1.02Presolar GrainsE. K. Zinner

Washington University, St. Louis, MO, USA

1.02.1 INTRODUCTION 17

1.02.2 HISTORICAL BACKGROUND 18

1.02.3 TYPES OF PRESOLAR GRAINS 19

1.02.4 ANALYSIS TECHNIQUES 19

1.02.5 ASTROPHYSICAL IMPLICATIONS OF THE STUDY OF PRESOLAR GRAINS 20

1.02.6 SILICON CARBIDE 201.02.6.1 Mainstream Grains 221.02.6.2 Type Y and Z Grains 251.02.6.3 Type A þ B Grains 261.02.6.4 Type X Grains 261.02.6.5 Nova Grains 281.02.6.6 Grain Size Effect 28

1.02.7 SILICON NITRIDE 29

1.02.8 GRAPHITE 291.02.8.1 Physical Properties 291.02.8.2 Isotopic Compositions 30

1.02.9 OXIDE GRAINS 32

1.02.10 DIAMOND 33

1.02.11 CONCLUSION AND FUTURE PROSPECTS 34

ACKNOWLEDGMENTS 34

REFERENCES 34

1.02.1 INTRODUCTION

Traditionally, astronomers have studied thestars by using, with rare exception, electromag-netic radiation received by telescopes on andabove the Earth. Since the mid-1980s, anadditional observational window has been openedin the form of microscopic presolar grains foundin primitive meteorites. These grains had appar-ently formed in stellar outflows of late-type starsand in the ejecta of stellar explosions andhad survived the formation of the solar system.They can be located in and extracted from theirparent meteorites and studied in detail in thelaboratory. Their stellar origin is recognizedby their isotopic compositions, which are comple-tely different from those of the solar system and,for some elements, cover extremely wide

ranges, leaving little doubt that the grains areancient stardust.

By the 1950s it had been conclusively estab-lished that the elements from carbon on up areproduced by nuclear reactions in stars and theclassic papers by Burbidge et al. (1957) andCameron (1957) provided a theoretical frameworkfor stellar nucleosynthesis. According to theseauthors, nuclear processes produce elements withvery different isotopic compositions, depending onthe specific stellar source. The newly producedelements are injected into the interstellar medium(ISM) by stellar winds or as supernova (SN) ejecta,enriching the galaxy in “metals” (all elementsheavier than helium) and after a long galactichistory the solar system is believed to have formedfrom a mix of this material. In fact, the originalwork by Burbidge et al. and Cameron was

17

stimulated by the observation of regularitiesin the abundance of the nuclides in the solarsystem as obtained by the study of meteorites(Suess and Urey, 1956). Although providing only agrand average of many stellar sources, the solarsystem abundances of the elements and isotopes(Anders and Grevesse, 1989; Grevesse et al., 1996;see Chapter 1.03; Lodders, 2003) remained animportant test for nucleosynthesis theory (e.g.,Timmes et al., 1995).

In contrast, the study of stellar grains permitsinformation to be obtained about individual stars,complementing astronomical observations ofelemental and isotopic abundances in stars (e.g.,Lambert, 1991), by extending measurements toelements that cannot be measured astronomically.In addition to nucleosynthesis and stellar evol-ution, presolar grains provide information aboutgalactic chemical evolution, physical properties instellar atmospheres, mixing of SN ejecta andconditions in the parent bodies of the meteorites inwhich the grains are found.

This new field of astronomy has grown to anextent that not all aspects of presolar grains canbe treated in detail in this chapter. The interestedreader is therefore referred to some recentreviews (Anders and Zinner, 1993; Ott, 1993;Zinner, 1998a,b; Hoppe and Zinner, 2000; Nittler,2003) and to the compilation of papers found inBernatowicz and Zinner (1997). The latter notonly contains several detailed review papers onpresolar dust grains but also a series of chapterson stellar nucleosynthesis. Further informationon nucleosynthesis can be obtained from thetextbooks by Clayton (1983b) and Arnett (1996),and from reviews by Kappeler et al. (1989),Meyer (1994), and Wallerstein et al. (1997).

1.02.2 HISTORICAL BACKGROUND

Although the work by Burbidge et al. (1957)and Cameron (1957), and subsequent work by

nuclear astrophysicists made it clear that manydifferent stellar sources must have contributed tothe material that formed the solar system andalthough astronomical observations indicate thatsome of this material was in the form of interstellar(IS) grains (e. g., Mathis, 1990), it was generallybelieved that it had been thoroughly homogenizedin a hot solar nebula (Cameron, 1962). The uniformisotopic composition of all available solar systemmaterial seemed to confirm this opinion.

The first evidence for isotopic heterogeneity ofthe solar nebula and a hint of the survival ofpresolar grains came from hydrogen (Boato, 1954)and the noble gases xenon (Reynolds and Turner,1964) and neon (Black and Pepin, 1969; Black,1972) but it was only after the discovery ofanomalies in oxygen, a rock-forming element(Clayton et al., 1973), that the concept of survivalof presolar material in primitive meteorites waswidely accepted. The finding of 16O excesses wasfollowed by the detection of isotopic anomalies inother elements such as magnesium, calcium,titanium, chromium, and barium in refractoryinclusions (CAIs for calcium- and aluminum-richinclusions) (Wasserburg, 1987; Clayton et al.,1988; Lee, 1988). Also large anomalies in carbon(Halbout et al., 1986) and nitrogen (Lewis et al.,1983) indicated the presence of presolar grains.However, it was the pursuit of the carriers of the“exotic” (i.e., isotopically anomalous) noble gascomponents of neon and xenon (Figure 1) by EdAnders and his colleagues at the University ofChicago that led to their ultimate isolation(see Anders and Zinner, 1993). The approachtaken by these scientists, “burning down thehaystack to find the needle,” consisted of trackingthe noble gas carriers through a series of increas-ingly harsher chemical dissolution and physicalseparation steps (Tang and Anders, 1988b;Amari et al., 1994). Their effort culminated inthe isolation and identification of diamond,the carrier of Xe-HL (Lewis et al., 1987), siliconcarbide, the carrier of Ne-E(H) and Xe-S

Figure 1 Exotic noble gas components present in presolar carbonaceous grains. Diamond is the carrier of Xe-HL,SiC the carrier of Xe-S and Ne-E(H), and graphite the carrier of Ne-E(L) (source Anders and Zinner, 1993).

Presolar Grains18

(Bernatowicz et al., 1987; Tang and Anders,1988b), and graphite, the carrier of Ne-E(L)(Amari et al., 1990).

Once isolated, SiC and graphite (for diamond,see below) were found to be anomalous in all theirisotopic ratios, and it is this feature that identifiesthem as presolar grains. This distinguishes themfrom other materials in meteorites such as CAIsthat also carry isotopic anomalies in some elementsbut, in contrast to bona fide stardust, formed inthe solar system. They apparently inherited theiranomalies from incompletely homogenized pre-solar material. Another distinguishing featureis that anomalies in presolar grains are up toseveral orders of magnitude larger than those inCAIs and match those expected for stellaratmospheres (Zinner, 1997).

1.02.3 TYPES OF PRESOLAR GRAINS

Table 1 shows the types of presolar grainsidentified as of early 2000s. It also lists the sizes,approximate abundances, and stellar sources. Inaddition to the three carbonaceous phases that werediscovered because they carry exotic noble gascomponents (Figure 1) and which can be isolatedfrom meteorites in almost pure form by chemicaland physical processing, presolar oxide, siliconnitride (Si3N4), and silicates were identified byisotopic measurements in the ion microprobe andthe number of such grains available for study ismuch smaller than for the carbonaceous phases.Most oxide grains are spinel (MgAl2O4) andcorundum (Al2O3), but hibonite (CaAl12O19) andpossibly titanium oxide have also been found(Hutcheon et al., 1994; Nittler et al., 1994; Nittlerand Alexander, 1999; Choi et al., 1998; Zinneret al., 2003). While all these grains, as well aspresolar Si3N4 (Nittler et al., 1995), were locatedby single grain analysis of acid residues, presolarsilicates were discovered by isotopic imaging ofchemically untreated interplanetary dust particles(IDPs) (Messenger et al., 2003).

Finally, titanium-, zirconium-, and molyb-denum-rich carbides, cohenite ((Fe,Ni)3C), kama-cite (Fe–Ni), and elemental iron were found as

tiny subgrains inside of graphite spheres(Bernatowicz et al., 1991, 1996, 1999; Croatet al., 2003). While TiC inside of an SiC grain(Bernatowicz et al., 1992) could have formed byexsolution, there can be little doubt that interiorgrains in graphite must have formed prior to thecondensation of the spherules.

1.02.4 ANALYSIS TECHNIQUES

Although the abundance of carbonaceous pre-solar grains in meteorites is low, once they areidentified, almost pure samples can be preparedand studied in detail. Enough material of thesephases can be obtained for “bulk” analysis, i.e.,analysis of collections of large numbers of grainseither by gas mass spectrometry (GMS) of carbon,nitrogen, and the noble gases (Lewis et al., 1994;Russell et al., 1996, 1997), by thermal ionizationmass spectrometry (TIMS) of strontium, barium,neodymium, samarium, dysprosium (Ott andBegemann, 1990; Prombo et al., 1993; Richteret al., 1993, 1994; Podosek et al., 2003) orsecondary ion mass spectrometry (SIMS) (Zinneret al., 1991; Amari et al., 2000). While onlyaverages over many grains are obtained in thisway, it allows the measurement of trace elementssuch as the noble gases and heavy elements thatcannot be analyzed otherwise.

However, because presolar grains come fromdifferent stellar sources, information on individualstars is obtained by the study of single grains. Thischallenge has been successfully taken up by theapplication of a series of microanalytical tech-niques. For isotopic analysis, the ion microprobehas become the instrument of choice. Whilemost SIMS measurements have been on grains1 mm in size or larger, a new type of ion probe,the NanoSIMS, allows measurements of grains anorder of magnitude smaller (e.g., Zinner et al.,2003). Ion probe analysis has led to the discoveryof new types of presolar grains such ascorundum (Hutcheon et al., 1994; Nittler et al.,1994) and silicon nitride (Nittler et al., 1995).It also has led to the identification of raresubpopulations of presolar dust such as SiC grains

Table 1 Types of presolar grains in primitive meteorites and IDPs.

Grain type Noble-gas components Size Abundance Stellar sources

Diamond Xe-HL 2 nm 1,000 ppm Supernovae?Silicon carbide Ne-E(H), Xe-S 0.1–20 mm 10 ppm AGB, SNe, J-stars, novaeGraphite Ne-E(L) 1–20 mm 1–2 ppm SNe, AGBOxides 0.15–3 mm 1 ppm RG, AGB, SNeSilicon nitride 0.3–1 mm ,3 ppb SNe, AGBTi-, Fe-, Zr-, Mo-carbides 10–200 nm SNeKamacite, iron ,10–20 nm SNeOlivine 0.1–0.3 mm

Analysis Techniques 19

of type X (Amari et al., 1992) and type Y (Hoppeet al., 1994). Searches for presolar oxide grains andrare subpopulations of SiC profited from theapplication of isotopic imaging in the ion probe,which allows the rapid analysis of a large numberof grains (Nittler et al., 1997). Laser ablation andresonant ionization mass spectrometry (RIMS)(Savina et al., 2003b) has been successfully appliedto isotopic analysis of the heavy elements stron-tium, zirconium, molybdenum, and barium inindividual SiC and graphite grains (Nicolussiet al., 1997, 1998a,b,c; Savina et al., 2003a).Single grain measurements of helium and neonhave been made by laser heating and gas massspectrometry (Nichols et al., 2003).

The surface morphology of grains has beenstudied by secondary electron microscopy (SEM)(Hoppe et al., 1995). Such studies have beenespecially useful for pristine SiC grains that havenot been subjected to any chemical treatment(Bernatowicz et al., 2003). Finally, the trans-mission electron microscope (TEM) played animportant role in the discovery of presolar SiC(Bernatowicz et al., 1987) and internal TiC andother subgrains in graphite (Bernatowicz et al.,1991). It has also been successfully applied to thestudy of diamonds (Daulton et al., 1996) and ofpolytypes of SiC (Daulton et al., 2002, 2003).

1.02.5 ASTROPHYSICAL IMPLICATIONS OFTHE STUDY OF PRESOLAR GRAINS

There are many stages in the long history ofpresolar grains from their stellar birth to theirincorporation into primitive meteorites and, inprinciple, the study of the grains can provideinformation on all of them.

The isotopic composition of a given circum-stellar grain reflects that of the stellar atmospherefrom which the grain condensed. The atmo-sphere’s composition in turn is determined byseveral factors: (i) by the galactic history of thematerial from which the star itself formed, (ii) bynucleosynthetic processes in the star’s interior,and (iii) by mixing episodes in which newlysynthesized material is dredged from the interiorinto the star’s envelope. In supernovae, mixing ofdifferent layers with different nucleosynthetichistory accompanies the explosion and the ejec-tion of material. The isotopic compositions ofgrains provide information on these processes.

Grain formation occurs when temperatures inthe expanding envelope of red giants (RGs) or inSN ejecta are low enough for the condensation ofminerals. Many late-type stars are observed to besurrounded by dust shells of grains whose mineralcompositions reflect the major chemistry of thegas (e.g., Little-Marenin, 1986). The study of mor-phological features of pristine grains, of internal

grains, and of trace-element abundances can giveinformation on the physical and chemical proper-ties of stellar atmospheres (Bernatowicz et al.,1996; Amari et al., 1995a; Lodders and Fegley,1998; Kashiv et al., 2001, 2002; Croat et al., 2003).

After their formation as circumstellar grains oras SN condensates, grains enter a long journeythrough the ISM. They should be distinguishedfrom true IS grains that form in the ISM, e.g., indense molecular clouds. Grains of stellar origin aremost likely to be covered by mantles of IS cloudmaterial. During their IS history, grains aresubjected to a variety of destructive processes,such as evaporation in SN shocks and sputtering byshocks and stellar winds. They are also exposed togalactic cosmic rays that leave a record in the formof cosmogenic nuclides (Tang and Anders, 1988a;Ott and Begemann, 2000).

Grains might go in and out of IS clouds beforesome were finally incorporated into the densemolecular cloud from which our solar systemformed. The final step in the complex history ofstellar grains is the formation of planetesimals andof the parent bodies of the meteorites in which wefind these presolar fossils. By far the largestfraction of the solids, even in primitive meteorites,formed in the solar system and the fraction ofsurviving presolar grains is small (see Table 1).Primitive meteorites experienced varying degreesof metamorphism on their parent bodies and thesemetamorphic processes affected different types ofpresolar grains in different ways. The abundanceof different grain types can thus give informationabout conditions in the solar nebula and aboutparent-body processes (Huss and Lewis, 1995;Mendybaev et al., 2002).

1.02.6 SILICON CARBIDE

Silicon carbide is the best-studied presolar graintype. It has been found in carbonaceous, unequi-librated ordinary, and enstatite chondrites withconcentrations ranging up to ,10 ppm (Huss andLewis, 1995). Most SiC grains are less than0.5 mm in diameter. Murchison is an exception inthat grain sizes are, on average, much larger thanthose in other meteorites (Amari et al., 1994; Husset al., 1997; Russell et al., 1997). This differenceis still not understood but it, and the fact thatplenty of Murchison is available, is the reason thatby far most measurements have been made onMurchison SiC. Many SiC grains show euhedralcrystal features (Figure 2) but there are largevariations. Morphological studies by high-resol-ution SEM (Bernatowicz et al., 2003) revealdetailed crystallographic features that give infor-mation about growth conditions. Such informationis also obtained from TEM studies that show thatonly the cubic (3C) (,80%) and hexagonal (2H)

Presolar Grains20

polytypes are present, indicating low pressuresand condensation temperatures in stellar outflows(Bernatowicz et al., 1987; Daulton et al., 2002,2003). A preponderance of cubic SiC has beenobserved astronomically in carbon stars (Specket al., 1999).

The availability of .1 mm SiC grains andrelatively high concentrations of trace elements(Amari et al., 1995a) allow the isotopic analysis ofthe major and of many trace elements in individualgrains. In addition to the major elements, carbon

and silicon, isotopic data are available for thediagnostic (in terms of nucleosynthesis and stellarorigin) elements nitrogen, magnesium, calcium,titanium, the noble gases, and the heavy refractoryelements strontium, zirconium, molybdenum,barium, neodymium, samarium, and dysprosium.Carbon, nitrogen, and silicon isotopic as wellas inferred 26Al/27Al ratios in a large number ofindividual grains (Figures 3–5) have led tothe classification into different populations(Hoppe and Ott, 1997): mainstream grains(,93% of the total), and the minor subtypes A,B, X, Y, Z, and nova grains.

Most of presolar SiC is believed to haveoriginated from carbon stars, late-type stars oflow mass (1–3M() in the thermally pulsing (TP)asymptotic giant branch (AGB) phase of evolution(Iben and Renzini, 1983). Dust from such stars hasbeen proposed already one decade prior toidentification of SiC to be a minor constituent ofprimitive meteorites (Clayton and Ward, 1978;Srinivasan and Anders, 1978; Clayton, 1983a).Several pieces of evidence point to such an origin.Mainstream grains have 12C/13C ratios similar tothose found in carbon stars (Figure 6), which areconsidered to be the most prolific injectors ofcarbonaceous dust grains into the ISM (Tielens,1990). Many carbon stars show the 11.3 mm

Figure 3 Nitrogen and carbon isotopic ratios ofindividual presolar SiC grains. Because rare grain typeswere located by automatic ion imaging, the number ofgrains of different types do not correspond to theirabundances in the meteorites; these abundances aregiven in the legend (sources Alexander, 1993; Hoppeet al., 1994, 1996a; Nittler et al., 1995; Huss et al.,

1997; Amari et al., 2001a,b,c).

Figure 2 Secondary electron micrographs of (a) pre-solar SiC, (b) presolar graphite (cauliflower type), and(c) presolar graphite (onion type). Photographs courtesy

of Sachiko Amari and Scott Messenger.

Silicon Carbide 21

emission feature typical of SiC (Treffers andCohen, 1974; Speck et al., 1997). Finally, AGBstars are believed to be the main source of thes-process (slow neutron capture nucleosynthesis)elements (e.g., Busso et al., 2001), and thes-process isotopic patterns of the heavy elementsexhibited by mainstream SiC provide the mostconvincing argument for their origin in carbonstars (see below).

1.02.6.1 Mainstream Grains

Mainstream grains have 12C/13C ratios between10 and 100 (Figure 2). They have carbon andnitrogen isotopic compositions (Zinner et al.,1989; Stone et al., 1991; Virag et al., 1992;Alexander, 1993; Hoppe et al., 1994, 1996a;Nittler et al., 1995; Huss et al., 1997; Amari et al.,2002; Nittler and Alexander, 2003) that areroughly in agreement with an AGB origin.

Figure 4 Si isotopic ratios of different types of presolar SiC grains plotted as d-values, deviations in per mil (‰)from the solar ratios: diSi/28Si ¼ [(iSi/28Si)meas/(

iSi/28Si)(21] £ 1,000. Mainstream grains plot along a line of slope1.4 (solid line). Symbols are the same as those in Figure 3. Sources same as in Figure 3.

Figure 5 Aluminum and carbon isotopic ratios ofindividual presolar SiC grains. Symbols and sources for

the data are the same as those for Figure 3.

Presolar Grains22

Carbon-13 and 15N excesses relative to solar arethe signature of hydrogen burning via the CNOcycle that occurred during the main sequencephase of the stars. This material is brought to thestar’s surface by the first (and second) dredge-up.The carbon isotopic ratios are also affected byshell helium burning and the third dredge-upduring the TP-AGB phase (Busso et al., 1999).This process adds 12C to the envelope, increasesthe 12C/13C ratio from the low values resultingfrom the first dredge-up, and, by making C . O,causes the star to become a carbon star.

Envelope 12C/13C ratios predicted by canonicalstellar evolution models range from ,20 after firstdredge-up in the RG phase to ,300 in the late TP-AGB phases (El Eid, 1994; Gallino et al., 1994;Amari et al., 2001b). Predicted 14N/15N ratios are600–1,600 (Becker and Iben, 1979; El Eid, 1994),falling short of the range observed in the grains.However, the assumption of deep mixing (“coolbottom processing”) of envelope material to deephot regions in M,2.5M( stars during their RG

and AGB phases (Charbonnel, 1995; Wasserburget al., 1995; Langer et al., 1999; Nollett et al.,2003) results in partial hydrogen burning, withhigher 14N/15N and lower 12C/13C ratios in theenvelope than in canonical models (see also Husset al., 1997).

Two other isotopes that are a signature for AGBstars are 26Al and 22Ne. Figure 5 shows inferred26Al/27Al ratios in different types of SiC grains.The existence of the short-lived radioisotope 26Al(T1/2 ¼ 7.3 £ 105 yr) is inferred from large 26Mgexcesses. Aluminiun-26 is produced in the hydro-gen shell by proton capture on 25Mg and mixed tothe surface by the third dredge-up (Forestini et al.,1991). It can also be produced during “hot bottomburning” (Lattanzio et al., 1997), but this processis believed to prevent carbon-star formation (Frostand Lattanzio, 1996). Neon-22, the main com-ponent in Ne-E, is produced in the helium shell by14N þ 2a. The neon isotopic ratios measured inSiC bulk samples (Lewis et al., 1990, 1994) arevery close to those expected for helium shellmaterial (Gallino et al., 1990). In contrast tokrypton and xenon and heavy refractory elements,neon as well as helium and argon show very littledilution of helium shell material with envelopematerial, indicating a special implantation mech-anism by an ionized wind. Another piece ofevidence that the Ne-E(H) component originatedfrom the helium shell of AGB stars and not fromthe decay of 22Na (Clayton, 1975) is the fact thatin individual grains, of which only ,5% carry22Ne, it is always accompanied by 4He (Nicholset al., 2003). Excesses in 21Ne in SiC relative tothe predicted helium-shell composition have beeninterpreted as being due to spallation by galacticcosmic rays (Tang and Anders, 1988a; Lewiset al., 1990, 1994), which allows the determi-nation of grain lifetimes in the IS medium.Inferred exposure ages depend on grain sizeand range from 10 Myr to 130 Myr (Lewis et al.,1994). However, this interpretation hasrecently been challenged (Ott and Begemann,2000) and the question of IS ages of SiC is notsettled.

The silicon isotopic compositions of mostmainstream grains are characterized by enrich-ments in the heavy silicon isotopes of up to 200‰relative to their solar abundances (Figure 4). In asilicon three-isotope plot the data fall along aline with slope 1.4, which is shifted slightly tothe right of the solar system composition. Incontrast to the light elements carbon, nitrogen,neon, and aluminum and the heavy elements(see below), the silicon isotopic ratios of main-stream grains cannot be explained by nuclearprocesses taking place within their parent stars. InAGB stars the silicon isotopes are affected byneutron capture in the helium shell leading toexcesses in 29Si and 30Si along a slope 0.2–0.5

Figure 6 The distributions of carbon isotopic ratiosmeasured in presolar SiC (Hoppe et al., 1994; Nittleret al., 1995) and graphite grains (Hoppe et al., 1995)from the Murchison meteorite are compared toastronomical measurements of the atmospheres of

carbon stars (Lambert et al., 1986).

Silicon Carbide 23

line in a d-value silicon three-isotope plot(Gallino et al., 1990, 1994; Brown and Clayton,1992; Lugaro et al., 1999; Amari et al., 2001b).Predicted excesses are only on the order of 20‰ inlow-mass AGB stars of close-to-solar metallicity(metallicity is the abundance of all elementsheavier than helium). This led to the proposal thatmany stars with varying initial silicon isotopiccompositions contributed SiC grains to the solarsystem (Clayton et al., 1991; Alexander, 1993) andthat neutron-capture nucleosynthesis in these starsonly plays a secondary role in modifying thesecompositions. Several explanations have beengiven for the initial silicon ratios in the parentstars, which in their late stages of evolution becamethe carbon stars that produced the SiC. One is theevolution of the silicon isotopic ratios throughgalactic history as different generations of super-novae produced silicon with increasing ratios ofthe secondary isotopes 29Si and 30Si to the primary28Si (Gallino et al., 1994; Timmes and Clayton,1996; Clayton and Timmes, 1997a,b). Clayton(1997) addressed the problem that most SiC grainshave higher than solar 29Si/28Si and 30Si/28Si ratiosby considering the possibility that the mainstreamgrains originated from stars that were born incentral, more metal-rich regions of the galaxy andmoved to the molecular cloud from which our sunformed. Alexander and Nittler (1999), alterna-tively, suggested that the Sun has an atypicalsilicon isotopic composition. Lugaro et al. (1999)explained the spread in the isotopic compositionsof the parent stars by local heterogeneities in thegalaxy caused by the stochastic nature of theadmixture of the ejecta from supernovae of varyingtype and mass.

Titanium isotopic ratios in single grains (Irelandet al., 1991; Hoppe et al., 1994; Alexander andNittler, 1999) and in bulk samples (Amari et al.,2000) show excesses in all isotopes relative to 48Ti,a result expected of neutron capture in AGB stars.However, as for silicon, theoretical models(Lugaro et al., 1999) cannot explain the range ofratios observed in single grains. Furthermore,titanium ratios are correlated with those of silicon,also indicating that the titanium isotopic compo-sitions are dominated by galactic evolution effects(Alexander and Nittler, 1999). Excesses of 42Caand 43Ca relative to 40Ca observed in bulk samples(Amari et al., 2000) agree with predictions forneutron capture. Large 44Ca excesses are appar-ently due to the presence of type X grains (seebelow). Iron isotopic ratios have been measuredby RIMS in single grains (Davis et al., 2002;Tripa et al., 2002). Depletions in 54Fe are muchlarger than predicted by neutron capture in AGBstars and 57Fe does not show the predictedexcesses. While it is quite possible that, as forsilicon and titanium, galactic evolution effectsdominate and while admixture of SN ejecta

results in the observed 54Fe depletions, the SNmixing model also predicts substantial 57Feexcesses, which are not observed (see also Claytonet al., 2002).

All heavy elements measured so far show thesignature of the s-process (Figure 7, see alsoFigure 10). They include the noble gases kryptonand xenon (Lewis et al., 1990, 1994) but also theheavy elements strontium (Podosek et al., 2003),barium (Ott and Begemann, 1990; Zinner et al.,1991; Prombo et al., 1993), neodymium andsamarium (Zinner et al., 1991; Richter et al.,1993), and dysprosium (Richter et al., 1994).Although most measurements were made on bulksamples, it is clear that mainstream grainsdominate. Single-grain measurements of stron-tium (Nicolussi et al., 1998b), zirconium(Nicolussi et al., 1997), molybdenum (Nicolussiet al., 1998a), and barium (Savina et al., 2003a)have been made with RIMS. Large enrichmentsof certain heavy elements such as yttrium,zirconium, barium, and cerium in single main-stream grains also indicate large overabundancesof s-process elements in the parent stars (Amariet al., 1995a). For all the isotopic compositionsof the elements listed above except for dyspro-sium there is good agreement with theoreticalmodels of the s-process in low-mass AGB stars(Gallino et al., 1993, 1997; Lugaro et al., 2003).Discrepancies with earlier model calculationswere caused by incorrect nuclear cross-sectionsand could be resolved by improved experimentaldeterminations (e.g., Guber et al., 1997; Wisshaket al., 1997; Koehler et al., 1998).

The s-process isotopic patterns observed ingrains allow the determination of different par-ameters affecting the s-process such as neutronexposure, temperature, and neutron density(Hoppe and Ott, 1997). Since these parametersdepend in turn on stellar mass and metallicity aswell as on the neutron source operating in AGBstars, they allow information to be obtained aboutthe parent stars of the grains. For example, thebarium isotopic ratios indicate a neutron exposurethat is only half of that inferred for the solarsystem (Ott and Begemann, 1990; Gallino et al.,1993). Another example is provided by theabundance of 96Zr in single grains, which issensitive to neutron density because of therelatively short half-life of 95Zr (,64 d). Whilethe 13C(a,n) source with its low neutron densitydestroys 96Zr, activation of the 22Ne(a,n) sourceduring later thermal pulses in AGB stars restoressome of this isotope, whose abundance thus varieswith pulse number. Some grains have essentiallyno 96Zr, indicating that the 22Ne(a,n) source wasweak in their parent stars, pointing to low-massAGB stars as the source of mainstream grains(Lugaro et al., 2003).

Presolar Grains24

1.02.6.2 Type Y and Z Grains

Type Y grains have 12C/13C . 100 and siliconisotopic compositions that lie to the right ofthe mainstream correlation line (Figures 3 and

4(b)) (Hoppe et al., 1994; Amari et al., 2001b).Type Z grains have even larger 30Si excessesrelative to 29Si and, on average, lower d 29Si valuesthan Y grains. However, they are distinguishedfrom Y grains by having 12C/13C , 100

Figure 7 Isotopic patterns measured in bulk samples of SiC extracted from the Murchison meteorite. Isotopic ratiosare relative to the reference isotope plotted as a solid circle and are normalized to the solar isotopic ratios. Dataare from Lewis et al. (1994) (Kr and Xe), Podosek et al. (2003) (Sr), Prombo et al. (1993) (Ba), Richter et al. (1993)

(Nd and Sm), and Richter et al. (1994) (Dy).

Silicon Carbide 25

(Alexander, 1993; Hoppe et al., 1997). Compari-son of the carbon, silicon, and titanium isotopicratios of Y grains with models of nucleosynthesisindicates an origin in low-to-intermediate-massAGB stars with approximately half the solarmetallicity (Amari et al., 2001b). Such stars dredgeup more 12C, and silicon and titanium thatexperienced neutron capture, from the heliumshell (see also Lugaro et al., 1999). According totheir silicon isotopic ratios, Z grains came fromlow-mass stars of even lower (,one-third solar)metallicity (Hoppe et al., 1997). In order to achievethe relatively low 12C/13C ratios of these grains, theparent stars must have experienced cool bottomprocessing (Wasserburg et al., 1995; Nollett et al.,2003) during their RG and AGB phase. From thetheoretically inferred metallicities and averagesilicon isotopic ratios of mainstream Y and Zgrains, Zinner et al. (2001) derived the galacticevolution of the silicon isotopic ratios as a functionof metallicity. This evolution differs from theresults of galactic evolution models based on theyields of supernovae (Timmes and Clayton, 1996)and has important implications concerning therelative contributions from type II and type Iasupernovae during the history of our galaxy.

1.02.6.3 Type A 1 B Grains

Grains of type A þ B have 12C/13C , 10, buttheir silicon isotopic ratios plot along the main-stream line (Figures 3 and 4). In contrast tomainstream grains, many A þ B grains havelower than solar 14N/15N ratios (Hoppe et al.,1995, 1996a; Huss et al., 1997; Amari et al.,2001c). While the isotopic ratios of mainstream, Yand Z grains find an explanation in nucleosyn-thetic models of AGB stars, a satisfactoryexplanation of the data in terms of stellarnucleosynthesis is more elusive for the A þ Bgrains. The low 12C/13C ratios of these grainscombined with the requirement for a carbon-richenvironment during their formation indicatehelium burning followed by limited hydrogenburning in their stellar sources. However, theastrophysical sites for this process are not wellknown. There might be two different kinds ofA þ B grains with corresponding different stellarsources (Amari et al., 2001c). Grains with nos-process enhancements (Amari et al., 1995a;Pellin et al., 2000b; Savina et al., 2003c) probablycome from J-type carbon stars that also have low12C/13C ratios (Lambert et al., 1986). Unfortu-nately, J stars are not well understood and thereare no astronomical observations of nitrogenisotopic ratios in such stars. Furthermore, thelow 14N/15N ratios observed in some of the grainsas well as the carbon-rich nature of their parentstars appear to be incompatible with the

consequences of hydrogen burning in the CNOcycle, which seems to be responsible for the low12C/13C ratios of J stars and the grains. A þ Bgrains with s-process enhancements might comefrom post-AGB stars that undergo a very latethermal pulse. An example of such a star isSakurai’s object (e.g., Asplund et al., 1999;Herwig, 2001). However, grains with low14N/15N ratios pose a problem. Huss et al.(1997) proposed that the currently used18O(p,a)15N reaction rate is too low by a factorof 1,000. This would result in low 12C/13C and14N/15N ratios if an appropriate level of coolbottom processing is considered.

1.02.6.4 Type X Grains

Although SiC grains of type X account for only1% of presolar SiC, a fairly large number can belocated by ion imaging (Nittler et al., 1997; Hoppeet al., 1996b, 2000; Lin et al., 2002; Besmehn andHoppe, 2003). X grains are characterized bymostly 12C and 15N excesses relative to solar(Figures 3 and 6), excesses in 28Si (Figure 4) andvery large 26Al/27Al ratios, ranging up to 0.6(Figure 5). About 10–20% of the grains showlarge 44Ca excesses, which must come from thedecay of short-lived 44Ti (T1/2 ¼ 60 yr) (Amariet al., 1992; Hoppe et al., 1996b, 2000; Nittleret al., 1996; Besmehn and Hoppe, 2003). Inferred44Ti/48Ti ratios range up to 0.6 (Figure 8). Incontrast to presolar graphite, which containssubgrains of TiC, titanium in SiC seems to occurin solid solution and radiogenic 44Ca is uniformlydistributed in most of the grains. Only in one Xgrains a pronounced heterogeneity points to atitanium-rich subgrain (Besmehn and Hoppe,2003). Because 44Ti can only be produced in SNexplosions (Timmes et al., 1996), grains withevidence for 44Ti, and by implications all Xgrains, must have an SN origin. In type IIsupernovae 44Ti is produced in the nickel- andsilicon-rich inner zones (see Figure 9) (Woosleyand Weaver, 1995; Timmes et al., 1996). Siliconin the Si/S zone consists of almost pure 28Si. Alsothe other isotopic signatures of X grains arecompatible with an origin in type II supernovae:high 12C/13C and low 14N/15N ratios are thesignature of helium burning (Figure 9) and high26Al/27Al ratios can be reached in the He/N zoneby hydrogen burning.

However, these isotopic signatures occur inmassive stars in very different stellar zones, whichexperienced different stages of nuclear burningbefore the SN explosion (Figure 9) (e.g., Woosleyand Weaver, 1995; Rauscher et al., 2002). Theisotopic signatures of the X grains suggest deepand inhomogeneous mixing of matter from thesedifferent zones in the SN ejecta. While thetitanium and silicon isotopic signature of the X

Presolar Grains26

grains requires contributions from the Ni, O/Siand Si/S zones, which experienced silicon-, neon-,and oxygen-burning, significant contributionsmust also come from the He/N and He/C zonesthat experienced hydrogen and incomplete heliumburning in order to achieve C . O, the conditionfor SiC condensation (Larimer and Bartholomay,1979; Lodders and Fegley, 1997). Furthermore,addition of material from the intermediateoxygen-rich layers must be severely limited.Astronomical observations indicate extensivemixing of SN ejecta (e.g., Ebisuzaki andShibazaki, 1988; Hughes et al., 2000) andhydrodynamic models of SN explosions predict

mixing in the ejecta initiated by the formation ofRayleigh–Taylor instabilities (e.g., Herant et al.,1994). However, it still has to be seen whethermixing can occur on a microscopic scale andwhether these instabilities allow mixing of matterfrom nonneighboring zones while excluding largecontributions from the intermediate oxygen-richzones. Clayton et al. (1999) and Deneault et al.(2003) suggested condensation of carbonaceousphases in type II SN ejecta even while C , Obecause of the destruction of CO in the high-radiation environment of the ejecta. While thismight work for graphite, it is doubtful whetherSiC can condense from a gas with C , O (Ebeland Grossman, 2001). Even for graphite, thepresence of subgrains of elemental iron inside ofgraphite grains whose isotopic signatures indicatean SN origin argues against formation in anoxygen-rich environment (Croat et al., 2003).

Although multizone mixing models can quali-tatively reproduce the isotopic signatures of Xgrains, several ratios, in particular the large 15Nexcesses and excesses of 29Si over 30Si found inmost grains, cannot be explained quantitativelyand indicate deficiencies in the existing models.The latter is a long-standing problem: SN modelscannot account for the solar 29Si/30Si ratio(Timmes and Clayton, 1996). Studies of SiC Xgrains isolated from the Qingzhen enstatitechondrite (Lin et al., 2002) suggest that there aretwo population of X grains with different trends inthe silicon isotopic ratios, the minor populationhaving lower-than-solar 29Si/30Si ratios. Recently,Clayton et al. (2002) and Deneault et al. (2003)have tried to account for isotopic signatures fromdifferent SN zones by considering implantation

Figure 8 44Ti/48Ti ratios inferred from 44Ca excesses in SiC grains of type X and graphite grains are plotted againstSi isotopic ratios. Except for one graphite, all grains with evidence for 44Ti have 28Si excesses (sources Amari et al.,

1992, unpublished; Hoppe et al., 1994, 1996b, 2000; Nittler et al., 1996; Besmehn and Hoppe, 2003).

Figure 9 Schematic structure of a massive star beforeits explosion as a type II supernova (source Woosley andWeaver, 1995). Such a star consists of different layers,which are labeled according to their most abundantelements that experienced different stages of nucleo-synthesis. Indicated are dominant nuclear reactionsin some layers and the layers in which isotopes abundant

in grains of an inferred SN origin are produced.

Silicon Carbide 27

into newly condensed grains as they pass throughdifferent regions of the ejecta, specifically throughzones with reverse shocks.

Some SiC X grains also show large excesses in49Ti (Amari et al., 1992; Nittler et al., 1996;Hoppe and Besmehn, 2002). The correlation ofthese excesses with the V/Ti ratio (Hoppe andBesmehn, 2002) indicates that they come from thedecay of short-lived 49V (T1/2 ¼ 330 d) and thatthe grains must have formed within a few monthsof the explosion. 49V is produced in the Si/S zone,which contains almost pure 28Si. RIMS isotopicmeasurements of iron, strontium, zirconium,molybdenum, and barium have been made on Xgrains (Pellin et al., 1999, 2000a; Davis et al.,2002). The most complete and interesting are themolybdenum measurements, which reveal largeexcesses in 95Mo and 97Mo. Figure 10 shows themolybdenum isotopic patterns of a mainstreamand an X grain. The mainstream grain has a typicals-process pattern, in agreement with bulkmeasurements of other heavy elements such asxenon, barium, and neodymium (Figure 7). Themolybdenum pattern of the X grain is completelydifferent and indicates neutron capture at muchhigher neutron densities. While it does not agreewith the pattern expected for the r-process, it issuccessfully explained by a neutron-burst model(Meyer et al., 2000). In the type II SN models byRauscher et al. (2002) an intense neutron burst ispredicted to occur in the oxygen layer just below

the He/C zone, accounting for the molybdenumisotopic patterns observed in X grains.

Type Ia supernovae offer an alternative expla-nation for the isotopic signature of X grains. In themodel by Clayton et al. (1997) nucleosynthesistakes place by explosive helium burning of ahelium cap on top of a white dwarf. This processproduces most of the isotopic signatures of the SNgrains. The isotopes 12C, 15N, 26Al, 28Si, and 44Tiare all made by helium burning during theexplosion, which makes the transport of 28Si and44Ti through the massive oxygen-rich zone intothe overlying carbon-rich zones of a type II SNunnecessary. Mixing is limited to material fromhelium burning and to matter that experiencedCNO processing. The best match with the X graindata, however, is achieved for mixing scenariosthat yield O . C (Amari et al., 1998). Otherproblems include the questions whether highenough gas densities can be achieved in the ejectafor the condensation of micrometer-sized grainsand whether type Ia supernovae can generate aneutron burst necessary for the molybdenumisotopic pattern. More work is needed to decidewhether a type Ia SN origin for X grains is arealistic alternative.

1.02.6.5 Nova Grains

A few grains have isotopic ratios that are bestexplained by a nova origin (Amari et al., 2001a).These grains have low 12C/13C and 14N/15N ratios(Figure 3), large 30Si excesses (Figure 4), and high26Al/27Al ratios (Figure 5). All these features arepredicted to be produced by explosive hydrogenburning taking place in classical novae (e.g.,Kovetz and Prialnik, 1997; Starrfield et al., 1998;Jose et al., 1999) but the predicted anomalies aremuch larger than those found in the grains, and thenova ejecta have to be mixed with material ofclose-to-solar isotopic compositions. A compari-son of the data with the models implicates ONenovae with a white dwarf mass of at least 1.25 M(

as the most likely sources (Amari et al., 2001a).

1.02.6.6 Grain Size Effect

Grain size distributions of SiC have beendetermined for several meteorites and while grainsizes vary from 0.1 mm to 20 mm, the distributionsare different for different meteorites. Murchisonappears to have, on average, the largest grains(Amari et al., 1994), while SiC from Indarch(Russell et al., 1997) and Orgueil (Huss et al.,1997) is much finer grained. Various isotopicand other properties vary with grain size. Boththe 22Ne-E(H)/130Xe-S and the 86Kr/82Kr ratiosincrease with grain size (Lewis et al., 1994)

Figure 10 Molybdenum isotopic patterns measuredby RIMS in a type X and a mainstream SiC grain

(source Pellin et al., 1999).

Presolar Grains28

and the first ratio has been used as a measure for theaverage grain size in meteorites for which nodetailed size distributions have been determined(Russell et al., 1997). The 86Kr/82Kr ratio is afunction of neutron exposure and the data indicatethat exposure decreases with increasing grains size.The 88Sr/86Sr and 138Ba/136Ba ratios also dependon grain size but the dependence of neutronexposure on grain size inferred from these isotopicratios is just the opposite of that inferred from the86Kr/82Kr ratio. This puzzle has not been resolved.A possible explanation is a different trappingmechanism for noble gases and refractoryelements, respectively (Zinner et al., 1991), ordifferent populations of carrier grains if, as for neon(Nichols et al., 2003), only a small fraction of thegrains carry krypton. Excesses in 21Ne relative tothe predicted helium-shell composition, inter-preted as being due to spallation by galactic cosmicrays, increase with grain size (Tang and Anders,1988a; Lewis et al., 1990, 1994). However, thecorrelation of the 21Ne/22Ne ratio with the s-process 86Kr/82Kr ratio (Hoppe and Ott, 1997) andthe recent determination of spallation recoil ranges(Ott and Begemann, 2000) cast doubt on achronological interpretation. Other grain sizeeffects are, on average, larger 14N/15N ratios forsmaller grains (Hoppe et al., 1996a) and anincreasing abundance of Z grains among smallerSiC grains (Hoppe et al., 1996a, 1997). There arealso differences in the distribution of different graintypes in SiC from different meteorites: whereas theabundance of X grains in SiC from Murchison andother carbonaceous chondrites is ,1%, it is only,0.1% in SiC from the enstatite chondrites Indarchand Qingzhen (Besmehn and Hoppe, 2001; Linet al., 2002).

1.02.7 SILICON NITRIDE

Presolar silicon nitride (Si3N4) grains areextremely rare (in Murchison SiC-rich separates,5% of SiC of type X), but automatic ionimaging has been successfully used to detectthose with large 28Si excesses (Nittler et al., 1995;Besmehn and Hoppe, 2001; Lin et al., 2002;Nittler and Alexander, 2003). The carbon, nitro-gen, aluminum, and silicon isotopic signatures ofthese grains are the same as those of SiC grains oftype X, i.e., large 15N and 28Si excesses and high26Al/27Al ratios (Figure 12). Although so far noresolvable 44Ca excesses have been detected(Besmehn and Hoppe, 2001), the similarity withX grains implies an SN origin for these grains.While Si3N4 grains in SiC-rich residues fromMurchison are extremely rare and, if present, are oftype X, enstatite chondrites contain much higherabundances of Si3N4 (Alexander et al., 1994;Besmehn and Hoppe, 2001; Amari et al., 2002).

Most of them have normal isotopic compositions.Recent measurements of small (0.25–0.65 mm)grains from Indarch revealed several Si3N4

grains with carbon and nitrogen isotopic ratiossimilar to those of mainstream SiC grains, butcontamination from attached SiC grains cannot beexcluded (Amari et al., 2002).

1.02.8 GRAPHITE

Graphite, the third type of carbonaceouspresolar grains, was isolated because it is thecarrier of Ne-E(L) (Amari et al., 1990; Amariet al., 1995b). Subsequent isotopic measurementsof individual grains revealed anomalies in manydifferent elements.

1.02.8.1 Physical Properties

Only grains $1 mm in diameter carry Ne-E(L)and only round grains, which range up to 20 mm insize, appear to be of presolar origin (Amari et al.,1990; Zinner et al., 1995). Presolar graphite has arange in density (1.6 –2.2 g cm23) and fourdifferent density fractions have been isolated(Amari et al., 1994). Average grain sizes decreasewith increasing density, and density fractionsdiffer in the distribution of their carbon and noblegas isotopic compositions (Amari et al., 1995b;Hoppe et al., 1995). SEM studies revealed twobasic morphologies (Hoppe et al., 1995): denseaggregates of small scales (“cauliflowers,”Figure 2(b)) and grains with smooth or shell-likeplaty surfaces (“onions,” Figure 2(c)). TEManalysis of microtomed sections of graphitespherules (Bernatowicz et al., 1991, 1996) foundthe surface morphology reflected in the internalstructure of the grains. Cauliflowers consist ofconcentrically packed scales of poorly crystallizedcarbon, whereas onions either consist of well-crystallized graphite throughout or of a core oftightly packed graphene sheets of only severalatomic layers surrounded by a mantle of well-crystallized graphite. Most graphite spherulescontain small (20–500 nm) internal grains ofmostly titanium carbide (TiC) (Bernatowiczet al., 1991); however, also zirconium- andmolybdenum-rich carbides have been found(Bernatowicz et al., 1996). Recent studies ofgraphite spherules whose oxygen and siliconisotopic compositions indicated an SN origin(see below) did not detect Zr–Mo-rich carbidesbut revealed internal kamacite, cohenite, and irongrains in addition to TiC (Bernatowicz et al.,1999; Croat et al., 2003). Both cauliflowers andonions contain internal grains, which must havecondensed before the graphite, and were appar-ently captured and included by the growing

Graphite 29

spherules. Some onions show TiC grains at theircenter that apparently acted as condensationnuclei for the graphite (Figure 11). Sizes ofinternal grains and graphite spherules and theirrelationship and chemical compositions provideinformation about physical properties such aspressure, temperature, and C/O ratio in the gasfrom which the grains condensed (Bernatowiczet al., 1996; Croat et al., 2003).

1.02.8.2 Isotopic Compositions

Noble gas measurements were made on bulksamples of four density fractions (Amari et al.,1995b). In contrast to SiC, a substantial fraction ofNe-E in graphite seems to come from the decayof short-lived (T1/2 ¼ 2.6 yr) 22Na (Clayton,1975). This is supported by the low 4He/22Neratios measured in individual grains (Nichols et al.,2003). Krypton in graphite has two s-processcomponents with apparent different neutronexposures, residing in different density fractions(Amari et al., 1995b).

Ion microprobe analyses of single grainsrevealed the same range of 12C/13C ratios as inSiC grains but the distribution is quite different(Figure 6). Most anomalous grains have 12Cexcesses, similar to SiC X grains. A substantial

fraction has low 12C/13C ratios like SiC A þ Bgrains. Most graphite grains have close-to-solarnitrogen isotopic ratios (Hoppe et al., 1995;Zinner et al., 1995). In view of the enormousrange in carbon isotopic ratios these normalnitrogen ratios cannot be intrinsic and mostlikely are the result of isotopic equilibration,either on the meteorite parent body or in thelaboratory. Apparently, elements such as nitrogenare much more mobile in graphite than in SiC.An exception are graphite grains of low density(#2.05 g cm23), which have anomalous nitrogen(Figure 12). Low-density (LD) graphite grainshave in general higher trace-element concen-trations than those with higher densities and forthis reason have been studied for their isotopiccompositions in detail (Travaglio et al., 1999).Those with nitrogen anomalies have 15N excesses(Figure 12). Many LD grains have large 18Oexcesses (Amari et al., 1995c) and high 26Al/27Alratios that almost reach those of SiC X grains(Figure 12) and are much higher than those ofmainstream SiC grains (Figure 5). 18O excessesare correlated with 12C/13C ratios. Many grains forwhich silicon isotopic ratios could be determinedwith sufficient precision show 28Si excesses,although large 29Si and 30Si excesses are alsoseen. The similarities of the isotopic signatureswith those of SiC X point to an SN origin of LDgraphite grains. The 18O excesses are compatiblewith such an origin. Helium burning produces 18Ofrom 14N, which dominates the CNO isotopesin material that had undergone hydrogen burningvia the CNO cycle. As a consequence, the H/C zonein pre-SNII massive stars (see Figure 9), whichexperienced partial helium burning, has a high 18Oabundance (Woosley and Weaver, 1995). Wolf-Rayet stars during the WN–WC transitions arepredicted to also show 12C, 15N, and 18O excessesand high 26Al/27Al ratios (Arnould et al., 1997) butalso large excesses in 29Si and 30Si and aretherefore excluded for LD graphite grains with28Si excesses.

There are additional features that indicate anSN origin of LD graphite grains. A few grainsshow evidence for 44Ti (Nittler et al., 1996),others have large excesses of 41K, whichmust be due to the decay of the radioisotope41Ca (T1/2 ¼ 1.05 £ 105 yr) (Amari et al., 1996).Inferred 41Ca/40Ca ratios are much higher(0.001–0.01) than those predicted for the envel-opes of AGB stars (Wasserburg et al., 1994) but arein the range expected for the carbon- and oxygen-rich zones of type II supernovae, where neutroncapture leads to the production of 41Ca (Woosleyand Weaver, 1995). Measurements of calciumisotopic ratios in grains without evidence for 44Tishow excesses in 42Ca, 43Ca, and 44Ca, with 43Cahaving the largest excess (Amari et al., 1996;Travaglio et al., 1999). This pattern is best

Figure 11 Transmission electron micrograph of aslice through a presolar graphite grain (onion). Thegrain in the center is TiC and apparently acted ascondensation nucleus for the growth of the graphite

spherule. Photo courtesy of Thomas Bernatowicz.

Presolar Grains30

explained by neutron capture in the He/C and O/Czones (Figure 9) of type II supernovae. In caseswhere titanium isotopic ratios have been measured(Amari et al., 1996; Nittler et al., 1996; Travaglioet al., 1999), they show large excesses in 49Ti andsmaller ones in 50Ti. This pattern also indicatesneutron capture and is well matched by predictionsfor the He/C zone (Amari et al., 1996). However,large 49Ti excesses in grains with relativelylow (10–100) 12C/13C ratios can only be explainedif contributions from the decay of 49V areconsidered (Travaglio et al., 1999). Nicolussiet al. (1998c) have reported RIMS measurementsof zirconium and molybdenum isotopic ratios

in graphite grains from the highest densityfraction (2.15–2.20 g cm23), but no other isotopicratios had been measured. Several grains shows-process patterns for zirconium and molybdenum,similar to those exhibited by mainstream SiCgrains, although two grains with a distincts-process pattern for zirconium have normalmolybdenum. Two grains have extreme 96Zrexcesses, indicating an SN origin, but the molyb-denum isotopes in one are almost normal. Molyb-denum, like nitrogen, might have suffered isotopicequilibration in graphite. High-density graphitegrains apparently come from AGB stars aspreviously indicated by the krypton data (Amariet al., 1995b) and from supernovae. It remains to beseen whether also LD grains have multiple stellarsources.

In order to obtain better constraints on theor-etical models of SN nucleosynthesis, Travaglioet al. (1999) tried to match the isotopic compo-sitions of LD graphite grains by performingmixing calculations of different type II SN layers(Woosley and Weaver, 1995). While the resultsreproduce the principal isotopic signatures of thegrains, there remain several problems. The modelsdo not produce enough 15N and yield too low29Si/30Si ratios. The models also cannot explainthe magnitude of 26Al/27Al, especially if SiC Xgrains are also considered, and give the wrongsign in the correlation of this ratio with the14N/15N ratio. Furthermore, large neutron-captureeffects observed in calcium and titanium can beonly achieved in a mix with O . C. Clayton et al.(1999) proposed a kinetic condensation model thatallows formation of graphite in the high-radiationenvironment of SN ejecta even when O . C,which relaxes the chemical constraint on mixing.However, it remains to be seen whether SiC andSi3N4 can also form under oxidizing conditions.Additional information about the formationenvironment of presolar graphite is, in principle,provided by the presence of indigenous polycyclicaromatic hydrocarbons (PAHs) (Messenger et al.,1998). PAHs with anomalous carbon ratios showdifferent mass envelopes, which indicate differentformation conditions.

A few graphite grains appear to come fromnovae. Laser extraction GMS of single grains showthat, like SiC grains, only a small fraction containsevidence for Ne-E. Two of these grains have20Ne/22Ne ratios that are lower than ratios pre-dicted to result from helium burning in any knownstellar sources, implying decay of 22Na (Nicholset al., 2003). Furthermore, their 22Ne is notaccompanied by 4He, expected if neon wasimplanted. The 12C/13C of these two grains are 4and 10, in the range of SiC grains with a putativenova origin. Another graphite grain with12C/13C ¼ 8.5 has a large 30Si excess of 760‰(Amari et al., 2001a).

Figure 12 Nitrogen, oxygen, carbon, and aluminumisotopic ratios measured in individual low-densitygraphite grains. Also shown are data for presolar Si3N4

and SiC grains of type X (source Zinner, 1998a).

Graphite 31

In summary, although a few graphite grainshave the isotopic signature expected for conden-sates from AGB stars (isotopically heavy carbonand light nitrogen, enhanced abundances ofs-process elements) and fewer still those of novagrains, the majority seems to have an SN origin(see also Figure 6). This remains an unsolvedpuzzle because stars that produce SiC are alsoexpected to form graphite and the apparentunderabundance of graphite from AGB starspoints to deficiencies in the current understandingof the condensation of carbonaceous phases incarbon-rich stellar atmospheres and the survivalof different grain types in the ISM.

1.02.9 OXIDE GRAINS

In contrast to the carbonaceous presolar phases,presolar oxide grains apparently do not carry any“exotic” noble gas component. They have beenidentified by ion microprobe oxygen isotopicmeasurements of single grains from acid residuesfree of silicates. In contrast to SiC, essentially allof which is of presolar origin, most oxide grainsfound in meteorites formed in the solar system andonly a small fraction is presolar. The oxygenisotopic compositions of the presolar oxideminerals identified so far are plotted in Figure 13.They include 198 corundum or likely corundumgrains (Huss et al., 1994; Hutcheon et al., 1994;Nittler et al., 1994, 1997, 1998; Nittler andAlexander, 1999; Strebel et al., 1996; Choi et al.,1998, 1999; Krestina et al., 2002), 41 spinel grains(Nittler et al., 1997; Choi et al., 1998; Zinner et al.,2003), and 5 hibonite grains (Choi et al., 1999;Krestina et al., 2002). These numbers, however,

cannot be used to infer relative abundances ofthese mineral phases. Most presolar corundum andhibonite grains are $1 mm. In contrast, mostpresolar spinels were found by NanoSIMSanalysis of grains down to 0.15 mm in diameter.At this size, the abundance of presolar spinelamong all spinel grains is ,3%, whereas it is#0.5% among .1 mm spinel grains. Further-more, searches for presolar oxide grains have beenmade in different types of residues, some contain-ing spinel, others not. Another complication is thatmore than half of all presolar corundum grainshave been found by automatic direct 18O/16Oimaging searches in the ion microprobe (Nittleret al., 1997), a method that does not detect grainswith anomalies in the 17O/16O ratio but with close-to-normal 18O/16O. The oxygen isotopic distri-bution of corundum in Figure 13, therefore, doesnot reflect the true distribution. NanoSIMSoxygen isotopic raster imaging of tightly packedsubmicron grains from the Murray CM2 chondriteled to the identification of an additional 252presolar spinel and 32 presolar corundum grains(Nguyen et al., 2003).

Nittler et al. (1997) have classified presolar oxidegrains into four different groups according to theiroxygenisotopicratios.Grainswith17O/16O . solar(3.82 £ 1024) and 0.001 , 18O/16O , solar(2.01 £ 1023), comprising group 1, have oxygenisotopic ratios similar to those observed in RG andAGB stars (Harris and Lambert, 1984; Harris et al.,1987; Smith and Lambert, 1990), indicating suchan origin also for the grains. These compositionscan be explained by hydrogen burning in the coreof low-to-intermediate-mass stars followed bymixing of core material into the envelope duringthe first dredge-up (also second dredge-up in low-metallicity stars with M . 3M() (Boothroyd et al.,1994; Boothroyd and Sackmann, 1999). Variationsin 17O/16O ratios mainly correspond to differencesin stellar mass, while those in 18O/16O can beexplained by assuming that stars with differentmetallicities contributed oxide grains to the solarsystem. According to galactic chemical evolutionmodels, 17O/16O and 18O/16O ratios are expected toincrease as a function of stellar metallicity(Timmes et al., 1995). Grains with depletions inboth 17O and 18O (group 3) could thus comefrom low-mass stars (producing only small 17Oenrichments) with lower than solar metallicity(originally having lower than solar 17O/16O and18O/16O ratios). The oxygen isotopic ratios ofgroup 3 grains have been used to obtain an estimateof the age of the galaxy (Nittler and Cowsik, 1997).Group 2 grains have 17O excesses and large 18Odepletions (18O/16O , 0.001). Such depletionscannot be produced by the first and seconddredge-up but have been successfully explainedby extra mixing (cool bottom processing) of low-mass (M , 1.65M() stars during the AGB phase

Figure 13 Oxygen isotopic ratios in individual oxidegrains (sources Nittler et al., 1997, 1998; Chai et al.,1998, 1999; Krestina et al., 2002, unpublished data;

Zinner et al., 2003).

Presolar Grains32

that circulates material from the envelope throughregions close to the hydrogen-burning shell(Wasserburg et al., 1995; Denissenkov andWeiss, 1996; Nollett et al., 2003). Group 4 grainshave both 17O and 18O excesses. If they originatedfrom AGB stars they could either come from low-mass stars, in which 18O produced by heliumburning of 14N during early pulses was mixedinto the envelope by third dredge-up (Boothroydand Sackmann, 1988) or from stars with highmetallicity. More likely for the grains with thelargest 18O excesses is an SN origin as suggested byChoi et al. (1998) if 18O-rich material from theHe/C zone can be admixed to material fromoxygen-rich zones.

The only grain that has the typical isotopicsignature expected for SN condensates, namely, alarge 16O excess, is grain T84 (Figure 13) (Nittleret al., 1998). All oxygen-rich zones (O/C, O/Ne,O/Si—see Figure 9) are dominated by 16O(Woosley and Weaver, 1995; Thielemann et al.,1996; Rauscher et al., 2002). The paucity of suchgrains, whose abundance is expected to dominatethat of carbonaceous phases with an SN origin,remains an unsolved mystery. It has beensuggested that oxide grains from supernovae aresmaller than those from red giant stars, but recentmeasurements of submicron grains have notuncovered any additional oxides with large 16Oexcesses (Zinner et al., 2003; Nguyen et al.,2003). Another grain that does not fit into the fourgroups is T54 (Nittler et al., 1997). This graincould come from a star with .5M( thatexperienced hot bottom burning, a conditionduring which the convective envelope extendsinto the hydrogen-burning shell (Boothroyd et al.,1995; Lattanzio et al., 1997).

Some but not all grains in the four groups showevidence for initial 26Al (Nittler et al., 1997; Choiet al., 1998, 1999; Krestina et al., 2002). Because26Al is produced in the hydrogen-burning shell(Forestini et al., 1991), dredge-up of materialduring the TP AGB phase is required, and grainswithout 26Al must have formed before their parentstars reached this evolutionary stage. Initial26Al/27Al ratios are highest in group 2 grains(Nittler et al., 1997; Choi et al., 1998), which isexplained if these grains formed in the later stagesof the AGB phase when more 18O had beendestroyed by cool bottom processing and more26Al dredged up (Choi et al., 1998). Titaniumisotopic ratios have been determined in a fewpresolar corundum grains (Choi et al., 1998). Theobserved 50Ti excesses agree with those predictedto result from neutron capture in AGB stars. Thedepletions in all titanium isotopes relative to 48Tifound in one grain indicate that, just as for SiCgrains, galactic evolution affects the isotopiccompositions of the parent stars of oxide grains.One hibonite grain was found to have a large 41K

excess, corresponding to a 41Ca/40Ca ratio of1.5 £ 1024, within the range of values predictedfor the envelope of AGB stars (Wasserburg et al.,1994).

To date, all attempts to identify presolarsilicates in primitive meteorites have been unsuc-cessful (Nittler et al., 1997; Messenger andBernatowicz, 2000). However, Messenger et al.(2003) have discovered presolar grains, includingsilicates, in IDPs. These grains are 0.2–1 mm insize and the discovery was made possible by thehigh spatial resolution of the NanoSIMS. Itremains to be seen whether meteorites alsocontain presolar silicates in this size range(previous searches have been made mostly on$1 mm grains) or whether such grains werepreserved only in IDPs.

1.02.10 DIAMOND

Although diamond is the most abundantpresolar grain species (,500 ppm) and was thefirst to be isolated (Lewis et al., 1987), it remainsthe least understood. The only presolar isotopicsignatures (indicating an SN origin) are those ofXe-HL and tellurium (Richter et al., 1998), to amarginal extent also those of strontium andbarium (Lewis et al., 1991). However, the carbonisotopic composition of bulk diamonds is essen-tially the same as that of the solar system (Russellet al., 1991, 1996) and diamonds are too small(the average size is ,2.6 nm—hence nanodia-monds) to be analyzed as single grains. Atpresent, it is not known whether or not thisnormal carbon isotopic composition is the resultof averaging over grains that have large carbonisotopic anomalies, and whether all nanodia-monds are of presolar origin. Nitrogen shows a15N depletion of 343‰, but isotopically lightnitrogen is produced by the CN cycle in all starsand is therefore not very diagnostic. More recentmeasurements have shown that the nitrogenisotopic ratio of Jupiter (Owen et al., 2001) isvery similar to that of the nanodiamonds, whichtherefore is not necessarily a presolar signature.Furthermore, the concentration of Xe-HL is suchthat only one diamond grain in a million containsa xenon atom. As of early 2000s, all attempts toseparate different, isotopically distinct, com-ponents among nanodiamonds have met withonly limited success. Stepped pyrolysis indicatesthat nitrogen and the noble gas components Xe-HL and Ar-HL are decoupled, with nitrogenbeing released at lower temperature (Verchovskyet al., 1993a,b), and it is likely that nitrogen andthe exotic gases are located in different carriers.A solar origin of a large fraction of thenanodiamonds remains a distinct possibility(Dai et al., 2002).

Diamond 33

The light and heavy isotope enrichment in Xe-HL has been interpreted as being due to thep- and r-process, and thus requires an SN origin(Heymann and Dziczkaniec, 1979, 1980; Clayton,1989). In one model, Xe-H is made by a shortneutron burst, with neutron densities intermediatebetween those characteristic for the r- ands-processes (Clayton, 1989; Howard et al.,1992). Ott (1996) kept the standard r-process butproposed that xenon is separated from iodine andtellurium precursors on a timescale of a few hoursafter their production. Measurements of telluriumisotopes in nanodiamonds show almost completeabsence of the isotopes 120Te, 122 – 126Te, and aslight excess of 128Te relative to 130Te (Richteret al., 1998). This pattern agrees much better witha standard r-process and early element separationthan with the neutron burst model. Clayton andco-workers (Clayton, 1989; Clayton et al., 1995)have also tried to attribute the diamonds and theircarbon and nitrogen isotopic compositions to atype II supernova. This requires mixing ofcontributions from different SN zones. In contrast,Jørgensen (1988) proposed that diamond and Xe-HL were produced by different members of abinary system of low-mass (1 – 2M() stars,diamond in the winds of one member, a carbonstar, while Xe-HL by the other, which exploded asa type Ia supernova. However, at present we donot have an unambiguous identification of theorigin of the Xe-HL and tellurium, and of thediamonds (in case they have a different origin).

1.02.11 CONCLUSION AND FUTUREPROSPECTS

The study of presolar grains has provided awealth of information on galactic evolution,stellar nucleosynthesis, physical properties ofstellar atmospheres, and conditions in the solarnebula and on meteoritic parent bodies. However,there are still many features that are not wellunderstood with existing models of nucleosynth-esis and stellar evolution and stellar structure.Examples are the carbon and nitrogen isotopiccompositions of SiC A þ B grains, 15N and 29Siexcesses in SN grains, and the paucity of oxidegrains from supernovae. The grain data, especiallycorrelated isotopic ratios of many elements, thusprovide a challenge to nuclear astrophysicist intightening constraints on theoretical models.

Continuing instrumental developments allow usto make new and more measurements on thegrains and likely lead to new discoveries. Forexample, the NanoSIMS features high spatialresolution and sensitivity, making isotopic anal-ysis of small grains possible, and this capabilityhas already resulted in the discovery of presolarsilicate grains in IDPs (Messenger et al., 2003)

and the identification of a large number of presolarspinel grains (Zinner et al., 2003; Nguyen et al.,2003). The NanoSIMS will also make it possibleto analyze internal grains that have been studied indetail in the TEM (Stadermann et al., 2003).Another example is the application of RIMS tograin studies. As the number of elements that canbe analyzed is being expanded (e.g., to the rare-earth elements), unexpected discoveries such asthe molybdenum isotopic patterns in SiC X grains(Pellin et al., 1999) will probably result. RIMSmeasurements can also be made on grains, such asgraphites, for which the isotopic ratios of manyelements are measured with the ion microprobe.

It is clear that the discovery of presolar grainsand their detailed study have opened a new andfruitful field of astrophysical research.

ACKNOWLEDGMENTS

I thank Sachiko Amari, Peter Hoppe, NatashaKrestina, Larry Nittler, and Roger Strebel forproviding unpublished data, and Sachiko Amari,Tom Bernatowicz, and Scott Messenger forproviding micrographs. I am grateful for helpfrom and discussions with Sachiko Amari, PeterHoppe, Gary Huss, and Larry Nittler. Andy Davisprovided useful comments on the manuscript.

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