Solar system isotopic anomalies: Supernova neighbor or presolar carriers?

icAnus 32, 25,5--269 (1977)

Solar System Isotopic Anomalies: Supernova Neighbor or Presolar Carriers? D O N A L D D. CLAYTON Department of Space Physics and Astronomy, Rice University, ltouston, Texas 77001 Received November 30, 1976; revised February 21, 1977 I evaluate several nuclear and chemical problems related b o t h to the recent scenario suggesting t h a t the known isotopic anomalies in the solar system have resulted from a supernova near the protosolar nebula and to the model of extinct presolar carriers. Major features include : (1) Large quantities of extinct 248Cm and 38C1 are predicted from the C a m e r o n - T r u r a n model of a minor injection about 106 yr before condensation; (2) an extinct-carrier model of 26Mg is set forth in detail with a solid chemistry picture of the early solar system ; (3) a major thermonuclear supernova responsible for ~6A1, 244pu, and 4°K would have to have occurred several million years ( > 3 m.y.) before condensation and contributed a large fraction of the major stable chemical elements; (4) carbon isotope families are to be expected if the oxygen isotope families are due to a late injection of ~60; (5) the E a r t h and E meteorites m a y have condensed primarily in a carbon-rich nebula existing before admixtures of a major late ~SO-rich mixture ; (6) the extinct-presolar-carrier model is the single best explanation of all anomalies.

Cameron and Truran (1977) have discussed the idea that a single supernova triggered the collapse of the solar system and simultaneously injected an ensemble of isotopic inhomogeneities. Sabu and Manuel (1976) have advocated the same picture on the basis of noble-gas considerations; however, I will frame m y discussion on the Cameron and Truran paper, since it is that ensemble of anomalies that will also be my concern. Although motivated primarily by the 26A1 and 160 anomalies, they surmise that extinct 129I and 2a4pu and live 4°K owe their initial existence to the same event. I wish here to enlarge their discussion by outlining several additional important applications of their model. Some of these applications will be critical of specific details of their suggestion, and some will constitute new ideas for research on this general problem. At the same time I will criticize my own model of presolar carriers of extinct parents. These ideas are

divided for convenience into sections headed by the anomaly primarily under discussion. ~48CM/~44pU, "~AL

Amid the host of fascinating possibilities, one must evaluate the exciting role of ~48Cm. In conventional r-process theory, 248Cm is produced with about twice the initial abundance of 244pu, having two progenitors (:48Cm and 252Cf), whereas 244pu has only one (itself). Because the 248Cm lifetime is only slightly less than that of 2~A1, the fraction of ~48Cm surviving until meteorite formation can be almost as great as the fraction of 26A1 surviving. Seventy-four percent decays during each ~6A1 half-life (7.4 X 10 5 yr). If, for example, the condensation occurs after tl/2(248Cm) = 3.7 X 10 5 yr, the initial ratio 248Cm/~44pu ~ 2 will have shifted to 248Cm/244pu ---~0.5. Because the spontaneous-fission branch for 248Cm (9%) greatly exceeds that of 244pu (0.33v~o), the fission xenon from 248Cm in objects

255 Copyright O 1977 by Academic Press, Inc. All rights of reproduction in any form reserved.

ISSN 0019-1035

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I)ONAIA) 1). CLAYT()N

condensing then would exceed t h a t from e44pu b y roughly the factor 14. If all solar s y s t e m 244Pu were due to this triggering supernova, the fission of i n ,situ 24sCm would remain a m a j o r candidate for the origin of the so-called carbonaceous-chondrite fission (CCF) xenon. This possibility has been suggested previously by Rao and Gopalan (1973), and this new m o t i v a t i o n for a last-minute spike revives it. However, new m e a s u r e m e n t s show t h a t a correspondence of the xenon fission yields of 24~Cm with C C F Xe does not exist (Leich et al., 1977), and yet it should be present if these assumptions are correct. C a m e r o n and T r u r a n carefully distinguished their supern o v a from the traditional r-process event, but it still seems likely t h a t "~48Cm will be synthesized if ~dq)u is; however, one could easily relax their assumptions by taking the 24q)u to be primarily .t result of continuous galactic nucleosynthesis or by requiring the condensation to wait about eight or more ~-dsCm half-lives. The purest samples of 2441)u xenon in meteorites (e.g., Allende inclusions) m a t c h the measured s p e c t r u m of m a n - m a d e 244pu xenon so closely t h a t comparisons with the recently ineasured 24~Cm xenon allow (tuantitative limits to be placed on its admixture. I t e n t a t i v e l y conclude in this way t h a t 24q)u fission xenon in meteorites contains less t h a n 10% of 24sCm fission xenon. I t follows t h a t either the :dq~u nucleosynthesis occurred at least 2.8 X 10 (~ yr before condensation of the '-'44pu-bearing minerals or the C m has been chemically fractionated from the Pu. Only 6.8% of the synthesized '-'6A1 could survive 2.8 ) 1 0 ~ yr. Let us now suppose t h a t the explosive burning of carbon is the m a j o r source of '-'~A1. If one "~dopts t h a t time and the calculated production ratio 26A1 27A1 = 2 ;< 10 -:~ (Arnett, 1969) for those events t h a t synthesize 27A], and a measured (Lee et al., 1977) ratio in Allende () X 10 -~, one m a k e s the following conclusion: that late event ,synthesized /~dc~o of Allende's 27A1. An event synthesizing a

large fraction of |he stable mwh,i will hereafter be called "t major spike, to distinguish it from the minor spike proposed b y C a m e r o n and Truran. Clayton (1975a) presented the new and different picture t h a t C C F Xe was carried into the primordial solar system by presolar grains t h a t retained fission fragments from any short-lived nucleus t h a t could precipit a t e in minerals in the supernova expansion. T h e parent is not live in this model. H o w a r d et al. (1975) evaluated t h a t model for the specific cases :dS,2~°Cm. The 24SCm xenon should exceed 24q)u xenon in interstellar grains t h a t condensed in the supernow~ expansion by a factor of 54, given the simple ratios used previously. The m e a s u r e m e n t s of Leith et al. also suggest t h a t '-'4~Cm did not in fact do this, unless its fission xenon is either lost or greatly modified by yet other exotic components (Clayton, 1976). It therefore seems unlikely t h a t a large portion of the 2441)u xenon was carried in the way I suggested. In support of t h a t conclusion, I)rozd et al. (1977) have argued t h a t there is no longer a large discrepancy between the 24q)u xenon and the fission tracks, so that, most of the -~441)u was d e m o n s t r a b l y live in the solar system. However, if 24~Cm and 2441)u xenon were unable to survive in supernova minerals, it does not seem likely t h a t some fission xenon from some other nucleus would have been able to do so and to provide the C C F Xe c:m'iers. These arguments speak against having a m a j o r portion of the 24q)u xenon c.arried in by stardust. It m u s t he stated for clarity that, even if it yet proves true t h a t a minor portion of the '-'44pu xenon aml most of the '2"'Xe* were carried in by presolar grains, these need not be the same carriers as those of C C F Xe, which m a y have been carried in quite different minerals. The absence of "-48Cm xenon doe8 ~ot, moreover, rule out the survival oj' significa~d supernova stardust, but it does seem to rule out the survival <>ffis,sion xenon i n tho.se super,~ova comlensates carryin 9 244pu

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SOLAR SYSTEM SUPERNOVA TABLE I COMPARATIVE; SUMMARY OF ANOMALY ORIGINS

Feature

Free decay Fraction of AI 26Mg CmXe/PuXe < 0.1 Extinct 22Na Extinct ~:9I ~(146Sm/Pu) > 2 l~sCs/13~Ba < 2 × 10-4 4°K/~K ~ constant Little extinct 36C] Oxygen families ~t(12C/x3C) C/O ~ 1

Cameron-Truran neighbor Minor

Major

1 m.y. ~4% 26A1in situ NoNo Yes No~ Noo No~ Noo Yes Yes ~ ~6 Unlikely

3 m.y. ~40% 2~A1in situ Yes No Yes No~ N(, No~ Doubtful Yes Yes ~ ~ls Possible

Conventional gas

Presolar carriers

3 X 10-6 m.y.

100 m.y.

~9,5%

0

~6Mg in A1203 Maybe: Xe escape Yes Yes Yes Barely Yes Yes Yes Maybe Unlikely

Maybe: irradiation Yes Maybe : irradiation Yes No Yes Yes Yes No Maybe : fractionation No

- These negations canceled for He-shell alternative, which yields only oxygen families plus 26AI. are much stronger for the C a m e r o n - T r u r a n model, since the 24sCm is, in t h a t case, live. The foregoing considerations, along with others which follow, are summarized in Table I, which is intended to provide a rough comparative evaluation of four models for the origin of the isotopic anomalies in the solar s y s t e m : (1) the C a m e r o n T r u r a n model, a minor fraction (~4v/v) of solar elements from a neighboring supernova that exploted about 1 m.y. before solar system solid condensation began, (2) a variant of the first, in which a much larger fraction ( ~ 4 0 % ) of solar h e a v y elements arrive from a massive supernova occurring several million years before condensation, (3) condensed interstellar dust (presolar carriers) t h a t preserves anomalies from expanding supernova condensations through prior galactic history, and (4) the conventional homogeneous solar gas plus, where necessary, irradiation by energetic particles. Another variant of (1), described in the next section, is included as a footnote to Table I. At this point, one can see that the collapsing solar nebular must be t h o u g h t of and

24sCm. The negative constraints

as a four-component s y s t e m : interstellar gas, interstellar dust, gas injected b y the neighboring supernova, and dust injected by the neighboring supernova. Fluctuations in the relative mixture m a y account for some anomalies, once the subsequent accumulation chemistry is deciphered, but some of the anomalies m a y result from homogeneous distribution of special carriers or of live radioactivities. 2~Na, 26A1,A He-SHELL ALTERNATIVE, OXYGEN FAMILIES Perhaps the strongest case for supernova-condensate carriers of extinct radioactivities is 2'2Na, because its 2.6-yr halflife is much too short to allow it to be injected live into the nebula, but is about right for supernova condensation (Clayton, 1975b) leading to sodium-sited 22Ne carriers for Ne-E. A neighboring supernova could, of course, be the site of the needed supernova condensation, but t h a t special event is in no obvious way superior to the ubiquitous interstellar dust condensates from prior supernovas. Since presolar dust carriers "u'e needed, even if from /.lie neighboring supernova, i have negated the

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special events in Table i as sources of 2~-Na. This is to be understood not as a denial t h a t it has happened, but as a denial of its being logically advantageous. This is in the spirit of seeking the minimal assumptions needed to account for the facts. Nonetheless, Clayton (1975c) argued t h a t explosively ejected He shells are a potentially prolific source of 22Na, and I note here t h a t t h e y can also t)e sources of '-'GAl. These facts lead me to consider a variation of the C a m e r o n - T r u r a n scenario t h a t gives up the a t t e m p t to inject heavy radioactivities and settles for oxygen anomalies plus 2~A1. Injection of the h e a v y radioactivities leads, in any case, to difficulties, such t~s '24sCm above and :~"(~1, 4°K, and ~:~Cs to follow. In this suggested variant, only an explosively ejected giant helium shell plus perhaps some underlying explosive carbon fuel are injected. Because the helium-burning shell was so thin, very little young s-process products would be injected if the presupernova had not undergone He-shell flashes. Howard et al. (1971) showed that, if this He shell is suddenly heated to T ~ 6 X 10S°K before ejection, the oxygen is isotopically puriiied to ~6(), while ~"F, :tNe, '-'"Na, and 26A1 are synthesized. The 2~Ne overabundance means t h a t trapping of Ne in condensates during the expansion of this zone are not the origin of Ne-E. The e~Na trod 26A1 are synthesized primarily by the sequence of ceactions,

~4N(a, "y)'SF(a, p)')~Ne(p, v) X 22Na(a, p)2:'Mg(p, V)'-'%kl, which is possible on the ~4N residue of CNO burning only in those portions of the He shell t h a t have not burned He significantly, because once He burning has begun, the 14N is converted to neutron-rich ~O and "'-'Ne. When the shock hits those preburned shells, the reactions ave neutron liberating and lead in,~lcad lo neutron-rivh isotol)es;

viz., 1s() (a, n).~lNe (a, n) X24Mg(n, ~,)2~Mg(n, 7)26Mg, and 22Na and 26A1 are not synthesized. Arnould and Beelen (1974) have studied this difference in detail. Concentrating on the shell's having 14N seed, I find t h a t the 26A1 production is growing rapidly with peak temperature, being small below 7 X 108°K. Near 7 X 108°K peak temperature, the 26A1 yield is near "+A1,/2tNe ~ 5 X 10 -2, so that, if such zones injected about 10-2 of solar 2~Ne, the average ratio 2~A1/27A1 = 6 X 10 5 would also be achieved in the mixed nebula. In the ejected m a t t e r itself, the 26AlffTA1 ratio is (luite high, perhaps even of the order of unity, an important point, to which I return in the next section. These hightemperature zones, in sum, e.m inject 1~*O and '-'~A], both at the rather modest level of l0 -3 g/g. The remainder is overwhelmingly tie. Whereas this is enough 2~+A1, it is probably not enough pure ~%) for the oxygen anomaly, which is + 5 % in the anhydrous minerals (Clayton et al., 1976). T u r n then to the portions of the He shell heated to less than 6 X 10S°K. T h e y eject vanishingly small '~"A1, but the oxygen is so ~() rich (~sO > 1+O) that it can be thought of as pure ~80. There is no 17(). Chemical systems having differing admixtures of this component will lead to displaced internal oxygen fractionation lines. On such a picture, for example, the C2 matrix received more ()i" this injecta than did the Earth, which in return received more t h a n (lid the L chondrites (Clayton et al., 1976). The anhydrous minerals of the C2-C3 metc<;rites, on the other hand, still require an admixture of an ~+O-rich component, which in this case would most plausibly be obtained from the supernova condensates. This gencrates a more complicated hybrid pictin'e, illustrating the ambiguity of interl)reting generic r(,l~lionshil)S I)ctw~wn the

SOLAR SYSTEM SUPERNOVA distinct oxygen families that R. Clayton et al. (1976) have delineated so beautifully. As a final touch, consider that the overlying hydrogen envelope may produce 170 from 160(p, 3,) 17F when it is heated by the shock. Arnould and Beelen (1974) have formally shown that linear admixtures of this zone (overproducing 170) with the underlying He shell (overproducing 1sO) can plausibly lead to a solar ~70/lsO ratio (with negligible ~O). The astrophysical possibility would be an ~60-rich prior solar nebula into which the solar ~70 and ~sO have been injected. Fluctuations in the relative amounts can lead to all of the oxygen families. Lee et al. (1977) have advocated the convenience of this type of admixture, while we have shown here to be a reasonable possibility, not requiring (but not excluding) presolar condensates. Simultaneous injection of 26A1 makes this alternative a viable model, as far as I can presently tell. It is quite in the spirit of the Cameron-Truran model, but less ambitious in its nuclear products. 2~MgCARRIER, SOLID CHEMISTRY OF THE EARLY SOLAR SYSTEM An entirely different possibility was predicted (Clayton, 1975h) from astrophysical principles, even before I knew of the first detection of 26Mg anomalies; viz., that the 26A1 was not live in situ, but had instead decayed in interstellar grains. That model is still attractive, since it follows from two independent astrophysical plausibilities : (1) Most interstellar A1 resides in interstellar grains, (2) condensation in the supernova expansion where it was synthesized was probably the main condensation venue for very refractory A1. These condensates trap 26A1/~VA1at its production ratio, which, in the example of explosive carbon burning, has been calculated by Arnett (1969) to be 2 X 10-~. In the interstellar medium, the condensates carry excess :SMg*. In the early nebula, these ("~rriev AI oxides were envisioned ~o re~wt

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in a solid-state chemistry with other condensates, leading to 26Mg*-enriched minerals. Lee et al. (1977) have presented a case that they believe dooms this model by arguing that separate mineral phases within an Allende inclusion have been differentiated from a common homogenized melt. I suspect that this totally homogenized melt never existed in the parent of this inclusion, and to focus on the key issues in defense of my picture I will describe an excathedra scenario for this inclusion. My picture does not have the solar system solids condensing from a hot solar gas, which was unable to play the major chemical role beyond about 1 AU. The accumulation of solar system objects proceeds instead with interstellar dust directly (Cameron, 1975), although modified by some further growth from the solar gas and some further chemistry with it. Figure 1 of Clayton (1975a) illustrates how that fraction of condensed Si, for example, that had already been condensed in interstellar gas falls from 100% initially to around 20% by the time condensation is complete. A maior fraction of refractories was never in a gaseous state, depending on the specific element and ranging from 90% (A1, Ca, Ti) to 20% (Si) to about 1% (O). The solid accumulation of modified presolar condensates then reacts in a mechanical mixture via a solid-state chemistry. For definiteness only, consider that A1203 supernova condensate carries the ~6Mg* into the solar nebula. Grains of this Al~O~ stardust are mechanically compacted into a matrix with other small grains of varying mineralogy, but including the remainder' of the refractory A1, which is totally condensed into other small grains. The bulk assembly is envisioned to be Ca, Al-rich, as befits a potential parent of the Allende inclusions. If the grains are micron-sized, fluctuations in the ratio of carrier A1 to solar A] will exist on that small scale; but, on a scale of 10urn or more, the carrier phase of the stardust will contril)ute an

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almost-constant fraction (3%, for example) of the total A1, so t h a t the average ratio 2~Mg*/27A1 is diluted from 2 X 10 -~ in the stardust to 6 X 10-5 in any 10-tLm assembly of the mechanical mixture. The argument would be easier if a much larger fraction of Al were assembled as presolar A1203, but communicating the chemical idea is the objective of the following argument. Now the matrix is heated su.[ficiently for solid-state recrystallization to occur, but not enough to melt and homogenize the mixture. The key result, here postulated, is t h a t the 26Mg* within the AI20:~ will not be chemically differentiated from the A1, but will be carried along with it into other minerals formed from the heated mixture. In effect, I postulate either AI cages for the '~6Mg* or insufficient diffusion to separate the 26Mg* atoms from the ~ 10 :~ times more a b u n d a n t ~TA1 atoms in which they are imbedded. The A12():~ is only added to other minerals, and the ~Mg* is carried along. Gradients in the bulk composition of the other constituents of the assembly (O, Mg, Si, Ca) are postulated to lead to local differences in the mineralogy after the heating. Crystallization of melilite and anorthite from a local assembly, to be sure, does involve some reproportion and movement of Mg, but not necessarily through the A12():, stardust to the degree necessary to scour out the 26Mg*. The question is whether chemistry of this t y p e is possible, i.e., if one considers recrystallization requiring ingestion of A1 carrying a small trace of 2~;Mg, will the "~Mg follow the Al? This seems more plausible if the ingested A1 is the tota.1 source of A1 than it. does if it has to be mixed with solar A1 from surrounding minerals. Experiments may be possible. As a Gedanken experiment, I imagine trying to fuse new minerals by heating (only sufficiently for reaction) an inhomogeneous mixture of these powders: A1203 doped with a recognizable Mg anomaly trod grown in :momalous oxygen, normal Mg(),

and a normal Ca-rich pyroxene. The extent to which the peculiar Mg is carried into the new minerals could then be measured, along with the redistribution of oxygen. I can see puzzling suggestions of this point of view in the data of Lee et al. (1977). Instead of a single best line, suppose we think of the two extreme anorthite points as comprising one system and the melilite-normal Mg points as comprising another. The slope through the two anorthite points yields ~6Mg*/27A1 = 4.2 X 10-~, whereas the slope from normal Mg through melilite yields 2~Mg*/27Al = 6.0 X 10-'~'. Perhaps the difference is not statistically significant, but let me, for a point of principle, assume t h a t it is. A smaller 26Mg* in the anorthite suggests t h a t part of it actually was lost in forming that highly Mg-deficient mineral. Some loss might be chemically expected, even with the chemistry postulated here. ()n the live-2~A1 picture, on tile other hand, the anorthite anti melilite seem to have formed at times separated by roughly h'df a million years, which m a y I)e a very long time difference for recrystallizing such primitive inclusions. A key question is petrological, and has not been adequately evaluated. Is this inclusion actually the solidification of -~ homogenized melt, or does its morphological texture indicate instead solid recrystallization with only a limited amount of disproportionation? Intuitively, one feels that crystallization of a homogeneous equilibr'~ted melt will result in regular patterns of minerals, rather than in the tortured and convoluted mineral shapes within m a n y such inclusions. Even if some flow of Mg is able to occur, is it possible t h a t some of the 2~Mg* m a y remain trapped in minute corundum crystals within the partial melt? If the last is possible, we m a y have high-T corundum cages for the ~'6Mg* anomaly, just as Clayton (1975a, 1977) has :~dvanced and defcnde(l in the 'z~'Xe correl:ltion with

SOLAR SYSTEM SUPERNOVA iodine. On this view, this inclusion is definitely not a high-temperature gaseous condensate nor a differentiation from a melt. It is a peculiar solid-state chemistry characteristic of the early solar nebula. The inclusion looks like a high-T condensate only because interstellar dust itself is a high-T condensate. No 2~Mg*-corrclated anomalies, except with oxygen, are specifically expected in this inclusion. The supernova oxygen should be anomalous, but it may be diluted by redistribution in the heated assembly. Heavy elements are almost exclusively in the solar-grown minerals within the preheated mechanical mixture, because the Al~O~ is envisioned to have condensed in ejecta of explosive carbon burning, where (Os, Pb)/A1 are 10-2 of the solar ratio due to the vastly enriched concentration of A1 there. The (A1/Ca) ratio in explosive carbon burning also exceeds solar by at least 102, so that relatively little atomic Ca is available for trace incorporation into the Al~O3. Trace Ca is not without interest, however, because Howard et al. (1972) showed that large enrichments of 43'46'48Ca are anticipated in this same burning shell. The largest of these is 46Ca, whose overabundance is roughly 30 times greater than that of the major thermonuclear products (Na, Mg, A1). If the Ca is chemically depleted by a factor of 30 relative to A1 in the condensation of corundum, the 4~Ca/'27A1 ratio in the corundum would be solar, whereas all other Ca isotopes would be negligible. If 3% of the A1 in the anorthite were stardust, therefore, the 46Ca/4°Ca ratio in the anorthite would be anomalous by +50-/0. Lee et al. (1977) saw no effect that large, but their limit does not exclude interesting and detectable effects. Lest the accuracy of our calculations be taken too seriously, however, I must add that a 46Ca effect this large is not really expected. I think that explosive carbon burning is the major nucleosynthetic source of aluminum, in which case the overproduction of 4"Ca

261

cannot exceed that of A1 by a factor of 30, or the natural abundance of 4~Ca would be 30 times greater than it is. On the compensating side, however, the stardust A1 could exceed 30/o and carry a smaller ~6Mg* anomaly, due to an overestimate of 26A1 nucleosynthesis. I hold it likely that the A1 is primarily stardust, but only 3% of it resides in those Al-rich cages that have carried the 26Mg* through the solid-state chemistry. The remainder of the excess 26Mg has been homogenized so as to be locally inseparable from normal Mg, of which it is a part. Therefore, effects of the order of 1% excess in 46Ca could be sought. It may be best to compare anorthitic Ca to Ca from an Al-poor, Ca-rich mineral from an Allende inclusion. As an important clarification of this model, let me emphasize that the production ratio ~6A1/27A1 = 2 X 10-3 and the complementing 3% fraction of carrier A1 oxides are only illustrative numbers. It is much too early to identify the exact origin of the 26A1that went into the carriers. The He-shell alternative for 26A1 makes a much larger 26A1/27A1 production ratio (take unity, for example), so that the carrier A1 from that zone need only be 6 X 10-'~ of the total AI. Corundum grains from that zone could turn into small spinels carrying huge 26Mg anomalies. Even spine!s condensing there will carry anomalies large enough to be the carriers. Clayton and Hoyle (1976) proposed a nova origin for the carriers. Still other exciting possibilities exist; viz., that explosive oxygen burning terminating before the initial 24Mg has been converted to 2sSi will eject large 26A1/27A1 ratios. Therefore, one should he imaginative in contemplating the required carrier chemistry. A production ratio 2~A1/27A1= 0.1 would, for example, be potentially quite important for the presolar carriers, even if it occurs in a site responsible for only l0 3 of the total A1 nucleosynthesis. ()n top of this discussion, it is worth reiterating how 5% anomalies in "~(_)remain,

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I)ONALI) 1). CLAYTON

when those in heavier elements are practically homogenized b y the mechanical mixture. Clayton (1975a, b) was first to suggest that, 160 carriers condensed within the supernova interiors. Only several percent of solar oxygen is condensed, because t h a t is the limit imposed by the abundances ()f refractory dements, most of which condensed in the supernova expansions, where the oxygen is t"(). Therefore, the interstellar grains are more ~O rich than the solar gas. The eondensates mechanically separate the two. Fluctuations richer than "tverage in pristine grains have been the parents of the anhydrous minerals in Allende inclusions. As the remainder of the solar condensation proceeded, the presolar condensed oxygen was diluted t)y exchange with gaseous oxygen and by further condensation of Si and O; schematically, for example,

the Allende inclusion than in Juvinas, and possibly much more. Seheinin et al. mention the classic interpretation t h a t tile Allende inclusions predate Juvinas by at least 10 s y r ; however, we wish to suggest, instead, applications to the problem of the neighboring supernova or of presolar carriers of ah'eady extinct 146Sm. Its half-life (1.0 X 108 yr) renders it essentially stable during the C a m e r o n - T r u r a n scenario (minor or major). These two isotopes of Sm are expected to be produced by the p process in the rough ratio P~48,/P144 ~ - 0.4, according to Audouze and Sehramm (1972), although I personally suspect that, the ratio m a y be still smaller. Taking that value for argument, and excluding galactogenie ~46Sm, one sees t h a t ~4~Sm/mSm = 0.014 if 3.5¢7o of the solar system p nuclei originated in the neighboring supernova. The dilution factor dis( C a O ) s ~ . a r d u s t -]- (SiO),,,~ --~ CaSi()a, etc. cussed by Cameron and T r u r a n would then At this point, it m a y seem to some readers be 30, a value in reasonable agreement t h a t I have speculated too much. M y objec- with the other dilution factors of their tive in this section was to outline more scenario. ()n this view, one m a y say either clearly how m y presolar-carrier model, that Juvinas formed at least 10 s yr after which is extreme only in m y suggestions of the Allende inclusion or t h a t the Allende inthe required solid-state chemistry (if there), clusion got more than its share of ~4~Sm, can account for the data of Lee et al. (1977). which was inhomogeneously distributed To immediately adopt their conclusion of in the meteorites. A large age difference live '-'~AI has far-reaching impact on one's m a y make no sense, however, because Marti view of the origin of the solar nelmla, as el al. (1977) show t h a t the 2441)u/Nd ratios Cameron and T r u r a n (1977) have indicated. obtained from a melilite separate of a (!ourse-gr.~ined Allende inclusion and from 146SM , xaa(1S the Juvinas basaltic achondrite agree Seheinin et aI. (1977) claim to have de- within error limits. Clearly a factor of 2 to tected extinct 146Sm through an excess of 3 difference in ~46Sm, due to age difference, 1 part in 104 in the r'ttio ~4"Nd/mNd, en- should correspond to a comparably large abling them to conclude that ~4'~Sm/~44Sm difference in -~4q)u abundance. Unfortu= 0.014 4- 0.009 at the time of solidification nately, the P-'gXe* I correlation has not been found in Juvin'~s, and, "~s t h a t (.orrelaof the Allende inclusion that they studied. This value is ahnost three times the sta- tion has not been found in other basaltic tistically marginal best slope 141~Sm/ta4Sm achondrites (eucrites), it will prot)ably not = (5.4 =t= 7.2) X 10 -3 t h a t Lugmair et al. be detectable in .luvinas. Clearly, it should (1975) found in Juvinas. Ignoring for a be searched for very carefully, t)ut its moment, the overlap of the error bars, h,t :d)sence may reflect metamorphism rather me suppose to clarify my ideas |,]lal, II';SIn lhan age difference if tn'esolar carriers are '*t~qnl was at l(':~st :t f:u'lor of '2 gl'(':tt('l" in lhe true source of ~e~'Xe* ((!layion, 1975a,

SOLAR SYSTEM SUPERNOVA 1977). If it could be found and if Juvinas differs from Allende in 14eSm/144Sm, the repercussions would be large and would probably swing the balance in favor of my presolar-carrier hypothesis. I wish to emphasize that a mineral need not be a primary condensate in order to contain the 129Xe*-I correlation from presolar carriers. All that is required is that the even-smaller cohost cages (Clayton, 1977) have been incorporated within the mineral when it formed during the warm-solid chemistry of accumulation. For the special case of Allende inclusions, it is certainly of interest that a unique high-T correlation line is not obtained (Podosek and Lewis, 1972). T hat inclusion corresponds to several lines, having an age spread of 4 m.y. If the ~6Mg*bearing inclusion is the same, it may also spell trouble for the very sharp 26A1 isochronism between separate minerals (assuming that both the 129I and 26A1are to be regarded as live). Viewed as an extinct residual of presolar grains (Clayton, 1975a), the 14~Nd excess has resulted from a portion of the presolar grains that was never vaporized. Variations in the fraction fsm of samarium-bearing grains that predated the solar system would account for the observed differences between unequilibrated and differentiated objects. The factor-of-2 variations in 142Nd* would bear resemblance to the factor-of-2 variations in '29Xe*/'2H. Taking the same production ratio as before, the supernova condensates would entrain 146Sm/144Sm 0.4, resulting in 142Nd*/144Sm ~'~ 0.4 in that fraction of interstellar grains. The Allende excess of Scheinin et al. would then be interpreted as having accumulated from a region 3.5% richer in presolar grains than the average solar condensates. Some such variations are likely, and should not be regarded as an extreme hypothesis. Furthermore, variations in the ratio ~4~Nd*/~Pu are expected in this model, because the "~44pu xenon resulted from primarily live 2441'u,

263

whereas the 142Nd* excess results primarily from extinct 146Sm in the carriers. Another aspect of the attempt to decide between a late spike and presolar carriers is the isotopic analyses of barium in Allende by McCulloch et al. (1977). They find that any enrichment of l~Ba due to 135Cs, having ti/2 = 2.3 X 108 yr, is less than 2 parts in 104. The 135Ba synthesis is largely due to the r process, and if we assume that 5% of the r process in the solar system was due to the late spike (a dilution factor of 20), the l~SBa excess would be an order of magnitude greater than the limits established by the experiment of McCulloch et al. On the other hand, Cameron and Truran (1977) argued carefully that the neighboring event w a s not a conventional r process, and they did this in order to avoid an overproduction of 1291 in their scenario. One would therefore expect t~ similar argument to apply to ~35Cs, but numerically the situation is different. The even-even s yield at 134Ba is about half of the r yield at 135Cs, as opposed to the factor 1 / 2 0 advocated by Cameron and Truran for the '29I case. They suggested that a dilution of 100 was adequate to lower 129I,/'27I to a value of 1 X 10-4 . It follows that a dilution of about 500 is needed to reduce l~Ba below 2 X 10-4. This seems to present another problenl for this scenario, considering that the la~Cs would have been "live" while the solar condensation was occurring. This absence is a problem also for the major event, because the 13~Cs half-life is too long for substantial decay. The neighboring supernova models are therefore negated in Table I, with the exception only of the He-shell alternative. Continued absence of '~Ba anomalies also presents problems for the picture of presolar carriers from supernovas, but they are of a different kind. The excess of '35Ba would be correlated with Cs in that portion that had condensed during the stellar expansion following r-process nucleosynthesis. But, on this picture, many anomalies are

264

I)()NAIA) 1). CLAYTON

possible, because supernow~ grains will naturally segregate s, r, and p nuclei, so

that large variations at all isotopes of Ba might be expected on a picture of fluctuations in the fraction f of presolar grains. Fine-scale mechanical mixtures have evidently provided an effective homogenization of most such anomalies, which would nonetheless be present if one had sufliciently small primitive samples. Such vari'~tions have also been searched for unsuccessfully in the element osmium (Grossman and G a n a p a t h y , 1976; T a k a h a s h i et al. 1977) in Allende inclusions and mineral separates. The absence of these variations suggests t h a t the presolar grains are overwhelmingly composed of those light elements (O, Mg, A1, Si, S, Ca) t h a t are synthesized in a b u n d a n c e in supernow~s, and t h a t these small grains have been mechanically Inixed with solar Ba amt Os during their cont inuing growth and accumulation in the solar system. Since the ~3-'Cs ix extinct in this model, it ix easier to avoi(l anomalies due to it. In support of this contention, i remind the reader t h a t l;~Ba'-"4Mg, for example, has a value l0 -2 smaller in explosive cart)on-burning ejecta than in solar material, and this difference is due to the overwhelming enrichment of Mg in t h a t cjecta, which is where l~O-rich presolar spinels (-"4,'-':',~-~Mg, 2~,'-'7A12, "~()4), melilites, forsterites, and ensta~ites should condense in al)un(lance. A large, say 10%, enrichm e n t in supernova M g would then /)e expected on simple a priori grounds to carry only a 0.1% enrichment in supernov't Ba. Locating the presolar Ba an}l ()s shouht be expected to be quite "t (tifferent problem from t h a t of M g or (), as the smallness of the r-)~Xe a n o m a l y demonstrates. Its value is characteristie'dly 12~'~Xe* "~I ~ 10 4, even though a sizable fraction of solar refra('tories was initially condensed. The refractory presolar carriers of this a n o m a l y :,re, p r o b a b l y (luite low in I c,m('entration. The l-correl'~ted "-"Xc excess remains (wen il' :~ [inc me('hanic;d mixture has el'l'e(.lively

homogenized bulk samples, whereas th~ '3'~Ba excess can be lost by the homogenization. And factor-of-2 fluctuations in the presolar carrier components m a k e only very small changes in mBa/i3SBa, but they m a k e factor-of-2 changes in 129Xe* JI. 4oK I wish also to t~dd to Caineron's and T r u r a n ' s a d v o c a c y of 4°K as coming from the neighboring supernova, because we have two papers in the literature bearing explicitly on the synthesis of 4°K. I show t h a t it is more likely gatactogenic. 1)eter,~ et al. 0972) showed t h a t if a weak s process made all of solar 4°K, it would also have made 19c~o of solar 4'K as radioactive 4'Ca at an s-process neutron flux r = 0.007, and it would have m a d e 200-/0 of solar '~SFe at r = 0.1. In the former case, large Ca-correlated excesses of 4~K m a y he expected on grounds explained I)y Clayton (1975b), an(t, in the latter case, much of the '~SFe would have resulted from the l'~st spike. If 4"K resulted from an s-process late spike, therefore, at least 20c~ of a stable m a j o r nuclide came from t h a t same last event. One should not totally discount t h a t possibility; however, fluctuation anomalies in :'SFe shouhl also t)e present. H o w a r d el al. (1972) studied the produ(.tion of 4°K in explosive carbon 1)urning, another possibility mentioned by Cameron an(l Truran. We showed there t h a t the <)verabundances of 4°K are interesting, but not really large eomp.~re(l to other over•~t)undances. For example, ttt T9 = 2.05, we see from their Table 2 that, even if the late event m'tde all of the ~4Mg, it m a d e only 2S/S2 = 34% of the 4°K. Both of these studies show t h a t if the solar supply of 4°K were due to a late event, t h a t same event probat)ly eontribute(t a large portion of some stable nucleus; i.e., it was a major late event rather t h a n a ininor one. The idea of :momalics (hw to inhomog(,neous [hwluati(ms, as suggested I)y (!ameron and T r u r a n for the '"() anom:dy,

SOLAR SYSTEM SUPERNOVA

265

ratio 3°C1/:~CI-~- 0.2, which is a large a m o u n t of 3°C1 in the ejecta. Comparing, alternatively, the two long-lived radioactivities, one sees 3°C1/4°K ~ 1. If a significant fraction of 4°K were from the late event, therefore, an interestingly large concentration of ~°C1 was also injected. If this 3°C1 remains live, SOAr excess correlated with C1 should be detectable, because C1 is roughly 6 orders of magnitude more a b u n d a n t than Ar in carbonaceous meteorites. I do not know of the meteorites revealing anywhere the large excesses of S°Ar t h a t might be expected in the C a m e r o n T r u r a n model, although the 36Ar/3SAr ratio in the 600-800°C fraction of Allende (Manuel et al., 1972) rises to about 6.0, definitely in excess of the normal ratio. Perhaps, this small excess could be due to extinct 36C1, but the puzzle is t h a t it is not much larger if the 3°Cl were injected live. I therefore call for new searches for extinct 3°C1. Either it will be found or its absence will place severe constraints on the C a m e r o n - T r u r a n model. What is needed is experimental correlation of excess 36Ar with in situ chlorine, in exact analogy to '29Xe-I. Perhaps, t e m p e r a t u r e fractions following neutron irradiation m a y also work in this case, but the lack of a good reference isotope in Ar m a y present difficulties. Discoveries of excess S°Ar in bulk are not adequate, because spallation and neutron capture by 3~C1 can produce excess 36At in meteorites. One program might be as follows: Divide the meteoritic sample into two samples as nearly identical as possible, irradiate the second sample with 3~CL neutrons converting 37C1 into 3BAr, and then The weak s process also makes 3°C1 compare 36Ar//3SAr temperature fractions (tl/2 = 3.1 X 10 ~ yr) as effectively from in the two samples. The role of prior ~sC1 as it makes 4°K from ~gK. This nucleus neutron irradiation in the meteorite could also lives almost as long as 2°Al, so it offers be evaluated through Kr and Xe anomalies potential for leaving 36Ar anomalies in con- in the same samples. densations occurring within a million years Presolar carriers provide a small (because of the event. Taking ~n.~(35C1) to be 15 36C1 nucleosynthesis is small) source of mbar, one sees that the s-process neutron ~°Ar* (analogously to '29Xe*), however, so flux r = 0.01(X10~Tn cm -2) results in the t h a t positive detection of extinct 3°C1

appears to present a serious flaw to 4°K injection. To high accuracy, the K isotopic composition in an Allende inclusion (Begem a n n and Stegmann, 1976) equals the terrestrial composition. If all 4°K were due to the last event, it becomes necessary to mix it with the other potassium isotopes very homogeneously before forming such diverse objects as the E a r t h and Allende inclusions. One must question the plausibility of having 5 % fluctuations in late ~60 exist in the apparent absence of fluctuations in 4°K. The live ~29I is effectively stable over the million-year period postulated for condensation, so the factor-of-2 variations of ~29I/'27I within meteorites seemingly should also be interpreted in the C a m e r o n - T r u r a n model as fluctuations in the late-injection component. But how can '~9I/~27I have large fluctuations, while *°K has only small ones, considering t h a t both '29I and 4°K are supposed to have been injected by the neighboring supernova? In raising this puzzle, I note t h a t m y presolar grain model for ~29Xe* does not suffer from the same problem, and therefore m a y be superior. The '29Xe* excesses are entirely within the presolar grains, whereas all three isotopes of K coexist in both grains and gas. The isotopic percentage of *°K in grains and in gas can even be expected to be equal if the age distribution of interstellar grains approximately equals the age distribution of interstellar atoms. Thus the '~gXe* and ~60, which are both in large excess in presolar grains, can both fluctuate without noticeably causing 4°K to fluctuate.

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DONAL1) D. CLAYTON

would still leave both models as candidates. Lack of extinet-asC1 '~t~Ar would make it tough for late spike models, howew~r, except perhaps for the He-shell alternative. Note the double negatives required in the logistics of Table I.

that haw~ 'w~cumulated in the ensi.atite meteorites (Larimer, 1975). ltowever, in t h a t case the enstatite meteorites might he expected to be '-°C rich, whereas, more likely, the opposite is the true case. Vdovykin and Moore (1971) summarize m a n y researches into the puzzling isotopic ~C/'~C, C/O composition of carbon in meteorites, but Although the hydrogen envelope of a the enstatites tend to be about 2 % richer massive star m a y be quite rich in ':~C, in ~:~C than "~re the ordinary chondrites. approaching ~2C/~aC -- 4, it seems astro- ()ne might therefore try to solve this the physieally likely t h a t a neighboring super- other way around, supposing that the late nova injecting much of the solar system event eontributed much of the C and O, oxygen as isotopieally rich '~O would also but with C/() < 1/2, so that the oxygen have injected a significant fraction of solar richness is increased. Fluctuations rich in system carbon as isotopieally rich ~2C. This this late component would then form picture could explain why interstellar me'~- '2C-rich oxidized minerals, whereas those surements of ~C/"aC yieht values near 50, rich in the earlier-nehula component might rather than near the solar value near 89. have condensed the ':~C-rieher enstatite Although this difference is usually inter- minerals and earbon'~tes. The E a r t h itself preted as an aging effeet in the chemical m a y have condensed primarily as a reevolution of the galaxy, brought about by (lueed, rather than as an oxidized, minercontinuous nueleosynthesis, one should per- alogy if it was primarily from the earlier haps imagine the staggering possibility component. Indeed, the generic kinship of t h a t of the order of half of the solar heavy E a r t h and E meteorites is suggested by elements came from the neighboring super- oxygen isotopes (Clayton et al., 1976), nova[ The solar system would then be which also show that the anhydrous C2-C3 '2C rich in bulk. The much-larger dilution minerals were oxidized in regions much enriched in ~'(). This model also explains factors estimated hy Cameron and T r u r a n would then be interpreted as the fluctua- easily why the solar wind should be lions in the dilution factors, rather than the '3C rich, if in fact such a situation proves dilution factors themselves. This wouhl necessary to account for the ~aC richness then suggest a much-longer free decay in- of lunar soils (Epstein and Taylor, 1972). terval for 26AI, reducing it from '2~Al,. The ahnost-10% increase in '2C/'laC in ~¢AI~ 10 -:~ to ahout 6 X 10-:' when the organic material in carbonaceous chonsolar condensation begins, in which case drites relative to carbonates m a y relate to the problems of 24sCm and aaCd would be this late injection, although this difference ameliorated, since they woul O. The large ~aC t h a n is terrestrial carbon (Clayton, fluctuations containing gaseous C -~ () 1963). The point of the present discussion might be expected in the solar nebula; and is that anomMous carbon fluctuations are rapid condensation in these regions could to be expeeted along with those of anomalous oxygen, and not tLs an extra hypothesis. be the major source of the reduced minerals

SOLAR SYSTEM SUPERNOVA Nucleosynthesis of ~2C norm'~lly accompanies nucleosynthesis of 16(), so that, if the argument of Clayton et al. (1976) against chemical origin of the oxygen families is accepted, one should not attempt to explain carbon families solely by chemical arguments. It is to the simultaneous occurrence of chemical fractionation plus distinct genetic pools that I call attention. If these differing pools can have shifted the C/O condensation balance (Larimer, 1975), it seems necessary to again appeal to the low heavy-element concentration in the ~60-rich injecta to minimize the necessity of heavy-element isotopic differences between Earth and carbonaceous chondrites. In explosive carbon ejecta, for example, the 160/heavy element ratio has been increased by a factor of 102 over solar. TRANSBISMUTH CHRONOLOGIES In order that a major late event not destroy the classical conclusion of nuclear cosmochronology, it would seem that it must have been relatively weak in producing transbismuth nuclides. Then only a minor fraction of 235U was due to the late event, and it remains possible for 235U to be roughly five times less abundant than 2asU when compared to their production ratio. Th at is, although a major neighborhing event may have contributed as much as half, say, of 12C, 160, 24Mg, etc., it has contributed only several percent of U and Th. There is nothing obviously wrong astrophysically with this idea, because the large thermonuclear supernovas may not provide the sites for the traditional r process. And if the late injection were large, but almost homogenized with the earlier nebula, the fluctuation problems discussed earlier would be minimized. FLYPAPER MODEL Finally, one must reconsider whether the supernova need be regarded as "the cause" of the solar condensation. It is easy to see

267

that if 100 stars, say, form during the collapse of a 1000-M o molecular cloud, one of them may have exploded and dirtied the solar nebula, especially if the latter has lagged somewhat in its own collapse. One should consider that the debris m ay have been caught by the planetary disk around the early Sun, an idea advanced by T. Gold at the Gregynog Workshop as a "fly-paper model." I propose here that such an idea not only affords the possibility of injecting anomalies into the disk, but that it can also provide anomalies through low-energy nuclear reactions. The average kinetic energy of a Type II supernova surface layer is about 0.5 MeV/nucleon, corresponding to 104 km/sec. That energy is too low for nuclear reactions, but suppose that the outer 10-2 M o were ejected at v > 3 )< 104 km/see from a supernova a distance d (1.y.) from the Sun. The proton fluence in the disk is then, for linear rays, CpT = l0 TM cm -2 d-2(1.y.), which is adequate to produce 22Ne anomalies in unshielded grains if d < 10 1.y., but not to produce the 26Mg anomalies (Clayton et al., 1977), which would instead have to be injected as live 26A1. 1sO-Rich portions of the disk atmosphere could then perhaps condense the 160-rich grains, if they cannot be carried in directly at high speeds without being vaporized while stopping. The inner zones where the 160 grains condense may, however, expand at v - ~ 10~ km/sec, instead of the 104 km/sec characteristic of the surface layer. CONCLUSION The scorecard in Table I was constructed as a reminder of the arguments, and in such a way that the entry N o in a column negates that particular model. Without coming to any decisive conclusion relative to the Cameron-Truran scenario, I have simultaneously criticized it and also found new merit in it. I showed that if the late event had synthesized the radioactivities ascribed to it, it would have occurred several million years before the

2GS

I)()NALI) I). ('I,AYT()N

s o l a r c o n d e n s a t i o n a n d been able t o cont r i b u t e a m a j o r f r a c t i o n of t h e solar h e a v y e l e m e n t s , r a t h e r t h a n o c c u r r i n g 106 y e a r s before a n d c o n t r i b u t i n g o n l y a m i n o r f r a c t i o n . I f one a d h e r e s t o t h e m i n o r l a t e event advocated by Cameron and Truran (1977), o n l y t h e ~*0 a n o m a l y a n d 2*A1 anomaly from the He-shell alternative can be a d v o c a t e d w i t h o u t serious n u c l e o s y n t h e t i c difficulty. I also find t h a t m y m o d e l of p r e s o l a r c a r r i e r s of e x t i n c t a n o m a lies r e m a i n s viable, e v e n in t h e crucial case of e x t i n c t 2~A1. ACKNOWLEDGMENTS I thank Dieter Heymann for discussing many of the ideas of this paper as it developed, A.G.W. Cameron for discussing his model and for urging me to include my interpretation of the 26Mg anomalies, and Edward Anders for constructive criticism as a referee. Till Kirsten and Oliver Schaeffer provided useful discussions at the Max-Planck-Institut ffir Kernphysik (Heidelberg), which is thanked for preparing the revised manuscript during my stay there as a Humboldt Awardee. This research was supported by the National Science Foundation under Grant AST 74-20076. REFERENCES

ARNETT, W. 1). (1969). Explosive nucleosynthesis in stars. Astrophys. J. 157, 1369-1380. ARNOULD, M., AND BEELEN, W. (1974). More about the nucleosynthesis of nuclei between carbon and neon. Astron. Astrophys. 33, 215-230. ACDOUZE, J., AND SCHm~MM, D. N. (1972). 146Sm: A chronometer for p-process nucleosynthesis. Nature 237, 447-449. BEGEMANN, F., AND STEGMANN,W. (1976). hnplications from the absence of a 4~K anomaly in art Allende inclusion. Nature 259, 549-550. CAMERON, A. G. W. (1975). Clumping of interstellar grains during formation of the primitive solar nebula. Icarus 24, 128-133. CAmeroON, A. G. W., AND TRUUAN, J. W. (1977). The supernova trigger for formation of the solar" system. Icarus 30, 447-461. CL:YYTON~ 1). D. (1975a). Extinct radioactivities: Trapped residuals of presolar grains. Astrophys. J. 199, 765-769. CLAYTON, 1). D. (1975b). ~-2Na, Ne-E, extinct radiuactive anomalies and unsupported 4°Ar. Nature 257, 36-37.

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