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1. Introduction THE ASTROPHYSICAL JOURNAL, 502:284È295, 1998 July 20 ( 1998. The American Astronomical Society. All rights reserved. Printed in U.S.A. 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 aro- matic 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 aro- matic 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
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