Presolar Silicon Carbide Grains in Meteorites

Peter Hoppe

Max-Planck-Institut für Chemie, Abteilung Kosmochemie, P.O. Box 3060, D-55020 Mainz, Germany

hoppe@mpch-mainz.mpg.de

1. Introduction

Primitive meteorites contain small concen-trations (ppb to ppm) of presolar dust grainsthat have survived largely unaltered theprocesses that led to the formation of the solarsystem. The most important presolar mineralsidentified to date are diamonds, silicon carbide(SiC), graphite, silicon nitride (Si3N4), andcorundum (Al2O3). Diamonds are most abun-dant (with concentrations of up to 1000 ppm),followed by SiC and graphite (several ppm),corundum (< ppm), and Si3N4 (ppb). Diamondsare only 2 nm in size. All other types of grainsare larger and range from ≈ 0.2 to 20 µm (Fig.1). The isotopic compositions of presolar grainsare highly anomalous and vary over manyorders of magnitude, indicative of a stellarorigin of the grains. The laboratory study ofpresolar grains can thus provide information onstellar nucleosynthesis and evolution, thegalactic chemical evolution, grain growth instellar atmospheres or in the ejecta of stellarexplosions, and on the inventory of stars thatcontributed dust to the solar system (see Fig. 2).

Figure 1. Presolar SiC grain separated from theMurchison meteorite. The grain diameter isabout 0.5 µm.

All types of presolar dust contain the decayproducts of radionuclides that were incor-

porated in the host minerals at the time of grainformation. Most important are 26Al and 44Ti thatare diagnostic with respect to stellar nucleo-synthesis and stellar source. It is thus possibleto get information on the presence andabundance of radionuclides in certain types ofstars.

Figure 2. Path of presolar dust grains fromtheir stellar source to the laboratory. Theirisotopic compositions are determined by thestarting composition of and internal stellarnucleosynthesis in the parent stars. Afterpassage through the ISM such grains becamepart of the protosolar nebula. They survived theepisode of solar system formation inside smallplanetary bodies and they were carried to theEarth by meteorites from which they can beseparated by chemical and physical procedures.Finally, they can be analyzed for their isotopicand structural properties in the laboratory.

Silicon carbide is the best studied presolarmineral phase. The reason for this is itsrelatively high abundance in primitivemeteorites and its comparatively large grainsize that allows isotopic analyses of the majorand of many trace elements in individual

Mass

Proto-solar

nebula

Formation ofsolar system

Meteorites

Chemical & physicalseparation

of dust grains

Laboratoryanalyses

4He

Stellar nucleosynthesisand formation of

dust grains

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Star 12C4He

4He

grains. In addition to the major elements C andSi isotope data exist for N, Mg, Ca, Ti, thenoble gases, and heavy refractory elements(e.g., Sr, Zr, Mo, Ba, Nd, Sm). On the basis ofthe isotopic compositions of C, N, Si, and theabundance of radiogenic 26Mg six differentpopulations of SiC grains can be discerned: Themainstream grains, which make up the majorityof the grains (≈ 90% of the total), and the minortypes A, B, X, Y, and Z. The mainstream andtype X grains are of particular interest and thispaper will focus mainly on those grains.

AGB stars, or more specifically carbon stars,are believed to be source of the SiC mainstreamgrains. Arguments in favour of C stars are: (i)the SiC mainstream grains have 12C/13C ratiossimilar to those found in the atmosphere of Cstars (see Fig. 3), (ii) many trace elementspresent in the SiC mainstream grains carry thesignature of the s-process, (iii) C stars havebeen successfully invoked to explain theabundances and isotopic patterns of the grains’snobel gases, (iv) they are considered to be themost prolific injectors of carbonaceous dustinto the ISM, and (v) they are observed to showthe 11.3 µm emission feature typical of SiC. Onthe other hand, based on their C-, N-, and Si-isotopic compositions, presence of now extinct44Ti at the time of grain formation, and thesignature of r-process nucleosynthesis in theisotopic patterns of heavy trace elements, the Xgrains are believed to have formed in the ejectaof type II supernova explosions.

For more detailed informations the reader isreferred to some recent reviews on the subjectof presolar grains found in meteorites [1-6].

2. Carbon-, Nitrogen-, and Silicon-isotopicCompositions of Presolar SiC

Mainstream Grains: The SiC mainstreamgrains have 12C/13C ratios between 10 and 100and 14N/15N ratios between 50 and 20,000 (Fig.3). Most of these grains show the imprint of theCNO cycle, i.e., enrichments in 13C and 14Nrelative to solar abundances. The Si-isotopiccompositions of most mainstream grains arecharacterized by enrichments in the heavy Siisotopes of up to 200 ‰ relative to their solarabundances, falling along a line with slope ≈1.3 in a three-isotope-representation (Fig. 4). Itis the preferred interpretation today that theslope 1.3 Si correlation line primarily reflectsthe GCE of the Si isotopes, both in time and

space, and represents the starting compositionsof a large number of C stars.

Figure 3. C- and N-isotopic compositions ofpresolar SiC grains.

Figure 4. Si-isotopic compositions of presolarSiC grains. The iSi/28Si ratios are given aspermil deviation from the solar system ratios.

Type X grains: The X grains typically show theopposite isotopic signatures of those observedin the mainstream grains (Figs. 3 and 4). Their12C/13C ratios range from 18 to 7000 but mostof them have higher than solar 12C/13C ratios.With one exception all X grains show lowerthan solar 14N/15N ratios (down to 0.05x solar).From the nucleosynthetic point of view high12C/13C and low 14N/15N ratios are the signatureof He burning. Silicon generally shows

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SiC Mainstr.SiC A&BSiC XSiC YSiC Z

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He burningin massive stars

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C stars

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slope 1.34line

To pure28Si

depletion in the n-rich isotopes 29Si and 30Si (oralternatively enrichment in 28Si) of up to afactor of 5 relative to solar, the signature ofadvanced nuclear burning stages. The C- andSi-isotopic compositions of the X grains arewell explained by mixing of matter from the C-and Si-rich zones (which experienced H and Heburning and, respectively, Ne and O burning) ina type II SN. On the other hand, theenrichments in 15N and very high N contents ofup to several wt% in the X grains are hard tounderstand in view of current SN models. Thismight point to deficiencies in the currentunderstanding of the production of 15N in SNeor of the condensation behaviour of N in SNejecta.

3. Extinct Radioactivities

After nitrogen, aluminum and titanium are themost abundant trace elements contained inpresolar SiC (with concentrations on the orderof permil to percent). Since the proposed stellarsources of the mainstream and type X grainsproduce 26Al (C stars, SNe) and 44Ti (SNe),radiogenic 26Mg and 44Ca can be expected to bepresent in presolar SiC.

Figure 5. Inferred initial 26Al/27Al ratios ofpresolar SiC grains.

Aluminum-26: During condensation of SiCtrace elements like Mg and Al are incorporatedinto the growing grains. Due to differentcondensation behaviours Al is stronglyfavoured over Mg leading to high Al/Mg ratiosin the SiC grains. Magnesium isotope analyses

performed in the laboratory reveal strongexcesses in 26Mg and in some cases Mg evenconsists of monoisotopic 26Mg. Because 26Mgexcesses are very large and because 25Mg/24Mgratios are close to solar it is widely acceptedthat the 26Mg excesses found in presolar SiCare in fact due to the decay of radioactive 26Al.The inferred initial 26Al/27Al ratios of themainstream grains are typically between 10-4

and 10-2, roughly compatible with theexpectations for AGB stars (Fig. 5). The type Xgrains have very high inferred initial 26Alabundances with 26Al/27Al ratios of up to 0.6and they are clearly distinguished from themainstream grains (Fig. 5). When comparedwith expectations from type II SN mixingmodels the Al- and C-isotopic ratios of many Xgrains are adequately reproduced by the modelsalthough an explanation for the highest26Al/27Al is still lacking.

Figure 6. Inferred initial 44Ti/48Ti and 44Ti/Siratios of type X SiC grains.

Titanium-44: Similar to Mg and Al, Ca and Ticondense along with the growing SiC grains.While all mainstream grains have close to solar44Ca/40Ca ratios, some of the X grains (≈ 20%)show large excesses in 44Ca of up to a factor of20 relative to its solar abundance. Because the44Ca excesses are very large and since other Caisotope ratios are close to solar the 44Caexcesses are best explained as being due to thedecay of radioactive 44Ti. The inferred initial44Ti/48Ti ratios range up to 0.6 and 44Ticoncentrations can be as high as 0.1wt% (Fig.6). The data on 44Ti are in reasonableagreement with the predictions from type II SN

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44Ti-rich SiC X grains

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/48Ti

SN mixing

mixing models. Since any mixture of the C-and Si-rich zones (that supply the C and Si forthe growing SiC grains) cannot account for the44Ti/Si ratios of the X grains contributions fromthe innermost Ni-rich zone in the SN, which isrichest in 44Ti, appear to be necessary, implyingdeep mixing of the SN ejecta.

4. Outlook

Future isotope studies on presolar SiC grainswill focus on smaller grains (≈ 0.1-0.5 µm)since grains in this size range make up themajority of presolar SiC. In addition, we willinvestigate the homogeneity of the distributionof extinct 26Al and 44Ti in micrometer-sizedgrains. This aspect is closely related to thequestion on the presence of 26Al- and 44Ti-richsubgrains and it might be possible to get newinsights into the mixing processes in SN ejecta.Such studies will be possible with a newsecondary ion mass spectrometer, the Nanosims50, which will allow us to measure isotopiccompositions with a lateral resolution of downto 50 nm. This intrument will be delivered tothe Washington University at St. Louis and tothe Max-Planck-Institute for Chemistry atMainz in the year 2000.

References

[1] E. Anders and E. Zinner, Interstellar grainsin primitive meteorites: Diamond, siliconcarbide, and graphite, Meteoritics 28, 490-514,1993.[2] U. Ott, Interstellar grains in meteorites,Nature 364, 25-33, 1993.[3] P. Hoppe and U. Ott, Mainstream siliconcarbide grains from meteorites, in:Astrophysical Implications of the LaboratoryStudy of Presolar Materials, T.J. Bernatowiczand E. Zinner, eds., pp. 27-58, AIP, New York,1997.[4] S. Amari and E. Zinner, Supernova grainsfrom meteorites, in: Astrophysical Implicationsof the Laboratory Study of Presolar Materials,T.J. Bernatowicz and E. Zinner, eds., pp. 287-305, AIP, New York, 1997.[5] E. Zinner, Stellar nucleosynthesis and theisotopic composition of presolar grains fromprimitive meteorites, Ann. Rev. Earth andPlanet. Sci. 26, 147-188, 1998.[6] P. Hoppe and E. Zinner, Presolar dustgrains from meteorites and their stellar sources,Journal of Geophysical Research-SpacePhysics, in press, 1999.