Did Solar Energetic Particles Produce the Short-lived Nuclides

THE ASTROPHYSICAL JOURNAL, 549 : 1151È1159, 2001 March 10 ( 2001. The American Astronomical Society. All rights reserved. Printed in U.S.A.

DID SOLAR ENERGETIC PARTICLES PRODUCE THE SHORT-LIVED NUCLIDES PRESENT IN THE EARLY SOLAR SYSTEM ? J. N. GOSWAMI, K. K. MARHAS, AND S. SAHIJPAL1 Physical Research Laboratory, Ahmedabad 380 009, India ; goswami=prl.ernet.in, jitti=prl.ernet.in, sahijpal–sandeep=yahoo.com Received 2000 April 5 ; accepted 2000 November 9

ABSTRACT Production of the short-lived nuclides 41Ca, 36Cl, 26Al, and 53Mn by solar energetic particles (SEP) interacting with dust grains of chondritic (\solar) composition is estimated considering a broad range of spectral parameters for the SEP and appropriate nuclear reaction cross sections. The dust grains are assumed to follow a power-law size distribution and to range in size from 10 km to 1 cm. The possibility that an enhanced Ñux of SEP from an active early (T Tauri) Sun could have been responsible for the production of these short-lived nuclides in the early solar system is investigated. SEP production of 41Ca and 36Cl will match their abundances in the early solar system inferred from meteorite data if the SEP irradiation duration was D 5 ] 105È106 yr and the SEP Ñux was higher by a factor of more than 5 ] 103 than the contemporary long-term averaged value of N (E [ 10 MeV) D 100 cm~2 s~1. However, corresponding production of 26Al will be much below proton the level needed to match its inferred abundance in the early solar system. SEP production, therefore, fails to explain the observed correlated presence of 41Ca and 26Al with canonical initial abundances in early solar system solids. The abundance of 53Mn in the early solar system is not tightly constrained by the meteorite data, and the various estimates di†er by a factor of 5. Coproduction of 41Ca, 36Cl, and 53Mn that will match the meteorite data for the higher initial abundance of 53Mn is possible if the SEP irradiation persisted for about a million years or more with a Ñux enhancement factor of D 5000È10,000. On the other hand, the lower initial value of 53Mn can also be matched by a Ñux enhancement factor of D1000 and an irradiation duration of a few million years ; the corresponding production of the other nuclides will be ¹10% of the level needed to match their abundances in the early solar system. Target abundance consideration rules out the possibility of SEP production of 60Fe, another short-lived nuclide present in the early solar system. Thus, SEP production as the primary source of the short-lived nuclides in the early solar system appears to be unlikely. However, the possibility that SEP production could be an important source of 53Mn as well as of the short-lived nuclide 10Be, whose presence in the early solar system solids has been recently reported, makes it difficult to completely rule out any contribution from this source to the inventory of these nuclides in the early solar system. Subject headings : meteors, meteoroids È nuclear reactions, nucleosynthesis, abundances È solar system : formation È solar system : general È Sun : Ñares 1.

INTRODUCTION

with gas and dust in the solar nebula soon after the presence of this short-lived nuclide in the early solar system was established from meteorite studies (Lee, Papanastassiou, & Wasserburg 1976). Clayton (1994) proposed production of several short-lived nuclides via interactions of low-energy particles within the protosolar molecular cloud itself (see also Clayton & Jin 1995a ; Ramaty, Kozlovsky, & Lingenfelter 1996). SEP production of short-lived nuclides during the infall of solar nebula material into the evolving protoSun has also been proposed (Shu et al. 1997 ; Lee et al. 1998). It is important to identify the exact source(s) of the shortlived nuclides present in the early solar system as they can serve as time markers of processes that led to the formation of the solar system. If the short-lived nuclides were injected into the protosolar molecular cloud from a stellar source, their presence in early solar system solids puts very strong constraints on the time interval between the production of these nuclides in the stellar source and the formation of early solar system solids. In fact, analytical studies indicate this timescale to be less than a million years (Wasserburg et al. 1995 ; Cameron et al. 1995 ; Arnould et al. 1997). This short timescale led to the revival of the suggestion made by

Isotopic studies of meteorites provide evidence for the presence of several now-extinct short-lived nuclides with half-lives ranging from D105 (41Ca) to D 8.2 ] 107 yr (244Pu) in the early solar system. Most of these nuclides are considered to be freshly synthesized stellar products that were injected into the protosolar molecular cloud prior to or during its collapse (see, e.g., Cameron 1993). In particular, it has been proposed that some of the relatively shorterlived nuclides, such as 41Ca, 36Cl, 26Al, 60Fe, 53Mn, and 107Pd, could have been derived from a single stellar source (Wasserburg et al. 1994, 1995 ; Cameron et al. 1995 ; Gallino et al. 1996 ; Arnould, Paulus, & Meynet 1997). However, the possibility that some of these nuclides could be products of energetic particle interactions taking place within the protosolar molecular cloud itself or later in the solar nebula has also been proposed. Heymann & Dziczkaniec (1976) pointed out the possibility that 26Al (half-life \ 7 ] 105 yr) is a product of interactions of solar energetic particles (SEP) 1 Current Address : Department of Physics, Panjab University, Chandigarh 160014, India.

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Cameron & Truran (1977) of a triggered origin of the solar system (see, e.g., Boss & Foster 1997 ; Foster & Boss 1997 ; Vanhala 1998 ; Vanhala & Cameron 1998 ; Goswami & Vanhala 2000). On the other hand, if the short-lived nuclides are products of SEP interactions with material in the solar nebula, they cannot be used as time markers of presolar processes ; their presence provides us with speciÐc information about the energetic environment in the early solar system. The suggestion for SEP production of the short-lived nuclide 26Al present in the early solar system (Heymann & Dziczkaniec 1976) was followed by detailed calculations of the production rate of this nuclide for various irradiation scenarios and di†erent SEP spectra and target characteristics (Clayton, Dwek, & Woosley 1977 ; Heymann et al. 1978 ; Lee 1978). The results obtained in these studies showed that an extremely high Ñuence ([1020 cm~2) of energetic ([MeV) solar protons with speciÐc spectral characteristic is required to match the meteorite data for the initial abundance of 26Al in the early solar system. These studies were complemented by the work of Wasserburg & Arnould (1987), who estimated SEP production of 26Al and 53Mn (half-life \ 3.7 ] 106 yr) following the evidence for the presence of 53Mn in the early solar system obtained from meteorite studies (Birck & Allegre 1985). The results obtained by Wasserburg & Arnould (1987) showed that it is not possible to match the meteorite data to the initial abundances of these two short-lived nuclides with a single set of spectral parameters for the SEP. During the last decade, meteorite studies have provided evidence for the presence of two other short-lived nuclides in the early solar system, 60Fe (half-life \ 1.5 ] 106 yr ; Shukolyukov & Lugmair 1993a, 1993b) and 41Ca (half-life \ 1.03 ] 105 yr ; Srinivasan, Ulyanov, & Goswami 1994 ; Srinivasan et al. 1996), and a strong hint for the possible presence of a third one, 36Cl (half-life \ 3 ] 105 yr ; Murty, Goswami, & Shukolyukov 1997). These observations led to a renewed interest in SEP production of the short-lived nuclides in the early solar system. Recently, Shu et al. (1997) and Lee et al. (1998) considered SEP production of several short-lived nuclides within the framework of the ““ X-wind ÏÏ model (Shu, Shang, & Lee 1996) to address this question. In their model, the SEP interactions with nebular material take place very close to the proto-Sun during the infall of material from the nebular disk into the evolving proto-Sun and the irradiation timescales could be a few years to a few tens of years. By contrast, in the conventional scenarios proposed earlier, SEP interactions with gas and dust in the solar nebula are considered to be taking place in the meteorite-forming zone, i.e., at 2È4 AU, for a much longer duration. In this paper we follow the conventional scenario and present the results obtained from a study of SEP production of the short-lived nuclides 41Ca, 36Cl, 26Al, and 53Mn. Production of 60Fe is not considered as it can be ruled out from elementary considerations (lack of suitable targets with signiÐcant abundance in nebular material of solar composition). In the next section we present the details of the analytical model adopted by us and also the input parameters used in the calculations. We then present the results and discuss their implications for SEP production as a source of the shortlived nuclides present in the early solar system. Initial results obtained from this work were presented at a Lunar and Planetary Science Conference (Goswami, Marhas, & Sahijpal 1997).

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ANALYTICAL MODEL AND INPUT PARAMETERS

The analytical expression that we use for calculation of production rate of any short-lived nuclide is P \& i j

P

F(E) ] N ] p( j ] i, E)dE , j

(1)

where P is the production rate of radionuclide i, F(E) repi resents the Ñux of the solar energetic particles as a function of energy, N is the number of target nuclide j, and p(E) is j the relevant nuclear reaction cross section as a function of energy. 2.1. Flux of the Solar Energetic Particles SEP Ñux may be represented by either a power law in kinetic energy, dN P E~cdE, or an exponential in rigidity, dN P exp([R/R )dR, where c represents the power-law 0 spectral index and R the characteristic rigidity. In this 0 study we have considered both these forms of representations. The values for the power-law index c and the characteristic rigidity R were varied from 2 to 5 and from 50 to 0 ; these numbers encompass the broad 400 MV, respectively range of values seen in contemporary solar Ñares. We adopt a Ñux normalization of N (E [ 10 MeV) \ 100 cm~2 protonthe long-term averaged SEP s~1 ; this adequately represents Ñux over the last million years as inferred from studies of lunar samples (see, e.g., Reedy 1998). Both proton- and alpha-particleÈinduced reactions were considered, and the alpha/proton ratio was varied between 0.05 and 0.1 ; the value for this ratio averaged over the solar cycles 20 and 21 was found to be D0.06 (Goswami et al. 1988). We have not considered production by 3He as the occasional 3He-rich Ñares are mainly impulsive weak Ñares (see, e.g., Kocharov & Kocharov 1984 ; Reams 1999) that contribute very little to the total Ñuence of SEP integrated over a solar cycle. 2.2. T arget Characteristics and Irradiation Scenario Various irradiation scenarios and target characteristics were considered in the previous studies of SEP production of the short-lived nuclides 26Al and 53Mn (Heymann & Dziczkaniec 1976 ; Clayton et al. 1977 ; Heymann et al. 1978 ; Lee 1978 ; Wasserburg & Arnould 1987). Both nebular gas and solids of solar composition were considered as targets, and the production rates of these two nuclides were estimated considering the following scenarios : (1) irradiation of nebular gas without shielding ; (2) irradiation of nebular gas, where condensation of solids is taking place ; and (3) irradiation of nebular solids (thin target approximation). Even though self-shielding of SEP by nebular gas was not explicitly taken into account, its e†ect was considered through certain ad hoc assumptions (Lee 1978 ; see also Clayton & Jin 1995b). In the present study, we consider early solar system solids of chondritic (\solar) composition ranging in sizes from 10 km to 1 cm and following a size distribution of the type dn P r~bdr as targets ; the values of b considered in the calculations are 3, 4, and 5. We postulate these solids to be the precursor of the refractory CAIs (Ca-Al inclusions) found in primitive meteorites, which almost exclusively contain the records of the two shortest-lived nuclides, 41Ca and 26Al. We ignore shielding by nebular gas and assume that SEP particles will have free access up to 2È4 AU ; this will maximize SEP production of the short-lived nuclides.

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SHORT-LIVED NUCLIDES IN EARLY SOLAR SYSTEM

2.3. Nuclear Reaction Cross Sections The reaction cross sections for the production of 41Ca, 26Al, and 53Mn from di†erent targets by low-energy protons and alpha particles have been compiled recently by Ramaty et al. (1996), and we have used these cross sections in our calculations. In the case of 36Cl, cross sections for production from Ca and K [Ca(p, x)36Cl and K(p, x)36Cl] have recently become available (Imamura et al. 1997 ; Schiekel et al. 1996 ; Sisterson et al. 1997b). However, cross sections for individual proton- and alpha-particleÈinduced reactions involving S, Cl, and K as targets are not available at present, and we have estimated these by considering equivalent reactions. In particular, the reaction cross section for 33S(a, p)36Cl was based on data for a set of equivalent reactions in the compilation of Lorenzen & Brune (1974). We estimated the cross section for the reaction 34S(a, pn)36Cl from the equivalent reactions

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39K(a, pn)41Ca and 24Mg(a, pn)26Al, and for 36S(p, n)36Cl, we considered the reaction 41K(p, n)41Ca. The equivalent reactions considered to obtain the cross section for the reaction 37Cl(p, pn)36Cl are 27Al(p, pn)26Al, 42Ca(p, pn)41Ca, and 54Fe(p, pn)53Fe. Intermediate values of cross sections for 44Ca(p, p3n)41Ca and 28Si(p, 2pn)26Al were chosen for the reaction 39K(p, 3pn)36Cl. The values adopted by us are close to those reported for production of 36Cl from K (see Fig. 1). Finally, for the reaction 35Cl(a, 3He)36Cl, we considered the equivalent reaction 40Ca(a, 3He)41Ca. However, as the threshold energy for the former reaction is D2.5 MeV amu~1 compared to D7 MeV amu~1 for the latter, the reaction cross section for 36Cl production was adjusted by scaling down the energy by a factor of 2.8. In Figure 1 we show the reaction cross sections used to estimate the SEP production of the di†erent short-lived nuclides.

FIG. 1. ÈCross sections of proton- and alpha-particleÈinduced reactions leading to production of the short-lived nuclides 26Al, 41Ca, 36Cl, and 53Mn. Data for 26Al, 41Ca, and 53Mn are from the compilation of Ramaty et al. (1996). Reaction cross sections for production of 36Cl are estimated by considering equivalent reactions (see text for details). Also shown are the measured cross sections for the K(p, x) 36Cl and Ca(p, x) 36Cl reactions compiled by Sisterson et al. (1997b).

FIG. 2.ÈProduction rates of the short-lived nuclides 26Al, 41Ca, 36Cl, and 53Mn in atoms minute~1 kg~1 due to SEP interactions with spherical grains of chondritic (\solar) composition shown as a function of shielding depth. Results obtained for grains of two di†erent sizes are shown for two spectral representations of the SEP and three values for the spectral parameters (c and R ). Surface production rates are plotted at a shielding depth of 10~4 g cm~2. 0 s~1. The results are for an SEP Ñux normalization of N (E [ 10 MeV) \ 100 cm~2 proton

SHORT-LIVED NUCLIDES IN EARLY SOLAR SYSTEM

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the relation (2) may be expressed as (in the case of a powerlaw energy spectrum, dN P E~cdE) (dN/dE) \ const.[Eg ] Eg](1~g~c)@gEg~1 , (4) x (s/x) where E represents energy of an SEP with residual (s/x) range x. A similar approach can be used to obtain an appropriate expression when the Ñux of SEP is represented by an exponential in rigidity. The production rates of the di†erent nuclides at di†erent shielding depths within grains of di†erent radii were obtained by using numerical approach to evaluate equation (1) incorporating appropriate input parameters. A software package developed by us for calculating SEP production rates in lunar samples was suitably modiÐed for the present purpose. On the basis of the production rate proÐles as a function of depth within grains of di†erent sizes (Fig. 2), we estimated the integrated production rate for each nuclide for grains of di†erent sizes. The dependence of production rate on grain radius (Fig. 3) was approximated by an analytical expression in each case and was used to estimate the e†ective production rates of the short-lived nuclides for various grain ensembles following di†erent size distributions. 3.

FIG. 3.ÈProduction rates of the short-lived nuclides 26Al, 53Mn, 36Cl, and 41Ca as a function of grain size due to SEP interactions with grains of chondritic composition [Ñux normalization : N (E [ 10) MeV \ 100 proton cm~2 s~1]. Results obtained for two spectral representation of the SEP Ñux are shown.

2.4. Calculation of Production Rate SEP production rates of the di†erent short-lived nuclides are estimated as a function of shielding depth within a target of given size and with chondritic composition using equation (1). The energy-degraded SEP spectrum at a shielding depth x within the target, (dN/dE) , is obtained by considering only ionization energy loss,x following the approach of Lal (1972) : (dN/dE) \ (dN/dE) [(dE/dS) /(dE/dS) ] , (2) x x/0 0 x where (dN/dE) represents the primary energy spectrum x/0 of the SEP incident on the target and S represents the residual range. If we assume that the range-energy relations for the proton and alpha particles in chondritic targets can be approximated as S \ const.Eg ,

(3)

RESULTS

The production rates of the short-lived nuclides 41Ca, 36Cl, 26Al, and 53Mn at di†erent depths within spherical target grains of chondritic (\solar) composition are shown in Figure 2 for two di†erent grain sizes and for di†erent SEP spectral parameters. The production rates are given in units of atoms minute~1 kg~1 of target, and the size and depth are expressed in g cm~2, assuming a density of 3.4 g cm~3 for the target grains of chondritic composition. Production from both proton- and alpha-particleÈinduced reactions is considered. The results presented here are for a value of 0.1 for the alpha/proton ratio. The contribution from the alpha particles to the total production depends on the spectral characteristics of the SEP and becomes signiÐcant for steeper energy spectra (lower R and higher c values) and varies from D15% to D50% for0 26Al and 41Ca. In the case of 36Cl, contribution from alpha particles exceeds that from protons for R ¹ 100 MV and c º 1.5. In contrast, 53Mn is primarily a 0product of proton-induced reactions with less than 5% contribution from alpha particles, and for the Ñux normalization used in this study its production is not sensitively dependent on the spectral parameters (see Fig. 2 and Table 1). It can be noted from Figure 2 that the production rate proÐles fall steeply with depth except in the case of the smaller grains, where the production rates are relatively high and are nearly independent of depth ; this is also consistent with an e†ectively isotropic SEP irradiation of the grains. We have used the data for the depth dependence of production rate to estimate the integrated production rate for individual grains of di†erent sizes. The results are shown in Figure 3 for two spectral representations of the SEP (power law in kinetic energy [c \ 3] and exponential in rigidity [R \ 100 MV]). We have approximated the dependence of the0 production rate on grain size by an analytical expression to estimate the production rate for ensembles of grains following speciÐc size distributions. In Table 1 we show the results obtained for six di†erent SEP spectral parameters ; data for two grain-size distribution parameters are presented in each case. Because of the higher production rates in

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TABLE 1 PRODUCTION RATE (ATOMS MINUTE~1 kg~1) OF SHORT-LIVED NUCLIDES BY SOLAR ENERGETIC PARTICLES IN TARGET OF CI (\SOLAR) COMPOSITION NUCLIDE PARAMETER

26Al

36Cl

41Ca

53Mn

Half-life (yr) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 ] 105 3 ] 105 1.03 ] 105 3.7 ] 106 Initial abundancea (atoms kg~1) . . . . . . . . . . . . . . . . 9.7 ] 1018 1.3 ] 1016 2.0 ] 1015 9.6 ] 1017(H) ; 2.0 ] 1017(L) Production rate (atoms minute~1 kg~1) : . . . . . . c \ 2; b \ 3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 366 5.7 3.6 175 c \ 2; b \ 4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 381 6.1 3.8 182 c \ 3; b \ 3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 469 13.1 6.8 185 c \ 3; b \ 4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 505 14.3 7.5 196 c \ 4; b \ 3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 790 31.5 13.7 195 c \ 4; b \ 4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 875 35.3 15.2 210 R \ 50 ; b \ 3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 277 4.5 3.1 144 0 R \ 50 ; b \ 4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 298 4.9 3.4 154 0 R \ 100 ; b \ 3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 316 3.4 2.3 158 0 R \ 100 ; b \ 4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 331 3.6 2.5 166 0 R \ 200 ; b \ 3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 343 3.0 1.9 157 0 R \ 200 ; b \ 4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 354 3.1 2.0 163 0 NOTE.ÈData are given for two ensembles of grains following di†erent size distributions (b \ 3 and 4) and for three sets of values for the spectral parameters c and R . Flux normalization : N (oE[10 MeVo)\100 cm~2 s~1. 0 protonsnuclides [26Al/27Al \ 5 ] 10~5 ; 36Cl/35Cl \ 1.4 ] 10~6 ; a Based on meteorite data for initial abundances of the short-lived 41Ca/40Ca \ 1.5 ] 10~8 ; 53Mn/55Mn \ 4.4 ] 10~5 (H) and 9 ] 10~6 (L)] ; stable nuclide abundances in CI meteorites are from Anders & Grevesse 1989.

the smaller grains and the steeply falling power-law size distributions, contributions from the smaller grains dominate the ensemble-averaged production rates. 4.

DISCUSSION

The results obtained by us (Figs. 2 and 3 and Table 1) show that SEP interactions with 10 km to 1cm sized grains of chondritic (\solar) composition can lead to signiÐcant production of the four short-lived nuclides 41Ca, 36Cl, 26Al, and 53Mn. On the basis of the estimated production rates it is possible to determine the SEP Ñux required to produce these nuclides in amounts that will match their initial abundances in the early solar system inferred from meteorite data. The initial abundances considered by us are as follows : 41Ca/40Ca \ 1.5 ] 10~8 (Srinivasan et al. 1994, 1996), 36Cl/35Cl \ 1.4 ] 10~6 (Murty et al. 1997), 26Al/ 27Al \ 5 ] 10~5 (Lee et al. 1976 ; see also MacPherson, Davis, & Zinner 1995), and two values for 53Mn/55Mn (4.4 ] 10~5 [Birck & Allegre 1985] ; 9 ] 10~6 [Lugmair & Shukolyukov 1998]). 4.1. Constraint on the Flux of SEP from an Active Early Sun The enhancement in the SEP Ñux from the early Sun required to produce the short-lived nuclides 41Ca, 36Cl, 26Al, and 53Mn in amounts necessary to match the meteorite data are plotted in Figure 4 as a function of the SEP irradiation duration. The enhancement factors are relative to the long-term averaged proton Ñux for the contemporary Sun, taken as 100 protons cm~2 s~1 for energy above 10 MeV (see, e.g., Reedy 1998). Results obtained for SEP Ñux represented as both a power law in kinetic energy (c \ 3) and exponential in rigidity (R \ 100 MV) are shown in this 0 Ðgure for a speciÐc power-law size distributions of the target grains (b \ 3) and for SEP irradiation duration varying from 1000 to 108 yr. Similar plots may be constructed from the data given in Table 1 for other values of spectral parameters c and R and for di†erent grain size 0

distributions (b \ 3 and 4). The required Ñux-enhancement factors for each of the nuclides initially decrease linearly with time and then become independent of time when the production reaches an equilibrium as the irradiation time exceeds the mean lifetime of a nuclide. The enhancement factors for any given irradiation time shown in this Ðgure and also noted below are considered to be lower limits for two reasons. First, we have ignored possible self-shielding of SEP by nebular gas ; this will reduce the e†ective SEP Ñux and hence push up the enhancement factor. Second, the estimated enhancement factors are valid for irradiation at 1 AU since the contemporary long-term averaged SEP Ñux considered by us is based on studies of lunar samples. However, the irradiation of the meteoritic grains took place at 2È4 AU, and the estimated enhancement factors will go up if we take into account the radial gradient in SEP Ñux. Several salient features may be discerned from the results shown in Figure 4. These are as follows : 1. A minimum Ñux enhancement factor of almost 105 and an irradiation timescale of D106 yr are needed to produce 26Al in amounts necessary to match the meteorite data. 2. No combination of Ñux enhancement factor and irradiation time can lead to coproduction of 26Al with any of the other nuclides that will match the meteorite data. 3. 41Ca and 36Cl may be coproduced to match the meteorite data if the Ñux enhancement factor is D 5 ] 103È104 and the irradiation timescale is D 5 ] 105È106 yr. 4. Coproduction of 53Mn (lower initial abundance) with 41Ca and 36Cl in the required amount would require similar Ñux enhancement factors and irradiation time of less than 5 ] 105 yr. 5. Coproduction of 53Mn (higher initial abundance) with 41Ca and 36Cl would, however, require longer irradiation timescales of about a million to several million years. One may also note that a much lower Ñux enhancement factor of D103 is needed to match the lower 53Mn initial

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SHORT-LIVED NUCLIDES IN EARLY SOLAR SYSTEM

FIG. 4. SEP Ñux enhancement factors relative to the contemporary long-term averaged SEP Ñux of N (E [ 10 MeV) \ 100 cm~2 s~1 proton required to produce the short-lived nuclides 26Al, 53Mn, 41Ca, and 36Cl in amounts that will match their inferred abundances in the early solar system are plotted as a function of the SEP irradiation duration. Results obtained for two SEP spectral representations and a particular size distribution of the targets (chondritic grains) are shown. For 53Mn, the results for two extreme values of initial abundances, inferred from meteorite data, are shown ; these are labeled as 53Mn-H and 53Mn-L, respectively.

abundance if the irradiation timescale is a few million years ; the corresponding production of 26Al is negligible, and that of 41Ca and 36Cl is less than 20% compared to the amounts needed to match the meteorite data. We should emphasize here that the speciÐc values of the enhancement factors and irradiation time will vary depending on the SEP spectral parameters and size distributions used in the calculations ; however, the general trend remains similar, and the above conclusions essentially remain valid. 4.2. SEP Production as a Source of Short-lived Nuclides Low-mass solar-type stars go through a highly active (T Tauri) phase during their early evolution. Observed emissions in UV and X-rays from such young stars are orders of magnitude higher than those from the contemporary Sun

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(see, e.g., Feigelson, Giampapa, & Vrba 1991 ; Feigelson & Montmerle 1999), and one would expect this to be true for energetic particle emission as well. The Sun is believed to have evolved through such an active early phase, and the Ñux of energetic particles from the early Sun is expected to be much higher than the contemporary value. Studies of SEP-produced noble gases in silicate grains from primitive meteorites do provide evidence for an enhanced Ñux of SEP from the early Sun (Ca†ee et al. 1987 ; Hohenberg et al. 1990 ; see also Ca†ee et al. 1991). Flux enhancement factors of D 100È1000 have been estimated from these studies. Even though the formation of the CAIs hosting the daughter products of the two shortest-lived nuclides 41Ca and 26Al has preceded the formation of the more abundant silicate grains, it is doubtful if the SEP Ñux could have been much higher at the time of irradiation of the CAI precursor solids and reached a value of greater than 105 needed for production of 26Al. Even if we allow for the possibility of such an extreme enhancement of SEP to produce the required amount of 26Al, it will lead to overproduction of 41Ca, 36Cl, and 53Mn, which will be at variance with the meteorite data. On the other hand, the SEP Ñux enhancement factors of D10,000 required to match the meteorite data for 41Ca, 36Cl, and 53Mn (higher abundance) and of D1000 to match the data for 53Mn only (lower abundance) cannot be ruled out completely. A solution to the problem of overproduction noted above, and in particular for 41Ca, was suggested by Shu et al. (1997) and Lee et al. (1998) within the framework of the ““ X-wind ÏÏ model. In this model, the SEP irradiation of refractory solids takes place very close to the proto-Sun, and a special conÐguration for these solids, characterized by a Ca-AlÈrich core surrounded by an Mg-rich mantle, is proposed. In such a target conÐguration, one can expect a relative enrichment in 26Al because of its preferential production by low-energy solar particles within the Mg-rich mantle, while only higher energy particles that reach the core region will initiate nuclear interactions and produce 41Ca. The magnitude of this enrichment will depend on the thickness of the mantle as well as the size of the core and their exact compositions. Unfortunately, meteoritic CAIs that almost exclusively contain the records of these two nuclides come in many forms, shapes, and compositions and vary in size from a few tens of microns to a couple of centimeters, and it is difficult to support such an ad hoc hypothesis for very speciÐc target conÐgurations for the CAI precursors. Further, one would also expect a wide spread in the initial abundance of 26Al and 41Ca in wellpreserved early solar system solids rather than the welldeÐned abundances inferred from the presently available meteorite data (see, e.g., MacPherson et al. 1995 ; Sahijpal, Goswami, & Davis 2000). The only alternative, therefore, is to consider the possibility of multiple sources for the shortlived nuclides in the early solar system with a dominant contribution from a stellar source for 26Al and 60Fe and SEP production accounting for the other nuclides, 41Ca, 36Cl, and 53Mn. Such a proposal, however, is in conÑict with both analytical as well as experimental data. Studies of nucleosynthetic yields of various short-lived nuclides from a thermally pulsingÈasymptotic giant branch (TP-AGB) star suggest that such a source can provide the required amount of the four short-lived nuclides 41Ca, 26Al, 60Fe, and 36Cl, and there is no need for an additional source (e.g., SEP production) to explain the meteorite data for these nuclides

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(Wasserburg et al. 1995 ; Gallino et al. 1996). Similarly, a supernova could also be a source of all Ðve short-lived nuclides 41Ca, 26Al, 60Fe, 36Cl, and 53Mn (Cameron et al. 1995). Further, recent experimental data obtained from isotopic studies of refractory early solar system solids (Sahijpal et al. 1998, 2000) demonstrate the correlated presence of 41Ca and 26Al at a microscopic scale with abundances close to their canonical values (41Ca/40Ca D 1.5 ] 10~8 ; 26Al/ 27Al D 5 ] 10~5). This observation can be best explained by postulating that both 41Ca and 26Al were injected into the collapsing protosolar cloud from a stellar source and that they followed the same pathways before getting incorporated into the refractory early solar system solids (CAIs) at the time of their formation. Another interesting feature is the absence of both 26Al and 41Ca in some refractory early solar system solids that have other characteristics (trace element abundance, stable isotopic anomalies) suggesting that these are the earliest solids to form in the solar system. In a stellar origin model, this particular feature may be explained as being due to the very early formation of these solids near the central region of the collapsing protosolar cloud prior to the arrival of the short-lived nuclides injected into the cloud from a stellar source (Sahijpal & Goswami 1998 ; Sahijpal et al. 2000). On the other hand, if we consider SEP as the dominant source for 41Ca and a stellar source for 26Al, we need to invoke several ad hoc hypotheses to explain these observations. The possibility that a few of these short-lived nuclides could be the product of continuous Galactic nucleosynthesis rather than freshly synthesized material from a single stellar source has also been proposed. In particular, the observation of the Galactic 26Al line at 1.809 MeV (see, e.g., Mahoney et al. 1984 ; Diehl et al. 1995) hinted at the possibility that ambient 26Al present in the protosolar molecular cloud could account for the presence of this short-lived nuclide in the early solar system solids (Mahoney et al. 1984). However, the measured Ñux of the 26Al line corresponds to a Galactic 26Al/27Al ratio of 2È5 ] 10~6 (Clayton, Hartmann, & Leising 1993 ; Meynet 1994 ; Diehl et al. 1995), which is an order of magnitude below the value of 5 ] 10~5 for this ratio found in early solar system solids. Thus, one can rule out the possibility that ambient Galactic 26Al could be the source of this short-lived nuclide in the early solar system. Another short-lived nuclide for which contribution from continuous Galactic nucleosynthesis may be important is 53Mn (Busso, Gallino, & Wasserburg 1999) as this nuclide cannot be synthesized in a TP-AGB star, a potential source of several other short-lived nuclides (26Al, 36Cl, 41Ca, 60Fe, and 107Pd) in the early solar system (Wasserburg et al. 1995 ; Gallino et al. 1996). Although a single stellar source for the short-lived nuclides in the early solar system appears to be more likely, it is not possible to completely rule out any contribution from SEP production. Possible evidence for this comes from the recent observation of excess 10B in early solar system objects that is attributed to in situ decay of the short-lived nuclide 10Be (half-life \ 1.5 ] 106 yr) that was incorporated into these objects at the time of their formation (McKeegan, Chaussidon, & Robert 2000). 10Be is not a product of stellar nucleosynthesis, and its presence in the early solar system has to be explained in terms of energetic particle production, either in a presolar or a solar nebula environment. It may be noted here that the stable isotopes of Be and B are themselves products of energetic particle

Vol. 549

interactions (Reeves, Fowler, & Hoyle 1970 ; Meneguzzi, Audouze, & Reeves 1971 ; see also Reeves 1994) and the solar system 11B/10B ratio of 4.0 ^ 0.1 (Zhai et al. 1996) is higher than the value of 2.5 expected from production because of high-energy galactic cosmic-ray interactions with interstellar matter. Thus, additional source(s) for preferential production of 11B is needed to make up for this shortfall. This requirement need not be particular to the solar system alone and could be more general in nature as indicated by the 11B/10B ratio of 3.4 ^ 0.7 for the local interstellar di†use clouds (Lambert et al. 1998). Two suggested possibilities for preferential production of 11B are low-energy particle interactions in a presolar setting (Meneguzzi & Audouze 1975 ; Reeves & Meyer 1978 ; Walker, Mathews, & Viola 1985) and neutrino-induced spallation reactions during Type II supernova events (Woosley et al. 1990). Unfortunately, the exact setting and duration of the proposed low-energy particle interactions for preferential production of 11B is not well deÐned, and at present it is not clear if this can also lead to production of 10Be, sufficiently close to the time of protosolar cloud collapse, to account for the presence of this short-lived nuclide in the early solar system. On the other hand, if we consider 10Be as a product of SEP interactions, the initial 10Be/9Be ratio of D 9 ] 10~4 obtained by McKeegan et al. (2000) may be used to estimate the SEP Ñux needed for production of the required amount of 10Be, which in turn allows us to infer the corresponding production of the other short-lived nuclides. The e†ective production of 10Be, however, needs high-energy ([50 MeV) protons (see, e.g., Sisterson et al. 1997a) while the other short-lived nuclides are produced mainly by low-energy (\30 MeV) particles (Fig. 1). Because of this di†erence, SEP production of 10Be, relative to the other short-lived nuclides, depends very sensitively on the spectral parameters used in the calculations. If we consider the SEP Ñux normalization used in this study, production of 10Be will increase with increasing spectral hardness (lower c) while the reverse will be the case for the other short-lived nuclides. Nonetheless, it is possible to make several general observations when we consider SEP irradiation of solids of chondritic (\solar) composition : 1. Irrespective of the choice of spectral parameter and irradiation duration, SEP Ñux needed to produce 10Be to match its initial abundance inferred from meteorite data will fall far short of matching the data for the short-lived nuclide 26Al. 2. If we consider an exponential representation for the SEP [dN P exp([R/R )dR], the production of the other 0 and 53Mn) will be much below three nuclides (41Ca, 36Cl, the required level, except for extremely soft spectra (R ¹ 50 0 MV). 3. A power-law representation for the SEP (dN P E~cdE) may allow us to match the data for 41Ca, 36Cl, 53Mn, and 10Be for certain speciÐc choice of spectral parameter, Ñux enhancement factor, and irradiation duration. Finally, if we consider a very short duration (a few tens of years) SEP irradiation as responsible for production of 10Be as advocated by McKeegan et al. (2000) in the framework of the X-wind model, it is not possible to match the data for the other short-lived nuclides. In summary, even though SEP production of the shortlived nuclides 41Ca, 36Cl, 26Al, and 53Mn in the early solar

No. 2, 2001

SHORT-LIVED NUCLIDES IN EARLY SOLAR SYSTEM

system seems plausible, it fails to explain the initial abundances of these nuclides inferred from meteorite data in a self consistent manner. Production of 26Al in the required amount to match its abundance in the early solar system will lead to an order of magnitude higher abundances for the other nuclides than what was inferred from meteorite studies. SEP production of 60Fe can also be ruled out. The possibility that 26Al and 60Fe were added from a stellar source and that the other nuclides were locally produced by SEP appears unlikely in view of the observed correlation in the initial abundances of 26 Al and 41Ca in early solar system solids at a microscopic scale. Further, any stellar source contributing to the inventory of 26Al and 60Fe in the early solar system will also add the requisite amount of 41Ca and 36Cl, making any contribution from SEP production unnecessary. However, contribution from SEP production to the inventory of the short-lived nuclides in the

1159

early solar system cannot be ruled out completely and is indicated by the recent evidence for the presence of 10Be in the early solar system. SEP production could also be a signiÐcant source for 53Mn if the recently proposed lower initial abundance of this nuclide is conÐrmed by future experiments. Nonetheless, a presolar origin of the shortlived nuclides 26Al, 41Ca, 53Mn, 60Fe, 107Pd, and 36Cl in the early solar system remains the most viable proposition at present and bolsters the proposal for a triggered origin of the solar system. We thank R. C. Reedy for discussion on 10Be production and E. Zinner for helpful comments. J. N. G. wishes to acknowledge hospitality provided by the Max-PlanckInstitut fuŽr Chemie, Mainz, during the preparation of the revised version of the paper.

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