Presolar dust grains from meteorites and their stellar sources

Transcription

1 , JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 105, NO. A5, PAGES 10,37110,385, MAY 1, 2000 Presolar dust grains from meteorites and their stellar sources Peter Hoppe MaxPlanckInstitute for Chemistry, Cosmochemistry Department, Mainz, Germany Ernst Zinner McDonnell Center for the Space Sciences and Physics Department, Washington University, St. Louis, Missouri Abstract. Primitive meteorites contain small concentrations (ppb to ppm) of presolar dust grains that have survived largely unaltered the processes that led to the formation of the solar system. Minerals identified to date include diamond, silicon carbide (SIC), graphite, silicon nitride (Si3N4), corundum (A1203), spinel (MgA1204), and possibly titanium oxide (TiO2). These grains exhibit large isotopic anomalies indicative of a stellar origin. Variations in the isotopic ratios of the major elements and of many trace elements contained in the grains range over more than 4 orders of magnitude. The presolar dust grains preserve memories of both nucleosynthesis in the parent stars and galactic chemical evolution. Most of the silicon carbide and corundum grains formed in the winds of red giant and asymptotic giant branch stars. Most graphite grains, some SiC and corundum grains, and all silicon nitride grains originated most likely in supemova ejecta. A few SiC and graphite grains appear to have a nova origin. The origin of the diamonds is still unknown but at least a small fraction apparently comes from supernovae. Diamonds are only 2 nm in size. The other types of presolar grains are larger and range from 0.2 to 20 gm. These sizes are larger than those inferred for dust in the interstellar medium but are comparable to the sizes of interstellar dust in the heliosphere identified by the Gallileo and Ulysses spacecraft missions. 1. Introduction incorporated into certain types of meteorites. Because they must have experienced a long history in the interstellar Until recently, all the information about interstellar (IS) medium (ISM) as IS dust, these grains provide information on dust has been obtained by astronomical observation [e,g., aspects of IS dust that cannot be obtained in other ways. Mathis, 1990]. Measurements of extinction, scattering, and However, the most important information extracted from the polarization at different wavelengths, ranging from the UV to study of presolar dust grains is on their stellar sources. In the far IR, are used to obtain information on the size particular, the isotopic compositions of the grains yield distribution and, to a more limited extent, the composition of information on nucleosynthesis and stellar mixing, and their IS dust. Comparison with observations of circumstellar dust mineralogy and trace element concentrations can be used to shells, undoubtedly major sources of refractory IS dust, makes infer the physical and chemical properties of the stellar it possible to tie IS dust to its stellar sources [Gehrz, 1989]. atmospheres where they formed. While previously it has been believed that IS dust grains In this paper we give a survey of the types of presolar could not enter the solar system, particles impacting the grains, their physical, chemical, and isotopic compositions, Ulysses and Galileo spacecraft have been identified as having and the implications of these properties for the grains' stellar an extrasolar origin based on their direction and velocity sources. We also discuss the relationship of presolar and IS [Griin et al., 1993, 1996]. The characteristic size of these dust. For more detailed information the reader is referred to particles is several tenths to more than 1 gm [Frisch et al., some recent reviews [Anders and Zinner, 1993; Ott, 1993; 1999]; no compositional information has been obtained to Zinner, 1998] and to the compilation of papers found in date. Even larger sizes are implied for highvelocity particles Astrophysical Implications of the Laboratory Studies of of inferred extrasolar origin giving rise to radar meteors Presolar Material [Bernatowicz and Zinner, 1997]. [Taylor et al., 1996]. Within the last decade a new source of information about 1.1. Historical Background IS dust has been identified in the form of presolar dust grains from primitive meteorites. These grains, whose extrasolar Presolar grains in meteorites are recognized by their origin is indicated by their isotopic compositions, which are isotopically anomalous composition. Although it has been completely different than those of solar system material, realized that different stars produce the various elements with apparently formed in stellar outflows and supernova ejecta, very different isotopic ratios and that the solar system survived the formation of the solar system and were constitutes a mixture of material from many stellar sources [Burbidge et al., 1957], in the 1960s it was still believed that this material had been thoroughly homogenized in a hot solar Copyright 2000 by the American Geophysical Union. nebula [Cameron, 1962], resulting in uniform isotopic ratios. The first hint that presolar isotopic signatures had indeed survived solar system formation came from H [Boato, 1954] Paper number 1999JA /00/1999JA ,371

2 10,372 HOPPE AND ZINNER: PRESOLAR DUST GRAINS and the noble gases Xe [Reynolds and Turner, 1964] and Ne [Black and Pepin, 1969]. However, it was only after the discovery of anomalies in O, a rockforming element [Clayton et al., 1973], that the idea of relict presolar material in meteorites found wide acceptance. After the finding of 160 excesses, discoveries of isotopic anomalies in elements such as C, N, Mg, Ca, Ti, etc. proliferated [see, e.g., Lee, 1988; Wasserburg, 1987], but it was the search for the carriers of the "exotic" (i.e., is topically anomalous) noble gas components XeHL [Reynolds and Turner, 1964], NeE [Black and Pepin, 1969] and XeS [Srinivasan and Anders, 1978] that led to the isolation of presolar grains by Ed Anders and his colleagues at the University of Chicago 20 years later [see Anders and Zinner, 1993]. 3 Solar pm...i!i!i:i:i:i:i:l......::::::::::::::::: Graphite... :.:..., 120 pm NeE(L)!ii SN, AGB stars, Novae SiC type X!iii!i!11ilililili!' ''l i':i!i::?:!ililiiiiii! I Corundum and I::::::::::::::i:!:... spinel 0.53 lam iiiiiii!iii!i!ii!!i! ::::::::::::::::::::::::::::::::: RG... stars,, AGB stars 1 Si3N4 lam...'2...' SN Bulk abundances in primitive meteorites 1.2. Isotopic Anomalies in Meteorites Figure 2. Types of presolar grains discovered in primitive meteorites. Given are their abundances (mass fraction), sizes, Once isolated, these carrier grains were found to be inferred stellar sources, and the anomalous noble gas anomalous in all their isotopic ratios and it is this feature that components carried by some of them. SN, supernovae; RG, identifies them as presolar grains. Since the term "isotopic red giant. anomalies" in meteorites is used somewhat loosely, one has to be careful to distinguish presolar grains, which formed in the atmospheres of other stars, from solar system material that rich inclusions (CAIs),(2) solids that have isotopic effects carries isotopic anomalies but formed in the solar system. from the decay of shortlived isotopesuch as 41 Ca, ~ 96 AI, and In addition to presolar grains, specks of stardust, we can 53Mn that were still alive when the earliest solids formed, and distinguish three other types of material with isotopic (3) material with anomalies in elements such as H and N as anomalies in primitive meteorites: (1) Calciumaluminumwell as noble gases, some of which were apparently inherited from incompletely homogenized presolar material. Some of the isotopic signatures such as D and SN excesses have probably a chemical origin in molecular clouds [Messenger and Walker, 1997], the origin of others is not well understood ß Presolar Oxide [Alexander et al., 1998; Sugiura et al., 1998; Wieler et al., 10s I 1991]. 104 ] o Presolar Graphite CAIs have isotopic anomalies of nuclear origin in elements such as O, Mg, Ca, Ti, Cr, Fe, Ni, Sr, Ba, Nd, and Sm ß [Clayton et al., 1988; Lee, 1988]. These solids must have 10a formed in the solar nebul and incorporated incompletely / mixed but heavily processed presolar material. Important / differences between CAIs and true presolar grains includes 102, their size (CAIs are in the range of millimeters to centimeters, whereas presolar grains are of the order of micrometers) and //// the magnitude of the isotopic anomalies (Figure 1). While the 1 I... // ß... '... range of O isotopic ratios in CAIs is at most 10%, O isotopic 100,f l04 x 10s ratios in presolar grains range over more than four orders of, 160/18 O, magnitude. The situation is similar for 26A1/27AI ratios inferred from 26Mg excesses [MacPherson et al., 1995] and isotopic anomalies in heavy elements such as Ba, Nd, and Sm [Begemann, 1993]. Presolar grainshow isotopic ratios that are expected for stellar atmospheres. In contrast, the isotopic anomalies in solar syste material indicate some presolar signatures but are the result of extensive mixing and processing of presolar material in the solar system Types of Presolar Grains Figure 2 shows the types of presolar grains that have been identified to date. Also given in the figure are their sizes and most likely stellar sources. Diamond [Lewis et al., 1987], 160/18 O silicon carbide [Bernatowicz et al., 1987; Tang and Anders, Figure 1. Oxygen isotopic ratios measured in calcium 1988] and graphite [Amari et al., 1990] were discovered aluminumrich inclusions (CAIs) and in individual presolar because they are tagged with the noble gas components ains. ereas ratios in CAIs va by at most 10%, those in indicated Figure 2. They can be separated from the presolar aphite and oxide ains range over more than four meteorites in essentially pure form by chemical processing orders of ma itude. [Amari et al., 1994]. The other types of presolar grains, 103

3 : HOPPE AND ZIN,TER: PRESOLAR DUST GRAINS 10,373. 'i : :' :'...;...,. ::. ::... '..,... "'';;,, '..: :...:..:;.:.½ '*... i... "'i" :: :.i": :.' :,::.. :..>. '";:"::*,':i:':$:'*'..'. ii.. *::!:i,." ::';.!;:,:'... ::..""./".;':'*'*:?: ß ;'3?;?:'!i!: :. Figure 3 shows secondary electron micrographs of SiC (Figure 3a) and graphite grains ( Figures 3b and 3c). Most SiC grains have euhedral crystal features indicating that these grains did not experienc extensive processing during their IS history. Transmission electron microscopy studies reveal the presence of both tx and 13 polytypes [Daulton et al., 1998]. Presolar graphite grains are found in two main morphologies: "cauliflowers," aggregates of small scales (Figure 3b), and "onions," concentric layers of well crystallized graphite (Figure 3c). The surface morphologies of the carbon grains is also reflected in their interior structure [Bernatowicz et al., 1996]. Onions contain either cores of disordered graphene sheets or small refractory (Ti, Zr, Mo) carbide grains that acted as seeds for carbon condensation. The fact that these carbide grains must have condensed before graphite provides information on densities, temperatures, and C/O ratios of the stellar source gas [Bernatowicz eta/., 1996]. In this paper we concentrate on SiC, silicon nitride, graphite, and corundum grains. Diamond is too small for singlegrain analysis, and, although it is the most abundant grain species, comparatively little information exists on this grain type. The XeHL component [Huss and Lewis, 1994; Lewis et al., 1987] and the Te isotopic composition [Richter et a/., 1997] indicate a supernova source but the C isotopic ratio is essentially solar [Russell et al., 1996]. Since only one out of 106 diamond crystals contains a single Xe atom, it is not clear whether all diamonds are of presolar origin. For more details, see Anders and Zinner [ 1993]. Figure 3. Secondary electron micrographs of (a) presolar SiC and graphite of two different morphologies, (b) cauliflower and (c) onion. These grains are unusually large, typical diameters are several tenth pm for SiC and 12 pm for graphite. Scale bar of 1 pm is shown at the lower fight of the photographs. Pictures courtesy of Sachiko Amafi Techniques for Presolar Grains Analysis Isotopic analyses are the most diagnostic measurements on presolar grains for identification of their stellar sources. Bulk analyses are made on collections of large numbers of grains either by gas mass spectrometry (C, N, noble gases) or by thermal ionization mass spectrometry (Sr, Ba, Nd, Sm, Dy). They allow the measurement of trace elements, but only averages over many grains are obtained in this way. Presolar SiC, graphite, Si3N4, and oxide grains are large enough (Figure 2) to allow single grains analysis. Most analyses have been made by secondary ion mass spectrometry (SIMS) with the ion microprobe. This technique allows isotopic measurements of major elements in grains down to = 0.5 pm and of many minor elements in larger grains. Recently, single grains have been analyzed for their Sr, Zr, and Mo isotopic ratios by laser ablation and resonance ionization mass spectrometry (RIMS) [Nicolussi et al., 1997, 1998a, b]. We also want to mention isotopic measurements of He and Ne released from single SiC and graphite grains by laser heating [Nichols et al., 1992, 1994] Astrophysical Implications of Presolar Grain Analysis Figure 4 shows in cartoon form the history of presolar grains from their birth in stars to their study in terrestrial corundum (A1203), spinel (Mgh1204), silicon nitride (Si3N4), laboratories. There are many stages in this long history and, in and possibly titanium oxide (TiO2), were identified by single principle, the study of the grains should provide information grain isotopic measurements in the ion microprobe [Hutcheon about most of them. et al., 1994; Nittler and Alexander, 1999; Nittier et al., 1994, The isotopic composition of a given circumstellar grain 1995]. in addition to these types, small grains of Ti, Zr, and reflects that of the stellar atmosphere in which the grain Morich carbides, kamacite (FeNi), and cohenitc ((FeNi)C) condensed. The atmosphere's composition in turn is are found within SiC (only TiC) and graphite grains determined by several factors: (1) by the galactic history of [Bernatowicz eta/., 1991, 1996, 1999]. the material from which the star itself formed, (2) by the

4 . 10,374 HOPPE AND ZINNER: PRESOLAR DUST GRAINS Red Giant / x Molecular ø. ø o Supernova Cloud lowtemperature phases, implying that, while some regions reached high temperatures, others remained relatively cool. The final step in the complex history of stellar grains is the formation of planitesimals and of the parent bodies of the meteorites in which we find these presolar fossils. By far the largest fraction of the solids even in primitive meteorites formed in the solar system and the fraction of surviving presolar grains is small (see Figure 2). Primitive meteorites experienced varying degrees of metamorphism on their parent bodies and these metamorphic processes affected different types of presolar grains in different ways. The abundance of different grain types can thus give information about parent body processes [Huss, 1997]. 2. Isotopic and Physical Properties and Stellar Sources of Presolar Grains 2.1. Silicon Carbide?' Solar System Silicon carbide is the best studied presolar mineral phase in meteorites. The reason for this is its relatively high abundance (several ppm) in primitive meteorites and its comparatively large grain size (see below) that allows isotopic analysis of the major and of many trace elements in individual grains. In addition to the major elements C and Si, isotopic data are available for the diagnostic (in terms of nucleosynthesis and stellar origin) elements N, Mg, Ca, Ti, the noble gases, and ß ' ' { Presolar G ins heavy refractory elements (e.g., Sr, Zr, Mo, Ba, Nd, Sm). meteorite Large anomalies and variations in the isotopic Figure 4. Cartoon of history of presolar dust grains from compositions have been found for all these elements, their birth in expanding red giant atmospheres and supernova indicative of different nucleosynthetic processes and different ejecta to their study in terrestrial laboratories. Figure courtesy stellar sources. On the basis of isotopic compositions of C, N, of Larry Nittler. Si, and the abundance of radiogenic 26Mg six different populations of SiC grains can be discerned [Hoppe and Ott, 1997]: The mainstream grains, which make up the majority of nucleosynthesis in the star's inner layers, and (3) by mixing the SiC grains ( 90% of the total), and the minor types A, B, X, Y, and Z. episodes in which newly synthesized material is dredged from the interior into the star's envelope. In supernovae, mixing of 15 different layers with different nucleosynthetic history Carbon stars accompanies the explosion and the ejection of material. Grain formation occurs when temperatures in the 10 expanding envelope of red giants or in supernova ejecta are low enough for minerals to condense from the gas phase. Many latetype stars are observed to be surrounded by dust 5 shells of grains whose mineral compositions reflec the major chemistry of the gas [LittleMarenin, 1986]. ' 600 After their formation in stellar environments as circumstellar grains, or as supernova condensates, grains enter I Murchison a long journey through the interstellar medium as IS grains. They should be distinguished from true interstellar grains that 400! SiC form in the interstellar medium, e.g., in dense molecular clouds. However, grains of stellar origin are most likely to be covered by mantles of interstellar cloud material. During their 200 IS history, grains are subjected to a variety of destructive processes, such as evaporation in supernova shocks and 0 sputtering by shocks and stellar winds. They are also exposed to galactic cosmic rays that leave a record in form of cosmogenic nuclides. 12C/13 C Grains might go in and out of interstellar clouds before Figure 5. Distributions of 12C/13C ratios measured in the they are finally incorporated into the dense molecular cloud atmosphere of carbon stars [Lambert et al., 1986] and of,from which our solar system formed. The formation of the presolar SiC grains from the Murchison meteorite [Hoppe et solar system is a complicated process that is presently still not al., 1994, 1996b]. The dashed line represents the solar system fully understood. Primitive meteorites contain both high and 12C/13C ratio.

5 _ HOPPE AND ZINNER: PRESOLAR DUST GRAINS 10,375 On the basis of the signature of sprocess nucleosynthesis (slow neutron capture) found in acidresistant meteoritic residues, dust from carbon stars, latetype stars of low and intermediate mass (18 Ms) in the thermally pulsing asymptotic giant branch (AGB) phase of evolution [Iben and Renzini, 1983], has been considered already one decade prior to identification of SiC to be a potential minor constituent of primitive meteorites [Clayton, 1983; Clayton and Ward, 1978; Srinivasan and Anders, 1978]. As has been shown later, many trace elements present in meteoritic SiC grains indeed carry the signature of the sprocess, but this is not the only indicator of a carbon stars origin for the majority of the meteoritic SiC grains. In addition, (1) mainstream grains have 2C/ 3C ratiosimilar to those found in carbon stars (Figure 5), (2) carbon stars are considered to be the most prolific 100,',, u u injectors of carbonaceous dust grains into the ISM [Gehrz, /' io qbc øno o / : o o o 1989; Tielens, 1990], (3) they are observed to show the 11.3 tm emission feature typical of SiC [Treffers and Cohen, 1974], and (4) they have been successfully invoked to explain the abundances and isotopic patterns of the the grains' noble,5 osi/assi (%0} gases [Gallino et al., 1990; Lewis et al., 1990]. While a carbon star origin is strongly favored for the mainstream and Yig. re?. Siliconisotopic compositions of presolar SiC type Y and Z grains, the X grains must have formed in the ains given as the permil deviations from the solar system I 28! 28 i 28 Si/ Si and Si/ Si ratios. 15'Si/ = [('Si/ Si)/('Si/ Si)s1] ejecta of supernova explosions. x Mainstream grains plot along a line with slope 1.34, Mainstream grains. SiC mainstream grains have believed to reflect the galatichemical evolution (GCE) of the 12C/13C ratios between 10 and 100 and 14N/15N ratios between Si isotopes. Grains that fall off the slope 1.34 correlation line 50 and 20,000 (Figure 6) [Alexander, 1993; Hoppe et al., make up the minor types Y and Z. Theoretical predictions for 1994, 1996b; Huss et al., 1997; Virag et al., 1992; Zinner et the evolution of the Siisotopic compositions in the envelope al., 1989]. Most ( 70%) of these grain show the imprint of of lowmass AGB stars due to successive dredgeup of He the CNO cycle, i.e., 13C and InN excesses relative to solar shell matter are shown for comparison. Data for the mainstream and Y grains are from Amari et a/.[1997a] and Hoppe et al. [1994], those for the Z grains from Alexander [ 1993], Gao and Nittler [ 1997], ttoppet al. [ 1996b, 1997], 105: and Huss et al. [ 1997]. ß SiC Mainstr. CNO cycle SiC A&B + [3 SiC x. % 'o 104 i ß SiC Y _; A Si3N :. ß Extramixing os,cz Z 103 ' ':i C stars (e.g., CBP) ß. 102, : ' " ld i lq :Om". 10 '*' 0 He burning N'o. in massive stars 100 ' " "1 '"i'"l ' 10 ' s 600 ß Mainstream ß type Y 500 O typez 12C/13 c 2 Ms) that had experienced the first dredgeup. This problem led to proposals of extra mixing of CNOprocessed matter Figure 6. Nitrogen and Cisotopicompositions of presolar into the star's envelope subsequent to the first dredgeup [e.g., SiC a d Si3N4 ains. The dashed lines represent the solar system isotopic ratios. Theoretical expectations for carbon Charbonnel, 1994]. A parametric model for extra mixing, named cool bottom processing, was introduced by stars (white box), novae, the massive stars, and the effect of cool bottom processbg Wasserburg et al. [1995]. During cool bottom processing, ("CBP") in carbon stars on the / ratio are shown for deep circulation currents transport matter from the envelope comparison. Dma are from l n r []993], m ri t l. throughot regions near the H burning zone. This model can account for 4N/l q ratios of up to 6000 and cool bottom [] 7], Ht/ r t L [] 5, ] 6], a d Washin.on University processing has been proposed for resolving the discrepancy (unpublishedata,] 996). between the SiC grain data and predictions from the canonical 400 3OO 200 $,,," 100 ß ß _ o.,; ø Presolar SiC / slope 1.34 " line To AGB. He shell abundances, with 12C/13C ratios lying between 40 and 80 and 4N/ xt ratios between 500 and The nucleosynthetic processes important for carbon stars are H burning via the CNO cycle, He burning, and the sprocess. Current models of lowmasstars [Iben and Renzini, 1983] proceed from the assumption that the products of the nuclear burning are brought to the stellar envelope in three dredgeup episodes, leading to 2C/13C ratios between 30 and 200 [El Eld, 1994; Gallino et al., 1994; Iben, 1977; Sweigart et al., 1989] and 4N/ SN between 600 and 1600 [Becker and Iben, 1979; E1 Eld, 1994] during the carbonstar phase. These models satisfactorily explain the Cisotopi compositions of most mainstream grains but are unable to account for the high 4N/ xt ratios often observed. It is well known that the canonical evolutionary models cannot account for the low :C/13C ratiospectroscopically observed in lowmass stars (<

6 10,376 HOPPE AND ZINNER: PRESOLAR DUST GRAINS threedredgeup models [Gallino et al., 1994; Hoppe et al., b; Huss et al., 1997]. The Siisotopic compositions of most mainstream grains are characterized by enrichments in the heavy Si isotopes of up to 200%o relative to their solar abundances (Figure 7) [Alexander, 1993; Hoppe et al., 1994, 1996b; Huss et al., 1997]. In a Sithreeisotope plot the data fall along a line with slope 1.34, suggestive of simple twocomponent mixing. However, this view is incompatible with expectations from current AGB star models. In AGB stars Si is affected by the s process in the He shell, leading to enrichments of 29Si and 3øSi at the expense of 28Si [Brown and Clayton, 1992; Gallino et al., 1990, 1994]. With successive third dredgeups of matter from the He shell, the Siisotopic composition in the star's envelope is expected to evolve along a line with slope (in a 529Si/28Si versus 53øSi/28Si representation), variance 0 with the Si correlation line of the mainstream grains. It has been argued by Clayton [1988] that meteoritic presolar dust preserves a memory of the galactic chemical evolution (GCE). It is the preferred interpretation today that changes in size separates the surface Si isotopic ratios due to the third dredgeup are 8øKr/82Kr only of minor importance and that the slope 1.34 Si correlation line primarily reflects the GCE of the Si isotopes. to discriminate In this view the mainstream grains represent the starting compositions of a large number of AGB stars (several tens) [Alexander, 1993; Gallino et al., 1994; Timroes and Clayton, metallicity 1996]. According to the model of Timroes and Clayton [ 1996] buming ß sic Core Heburning in massive stars neutronpulse o AGB Heshell r=0.6 [] AGB Heshell r=0.8 solar system sprocess AGB: larger Fe/H ß oo larger grains i (86Kr/82Kr)s = f(n) Figure 8. Kryptonisotopic compositions of different grain of presolar SiC [Lewis et al., 1990, 1994]. The and 86Kr/82Kr ratios are sensitive monitors for neutron density and temperature and these ratios can be used between differentypes of stellar sources. The grain data show excellent agreement with predictions for the He shell of lowmass (13 Ms) AGB stars of variable (expressed as Fe/H) and overlap factor [Gallino et al., 1990] but are distinct from Kr expected from core He 29Si/2aSi and 3øSi/2aSi ratios are expected to increase in massive stars. Figure courtesy of Edward Anders. throughout galactic history. Because the parent stars of the SiC grains must be older than the solar system one expects to find lower than solar 29Si/28Si and 3øSi/2aSi ratios in the SiC grains, contrary to observation. Possible solutions to this data and theoretical predictions for the sprocess in lowmass puzzle include the suggestion (1) that the parent stars of AGB stars. It should be noted that considerable disagreements mainstream SiC grains were born in metalrich central regions between the Ba, Nd, and Sm data and earlier model of our galaxy (where the 29Si/28Si and 3øSi/28Si ratios are predictions were resolved by remeasurement of the relevant expected to be higher than solar) and moved to the location of nuclear reaction cross sections. the molecular cloud from which the solar system formed Type Y and Z grains. Grains that fall to the 3øSi [Clayton, 1997], and (2) that the solar system has an atypical rich side of the Si mainstream line make up the SiC grains of Siisotopic composition compared with that of the ISM at type Y (Figure 7) [Amari et al., 1997a; Hoppe et al., 1994] solar metallicity [Alexander and Nittler, 1999]. and Z [Alexander, 1993; Hoppe et al., 1996b, 1997]. Most Y Silicon carbide is the carrier of NeE(H), a Ne component grains have50%o < õ29si/28si <50 %o and all of them have consisting of almost pure 22Ne and of sprocess Kr and Xe 53øSi/28Si > 0 (Figure 7) and 12C/13C > 100 (Figure 6). The C [Lewis et al., 1990, 1994]. The isotopicompositions of the and Siisotopic signatures of the Y grains are compatible with noble gases (measured in SiC bulk samples) can be resolved an origin from lowmass AGB stars that experienced strong into two components, one with approximately solar He shell dredgeup. Most Z grains have lower than solar composition and one with anomalous isotopicomposition. In 29Si/28Si and 12C/13C ratios (Figures 6 and 7), and they terms of an AGB star origin of most SiC grains these two generally exhibit larger deviations from the Si mainstream components are interpreted to represent the noble gases from line than the Y grains. The most likely stellar sources of the the envelope and those from the He shell. The isotopic majority of Z grains are lowmass, lowmetallicity AGB stars compositions of the anomalousprocess components agree that experienced strong cool bottom processing during the red quite well with predictions for lowmass (13 Ms) AGB stars giant phase and strong dredgeup of Si from the He shell (see Figure 8) [Gallino et al., 1990]. The analysis of He and [Hoppe et al., 1997]. A small fraction of the Z grains that are Ne in individual grain showed that only 4% of them carry characterized by low C/ C and 14N/ SN ratios (see below) significant amounts of 4He and 22Ne and that 22Ne is always and very large 3øSi excesses might be from novae. accompanied by 4He [Nichols et al., 1992], providing support Type A and B grains. Type A and B grains plot on to the view that 22Ne is made by 14N + 2or in the He shell of the Si mainstream line but are distinguished from the AGB stars. mainstream grains by their Cisotopic signature. Type A The signature of the sprocess is also found in the isotopic grains have 12C/13 C < 3.5 (the equilibrium value of the CNO patterns of Ba, Nd, and Sm measured in SiC bulk samples cycle) and type B grains have 3.5 < 2Cfl3C < 10 (Figure 6). [Ott and Begemann, 1990; Prombo et al., 1993; Richter, Mostype B grains have 4N/15N ratios in the range of the 1995; Zinner et al., 1991b] and of Sr, Zr, and Mo measured mainstream grains (i.e., 4N/ SN ratios larger than solar) but in single SiC grains [Nicolussi et al., 1997, 1998a, b]. For all many type A grains have lower than solar '*N/'SN ratios (as these elements, there is good agreement between the grain low as 50) [Amari et al., 1997b; Hoppe et al., 1994, 1996a, b;

7 HOPPE AND ZINNER: PRESOLAR DUST GRAINS 10,377 Huss et al., 1997; Washington University, unpublishe data, 1996]. Novae are expected to produce grains with low 2C/ 3C and 4N/ SN ratios, the signature of explosive H burning (Figure 6). However, only two SiC grains (one of which is of type B, the other of type Z) have 4N/ SN < 10 as expected for nova grains. The majority of type A and B grains apparently did not form in the ejecta of nova explosions. Jtype carbon stars have 2C/ 3C ratios in the range of the type A and B grains [Lambert et al., 1986] and these stars have been suggested as possible stellar sources of type A and B grains [Hoppe et al., 1994, 1996b]. Nitrogenisotopic data are rare for these objects but the observations of Wannier et al. [ 1991 ] indicate a SN enrichment for the Jtype carbon star Y CVn, similar to the excesses observed for many type A grains. Nitrogen15 enrichments are expected from hot CNO burning, a process difficult to reconcile with Jtype carbon stars. As a solution of the problem of grains with low 2C/ 3C and 14N/15N ratios, Huss et al. [1997] proposed that the currently used 80(p, x) SN reaction rate is too low by a factor of This would result in low 12C/13C and 4N/ SN ratios if an appropriate level of cool bottom processing is considered Type X grains. X grains make up only 1% of all meteoritic SiC [Amari et al., 1992; Hoppe et al., 1996c; Nittier et al., 1996]. X grains typically show the opposite Figure 10. Schematic cross section through a type II preisotopic signatures of those observed in the mainstream grains supernova star. According to theoretical models such a star (Figures 6 and 9). Their C/ C ratios range from 18 to 7000 consist of layers that experience different stages of nuclear 12, 13 but most of them have higher than solar C/ C ratios (up to burning. The layers are labeled according to the most 80x solar). With one exception all X grains show higher than abundant elements. Figure adapted from Meyer et al. [1995]. solar SN/ nn ratios (up to 22x solar). From the nucleosynthetic point of view high 2C/ 3C and low 4N/15N ratios are the signature of He burning (Figure 6). Silicon generally shows depletion in the nrich isotopes 29Si and 3øSi 44Ca excesses, respectively. Mg26 and 44Care the daughter (or alternativly enrichment in 28Si, up to a factor of 5 relative isotopes of the radionuclides 26A1 (halflife Tu2 = 716,000 to solar Si see Figure 9). Other features of the X grains are years) and 44Ti (Tu2 = 59 years) high A1/ A1 ratios (up to 0.61) and, in some cases, also high On the basis of the enrichment in 28Si and the presence of 44Ti/naTi ratios (up to 0.55), inferred from large 26Mg and 44Ti at the time of grain formation, type II supernovae have t , Si/28Si (%o) slope 1.34 line ß SiC Mainstream rising from the interior [] SiC X grains k Si3N 4 ß Graphite Figure 9. Siliconisotopicompositions of presolar SiC mainstream, SiC X, Si3N4, and graphite grains. The dashed lines representhe solar Siisotopic ratios. Data are from Hoppe et al. [ 1994, 1996b, c], Nitder et al. [ 1995, 1996], and Travaglio et al. [ 1999]. Supernova structure He/N OIC C>O O>>C 28Si 44Ti been proposed as the most likely stellar sources of these grains. Prior to explosion such stars are believed to consist of concentric layers that experiencedifferent stages of nuclear burning (Figure 10) [Meyer et al., 1995; Weaver and Woosley, 1993; Woosley and Weaver, 1995]. The isotopic signatures of the X grains suggest deep and inhomogeneous mixing of matter in the supernova ejecta. The Siisotopic signature of the X grains requires matter from interior zones that experienced Ne and Oburning (O/Si and Si/S zones). In order to achieve C > O, the condition for SiC condensation [Larirner and Bartholornay, 1979; Lattimer et al., 1978], significant contributions must come from two outer layers that experienced H and incomplete He burning (He/N and He/C zones), and addition of material from the intermediate Orich layers to the SiC condensation site in the ejecta must be severely limited. Hydrodynamic models of supernova explosions predict fingers and mushroomlike structures into the outer portions of the ejecta as a result of RayleighTaylor instabilities [Arnett et al., 1989; Ebisuzaki et al., 1989; Herant and Woosley, 1994]. These structures may allow mixing of matter from nonneighboring zones while excluding large contributions from the intermediate zones. However, since a type II supernova contains huge amounts of O, these mixing scenarios are not without problems. A possible solution has been recently presented by Clayton et al. [1999], who argued for condensation of carbonaceous phases in type II supernova

8 . 10,378 HOPPE AND ZINNER: PRESOLAR DUST GRAINS ejecta even while C < O. In this model, carbonaceous grains Grain sizes. The size distribution of SiC has been condense from initially gaseous C and O atoms because studied in several meteorites and differences between energetic electrons produced by radioactivity in the supernova different meteorites are evident. SiC from the carbonaceous cause dissociation of the molecule CO which in equilibrium chondrite Murchison tends to be larger than SiC from other chemistry limits the availability of C atoms. meteorites. About 20 wt% of all Murchison SiC grains have Excesses of 44Ca from the decay of * I'i have previously diameters > 1 gm and 4 wt% have diameters < 0.3 gm [Amari been predicted to occur in meteoritic samples [Clayton, 1975] et al., 1994]. SiC from the enstatite chondrite Indarch, which and it was pointed out that 44Ti can only be produced in is considered to be more representative for SiC in the supernovae [Woosley et al., 1973]. The radionuclide 44Ti is protosolar nebula [Russell et al., 1997], is finer grained: 60 most abundant in the innermost Nirich zone and the grain wt% of the grains are in the < 0.3 gm diameter fraction and data strongly argue for contributions from this zone to the SiC only 4 wt% in the > 1 gm fraction. Despite these differences condensation site in the ejecta (Figure 11). However, between meteorites a significant fraction of meteoritic SiC has quantitative comparison of the X grain data with predictions sizes of > 0.3 gm. These sizes are larger than the diameters of from mixing models reveal serious problems. In particular, gm inferred for dust in the ISM [Mathis et al., 1977] the large SN excesses and enrichments of 29Si relative to 3øSi but are comparable to those found by the Ulysses and Gallileo seen in X grains cannot be satisfactorily explained, pointing spacecraft missions for interstellar dust in the solar system either to deficiencies in currentype II supernova models or to [Frisch et al., 1999]. an alternative origin of the X grains, e.g., in the ejecta of type Ia supernovae [Clayton et al., 1997]. What speaks for a SNIa 2.2. Silicon Nitride origin is the fact that all characteristic isotopes of the X grains The presolar silicon nitride grains identified to date have are produced in a single process (explosive He burning) and sizes around 1 gm [Nittler et al., 1995]. However, these that mixing can be limited to material from He burning and to grains most likely represent larger sizes and are not matter that experienced CNO processing. The best match with representative of the true sizes of presolar Si3N4 in meteorites. the X grain data, however, is achieved for mixing scenarios The Si3N4 grains have isotopic signaturesimilar to those of that yield O > C [Amari et al., 1998], violating the condition the SiC X grains. 4N/ SN ratios range from 18 to 100 (i.e., are for SiC condensation. Recently, Pellin et al. [ 1999] measured lower than solar, cf. Figure 6), and Si is characterized by 288i Mo, an element whose isotopic composition is expected to be enrichments with õ29si/28si and õ3øsi/28si ranging down to different in type Ia and type II supernovae, in two X grains. 200 and 350 %o, respectively (Figure 9). Presolar Si3Nn However, their data do not allow an unambiguous distinction grains contain C at the percent level and measured 2C/ 3C between these sources and the final answer whether the X ratios range from 30 to 170 (Figure 6). In addition, these grains formed in the ejecta of type Ia or type II supernova grains carry radiogenic 26Mg with inferred 26A1/27A1 ratios of explosions is still outstanding. up to 0.2. The close isotopic relationship between Si3N4 and the SiC X grains indicates that both grain types have the same stellar sources, namely, supernovae. 10 _ 2.3. Graphite 44Tirich SiC X grains Like in the case of presolar SiC, the relatively high SN mixing abundance (several ppm) of graphite in primitive meteorites and the comparatively large grain size (most grains are > / Ni zone C and ' '* 1 gm) permit the study of the isotopic compositions in single Sirich grains not only of the major element C but also of many trace E zones I elements, namely, N, O, Ne, Mg, Si, K, Ca, and Ti. Important additional information comes from noble gas analyses of bulk I.!' samples. Large anomalies and variations were found for the 10 ' isotopic compositions of all analyzed elements and different types of stars apparently contributed presolar graphite grains _ I to the solar system. On the basis of the Cisotopic composition graphite has been divided into 4 different populations (Figure 12): Group I grains have 2C/13C ratios between 2 and 20 (similar to the SiC type A & B grains), group 2 grains between 20 to 80 (the range of most SiC grains), group 3 grains around 90 (close to the solar 2C/13C 10 '6 10 '5 10 '4 10 '3 10 '2 10 ' ratio), and group 4 grains have 2C/13C > 100 (similar to the 44Ti/Si SiC type X grains). Except group 3 grains, many of which might be condensates from the molecular cloud from which Figure 11. Inferred initial 44Ti/48Ti ratio as a function of our solar system formed [Zinner et al., 1995], group 4 grains 44Ti/Si in SiC X grains [Hoppet al., 1996c; Nittler et al., are most abundant (50% of all graphite grains) and these 1996; University of Bern (unpublishedata, 1996)]. Provided that the Ti/Si ratio is unfractionated during SiC condensation, grains most likely formed in the winds or ejecta of massive mixtures that include only matter from the C and Sirich stars (type II supernovae and, less likely, WolfRayet stars). zones cannot account for the high 44Ti/Si ratios observed in Such an origin is also most likely for many of the group 2 the majority of SiC X grains. This points to contributions grains (10% of all graphite) but some of these grains might from the innermost Nirich zone, whic has the highest 44Ti also have formed in the winds of AGB stars. Although the abundance. origin of group 1 grains (10% of all graphite) is largely

9 HOPPE AND ZINNER: PRESOLAR DUST GRAINS 10, the vast majority of the SiC grains. Many of the graphite Murchison grains, in particular the lowdensity grains, exhibit large Group 3 excesses in 180 with 160/180 ratios as low as 2.5 (0.005x graphite solar; see Figure 13) [Arnari et al., 1993, 1995b; Hoppe et al., 1995; Travaglio et al., 1999]. The 180 excesses are positively correlated with 15N excesses. The 80rich grains have both loo isotopically light and heavy C (Figure 13). Although there is no strong correlation between 60/ 80 and 12C/13C, the lowest 60/ 80 ratios are found in grains with high 12C/ 3C ratios. Many graphite grains from all groups carry radiogenic 26Mg with inferred 26A1/2?A1 ratios of up to 0.15 at the time of grain 5o formation [,4marl et al., 1993, 1995b; Hoppe et al., 1995]. Group 4 These ratios are higher than those of SiC mainstream grains Group 1 [Hoppe et al., 1994; Huss et al., 1997; Zinner et al., 1991a] and come close to the values found in SiC X grains. Large isotopic anomalies are also seen for Si [, rnari et al., 1995b; Hoppe et al., 1995; Nittler et al., 1996; Travaglio et al., o 1999]. Both excesses and deficits of the nrich isotopes 29Si and 3øSi are observed (Figure 9). 29Si/28Si and 3øSi/28Si ratios 12C/13 C range from 0.5x to 2.2x solar, but most grains have deficits in 298i and 3øSi, the signature of SiC grains of type X. Other Figure 12. Distribution of 12C/13C ratios in presolar graphite diagnostic isotopic features of presolar graphite grains include 41 44, grains from the Murchison meteorite. According to 12C/13C large excesses of K [Arnari et al., 1996b] and Ca [Nittler et ratios and density graphite has been divided into four distinct groups. Data are from Hoppe et al. [! 995]. al., 1996], due to radioactive decay of 41Ca (T1/ years) and 44Ti, respectively, and the presence of the noble gas component NeE(L) [Arnari et al., 1995a]. This Ne component consists of almost pure 22Ne and is released at uncertain, some of these grains appear to have formed in nova lower temperatures than the related NeE(H) component and supernova ejecta. The observation that most meteoritic carried by presolar SiC. graphite grains probably have a supernova origin and that Excesses in 12C, 15N, and 180 are expected to result from they have sizes in the micrometerrange is consistent with the model by Clayton et al. [1999] that predicts that for type II He burning in massive stars and WolfRayet stars and type!i supernovae are therefore considered potential sources for the supernova graphite grains most of the mass resides within majority of graphite grains. WolfRayet stars are massive (> large particles. 25 Ms), masslosing stars [e.g., Chiosi and Maeder, 1986], Physical properties. Presolar graphite grains have densities between 1.6 and 2.2 g/cm 3. Grains of lower density tend to have higher trace element concentrations. 104 Consequently, more isotopic information has been obtained Murchison for those grains. Grain sizes range from 0.8 to 20 gm. Most grains are round (see Figures 3b and 3c). Nonround grains are Graphite predominantly found among the lowdensity grains and are possibly molecular cloud condensates [Zinner et al., 1995]. The relationship between morphology (see section 1.3), density, and Cisotopic composition of graphite grains is complex [Hoppe et al.,!995; Zinner et al., 1995] but two feature stand out: (1) Only round grains have isotopically anomalous C, and (2) the relative proportions of the various graphite groups depend on density. Although not fully understood, this indicates a close relationship between 103, physical properties and stellar sources of the grains. 10 = Isotopic properties and stellar sources. Presolar I i i graphite grains have C/ C ratios between 2 and 7000 ß (Figure 12) [Arnari et al., 1993; Hoppe et al., 1995], similar to i incomplete He the range observed for SiC grains. But the distribution of [ in massive stars 10 o '"... I ''"... I ''"'" 1 ' '"'"'1 ' '"'"'1 ''""" 12C/13C ratios is clearly distinct from that of SiC (Figure 5), o indicating different relative proportions of their stellar sources among the populations of these two types of presolar grains. 12C/13 C Most graphite grains have isotopically light C (the group 4 Figure 13. The 160/180 and 12C/13C ratios grains), a characteristic that is rare among presolar SiC grains. graphite grains from the Murchison meteorite. The dashed Nitrogen isotopic variations are much smaller than those seen lines represent the solar system 60/180 and 2C/ 3C ratios. in SiC, 4N/15N ratios range from 30 to 700 [Amari et al., Expectations for incomplete He burning in massive stars are 1993; ttopp et al., 1995]. Most graphite grains have l N indicated for comparison. Data are from Arnari et al. [1995b], excesses relative to solar, the opposite of what is observed for Hoppe et al. [1995], and Travaglio et al. [1999]. bum!rig of presolar

10 10,380 HOPPE AND ZINNER: PRESOLAR DUST GRAINS characterized by three evolutionary stages with large enhancements of N (WN phase), C (WC phase), and O (WO phase) at their surface. Excesses of 2C, SN, and 80 are expected during the WNWC transition when the products of He burning appear at the star's surface. During He burning The isotopic and physical information on presolar 2C is produced from the triple alpha reaction, while N is corundum obtained thus far is more limited than that on SiC destroyed by 14N(a,y)lSF([ y)180 and by lsn(a,y) 9F. This and graphite. The reason for this is threefold: (1) the results in low N abundances, but faster destruction of 4N can abundance of presolar corundum is much lower (subppm) result in SN excesses (relative to the solar SN/ 4N ratio). than that of SiC and graphite, (2) unlike meteoritic SiC, Oxygen is expected to be 1SOrich due to (capture on 4N. essentially of which is of presolar origin, only about 1% of WolfRayet stars are also expected to show 26A1 at the surface meteoriti corundum is presolar, and (3) presolar oxides lack with 26AI/27A1 ratios of up to 0.1 [Prantzos et al., 1986]. a characteristic noble gas componenthat could serve as Problematic for a WolfRayet star origin of many graphite "beacon" in the procedures used to separate presolar grains. grains are the Si isotopic ratios and the presence of radiogenic Most presolar corundum grains have been identified by ion 44Ca, which has previously predicted for supernova grains imaging in the ion microprobe. Isotopic measurements have [Clayton, 1975]. In WolfRayet stars, Si is affected by the s been performed for the elements O, N, Mg, and Ti [Choi et process, which is expected to lead to largenrichments in 29Si al., 1998; Huss et al., 1994; Hutcheon et al., 1994; Nittler et and 3øSi [Busso and Gallino, 1985], the opposite signature of al., 1994, 1997, 1998; $trebel et al., 1996]. that found in most graphite grains. Radiogenic 44Ca is not As has been the case for presolar SiC and graphite, presolar expected to be present in grains from WolfRayet stars as its corundum grains can be classified into different groups that radioactive precursor, 44Ti, is exclusively made by explosive represent different nucleosynthetic histories and thus different nucleosynthesis at high temperatures. This leaves supernovae as the most likely stellar sources for the majority of the graphite grains, at least for those for which Si and Ca data 105 _ exist. Travaglio et al. [1999] performed mixing calculations for the ejecta of type II supernovae and their models are able to reproduce the observed 12C/13C, 160/180, and 3øSi/28Si ratios as well as the abundances of radiogenic 4 K and 44Ca in low density graphite grains. Because mixing in these models is constrained by C/O > 1 at the graphite condensation site in the ejecta, inhomogeneous mixing in the ejecta is required as had been concluded from the SiC X grain data (see section 2.1.4). Clayton et al. [1999] argue, on the other hand, that large supernova graphite particles condense even for O > C in which case the chemical constraints on mixing should be relaxed. Evidence for a nova origin of some graphite grains comes from NeE(L). Laser gas extraction mass spectrometry of single grains has shov, n that this component is contained only in a small fraction of the grains [Nichols et al., 1992, 1994]. Two of the gasrich grains (representing about 2% of all graphite grains analyzed for No) had 2øNe/22Ne ratios that phases in Crich stellar atmospheres and the survival of different grain types in the ISM Corundum io Presolar Corundum T84 i 1 st Dred ' L ß Group 1 & Group 2 O Group 3 II Group 4 Red Giants Supernovae....,%.. 1.,.., Processing I I I I I III I i i i iiiii were clearly lower than the ratio theoretically predicted to result from He burning in AGB stars, WolfRayet stars, or s supemovae. These low ratios imply an origin in novae where 22No is produced by the decay of the shortlived radionuclide Figure 14. Oxygenisotopic compositions of meteoritic 22Na (Tin = 2.6 years) [Clayton, 1975]. A nova origin is presolar corundum grains. The dashed lines represent the solar system isotopic ratios. Most of the presolar corundum supported by low 2C/ 3C ratios of 4 and 10, respectively, and grains have ratios similar to those measured in the atmosphere the fact that no He accompanied Ne in these grains. of red giant stars [Harris and Lambert, 1984; Harris et al., In contrasto SiC, only very few graphite grains have the 1987; Smith and Lambert, 1990]. The line labeled "GCE" isotopic signature expected for condensates from AGB stars represents the expected galactic chemical evolution line for O (isotopically heavy C and light N, enhanced abundances of s slope one in a loglog representation if the 160/ 80 and process elements). In addition, the high 26A1/27A1 ratios and 60/ 70 ratios scale with 1/metallicity [Clayton, 1988]), the observed O and Siisotopic signatures are incompatible adjusted to pass through the point of solar O. Model with an AGB star origin. The only evidence for an AGB predictions for the first dredgeup in red giant stars for three origin of some graphite grains comes from KrS observed in different metallicities Z.(different initial compositions along the GCE line with Z = 0.025, 0.02 (solar), and graphite bulk samples [Amari et al., 1995a; Lewis and Amari, [Boothroyd et al., 1994]), the effect of cool bottom processing 1992]. In principle, stars that produce SiC are also expected and hot bottom burning ("HBB") in AGB stars, and the O to form graphite. It is an unsolved puzzle why only very few isotopic ratios expected in supernovae and in WolfRayet graphite grains have the isotopic signatures typical of stars during the OfWN phase are shown for comparison. meteoritic SiC and AGB stars; the apparent underabundance Grain data are from Choiet al. [1998], Huss et al. [1994], of graphite from AGB stars points to deficiencies in the Nitder et al. [1994, 1997], Nitder and Gao [1998], and current understanding of the condensation of carbonaceous Strebel et al. [ 1996]. I! T54,,,I,,,,i ' ' ''"" asola80 WR stars AGB HBB

11 HOPPE AND ZINNER: PRESOLAR DUST GRAINS 10,381 stellar sources. Nittler et al. [ 1997] discerned four different groups of presolar corundum grains. Their classification scheme is based on Oisotopic composition and abundance of extinct 26A1. Stellar sources proposed for the four groups are red giant and AGB stars that differ in their initial Oisotopic compositions (which were inherited from the interstellar gas composition from which the parent stars formed, and which, according to the theory of galactic chemical evolution, in an oxygen threeisotope plot moved diagonally downward with time; cf. Figure 14) and their indigenous nucleosynthetic production (which is mainly a function of stellar mass). Besides these four grain groups the Oisotopic signatures of two unclassified grains (representing about 1% of all presolar corundum grains) are indicative of a supemova origin Grain sizes. The presolar corundum grains identified to date have grain sizes between 0.5 and 5 gm. Because most of these grains were found by ion imaging in the ion microprobe, a technique that allows only the identification of grains with sizes > 0.5 [tm, many smaller corundum grains might have remained undetected and the size distribution of the known presolar corundum grains is probably not representative for presolar corundum in meteorites Isotopic properties and stellar sources. Presolar corundum grains have 160/170 ratios between 70 and 30,000 ( x solar) and 60/180 ratios between 150 and 50,000 ( x solar; Figure 14). Most grains (group 1 and 2 grains) are characterized by lower than solar 160/170 and higher than solar 160/t80 ratios. Substantial 170 enrichments and moderate 180 depletions are the signature of H burning the interior of low to intermediatemasstars and mixing of partially Hburned material into the envelope during the socalled first dredgeup, which occurs when the star becomes a red giant. The magnitude of this effect depends on stellar mass. Most group 1 grains have Oisotopic compositions similar to those observed in the atmosphere of red giant stars (Fig. 14) [Harris and Lambert, 1984; Harris et al., 1987; Smith and Lambert, 1990] and are well explained by evolution models for stars of mass 1 to 9 Ms and metallicities 0.8 to 1.25x solar (cf. Figure 14) [Boothroyd et al., 1994; Boothroyd and Sackmann, 1999]. Many of these grains carry AGB or, alternatively, these grains formed in AGB stars of higher than solar metallicity. A distinct possibility is a supemova origin of some group 4 grains as proposed for the isotopically related unclassified grain SC122 (see below). The Oisotopic ratios of the unclassified grains T54 [Nittier et al., 1997], T84 [Nittier et al., 1998], and SC122 [Choi et al., 1998] cannot be explained by conventional red giant star models. Grain T54 could come from an AGB star of relatively high mass (> 45 Ms) that experienced hot bottom burning, a process in which the convective envelope extends into the H burning shell [Boothroyd et al., 1995], or from a massive WolfRayet star during the OfWN phase of evolution. Grain T84 has 160/170 and 160/180 ratios of 510x solar. Large 160 enrichments are expected for grains from supemovae. In type II supemovae O is most abundant in the O/Si, O/Ne, and O/C zones (Figure 10), all of which consist of almost pure 160. Mixing with matter from outer regions in the ejecta would contribute some 170 and 180 but the overall signature of supernova condensates would still be a 60 excess. A supemova origin was also proposed for grain SC122. The 80 excess and close to solar 160/ 70 ratio seen in this grain is well explained by a mixing model for a type II supernova [Choi et al., 1998]. Although 160 excesses are expected to be the dominant signature of supemova grains, 180 excesses can be obtained if an appropriate amount of matter from the He/C zone, which experienced incomplete He burning, is mixed to the corundum condensation site in the ejecta (which must fulfill the condition O > C). To date grain T84 is the only corundum grain that carries the expected predominant Oisotopic signature of a supemova origin. It represents less than 1% of all presolar corundum grains. The low abundance of such grains is puzzling because it is much lower than that of carbonaceousupemova grains and because corundum from supernovae is expected to make up several tens of percent of presolar oxide grains in meteorites. It has been suggested that supernova oxide grains are smaller than those from red giant stars and that the true ratio of supernova to red giant star grains in meteorites might be higher than what has been determined from grains in the size range of 1 pm [Nittler et al., 1998]. 3. Summary and Outlook Primitive meteorites contain small quantities of presolar dust grains that have survived largely unaltered the processes radiogenic 26Mg and inferred initial 26A1/27A1 ratios indicate before and during the formation of the solar system. These contributions from stars both prior to and during the AGB grains exhibit large anomalies and variations in the isotopic phase of evolution. The large 180 depletions of group 2 grains compositions of the major and of many trace elements. The point to an origin in < 1.5 Ms stars that experienced cool isotopic signatures can successfully be explained in terms of bottom processing during the AGB phase [Wasserburg et al., stellar nucleosynthesis and evolution and most of the presolar 1995]. Note that 160/180 ratios in the range observed in the dust grains are clearly of circumstellar origin. Presolar dust most 18 poor group 2 grains cannot be measured grains identified to date include diamond, SiC, graphite, spectroscopically in the atmosphere of stars, a limitation corundum (A1203), and silicon nitride (Si3N4)(Figure 2). In imposed by the measurementechnique, and it is thus not addition, a few spinel (MgA1204) grains, possibly a titanium surprising thathe highest 360/180 ratios of presolar corundum oxide (TiO2) grain, and carbides of Ti, Mo, and Zr, kamacite are not seen in the astronomical study of Oisotopic (FeNi) and cohenite ((FeNi)C) inside graphite grains were compositions of red giant stars. The group 3 grains are related identified to have a stellar origin. to the group 1 grains. The high 360/170 ratios of group 3 Table 1 lists the different types of presolar grains, their grains are indicative of an origin from red giant stars with low isotopic signatures and most likely stellar sources. Diamonds mass ( < 1.4 Ms). The Oisotopic signatures of the group 4 are most abundant but least understood. Their origin is still grains are also compatible with an origin from lowmass AGB unknown although they carry a supernova isotopic signature stars. The low 160/180 ratios of these grains requireither and at least some of the diamonds themselves probably dredgeup of 180rich material during early pulses on the formed in supernova ejecta. Most silicon carbide grains formed in the winds of lowmass AGB stars with different initial compositions established by galactic chemical evolution. A small fraction (1%) of the SiC grains has a supernova origin. Although still controversial, Jtype carbon

12 10,382 HOPPE AND Z1NNER: PRESOLAR DUST GRAINS Table 1. Presolar Dust Grains in Primitive Meteorites Mineral Isotopic Signatures a Stellar Source Contribution of Stellar Source Diamond Silicon carbide Graphite Corundum XeHL enhanced 3C, inn, 22Ne, sprocess elements low 2C/ 3C, often enhanced enhanced 2C, SN, 28Si; extinct 26A1, anti low 12C/1JC, low lan/lsn enhanced 2C, 5N, 80, 28Si; extinct 26A1, 41Ca, anti Krs low 12C/13C low 2C/ 3C; NeE(L) enhanced 70, moderate depletions in 80 enhanced 70, strong depletions in 80 enhanced O Silicon nitride enhanced 2C, 5N, 288i; extinct 26A1 arelative to solar system isotopic signatures. supemovae unknown AGB stars (13 Ms different metallicities) > 90% Jtype C stars (?) < 5 % supernovae 1% novae 0.1% supemovae, WolfRayet stars (?) > 80% AGB stars < 10% Jtype C stars (?) < 10% novae 2% RGB and AGB stars (19 Ms, different > 70% metallicities) 20% AGB stars (< 1.5 Ms, cool bottom processing) 1% supemovae supernovae 100% stars might also have contributed to the population of SiC grains and very few grains (0.1%) appear to have formed in the ejecta of nova explosions. In contrast to SiC, most graphite grains are probably from massive stars. Type II supernovae are favored as sources for the majority of the graphite grains although WolfRayet stars cannot be excluded in some cases. Similar to SiC, some graphite grains might have condensed in the winds of Jtype carbon stars and in the ejecta of novae. Almost all corundum grains (and the isotopically related spine! grains) formed in the winds of lowand intermediatemass red giant and AGB stars of different mass and metallicity. The contribution from supernovae is limited to about 1%, similar to the fraction of such grains among the meteoritic SiC grains. On the other hand, all minerals is closely related to the question of dust survival in the ISM and during solar system formation. On the basis of chemical and structural characteristics Bradley [1994] has argued that GEMS ("Glass with Embedded Metal and Sulfides") found in IDPs are interstellar silicates. The presolar nature of GEMS, however, remains uncertain as long as no isotopic data are available. Of particular interest is cometary matter since it represents the most primitive solar system material and is thus believed to be the best source of unaltered presolar dust in the solar system. Cometary particles have undoubtedly already been studied among IDPs collected in the upper atmosphere, however their identification relies on circumstantial evidence. Samples from a comet will be available in the near future by NASA's cometary sample silicon nitride grains carry the signature of supernova return mission STARDUST. In addition, contemporary nucleosynthesis. interstellar dust will be collected by STARDUST and it will Grain sizes vary over 4 orders of magnitude, from about 1 nm to 10 gm. If the diamonds are omitted then most grains have sizes > 0.3 gm. These sizes are larger than those assumed to be typical for the ISM but are comparable to the be extremely interesting to compare its physical and isotopic properties with those of the meteoritic (and cometary) grains that formed more than 4.6 billion years ago. Because the isotopicompositions of certain elements (e.g., Si and Ti) in sizes inferred for interstellar dust in the heliosphere by the dust grains from cool stars are expected to be largely Ulysses and Galileo spacecraft missions. unaffected by the nucleosynthesis inside the grains' parent The discovery and study of presolar grains in meteorites stars it should be possible to trace the evolution of the isotopic has opened a new and fruitful field of astrophysical research. composition of such elements in our galaxy during the past The laboratory study of presolar grains complements 5 billion years. As an example, galacti chemical evolution traditional methods of astronomy and astrophysics and has models predicthe 29Si/28Si and 3øSi/28Si ratios to increase provided new insights into nuclear and chemical processes in with the age of the galaxy, and it should be possible to verify and around stars and into the early history of the solar system. these predictions by comparison of Si in presolar and Although they are only minor constituents of primitive meteorites and are not necessarily representative of dust in the ISM 4.6 billion years ago, the same types of grains are likely to be present also in the ISM today. The physical properties of the identified presolar minerals predestine them to survive exposure to heliospheric plasmas and the solar wind and these minerals might be important constituents of interstellar dust in the heliosphere. Future work on presolar grains in meteorites will focus on the study of smaller grains, which will be facilitated by the availability of better instrumentation the near future, and on the search for new circumstellar minerals such as silicates and contemporary interstellar SiC grains. This is of high scientific interest because the concept of GCE is closely related to the question of dust and gas injection from different types of stars into the ISM, thus providing important information on the history of the local ISM. During passage through the ISM the presolar grains found in meteorites were subjected to bombardment with galactic cosmic rays and one can expecto find variable enrichments of rare nuclidesuch as the isotopes of Li, Be, and B, and 2 Ne, depending on the duration of this bombardment. The detection of cosmogenic nuclides in presolar grains is one (possibly the only) way to directly determine the life times of sulfides. The chance of successful identification of new dust in the ISM. First attempts to determine IS ages of

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