peatrn C Phenanthrene 215 0.038 178 1727

SOTOPICALLY 1 0.shows 10). grain Normal CGrains Exact Excess Light CGrains 12 C -Naphthalene 12 iue12 Figure 2 H C/ 12 13 (amu) Mass 13 C -Naphthalene ),

eltos h asecse esrdi hs oeue alwti h omldistribution normal the within fall molecules these in measured excesses mass The depletions. C 3 o ie oeuei nesl rprinlt h ubrof number the to proportional inversely is molecule given a for C A 13

NOMALOUS C -Naphthalene R

C-poor 4 grain .02 1 C 311 0.00027

P Phenanthrene N ( n AL 1 TABLE 2 ) M C1-Phenanthrene OLECULES

R C -Phenanthrene ihtedmnn Asbigtesm nbt spectra. mass both in di†erent same shows contrast, the in being distinctly K10, residue, PAHs Grain acid dominant the have the to total similarities with has integrated but M8.54 comparable Grain envelopes. have intensities M8.54 signal grain and K10 oeaems ag,fo 7 o20au essentially amu, 270 gener- to 178 certain from make range, span mass to molecules regions. moderate The possible PAHs. a spectral indigenous still the uncrowded about is molecules alizations fortuitously anomalous it in isotopically Nevertheless, those identify to with PAHs, ability normal limited the isotopically has interferences, of heteroaromatic with compounded presence appar- the ubiquitous discussed, previously ently As date. to identiÐed ecules of suite peaks. isotopomer extended overlapping an of tribution of expected behavior is this 13 what conclusive, with not consistent Although is study. this in sured a 2 molecule -nihdPH nwihec A hw iedis- wide a shows PAH each which in PAHs C-enriched greater al 1 Table C3-Phenanthrene I ETFE TO DENTIFIED a

C4-Phenanthrene pcrlcmlxt hnayohrganmea- grain other any than complexity spectral 3 2 1 umrzsteiooial nmlu mol- anomalous isotopically the summarizes Assignment crsn C -chrysene peatrn C -phenanthrene peatrn C -phenanthrene C5-Phenanthrene

D C6-Phenanthrene ATE Pyrene b

C1-Pyrene Structure 21 18 16 15 14 Chrysene no H H H H H 18 12 14 12 10

rmnn ek n has and peaks prominent Benzopyrene No. 1, 1998 POLYCYCLIC AROMATIC HYDROCARBONS 293

R = 2.4 Simulation R = 89 "Solar" Signal Intensity

176 178 180 182 184 186 188 190 Simulation Mass (amu) R = 18

R = 89 Signal Intensity Grain 7.25 Signal Intensity

176 178 180 182 184 186 188 190 Mass (amu) 153 155 157 R = 1700 Mass (amu)

FIG. 11.ÈEvidence for 13C enriched acenaphthalene in grain 7.25. This grain was measured by ion probe to have a large 13C enrichment (R \ 18).

Signal Intensity The extremely variable mass distributions and relative abundances of the normal PAHs observed from one graph- 176 178 180 182 184 186 188 190 ite grain to the next indicate that the normal PAHs are not Mass (amu) a unique contaminant introduced during the chemical treatments used to isolate the grains from their parent meteor- FIG. 10.ÈCalculated C isotopomer distributions for the molecule phenanthrene at three di†erent values of R (2.4, 89, 1700), representing the ites. The normal PAHs could have been introduced to the two extreme R-values observed in the grains in this study and the solar graphite grains at any number of stages in the complex value. history of the grains. Indeed, the variable nature of the normal PAHs suggests that di†erent processes have been operative on di†erent grains. Although the lack of distinc- covering the size range of PAHs observed in other extrater- tive isotopic signatures may make it difficult to determine restrial materials(Clemett 1996). The molecules are diverse the origins of the normal PAHs, important constraints may and include alkylated species extending up toC . However, emerge from work on the stability and siting of PAHs owing to limited statistics, it is presently impossible3 to within the grains. determine if the indigenous PAHs have identical mass patterns in di†erent circumstellar graphite grains.

4. ORIGIN OF THE OBSERVED MOLECULES Grain K10 RIonprobe = 10.5 We Ðrst make the distinction between those PAHs that exhibit correlated isotopic anomalies with their parent graphite grains and that are therefore truly indigenous to the Grain 8.54 grains and those whose isotopic compositions are indistin- RIonprobe = 89 guishable from the solar value, hereafter referred to as ““ normal ÏÏ PAHs. It should be emphasized that although isotopically indistinguishable from solar system materials, Signal Intensity Acid Residue many normal PAHs may still have a presolar origin. We note, for reference, that while ensembles of meteoritic microdiamonds also have solar 12C/13C values, they are generally believed to be presolar because they are the apparent carriers of the exotic noble gas component Xe-HL 100 150 200 250 300 350 400 450 (Anders& Zinner 1993 and references therein). In addition, Mass (amu) some of the PAHs identiÐed as normal could have a con- FIG. 12.ÈComparison of the PAH spectrum observed from the 13C siderable indigenous PAH component, which is obscured in enriched grain K10 with spectra from an isotopically normal grain and the crowded mass spectrum. Murchison acid residue. 294 MESSENGER ET AL. Vol. 502

Although the indigenous PAHs may also have been the grains may be a product of radiation processing, either formed at di†erent stages in the history of the grains, the while traversing the interstellar medium or during their isotopic linkage between the molecules and the grains pro- residence in the presolar molecular cloud.Starukhina et al. vides an important constraint on their formation mecha- (1990) have demonstrated that radiation-induced formation nism. of PAHs in H-saturated graphite under realistic astro- Certainly one possible origin of the indigenous PAHs is physical conditions is possible. For example, 10 KeV irra- in gas-phase reactions followed by incorporation into the diation of H-saturated graphite by H, He, and N ions all growing graphite grains. TEM studies of circumstellar lead to the generation of simple PAHs and their alkylated graphite provide independent, albeit indirect, evidence for homologs in the top 1000A of the exposed graphite surface. the presence of PAHs(Bernatowicz et al. 1996). TEM For the case of interstellar graphite, we estimate that the analysis shows that most grains (D80%) have a distinct total energy deposited in a typical grain by galactic cosmic nanocrystalline C core surrounded by well-graphitized C. rays during an assumed 107 yr exposure ([2 ] 1021 eV) These nanocrystalline C cores usually constitute a signiÐ- is sufficient to account for several ppm of PAHs (assuming cant fraction of the mass of the grains. Electron di†raction 100% production efficiency). In reality the actual radiation studies show that the core material is composed primarily exposure could be far higher and this is thus a potentially of randomly oriented hexagonal sheets of C atoms with sp2 signiÐcant process. bonding (aromatic), typically 3È4 nm in size (graphene). Still another process that cannot be excluded is the syn- This material can be considered part of a structural contin- thesis of PAHs in solid state reactions resulting from super- uum, with PAHs representing the small size limit. Indeed, nova shock wave interactions in the interstellar medium. electron di†raction patterns of the core material indicate Indeed, some workers have calculated rather large grain that there must be a large number of aromatic units less destruction rates from such interactions (e.g., Draine 1995) than 1 nm in size, perhaps accounting for as much as 25% and under these conditions conversion of a preexisting of the core material. The authors suggest that the graphene carbon-hydrogen mixture into PAHs must be considered. and ““ PAH-like material ÏÏ were produced in the gas phase It is conceivable that the indigenous PAHs formed as a prior to graphite formation, later condensing to provide result of hydrothermal processing of the graphite grains on nucleation centers for further graphite growth. their meteorite parent bodies. However, aqueous processes Bernatowiczet al. (1996) have modeled the formation of are more likely to be important for transporting PAHs than graphite in circumstellar atmospheres, providing con- for producing PAHs and as such should be considered as a straints on the temperature, pressure, and timescale for their potentially signiÐcant source of the normal PAHs. These growth based on the grain sizes and the presence of internal issues may be clariÐed by comparing the PAHs present in carbides. They argued that the nanocrystalline C cores graphite grains extracted from di†erent meteorite classes, formed under conditions of supersaturation in anomalously which have experienced a range of aqueous and thermal dense regions of the stellar atmosphere (density clumps) at a processing. temperature of at least 1550 K. CONCLUDING REMARKS Cherchne†,Barker, & Tielens (1992) have developed a 5. chemical kinetic model for PAH formation in C-rich cir- Several lines of future investigation deserve to be cumstellar outÑows, although with low predicted pro- pursued. In this work we have concentrated on searching duction rates. While the temperature constraint derived by for positive correlations between the carbon-isotopic com- Bernatowiczet al. (1996) is substantially higher than gener- position of PAHs and their parent graphite grains. Further ally expected for PAH formation (D1200 K), Cherchne†, studies should look for potential correlations between Barker, & Tielens(1991) argue that PAH chemistry is subsets of indigenous PAHs and probable stellar origins of dependent on the PAHÏs vibrational temperature and not the graphite grains as determined by isotopic analysis. An the gas kinetic temperature. Owing to the ability of PAH observed correspondence between the nature or abundance molecules to rapidly dissipate vibrational energy through of indigenous PAHs and the stellar origin of the parent Ñuorescence, the vibrational temperature may be several graphite would leave little doubt that these molecules hundred degrees below the gas kinetic temperature in coformed with the graphite in a circumstellar environment. typical stellar atmosphere conditions. By invoking this Potentially powerful constraints on the physical and chemi- ““ inverse greenhouse e†ect,ÏÏ PAH formation is expected to cal conditions of circumstellar graphite formation could be commence when the gas kinetic temperature drops to 1500 provided by complementary TEM analysis of the same K, e†ectively reconciling gas phase PAH formation with the grains studied by the dual techniques described here. This is temperature constraints imposed on circumstellar graphite not a trivial experiment, but it is possible. formation. The conditions of high-density supersaturation Further progress in understanding the origins of the dif- required for circumstellar graphite grain formation deter- ferent PAH species would be greatly helped if some method mined byBernatowicz et al. (1996) are more favorable for could be found to physically separate the normal from the PAH production than those assumed byCherchne† et al. indigenous PAHs. This might be achieved, for example, if (1991), possibly leading to substantially higher PAH yields the indigenous PAHs were more tightly bound to the in their model. graphite, allowing for simple heating of the grains to drive IR emission features attributed to PAHs are commonly o† the more easily volatilized normal PAHs. It is also pos- observed in C-rich planetary nebulae, formed from the mass sible that the normal PAHs are more susceptible to organic ejected in strong AGB stellar winds (e.g.,Cohen et al. 1986). solvents and the grains could be ““ cleaned ÏÏ of the normal Direct observation of these emission features in the atmo- component. The indigenous PAHs may very well be con- spheres of C-rich red giants may be difficult because of the centrated in the nanocrystalline cores common to most lack of UV photons necessary to induce PAH Ñuorescence. graphite grains. It is possible, though admittedly very diffi- Some fraction of the indigenous PAHs observed within cult, to separate the core material from the parent graphite No. 1, 1998 POLYCYCLIC AROMATIC HYDROCARBONS 295 grains for independent study. highly dependent on the distribution, nature and abun- Although we lean to the view that the indigenous PAHs dance of reacting species, such radiation, heating, and were produced in gas-phase reactions and incorporated into aqueous treatments should be applied to the circumstellar the graphite grains as they were forming, other mechanisms graphite grains themselves. cannot be ruled out. However, the PAHs produced by high- temperature gas-phase reactions should be distinct in their mass distributions and degrees of alkylation from those This paper was improved as a result of a review by X. produced by exposure of graphite to thermal, radiation or Tielens. We thank E. Zinner for critical readings of the aqueous processing. More detailed characterization of the manuscript during its preparation. This work was sup- indigenous PAHs, combined with laboratory investigations ported by grants from NASA (NAGW-3629) to the Stan- of analog systems may help determine the origin of these ford University group and NSF (EAR-9316328) to the molecules. However, since the molecules produced may be Washington University group.

REFERENCES Allamandola, L. J., Tielens, A. G. G. M., & Barker, J. R. 1985, ApJ, 290, Kroto,H. 1988, Science, 242, 1139 L25 Kroto, H. W., Heath, J. R., OÏBrien, S. C., Curl, R. F., & Smalley, R. E. ÈÈÈ.1989, ApJS, 71, 733 1985, Nature, 318, 162 Amari,S., Anders, E., Virag, A., & Zinner, E. 1990, Nature, 345, 238 Leger,A., & Puget, J. L. 1984, ApJ, 137, L5 Amari,S., Hoppe, P., Zinner, E., & Lewis, R. 1993a, Nature, 365, 806 Levy, R. L., Grayson, M. A., & Wolf, C. J. 1973, Geocim. Cosmochim. Amari, S., Lewis R., & Anders E. 1994, Geochim. Cosmochim. Acta, 58, Acta, 37, 467 459 Lewis, R. S., Tang, M., Wacker, J. F., Anders, E., & Steel, E. 1987, Nature, Amari,S., Zinner, E., & Lewis, R. S. 1993b, Meteoritics, 28, 316 326, 160 Anders,E., & Zinner E. 1993, Meteoritics, 340, 906 Maechling, C. R., Clemett, S. J., & Zare, R. N. 1995, Chem. Phys. Lett., 241, Bernatowicz,T., Amari, S., Zinner, E., & Lewis, R. S. 1991, ApJ, 373, L73 301 Bernatowicz, T., Cowsic, R., Gibbons, P. C., Lodders, K., Fegley, B., Jr., Newton, J., Bischo†, A., Arden, J. W., Franchi, I., Geiger, T., Greshake, A., Amari, S., & Lewis, R. S. 1996, ApJ, 472, 760 & Pillinger, C. 1995, Meteoritics, 30, 47 Bernatowicz, T., Fraundorf, G., Tang, M., Anders, E., Wopenka, B., Nittler, L. R., Alexander, C. M. OÏD., Gao, X., Walker, R. M., & Zinner, E. Zinner, E., & Fraundorf, P. 1987, Nature, 330, 728 1994, Nature, 370, 443 Bialkowski,S. E. 1987, Rev. Sci. Instrum., 58, 2338 Nittler,L. R., et al. 1995, ApJ, 453, L25 Bregman, J. D. 1989, in IAU Symp. 135, Interstellar Dust, ed. L. J. Alla- Pappas, D. L., Hurbowchak, D. M., Ervin, M. H., & Winograd, N. 1989, mandola & A. G. G. M. Tielens (Dordrecht: Kluwer), 109 Science, 243, 64 Carey, W., Zinner, E., Fraundorf, P., & Lewis, R. S. 1987, Meteoritics, 22, Pering,K. L., & Ponnamperuma, C. 1971, Science, 173, 237 349 Sagan, C., Khare, B. N., Thompson, W. R., McDonald, G. D., Wing, M. R., Cherchne†,I., Barker, J. R., & Tielens, A. G. G. M. 1991, ApJ, 377, 541 Bada, J. L., Vo-Dinh, T., & Arakawa, E. T. 1993, ApJ, 414, 399 ÈÈÈ.1992, ApJ, 401, 269 Salma, F., Bakes, E. L. O., Allamandola, L. J., & Tielens, A. G. G. M. 1996, Clemett,S. J. 1996, Ph.D. thesis, Stanford Univ. ApJ, 458, 621 Clemett, S. J., Maechling, C. R., Zare, R. N., & Alexander, C. M. OÏD. 1992, Shibanov, A. N. 1985, in Laser Analytical Spectroscopy, ed. V. S. Lekok- Lunar Planet. Sci., 23, 233 hov (Bristol: Adam Hilger) Clemett, S. J., Maechling, C. R., Zare, R. N., Swan, P. D., & Walker, R. M. Starukhina, L. V., Shkuratov, Yu. G., Kodina, L. A., Ogloblina, A. I., 1993, Science, 262, 721 Stankevich, N. P., Peregon, T. I., & Tishchenko, L. P. 1990, Lunar Clemett, S. J., & Zare, R. N. 1997, in IAU Symp. 178, Molecules in Astro- Planet. Sci., 21, 1192 physics: Probes and Processes, ed. E. F. van Dishoeck (Dordrecht: Tang,M., & Anders, E. 1988, Geochim. Cosmochim. Acta, 52, 1235 Kluwer), 305 Thomas,K. L., et al. 1995, Geochim. Cosmochim. Acta, 59, 2797 Cohen, M., Allamandola, L. J., Tielens, A. G. G. M., Bregman, J., Simpson, Wiley,W. C., & McLaren, I. H. 1955, Rev. Sci. Instrum., 26, 1150 J. P., Witteborn, F. C., Wooden, D., & Rank, D. 1986, ApJ, 302, 737 Willner, S. P., Soifer, B. T., Russell, R. W., Joyce, R. R., & Gillett, F. C. Cronin, J. R., Pizzarello, S., & Cruikshank, D. P. 1988, in Meteorites and 1977, ApJ, 217, L121 the Early Solar System, ed. J. F. Kerridge & M. S. Matthews (Tucson: Winograd, N., Baxter, J. P., & Kimock, F. M. 1982, Chem. Phys. Lett., 82, Univ. Arizona Press), 819 591 Draine,B. T. 1995, Ap&SS, 233, 111 Zenobi, R., Philippoz, J. -M., Buseck, P. R., & Zare, R. N. 1989, Science, Geballe, T. R., Lacy, J. H., Persson, S. E., McGregor, P. J., & Soifer, B. T. 246, 1026 1985, ApJ, 292, 500 Zenobi, R., Philippoz, R.-M., Zare, R. N., Wing, M. R., Bada, J. L., & Hahn,J. H., Zenobi, R., Bada, J. F., & Zare, R. N. 1988, Science, 239, 1523 Marti, K. 1992, Geochim. Cosmochim. Acta, 56, 2899 Hoppe, P., Strebel, R., Eberhardt, P., Amari, S., & Lewis, R. 1994, Lunar Zenobi, R., & Zare, R. N. 1991, in Advances in Multiphoton Processes and Planet. Sci., 25, 563 Spectroscopy, vol. 7, ed. S. H. Lin (Singapore: World ScientiÐc), 1 Huss,G., Fahey, A., Gallino, R., & Wasserberg, G. 1994, ApJ, 430, L81 Zinner, E., Tang, M., & Anders, E. 1989, Geochim. Cosmochim. Acta, 53, Kovalenko, L. J., Maechling, C. R., Clemett, S. J., Philippoz, J.-M., Zare, 3273 R. N., & Alexander, C. M. OÏD. 1992, Anal. Chem., 64, 682