The Astrophysical Journal, 594:605–616, 2003 September 1
# 2003. The American Astronomical Society. All rights reserved. Printed in U.S.A.
THE PREDICTABLE COLLATERAL CONSEQUENCES OF NUCLEOSYNTHESIS BY SPALLATION
REACTIONS IN THE EARLY SOLAR SYSTEM
I. Leya, A. N. Halliday, and R. Wieler
Institute for Isotope Geochemistry and Mineral Resources, ETH Zürich, NO C60.6, CH-8092 Zürich, Switzerland; [email protected]
Received 2003 April 2; accepted 2003 May 8
ABSTRACT
Ever since their first discovery in 1960, the origin of the relatively short-lived radionuclides, now extinct
but alive in the early solar system, has been under debate. Possible scenarios are either nucleosynthetic production in stellar sources, e.g., asymptotic giant branch stars, Wolf-Rayet stars, novae, and supernovae, with
subsequent injection into the solar nebula, or the production by spallation reactions in the early solar system.
Here we present model calculations for the second scenario, the production of the relatively short-lived radionuclides by solar energetic particle events at the start of the solar system. The model is based on our current
best knowledge of the nuclear reaction probabilities. In addition, the modeling depends on the relative fluence contribution of protons, 3He, and 4He in the solar particle events as well as on their energy distribution.
The relative fluence contribution is the only free parameter in the system. Finally, the modeling depends on
the chemical composition assumed for the irradiated target. The model simultaneously describes the
observed solar system initial ratios 7Be/9Be, 10Be/9Be, 26Al/27Al, 41Ca/40Ca, 53Mn/55Mn, and 92Nb/93Nb.
In the framework of the local production scenario, the concordance of measured and modeled data for
nuclides with half-lives ranging from 53 days up to 36 Myr enables us to put some stringent constraints on
possible calcium-aluminum–rich refractory inclusion (CAI) production and its timing. One important
requirement in such a scenario is that the material forming most of the CAIs must have experienced a surprisingly homogenous particle fluence. CAIs showing evidence for live 10Be, 26Al, 41Ca, 53Mn, and 92Nb close to
the inferred solar system initial ratios would have to have been irradiated within 1 Myr. Much more stringent would be the time constraint for the one CAI for which formerly live 7Be has been reported. Such CAIs
would have to have been irradiated for less than about 1 yr. Such a short timescale requires flux densities as
high as 1016 cm2 s1. To allow further tests of the local production scenario, we also predict solar system
initial ratios for 14C/12C, 22Na/23Na, 36Cl/35Cl, 44Ti/48Ti, 54Mn/55Mn, 63Ni/60Ni, and 91Nb/93Nb, whose
correlated shifts in the daughter isotopes would help to further test the local production scenario.
Subject headings: nuclear reactions, nucleosynthesis, abundances — solar system: formation
abundances in the early solar system is not straightforward,
since three different stellar events are needed, and their timing is crucial (Meyer & Clayton 2000). Another difficulty is
that this scenario requires that only the outermost layers of
a supernova, about 1 Myr prior to solar system formation,
were injected into the collapsing molecular cloud core.
However, such a scenario is so far not widely accepted. In
addition, recent experimental data yield evidence for the
existence of two short-lived radionuclides in the early solar
system that are difficult to explain within a stellar
production scenario.
In recent years 10B excesses resulting from the in situ
decay of 10Be in the early solar system have been reported
by various authors (e.g., McKeegan, Chaussidon, & Robert
2000; MacPherson & Huss 2001; Sugiura, Shuzou, &
Ulyanov 2001). The existence of live 10Be is difficult to
explain by nucleosynthetic production because in most
stellar events Be is destroyed rather than produced. 10Be
production in bursts emitted from a rapidly rotating neutron star has been proposed as an explanation for this
(Cameron 2001). More recently, a 7Li excess thought to be
from the in situ decay of 7Be has been reported for one CAI
(Chaussidon et al. 2001; Chaussidon, Robert, & McKeegan
2002). The easiest explanation for this finding in the context
of stellar production would be that stellar explosions carried
the nuclide into the solar system in so-called presolar grains,
which are believed to be direct condensates in late-stage
stellar outflows. However, Chaussidon and coworkers
1. INTRODUCTION
Numerous studies have unambiguously proved the existence of relatively short-lived radionuclides in early solar
system objects. Their former presence is indicated by
excesses of their daughter isotopes. The most promising
objects for such studies are refractory condensates found in
some types of meteorites, the so-called calcium-aluminum–
rich refractory inclusions (CAIs). Because of analytical
improvements, the number of relatively short-lived radionuclides that were known to be alive in the early solar system
has shown a dramatic increase since the first detection of
129I and 26Al (Reynolds 1960; Lee, Papanastassiou, &
Wasserburg 1977).
The relatively short-lived radionuclides offer great potential for chronological studies of early solar system events
with a high time resolution. However, before they can be
used for dating, the origin of the radionuclides has to be
known, and the question of whether they were homogeneously distributed throughout the early solar system has to
be answered. For many years the most promising hypothesis was that these radionuclides were produced by stellar
nucleosynthesis, e.g., asymptotic giant branch stars, WolfRayet stars, novae, or supernovae. Connected to this stellar
production scenario is the hypothesis that the source of the
nuclides may also have triggered the collapse of the molecular cloud core, from which the solar system formed. However, a self-consistent description of the radionuclide
605
606
LEYA, HALLIDAY, & WIELER
conclude from systematic mineralogical studies of the
respective CAI that it once contained live 7Be, and one
therefore has to consider the consequences if this nuclide
really was extant in the early solar system and related to the
formation of the other short-lived nuclides.
If 7Be was really alive in the early solar system (so far, 7Li
excesses have been found in only one CAI), the half-life of
only 53.25 days excludes scenarios of 7Be production by
stellar nucleosynthesis with subsequent injection of live 7Be
into the solar system. Therefore, the report of live 7,10Be in
the early solar system does reinforce the arguments based
on 10Be that at least some of the relatively short-lived radionuclides were produced within the early solar system by
spallation reactions. The projectiles may have been accelerated to high energies by various mechanisms in young stellar
objects (YSOs). For example, regions of projectile acceleration near the surface of the YSO (as for the recent Sun), in
the X-region as well as close to the accretion disk, are discussed by Feigelson, Garmire, & Pravdo (2002). In the
framework of the X-wind model, flare generation and acceleration are in the X-region (e.g., Russell, Gounelle, &
Hutchison 2001). Throughout this paper we refer to this as
the ‘‘ local production scenario.’’
The local production scenario is further supported by
astronomical observations. Systematic studies of X-ray
emissions from YSOs indicate that most of them eject significant amounts of their material as solar energetic particles
(SEPs) and/or bipolar outflows. In a recent systematic
study, Feigelson et al. (2002) show that the so-called T Tauri
phases are common for stellar objects with 0.8–1.4 M and
that the particle flux in the YSO phase is up to 105 times
higher than for recent solar SEP events. The energy of the
emitted particles, i.e., mostly protons, 3He, and 4He, is as
high as some tens of MeV nucleon1 in present-day SEP
events. Particles in this energy range induce nuclear reactions in the irradiated material. If SEP production is either
near the accretion disk or in the X-region, various kinds of
nuclear reactions within the disk material are induced. If the
SEP production is on the surface of the YSO, a magnetic
connection of the protostar with the accretion disk is
required for any local production scenario. In such nuclear
interactions, among others, short-lived radioactive isotopes
are produced. Depending on the mechanism for SEP production, the projectiles might be focused on small regions,
leading to high nuclide production rates. Therefore, nuclear
spallation reactions may well have contributed to the
nucleosynthesis of some of the relatively short-lived
radionuclides, e.g., 7,10Be, 26Al, 36Cl, 41Ca, and 53Mn.
In recent years various groups tested the local production
scenario by comparing the modeled nuclear spallation
yields with the inferred solar system initial ratios (e.g., Lee
et al. 1998; Gounelle et al. 2001; Goswami, Marhas, &
Sahijpal 2001). However, they all failed to simultaneously
describe the initial solar system radionuclide abundances
for targets having a common chemical composition (solar
or CAI). If the projectile spectrum was adjusted so that the
modeled 26Al/27Al was in agreement with the initial ratio
inferred from meteorite data, the modeled initial 41Ca/40Ca
was about 2 orders of magnitude larger than the experimental ratio (e.g., Lee et al. 1998). This problem was
circumvented in a later study by assuming a Ca-poor rim
around the irradiated material (Gounelle et al. 2001). However, such a layering is not supported by mineralogical
observations. Another independent approach to solve this
Vol. 594
problem was made by Goswami et al. (2001). These authors
studied whether the introduction of shielding effects in the
target could result in concordancy between measured and
modeled 26Al/27Al and 41Ca/40Ca ratios. However, as
expected from nuclear reaction analyses, shielding mostly
changes the total particle fluxes but only slightly affects relative proportions of the various nuclides, e.g., the 26Al/41Ca
ratio.
Here we present new model calculations for the production of 7,10Be, 14C, 22Na, 26Al, 36Cl, 41Ca, 44Ti, 53,54Mn, 60Fe,
63Ni, and 91,92Nb by spallation reactions in the early solar
system. In contrast to the earlier studies, our approach is
based on the particle spectra for protons, 3He, and 4He in
current SEP events; i.e., all three projectile types are
included. A further advantage of this approach is that both
types of SEP events (gradual and impulsive; see below) are
included, with their relative fluence as a free parameter. All
excitation functions used are based on our current best
knowledge of the relevant nuclear reactions. Assuming a
chemical composition of the irradiated material and adjusting the modeled production rate for 10Be to the inferred
solar system initial 10Be/9Be ratio of 103 (for a discussion, see below) enables one to estimate the particle fluence
needed to produce sufficient 10Be in the irradiated object
(the ‘‘ proto-CAI ’’ material). Using this fluence, the collateral production rates for the other short-lived radionuclides
are then determined. Assuming spectral parameters (energy
distribution, chemical composition) of the SEP events similar to those values for the recent Sun and assuming further
solar chemical composition for the target material enables
one to simultaneously describe most of the inferred solar
system initial abundances of the relatively short-lived radionuclides. Based on the agreement between measured and
inferred solar system initial ratios, stringent constraints are
placed on the CAI production scenario and its timing in the
framework of the local production scenario.
2. MODEL CALCULATIONS
The calculations of production rates for the relatively
short-lived radionuclides in the early solar system are based
on a model originally developed over many years to describe
solar and galactic cosmic-ray induced production of cosmogenic nuclides in meteorites and lunar rocks (e.g., Michel
& Neumann 1998; Leya et al. 2000, 2001). For these samples
the model calculations are now able to predict measured
production rates to within 12%. The model is based on the
differential projectile spectra, the excitation functions of the
relevant nuclear reactions, and the chemical composition of
the irradiated target. The production rates (g1 s1) for the
product nuclide, Pj(), are calculated using
X
XZ 1
ci A1
j;i;k ðE ÞJk ðE; ÞdE ; ð1Þ
Pj ðÞ ¼ NA
i
i
k
0
where NA is Avogadro’s number, Ai is the mass number of
the target element i, and ci is the concentration of i (g g1).
The excitation function is j,i,k(E), and Jk(E, ) is the differential flux density of the projectile type k. In the present Sun
the particles emitted from the solar surface and accelerated
via magnetic fields to energies up to a few hundred MeV
nucleon1 originate in either gradual (G-SEP) or impulsive
(I-SEP) solar particle events. The G-SEP events typically
have He/H and 3He/4He ratios similar to solar values and
No. 1, 2003
COLLATERAL CONSEQUENCES OF NUCLEOSYNTHESIS
are characterized by relatively high flux densities and moderately steep particle spectra. The I-SEP events are enriched
in He and 3He relative to solar H and 4He, respectively.
Besides these enrichments, the I-SEP events are characterized by relatively low flux densities and steep particle
spectra. Today the G-SEP events contribute considerably
more to the total fluence than I-SEP events. For modeling
we assume that the SEP events from the early Sun were similar in their spectral shapes and their chemical compositions
to those from the recent Sun. This means that we assume the
projectiles to be a mixture of recent I-SEP and G-SEP
events, both described by a power law in energy per nucleon,
E, with ¼ 2:7 for impulsive and ¼ 4:0 for gradual
events. However, to check the calculations for robustness
we varied between 2.5 and 3.0 for I-SEP events and
between 3.5 and 4.5 for G-SEP events. Since detailed information about the activity of the early Sun is lacking, we
consider the relative contributions of G-SEP and I-SEP
events to the total fluence as an adjustable parameter. The
total fluence, however, is adjusted in a way that the modeled
10Be/9Be ratio fits the experimental data of 1 103 (see
also discussion below). Note that our knowledge on the
early Sun is so limited that we do not even know the origin
of the SEP events in the YSO. For example, while in the
present Sun G-SEP and I-SEP events originate at the solar
surface, the X-wind model assumes particle acceleration
within the so-called X-region, i.e., 0.06 AU away from the
surface of the YSO (e.g., Shu, Shang, & Lee 1996; Lee et al.
1998; Ostriker & Shu 1995). In addition, an origin of SEP
events near the accretion disk has been discussed (e.g.,
Feigelson et al. 2002). Therefore, the spectral shapes of the
SEP events of the YSO might have been different from
present-day I-SEP and G-SEP events. Since we have no
information about the energy distribution of the particles
accelerated in the various possible regions in the early solar
system, we assume that the shapes of recent solar I-SEP and
G-SEP events are reasonable approximations. The particle
spectra are shown in Figure 1, and their parameters are
compiled in Table 1.
Fig. 1.—SEP spectra used for modeling. For the gradual solar particle
events [solid line; JðEÞ / E 2:7 ] the spectral shapes were varied between
E 2:5 and E 3:0 (shaded area). For the impulsive events [solid line;
JðEÞ / E 4:0 ] spectral shapes were varied between E 3:5 and E 4:5 (shaded
area).
607
TABLE 1
Parameters of the SEP Spectra Used for Modeling
Parameter
Impulsive Events
Gradual Events
Shape .....................
H/He .....................
He-3/He-4 .............
Weight ...................
Fluence (cm2) .......
E2.7
10
1
1
9.95 1022
E4.0
300
4 104
1000
9.95 1022
The production rates calculated with equation (1) require
for each product nuclide detailed excitation functions for all
relevant target elements and all relevant projectile types.
The model calculations are restricted to proton-, 3He-, and
4He-induced reactions. From meteorite studies it is known
that SEPs produce few secondary neutrons within the target
(e.g., Bodemann 1993; Neumann 1999; Rao et al. 1994). We
therefore do not include neutron-induced reactions. Note
that at the very beginning of the solar system (before deuterium is burned to 3He) 2H-induced reactions might also
have been important. However, as long as information
about possible deuterium flux densities and their temporal
variations is lacking, adding 2H to the list of projectiles
would mainly add another free parameter to the system.
Therefore, 2H-induced reactions are not included here.
Concerning the required cross section data, the situation
is best for proton-induced reactions because of many years
of effort modeling cosmogenic production rates in meteorites and the Moon (see, e.g., Leya et al. 2000, 2001). The
proton excitation functions adopted here are the same as
those used by Leya et al. (2000, 2001), Neumann (1999), and
Michel & Neumann (1998). For some product nuclides, e.g.,
92,91Nb, no measured proton-induced cross sections exist.
For these target product combinations we have to rely on
excitation functions determined using nuclear model codes.
Here we use a Hauser-Feshbach approach in the programmed version GNASH-FKK (Young, Arthur, &
Chadwick 1998).
For 3He- and 4He-induced reactions, essentially no measured cross sections exist, and we have to rely exclusively on
excitation functions determined using nuclear model codes.
Again the cross section data are calculated on the basis of a
Hauser-Feshbach model using the program GNASH-FKK.
However, the predictive power of nuclear model codes is
limited to within a factor of 2 at best (Michel & Nagel 1997).
We therefore assume the calculated excitation functions for
3He- and 4He-induced reactions to be rather uncertain. In
addition, the Hauser-Feshbach model is a statistical
approach, and an accurate description of nuclear interactions in light target elements cannot be expected. In order to
overcome some of these problems, the production rates by
3He- and 4He-induced reactions are not directly modeled.
We instead calculate the excitation functions for proton-,
3He-, and 4He-induced reactions using the GNASHFKK code. From these data and using equation (1), the production rates Pp,GNASH, PHe3,GNASH, and PHe4,GNASH are
modeled. From this the ratios PHe3,GNASH/Pp,GNASH and
PHe4,GNASH/Pp,GNASH are calculated. We expect that the
Hauser-Feshbach approach describes production rate ratios
more accurately than absolute production rates. The final
production rates used for further discussion, Pp,final, PHe3,final, and PHe4,final, are then determined by multiplying the
modeled GNASH-FKK production rate ratios with the
608
LEYA, HALLIDAY, & WIELER
TABLE 2
Target Elements Considered for Modeling the Production Rates
of the Relatively Short-lived Radionuclides
Isotope
Target Elements
Isotope
Target Elements
Be-7, 10 ...............
C-14 ....................
Na-22..................
Al-26...................
Cl-36 ...................
Ca-41 ..................
Be, B, C, O
O
Na, Mg, Al, Si
Mg, Al, Si
K, Cl
Ca
Ti-44 ............
Mn-53, 54 ....
Fe-60 ...........
Ni-63............
Nb-91, 92 .....
Ca, Ti, Fe
Mn, Fe, Ni
Fe, Ni
Ni, Cu
Zr, Nb, Mo
production rates due to protons, Pp,exp, determined
using the best available proton cross sections (usually
experimental values):
Pp;final ¼ Pp;exp ;
PHe3;GNASH
PHe3;final ¼
Pp;exp ;
Pp;GNASH
PHe4;GNASH
Pp;exp :
PHe4;final ¼
Pp;GNASH
ð2Þ
However, for some reactions, e.g., 44Ti from Ca, production
is possible by 3He- and 4He-induced reactions but not by
protons. In those cases we have to assume PHe3;final ¼
PHe3;GNASH and PHe4;final ¼ PHe4;GNASH .
The applied method reduces the uncertainties of the
model calculations due to the limited knowledge of the excitation functions as far as possible. However, as long as
experimental cross sections for 3He- and 4He-induced reactions are missing, we expect the uncertainties in the model
calculations from the cross section data alone to be about a
factor of 2. Table 2 summarizes the nuclear reactions and
target elements considered in our study. Figure 2 shows as
an example the cross sections for the production of 53Mn
from iron. The symbols indicate experimental data for the
proton-induced reaction (for references see Leya et al. 2000,
2001). The excitation functions given by lines are calculated
Vol. 594
using the GNASH-FKK code with five different input
parameter sets, i.e., different sets for the transmission coefficients (the variations of the five cross section data sets are
smaller than the line thickness in Fig. 2). The production
rates Pp,GNASH, PHe3,GNASH, and PHe4,GNASH are calculated
using the average of the five results.
3. MODELED ISOTOPIC RATIOS
In order to allow for a comparison of modeled and experimental data, we summarize in Table 3 the adopted solar
system initial ratios that have been inferred from meteorites.
Note that the value for the solar system initial 92Nb/93Nb
has been a matter of some debate, being either 1 105 as
given by Schönbächler et al. (2002) or 1 103 as measured
by Münker et al. (2000).
The finding of 7Li anomalies from live 7Be in the early
solar system suggests, in the context of the local production
scenario, very short timescales between nuclide production
and crystallization of the early condensates. If true, we
would expect isotopic anomalies from other relatively
short-lived radionuclides as well. We therefore modeled also
the collateral production rates for 14C, 22Na, 36Cl, 54Mn,
63Ni, and 91Nb. The nuclide 14C is included in our study
because the radioactive decay (T1=2 ¼ 5730 yr) might lead
to the production of excess 14N in carbon-rich early solar
system condensates. Assuming that the irradiated material
either had been degassed prior to or during the irradiation
or that the refractory CAIs crystallized essentially noble gas
free, the production of 22Na (T1=2 ¼ 2:603 yr) and 36Cl
(T1=2 ¼ 0:3 Myr) would lead to the buildup of pure 22Ne
(Ne-E) and 36Ar in early objects. Excess 44Ca and 54Cr in
early solar system condensates correlated with Ti/Ca and
Mn/Cr ratios would be expected if 44Ti (T1=2 ¼ 60:4 yr) and
54Mn (T
1=2 ¼ 312:2 yr) were alive in the early solar system.
Excesses in 54Cr, which seem to be correlated to the excesses
in 53Cr, have already been observed in meteorites
(Shukolyukov & Lugmair 2001). In a recent study 63Cu
excesses, which are suggested to be from the in situ decay of
63Ni (T
1=2 ¼ 100 yr), have been observed in carbonaceous
and ordinary chondrites (Luck et al. 2003). Finally, if 92Nb
were produced by spallation reactions in the early solar system, there would also be some collateral production of 91Nb
(T1=2 ¼ 680 yr). Consequently, the isotopic excesses in 91Zr
and 92Zr should be correlated. Since the relative production
rates of 91Nb and 92Nb are independent of the chemical
composition of the target, this radionuclide pair would be
very useful to further test the local production scenario and
would help to set time constraints on the formation of early
solar system objects.
3.1. Uncertainties
Fig. 2.—Excitation functions for the production of 53Mn from Fe. The
open symbols are experimental data from thin target experiments. The
results from theoretical model calculations for protons, 3He, and 4He are
obtained using a Hauser-Feshbach approach in the programmed version
GNASH-FKK.
The uncertainties of the model calculations are difficult to
estimate because of the large number of input data. A significant contribution to the overall uncertainty is from the
cross section data, i.e., the missing of experimental cross sections for 3He- and 4He-induced reactions. As discussed in
detail by Michel & Nagel (1997), the predictive power of
cross section data derived from theoretical nuclear model
codes is at best a factor of 2. However, using a statistical
model like the Hauser-Feshbach approach makes the resulting uncertainties strongly dependent on the target mass
number. For example, production rates of both 7Be and
No. 1, 2003
COLLATERAL CONSEQUENCES OF NUCLEOSYNTHESIS
609
TABLE 3
Short-lived Radionuclides, Their Half-Lives, and Their Inferred
Abundances in the Early Solar System
Radioactive Isotope
Half-Life
Reference
Isotope
Solar System Initial
Ratio
Be-7 ..............................
Be-10 ............................
C-14 ..............................
Na-22............................
Al-26.............................
Cl-36 .............................
Ca-41 ............................
Ti-44 .............................
Mn-53...........................
Mn-54...........................
Fe-60 ............................
53.29 days
1.6 Myr
5730 yr
2.6 yr
0.716 Myr
0.3 Myr
0.1043 Myr
60.4 yr
3.7 Myr
312.2 days
1.5 Myr
Be-9
Be-9
C-12
Na-23
Al-27
Cl-35
...
Ti-48
Mn-55
Mn-55
Fe-56
Ni-63.............................
Nb-91............................
Nb-92............................
100 yr
680 yr
36 Myr
Ni-58
Nb-93
Nb-93
0.22
1 103
...
...
5 105
106
1.5 108
...
2 105
...
4 109
>8 108
(1.1–3.5) 107
4 106
...
105
103
Reference
1, 2
3, 4, 5, 6
7
8
9
10
11
12
13
14
15
16
References.—(1) Chaussidon et al. 2001; (2) Chaussidon et al. 2002; (3) McKeegan
et al. 2000; (4) MacPherson & Huss 2001; (5) Sugiura et al. 2001; (6) Srinivasan 2002; (7)
MacPherson et al. 1995; (8) Murty et al. 1997; (9) Srinivasan, Ulyanov, & Goswami
1994; (10) Lugmair & Shukolyukov 1998; (11) Shukolyukov & Lugmair 1993; (12)
Mostefaoui et al. 2003; (13) Tachibana & Huss 2003; (14) Luck et al. 2003; (15)
Schönbächler et al. 2002; (16) Münker et al. 2000.
10Be
from oxygen by proton-induced reactions calculated
using either experimental cross sections or GNASH-FKK
results differ by about an order of magnitude. Fortunately,
at higher masses the predictive power of the GNASH-FKK
code is considerably higher. For example, the production
rates for 22Na and 26Al from Al and Si derived from
GNASH-FKK results differ by about a factor of 2 from the
data calculated using experimental cross sections. For 53Mn
and 55Mn the calculated excitation functions give production rates in reasonable agreement, i.e., to within 50%, with
those derived from measured cross sections. However,
despite the significant uncertainties in the excitation functions for 3He- and 4He-induced reactions, we estimate that
the contribution of the cross sections to the total uncertainty
should not be higher than about a factor of 2. This is
because the procedure used to calculate the total production
rates considerably reduces the uncertainties introduced by
the GNASH-FKK cross section data.
Additional uncertainties might arise from the fact that
our approach neglects shielding effects and production of
secondary particles in the target region. Assuming that the
region where the magnetic field lines from the YSO were
connected with the accretion disk was narrow and that the
material now forming most of the CAIs and chondrules originated in this region, self-shielding of the material might
have been significant. However, introducing shielding into
the system will have two effects. First, shielding will lower
the total flux densities of the SEP events. Since the total fluence is estimated by adjusting the modeled 10Be/9Be to the
inferred solar system initial ratio of 103, shielding mostly
changes the particle fluence but only slightly affects the
modeled relative proportions of the various nuclides, e.g.,
the 26Al/41Ca ratio. Second, shielding effects change the
shapes of the projectile spectra; i.e., the spectra become
harder. This in turn affects the calculated production rates.
However, the effect is similar for reactions with similar
threshold energies, and the relative proportions of the production rates are only slightly changed. To summarize,
shielding effects are neglected in the present version of the
model calculations because we expect them to be only of
minor importance and because their consideration would
add another free parameter to the system.
One of the major uncertainties arises from the fact that
the starting chemical composition of the irradiated material
(we assume that in the very beginning of the solar system
everything started with solar chemical composition) and its
possible chemical evolution with time are not well constrained. Throughout this paper we assume that the irradiated target material (the proto-CAI material) initially had
solar chemical composition. During the irradiation the
composition might eventually change to the refractory composition of the present-day CAIs, but it is unclear if and
when this change happened. We have taken this uncertainty
into account by modeling the solar system initial ratios of
the relatively short-lived radionuclide also for present-day
CAI composition. We expect that the values inferred from
meteorites should lie somewhere between both hypothetical
end-members. Unfortunately, the abundances of some crucial elements in the refractory end-member are not well
constrained since few data exist for bulk CAI chemistry. We
adopt values from the following sources: Be (Chaussidon
et al. 2001, 2002; Phinney, Whitehead, & Anderson 1979), B
(Chaussidon et al. 2001, 2002; McKeegan et al. 2000), Mg,
Al, Si, Ca, and Ti (Simon et al. 2002), Na, K, and Cr
(Srinivasan, Huss, & Wasserburg 2000), Mn (Shukolyukov
& Lugmair 2001), and Fe (Bischoff 1976; Grossmann &
Ganaphaty 1976). Since no data for Ni and Cu were found,
we assume that the Ni/Fe and Cu/Fe ratios are unaffected
by the chemical alteration, an assumption that is not critical
for 53,55Mn because their production from Ni is only minor.
610
LEYA, HALLIDAY, & WIELER
Vol. 594
TABLE 4
Chemical Composition (g g1) Assumed for Modeling
Element
Solar
CAI
Element
Solar
CAI
Be ...............
B .................
C.................
N ................
O.................
Na...............
Mg ..............
Al................
Si ................
Cl................
K ................
2.64 108
8.70 107
3.45 102
3.18 106
5.21 101
5.00 103
9.89 102
8.86 103
1.06 101
7.04 104
5.58 104
2.37 107
9.93 107
3.45 102
1.00 106
3.00 101
1.03 103
7.12 102
1.75 101
1.26 101
...
1.12 104
Ca ...............
Ti ................
Mn..............
Cr ...............
Fe ...............
Ni ...............
Cu...............
Zr................
Nb ..............
Mo..............
9.28 103
4.36 104
1.99 103
2.66 103
1.90 101
1.90 102
1.26 104
3.94 106
2.46 107
9.28 107
1.87 101
1.19 102
2.08 104
3.50 104
8.48 102
8.48 103
1.26 105
5.80 105
3.76 106
1.39 105
For 63Ni, however, this assumption significantly increases
the uncertainty of the modeled solar system initial
63Ni/58Ni. The C concentration is assumed to be unaffected
by the chemical evolution of the target, which is a critical
assumption for calculating the solar system initial 14C/12C
and the expected isotopic shifts in N; the (14N/15N) values
given for present-day CAI composition are only rough estimates. The concentrations for Zr and Nb are given by
Kornacki & Fegley (1986). Since we found no data for Mo,
we simply assume Mo to be enriched relative to solar by the
same factor as Zr and Nb. Again, this assumption is not crucial, since Mo contributes only very little to the production
of 91,92Nb. The chemical compositions used for modeling
are compiled in Table 4.
To summarize, a major part of the total uncertainties is
attributable to the excitation functions and the chemical
composition of the refractory end-member. Considering all
possible contributions to the overall uncertainties, we estimate the total uncertainty of the modeled solar system
initial ratios to be about a factor of 10. The relative production rates for similar target product combinations, e.g.,
7Be/9Be relative to 10Be/9B and 53Mn/55Mn relative to
54Mn/55Mn, should be more accurate, i.e., about a factor of
2. However, since the experimental data inferred from meteorites often vary substantially, the uncertainties of 1 order
of magnitude and a factor of 2 for the modeled absolute and
relative production rates, respectively, do not restrict the
major statements given below.
ferent in the YSO compared to the recent Sun, by treating
the ratio G-SEP/I-SEP as a free parameter. The results are
compiled in Table 5, where we distinguish between two irradiation scenarios. In the first scenario (second and third columns), we assume an irradiation time short enough to
neglect saturation effects for the production of any radionuclide considered. In the second (fourth and fifth columns),
an irradiation time of about 1 Myr is assumed, and saturation of the isotopes is taken into account. For further
discussion see x 4. Some of the modeled results for the first
scenario (no saturation) are plotted in Figure 3. Figure 3a
depicts the ratio of modeled to inferred solar system initial
ratios for 10Be/9Be (1, because of the normalization),
26Al/27Al, 41Ca/40Ca, and 53Mn/55Mn as a function of the
ratio G-SEP/I-SEP assuming solar composition for the target. In such a plot an ordinate value of 1 indicates perfect
agreement of experimental and modeled solar system initial
ratios. Figure 3b depicts the results for targets having CAI
composition throughout the irradiation. To adjust the fluence ratios G-SEP/I-SEP, we consider only 10Be, 26Al, 41Ca,
and 53Mn because the inferred solar system initial for
TABLE 5
Modeled Solar System Initial Ratios for the Relatively
Short-lived Radionuclides for Targets Having Solar or
Present-Day CAI Composition
Without Saturation
3.2. Modeling the Solar System Initial Ratios
7 Be=9 Be, 10 Be=9 Be, 26 Al=27 Al, 41 Ca=40 Ca,
53 Mn=55 Mn, and 60 Fe=56 Fe
For modeling the solar system initial ratios, the shape of
the particle spectra and their chemical composition, as well
as the fluence ratio of G-SEP to I-SEP events, have to be
assumed. The spectral shapes and the 3He/4He and H/He
ratios of G-SEP and I-SEP are summarized in Table 1. The
data are compiled from the results given by Gosling (1993),
Miller (1998), Mason, Mazur, & Dwyler (1999), Mason,
Dwyler, & Mazur (2000), and Reames et al. (1997). Note
that adopting these data from recent SEP events implies that
we assume that the energy distribution of the emitted particles as well as the chemical composition of the SEP events
did not change during the evolution of the Sun. However,
we do allow for the possibility that the relative fluence contributions of G-SEP and I-SEP events were significantly dif-
Ratio
10Be/9Be
..........
..........
14C/12C ............
22Na/23Na........
26Al/27Al..........
36Cl/35Cl ..........
41Ca/40Ca ........
44Ti/48Ti ..........
53Mn/55Mn ......
54Mn/53Mn ......
60Fe/56Fe .........
63Ni/58Ni .........
92Nb/93Nb .......
91Nb/92Nb .......
7Be/10Be
Tirr ¼ 1 Myr
Solar
CAI
Solar
CAI
1.0 103 !
70
2.0 108
1.4 105
1.2 105
1.3 104
1.5 107
1.5 108
6.5 105
0.20
3.0 1012
2.5 109
1.3 104
0.70
1 103 !
70
1.5 107
6.4 104
1.7 105
...
1.9 106
1.5 107
3.7 103
0.16
4.0 1011
6.3 1010
1.7 103
0.70
1.0 103 !
1.8 105
1.0 1010
6.3 1011
1.0 105
4.8 105
2.6 108
1.2 1013
7.0 105
1.9 107
2.8 1012
2.9 1012
1.6 104
1.1 107
1.0 103 !
1.8 105
7.5 109
2.9 109
1.4 105
...
3.3 107
1.2 1012
4.0 103
1.5 107
3.7 1011
7.4 1013
2.1 103
1.1 107
Note.—Two irradiation scenarios, a short-term irradiation without any
saturation effects and an irradiation lasting about 1 Myr, are distinguished.
No. 1, 2003
COLLATERAL CONSEQUENCES OF NUCLEOSYNTHESIS
Fig. 3.—Ratio of modeled and measured solar system initial ratios for
and 53Mn/55Mn as a function of the ratio gradual/
impulsive solar particle events for the YSO. The shaded area ranging from
0.1 to 10 indicates the uncertainties expected for the model calculations. (a)
Results for targets with solar chemical composition. Assuming a ratio of
gradual to impulsive solar particle events of >1000 enables us to simultaneously describe the solar system abundances of 10Be, 26Al, 41Ca, and 53Mn.
(b) Results for targets with present-day CAI composition. It can be seen
that a simultaneous description of the solar system initial abundances is not
possible.
26Al/27Al, 41Ca/40Ca,
92Nb/93Nb
is still under debate and 60Fe cannot be
explained by local production only (see below).
Figure 3a demonstrates that the model simultaneously
describes the inferred solar system initial abundances for
10Be, 26Al, 41Ca, and 53Mn within a factor of 10, i.e., within
the uncertainties expected for the model calculations, if the
ratio G-SEP/I-SEP is assumed to be greater than 1000 and
if the chemical composition of the irradiated material is
assumed to be solar during the entire irradiation. In contrast, if one assumes present-day CAI compositions for the
irradiated target, the model fails to simultaneously describe
the solar system initial abundances of 10Be, 26Al, 41Ca, and
53Mn (Fig. 3b). Assuming again G-SEP/I-SEP ¼ 1000
would bring 10Be/9Be and 26Al/27Al in concordance, i.e.,
within a factor of 3, but 41Ca and 53Mn would be overproduced by the model by factors of 130 and 190, respectively.
The differences can entirely be explained by the differences
between solar and present-day CAI chemistry. For example,
Be in CAIs is enriched relative to solar by a factor of 10.
Since the concentration of oxygen, which is the main target
element for 7,10Be production, is very similar for solar and
611
present-day CAI chemistry, the total fluence needed to produce 10Be/9Be ¼ 103 is also 10 times higher for material
with present-day CAI chemistry than for material having
solar composition. The 10 times higher fluence then again
brings measured and modeled 26Al/27Al in concordance.
The concentration of Si, which is the main target element
for 26Al production, does not vary substantially between
solar and CAI chemistry. Therefore, the 10 times higher fluence leads to a corresponding increase in the 26Al
production rate by about a factor of 10. Since Al is enriched
in CAIs relative to solar by about 1 order of magnitude, the
modeled 26Al/27Al still is close to the inferred solar system
initial ratio. In contrast, since the Fe/Mn ratio in CAIs is
about 4 times higher than solar and 53Mn production is
dominated by reactions on Fe, the 10 times higher fluence
leads to a 53Mn/55Mn ratio in CAIs about 40 times higher
than those modeled for solar chemical composition. The
production of 41Ca is dominated by reactions on Ca, and
increasing the fluence by a factor of 10 directly increases the
modeled solar system initial 41Ca/40Ca by the same factor
of 10.
To summarize, assuming solar chemical composition for
the irradiated target, a fluence ratio G-SEP/I-SEP of 1000,
and energy spectra as well as the relative proportions of protons, 3He, and 4He in the SEPs that are identical to those in
present-day SEP events permits for the first time a simultaneous description of the solar system initial abundances of
10Be, 26Al, 41Ca, and 53Mn in the framework of local production. In addition, the model gives a solar system initial
7Be/10Be of 70, in reasonable agreement with the experimental result of 230 130 given by Chaussidon et al.
(2002). The 7Be/10Be ratio is of great importance because it
is nearly independent of the chemical composition of the
target and can therefore be used to test the projectile spectra
used for modeling. In contrast, if one assumes present-day
CAI compositions for the irradiated material, the result is
an overproduction of 41Ca and 53Mn relative to 10Be and
26Al by more than 2 orders of magnitude. Therefore, in the
framework of the local production scenario all radionuclides considered so far must have been mostly produced
while the irradiated proto-CAI material maintained a solar
composition or, more exactly, had solar O/Be and Si/Al
ratios. The change to the more refractory CAI composition
had to occur after irradiation ended. The broad range of
half-lives of the radionuclides, from 53 days for 7Be up to 36
Myr for 92Nb, permits stringent constraints on the CAI production scenario and its timing. A discussion of a possible
CAI formation scenario is given in x 4. An important result
therefore is that the target material must have had solar
chemical composition throughout the irradiation, and we
have to answer the question of how the target material
becomes refractory. This is discussed in x 4.
If one assumes a solar composition for the irradiated
material, the calculated solar system initial 92Nb/93Nb is
1:3 104 . This is an order of magnitude higher than the
ratio given by Schönbächler et al. (2002) and an order of
magnitude lower than the value presented by Münker et al.
(2000). Hence, the modeled ratio agrees with both estimates
to within a factor of 10, which is within the uncertainties
expected for the model calculations. Note that the 92Nb production rates are exclusively based on theoretically derived
input data; i.e., there are no experimental cross sections
available for the production of 92Nb. For this reason
and because the calculated ratio lies in between both
612
LEYA, HALLIDAY, & WIELER
experimental data sets, the model does not provide a constraint on which of the initial ratios inferred by Schönbächler et al. (2002) and Münker et al. (2000) is correct. However,
we can conclude that the existence of 92Nb in the early solar
system is not necessarily proof for p-process nucleosynthesis
shortly before the formation of the solar system. The 92Nb
data can be explained by spallation reactions on Nb (with
small contributions from Zr and Mo). Whether the internal
production scenario can also explain some of the other
neutron-poor nuclides needs further study.
There are discrepancies between modeled and inferred
values of the solar system initial 60Fe/56Fe, a feature already
observed in earlier studies (Lee et al. 1998; Gounelle et al.
2001). The model underestimates the initial 60Fe abundance
inferred by Shukolyukov & Lugmair (1993) from achondrites by more than 3 and 2 orders of magnitude,
respectively, assuming either solar or present-day CAI composition. In general, the production of neutron-rich isotopes
by spallation reactions is very inefficient. Therefore, the
occurrence of the neutron-rich isotopes 60Fe and 182Hf
requires another production mechanism. The existence of
60Fe may therefore indicate r-process nucleosynthesis with
subsequent injection of material into the collapsing solar
nebula. However, Wasserburg, Gallino, & Busso (1998)
concluded that if a supernova is the source of the 26Al in the
solar system, then the initial 60Fe/56Fe would be in the range
of 3 107 to 1 105 , at least 2 orders of magnitude
above the achondrite value (4 109 ; Shukolyukov & Lugmair 1993). However, from analyses of chondrites two
groups recently reported evidence for a much higher solar
system initial 60Fe/56Fe ratio (Tachibana & Huss 2003;
Mostefaoui et al. 2003). The new values of ð1 7Þ 107 are
at the lower end of the range proposed by Wasserburg et al.
(1998). If confirmed, the new 60Fe data would also provide
support for a nucleosynthetic origin of 26Al and hence
would require the reassessment of the spallogenic origin of
some of the isotopes explained here.
3.3. Early Solar System Production of 14 C, 22 Na, 36 Cl, 44 Ti,
54 Mn, 63 Ni, and 91 Nb
In the previous section it was demonstrated that our
model calculations allow for a consistent description of the
abundances of most of the relatively short-lived radionuclides whose presence in the early solar system is established.
In the framework of the local production scenario, production by spallation is also expected for other radioactive and
stable nuclides. Here we discuss the radioactive isotopes
14C, 22Na, 36Cl, 44Ti, 54Mn, 63Ni, and 91Nb. Our interest in
these radionuclides is motivated by the recent hint of live
7Be in the early solar system in one CAI (Chaussidon et al.
2001, 2002). If the timescale for nuclide production and
CAI crystallization was short enough to allow for measurable isotopic anomalies due to the radioactive decay of 7Be,
we might expect to see effects from other very short-lived
radionuclides as well. Here we discuss the expected abundances of 14C, 22Na, 36Cl, 44Ti, 54Mn, 63Ni, and 91Nb for
particle fluences leading to 10Be/9Be ¼ 103 . Note that saturation effects are neglected for all isotopes discussed in this
section. If saturation effects were added, the radionuclide
abundances would decrease. Therefore, the values given
below are upper limits.
The modeled 14C/12C in the early solar system is
2 108 . Note that 14C production is entirely by spalla-
Vol. 594
tion reactions on oxygen. With a solar O/C ratio of 15,
this results in a total of 3:4 1013 g1 14C atoms at the end
of the irradiation. If one assumes that the crystallization of
the material occurred fast, i.e., that the decay of 14C to 14N
occurred entirely in already crystallized material, it would
result in ð14N/15NÞ 0:27G for samples with a solar nitrogen concentration of 3 ppm, an effect too low to be
detected. To make the discussion more realistic, we assume
that the CAIs crystallized essentially free of nitrogen, but we
consider typical present-day nitrogen blanks. If we assume a
blank level similar to that obtained in a recent study of N2 in
single lunar grains, about 1 pmol (Hashizume, Marty, &
Wieler 2002), a radiogenic signal of 100G would require
3 1013 isotopes of 14C, which can be translated into
1:5 1021 isotopes of the stable 12C using the modeled solar
system initial 12C/14C. Therefore, only 30 mg carbon is
needed to produce an isotopic shift of ð14N/15NÞ 100G in material originally free of nitrogen (except for the
blank contribution). Note that 7,10Be and 14C are produced
by spallation reactions on oxygen. Hence, the isotopic
effects produced by the radioactive decay of 7,10Be and 14C
should correlate. Therefore, it might be of interest to search
in early solar system condensates for 7Li and 10Be
enrichments correlated with high 14N/15N.
For the solar system initial 22Na/23Na, the model yields
1:5 105 . With a solar Na concentration of 5000 ppm
(Table 4), this would lead to a 22Ne concentration of
3 107 cm3 STP g1. This is very much larger than the
concentrations found in early solar system condensates.
However, whether CAIs contain any trapped Ne is unclear.
Whereas Shukolyukov et al. (2001) and Russell et al. (1998)
interpret 21Ne/22Ne ratios in an Efremofka and Vigarano
CAI, respectively, as possible evidence for the presence of
Ne-E, Vogel et al. (2003) showed that Ne in several CAIs
can entirely be explained by cosmogenic contributions from
Na- and Al-rich minerals. However, the absence of Ne-E
does not necessarily allow rejecting a local production scenario, because the absence of formerly present Ne-E can be
explained either by Na losses during the irradiation
(together with thermally induced losses of other volatile elements) or by noble gas losses during or after irradiation.
The latter possibility is indicated by the fact that CAIs are
also completely devoid of trapped Ne-HL or Ne-Q (Vogel
et al. 2003). Note that Na in CAIs typically is of secondary
origin and therefore cannot be used to estimate the expected
amount of Ne-E. Note also that Ne-E in meteorites known
to be from presolar grains is thought to be nucleosynthetic
(e.g., Nichols et al. 2002).
The solar system initial 36Cl/35Cl is modeled as
1:3 104 , about 2 orders of magnitude higher than the
value of 1 106 reported by Murty, Goswami, &
Shukolyukov (1997). As in the case of the ‘‘ missing ’’ Ne-E
in early solar system condensates, this discrepancy might
also be explained by volatile losses during or after the
irradiation.
For the solar system initial 44Ti/48Ti, the model gives
1:5 108 . Taking into account the large overabundance of
Ca relative to Ti in samples having a solar composition as
well as in refractory inclusions, any expected anomalies in
44Ca from the radioactive decay of 44Ti are below presentday analytical precision. For example, assuming solar
composition of the target, the expected anomaly in
44Ca/40Ca is less than 30 ppb. This is too low to be detected.
If we turn this argument around, to end up with
No. 1, 2003
COLLATERAL CONSEQUENCES OF NUCLEOSYNTHESIS
ð44Ca/40CaÞ ¼ 2 (where 1 is the isotopic shift of the sample relative to a standard in units of 104), which can be
detected with present-day analytical techniques, the sample
must have a Ti/Ca ratio 104 times higher than solar.
The modeled solar system initial 54Mn/55Mn is
1:3 105 , i.e., about 5 times lower than the modeled initial 53Mn/55Mn. Considering the isotopic abundances of
the daughter isotopes 53Cr and 54Cr of 9.5% and 2.36%,
respectively, the expected (54Cr) should therefore be about
the same as (53Cr). In contrast, the experimental data from
carbonaceous chondrites suggest (54Cr) values about 4
times higher than (53Cr) (Shukolyukov & Lugmair 2001).
Note that the model calculations predict the relative proportions of the 53Mn and 54Mn production rates to within a
factor of about 2, because a large part of the uncertainty
cancels out when production rate ratios of products from
the same target element are considered. Therefore, the
observed difference of a factor of 4 is not expected to be only
an artifact of the model calculations.
In a recent study on carbonaceous and ordinary chondrites, Luck et al. (2003) found excess 63Cu, which they suggested to be from the in situ decay of refractory 63Ni. From
their Figure 6 the solar system initial 63Ni/58Ni can be estimated to be 4 106 , about 3 orders of magnitude higher
than our modeled results of 2 109 . Despite the fact that
the cross sections for the production of 63Ni from Ni and Cu
are entirely based on theoretical model calculations, the
observed discrepancy of a factor of 1000 cannot be
explained as an artifact of the model calculations. Further,
the half-life of 63Ni (T1=2 ¼ 100 yr) makes stellar production
scenarios with subsequent injection into the forming solar
system highly unlikely. A possibility to explain the existence
of live 63Ni is CAI production in a supernova envelope, as
recently proposed by Cameron (2003). However, this scenario has difficulties explaining the rather ‘‘ normal ’’
isotopic composition of CAIs relative to accepted presolar
grain values.
For the solar system initial 91Nb/93Nb, the model gives
9 105 , i.e., 40% lower than the modeled 92Nb/93Nb
ratio. Considering the isotopic abundances of 91Zr and 92Zr,
the expected (91Zr) should be close to the reported (92Zr).
Unfortunately, the present data (Schönbächler et al. 2002)
are too uncertain to show any trend. Therefore, further tests
of the internal production scenario require more precise
isotopic data for 91Zr and 92Zr in early solar system
condensates.
To summarize, besides the established radionuclides discussed in the previous section, we also modeled the expected
solar system initial ratios 14C/12C, 22Na/23Na, 36Cl/35Cl,
54Mn/55Mn, 63Ni/58Ni, and 91Nb/93Nb. The main motivation for this additional modeling was the search for expected
collateral isotopic shifts, which might help to further test the
local production scenario and possibly to gain new information about the time constraints on the CAI production
scenario. Unfortunately, some of the predicted isotopic
anomalies might have been eradicated, e.g., 22Ne and 36Ar,
whereas others are too low to be detected, e.g., 44Ti and
91Nb. Only 14C, 54Mn, and 63Ni might be useful candidates
to test the local production scenario. However, for 54Mn
and 63Ni, the model results fall short by about a factor of 4
and 1000, respectively. Whether these differences indicate
that most of the relatively short-lived radionuclides were
not produced within the early solar system deserves further
study.
613
4. A SCENARIO FOR CAI PRODUCTION IN THE
FRAMEWORK OF THE X-WIND MODEL
Here we discuss the consequences for CAI production if
some of the short-lived radionuclides were produced by
energetic particles from the young Sun according to the
results derived in the previous section. In the following, CAI
production is considered in the framework of the X-wind
model only (e.g., Ostriker & Shu 1995; Shu et al. 1996; Lee
et al. 1998). One of the major challenges for a local production scenario is the experimental finding that the solar
system initial ratios in different samples show only modest
variations. For example, the initial 10Be/9Be measured so
far in 16 CAIs varies by less than a factor of 4 (McKeegan et
al. 2000; Chaussidon et al. 2001; MacPherson & Huss 2001;
Sugiura et al. 2001; Srinivasan 2002), and the initial
26Al/27Al in many CAIs falls close to the so-called canonical
value of 5 105 (MacPherson, Davis, & Zinner 1995).
This requires that the material now forming a large part of
the CAIs has seen a surprisingly homogeneous projectile fluence, despite a probably variable solar particle flux and
variable irradiation times and shielding conditions. In addition, the concordance of measured and modeled ratios for
nuclides with different half-lives can only be explained in
scenarios in which the time gap between the end of the irradiation and the crystallization is comparable with the
half-life of the shortest lived radionuclide considered.
Based on the results here, a CAI production scenario in
the framework of the X-wind model must fulfill the following requirements: (1) the irradiated material must have had
solar composition throughout the irradiation, (2) the irradiation of the material must have been very homogeneous,
and (3) the refractory CAIs must have crystallized out of
this homogeneously irradiated material on timescales comparable to the shortest half-life of the radionuclides
considered. A possible straightforward explanation is to
assume that the irradiation occurred on a gas with solar
composition and that the refractory CAIs crystallized out of
this gas directly after the irradiation. Such a scenario is
sketched in Figure 4. A gaseous target naturally results in a
homogeneous irradiation because convection and/or turbulent mixing of a gas near a YSO and/or in the X-region
should be very common. A further advantage of a gaseous
target might be that the chemical composition of the total
gas is not expected to change during the irradiation and/or
heating events as long as the gas is considered as a closed
system. After the irradiation, the gas was either shielded
from the young Sun or ejected from the X-region by the
X-winds and subsequently injected into the accretion disk at
planetary distances. The CAIs crystallized out of this homogeneously irradiated gas on timescales shorter than or at
least comparable to the half-life of the shortest lived radionuclide considered. In this scenario, the irradiation time is a
crucial parameter because radioactive saturation of the isotopes affects the relative proportions of the modeled solar
system initial ratios. Figure 5 shows the ratio of modeled to
inferred solar system initial abundances as a function of the
irradiation time. The calculations are adjusted such that the
modeled 10Be/9Be equals 1 103 (1 in the figure). Figure
5 demonstrates that the concordance of measured and modeled solar system initial abundances for 10Be, 26Al, 41Ca,
53Mn, and 92Nb holds for irradiation times of up to about 1
Myr. Longer irradiation times lead to lower relative proportions of the radionuclides with shorter half-lives. For
614
LEYA, HALLIDAY, & WIELER
Vol. 594
Fig. 4.—Production scenario explaining the concordance of spallogenic 7,10Be, 26Al, 41Ca, 53Mn, and 92Nb. A gaseous target with solar chemical composition is homogeneously irradiated in the X-region. After up to 1 Myr of irradiation, the material is ejected from the X-region and/or shielded from the SEPs.
Finally, the material is ejected by the X-wind and transported to planetary distances. The refractory CAIs crystallized out of this homogeneously irradiated
material.
example, 41Ca reaches 87.5% saturation after about 300 kyr,
whereas the level of 87.5% saturation for 10Be and 53Mn is
reached after 4.8 and 11.1 Myr, respectively. Consequently,
the 41Ca abundance monotonically decreases relative to
those of 10Be and 53Mn. Concerning the latter two nuclides,
the amount of 53Mn relative to 10Be increases for irradiation
times larger than 1 Myr, when 10Be is near its saturation
level but the amount of 53Mn still increases. However, the
time constraints become much more stringent if 7Be is added
to the list of radionuclides alive in the early solar system.
For CAIs that crystallized with 7Be/10Be 100, the irradiation time must have been shorter than about 1 yr to avoid
radioactive saturation of 7Be relative to 10Be. From Figure 5
we conclude that, in the framework of the local production
scenario, most of the material now forming the CAIs was
homogeneously irradiated over timescales not considerably
Fig. 5.—Ratio of modeled to measured solar system initial values for
and 92Nb/93Nb as a function
of the irradiation time. Because of their different half-lives, ranging from 53
days for 7Be up to 36 Myr for 92Nb, the relative proportions of the nuclide
abundances significantly depend on the length of the irradiation. From this
picture we conclude that irradiation times of up to 1 Myr are possible for
most of the CAIs but that in some rare occasions a high flux density lasting
no longer than about 1 yr leads to the formation of 7Be/10Be 100.
10Be/9Be, 26Al/27Al, 41Ca/40Ca, 53Mn/55Mn,
more than about 1 Myr. Some material, that from which
CAIs with evidence for 7Be crystallized, received flux densities about a factor of 106 times higher than the average,
during short times of less than 1 yr, however.
Note that in this hypothetical scenario a homogeneous
irradiation is accomplished because of two factors. First, a
gaseous target is assumed, which enables efficient mixing by
convection and/or turbulent mixing during the irradiation.
This should result in a homogeneous irradiation even if the
projectile spectra have varied during the irradiation. Second, the X-wind model assumes that the SEP events leading
to nuclide production have been produced within the
X-region itself, i.e., that the projectiles were accelerated in
the same region where the irradiation occurred (e.g., Russell
et al. 2001), which again should naturally result in a
homogeneous irradiation of the material within the
X-region.
To summarize, we conclude that for the local production
scenario to be correct, the irradiated material that formed
most of the CAIs was gaseous, with solar composition
throughout the irradiation. The irradiation would need to
have lasted not considerably more than about 1 Myr. Such
CAIs could have crystallized with live 10Be, 26Al, 41Ca,
53Mn, and 92Nb at levels now inferred from meteorite studies. The modeled 7Be/10Be of about 105 would not allow a
detection of any 7Li anomalies correlated to the beryllium
concentration. However, if some of the material was irradiated by high-flux SEP events, which lasted no longer than
about 1 yr, the CAIs would crystallize with 7Be/10Be of
about 100 and 10Be, 26Al, 41Ca, 53Mn, and 92Nb concentrations close to their inferred solar system initial ratios. In
both cases the irradiation would need to have been very
homogeneous, possibly because on one hand the irradiated
material was gaseous, which allows efficient mixing during
the irradiation, and on the other hand the SEP events originated at the same place where the irradiation occurred, i.e.,
in the X-region. After the irradiation the material would
need to have been ejected from the irradiation area and/or
shielded from the SEPs. The irradiated material would have
rapidly condensed, and the solar system initial abundances
would have been frozen into the earliest refractory
condensates.
No. 1, 2003
COLLATERAL CONSEQUENCES OF NUCLEOSYNTHESIS
5. CONSEQUENCES FOR THE PROPOSED CAI
PRODUCTION SCENARIO
In the following we distinguish between two widely different irradiation lengths. First, we assume a short-term
irradiation with a high flux density, which lasts less than
about 1 yr. As pointed out above, such a short irradiation
time is required to produce not only 10Be, 26Al, 41Ca, 53Mn,
and 92Nb but also 7Be/10Be in their inferred solar system initial proportions. Second, we assume an irradiation time of
1 Myr, which explains 10Be, 26Al, 41Ca, 53Mn, and 92Nb
but neglects the evidence of live 7Be in one Allende CAI,
since the modeled initial 7Be/10Be will be too low by about 7
orders of magnitude.
Comparing the flux densities needed to produce the
short-lived radionuclides with flux densities of SEPs from
the present Sun is a further test of the local production scenario. The total flux density, J(E), for the 1 yr and 1 Myr
scenarios is 1016 and 1010 cm2 s1, respectively. In the
literature, solar particle flux densities are usually given only
for particles with energies above 10 MeV, J(E > 10 MeV).
The values needed for successful modeling are then
JðE > 10 MeVÞ 2 1012 and 2 106 cm2 s1 for the 1
yr and 1 Myr scenarios, respectively [assuming
JðEÞ / E 2:7 and JðEÞ / E 4:0 for gradual and impulsive
events, respectively]. The long-term average flux density for
the present Sun is 102 cm2 s1 (Reedy 1977, 2002). With
the 105-fold enhanced flux densities in YSOs compared to
the present Sun (Feigelson et al. 2002), this results in
JðE > 10 MeVÞ 107 cm2 s1 for the young Sun, in concordance with our modeled estimate for the 1 Myr scenario.
Therefore, assuming an X-wind scenario and flux densities
105 times higher than for the present Sun according to
observations naturally results in 10Be/9Be, 26Al/27Al,
41Ca/40Ca, 53Mn/55Mn, and 92Nb/93Nb ratios close to the
values inferred from meteorites for the early solar system.
However, assuming on the other hand an irradiation time of
only 1 yr requires flux densities about 10 orders of magnitude above the average present-day value or about 5 orders
of magnitude above the values for YSOs reported by
Feigelson et al. (2002). Therefore, for proper modeling of
the solar system initial 7Be/10Be, a further short-term
increase of the flux of about 5 orders of magnitude is
needed. Individual solar flares in the present Sun may have
flux densities about 3 orders of magnitude higher than the
long-term average. Possibly, the material from which the
Allende CAI with hints for live 7Be crystallized has been
irradiated in such a hypothetical early ultra–high-flux SEP
event. Further, the flux given for the present Sun is for 4.
In the X-wind scenario the region of SEP production and
particle acceleration is not yet clear, but the SEP events in
the early Sun need not necessarily have been isotropic. Magnetic focusing of SEP events to small emission angles also
increases the flux densities and might enable simultaneous
modeling of the production of 10Be, 26Al, 41Ca, 53Mn, 92Nb,
and 7Be close to the inferred solar system initial values.
To conclude, if we assume in the framework of the local
production scenario that most of the material now forming
the CAIs has been irradiated not substantially longer than
about 1 Myr by low flux density events, CAIs could crystallize with 10Be, 26Al, 41Ca, 53Mn, and 92Nb abundances close
to the values inferred from meteorites for the early solar system. However, the 7Be/10Be in those CAIs would be too low
to produce detectable shifts in 7Li/6Li. This argument also
615
holds for 14C, 22Na, 44Ti, 54Mn, 63Ni, and 91Nb. Therefore,
if the evidence for 7Be is substantiated, there must also have
been some short-term SEP events with 105 times higher flux
densities by which 7Be at levels 7Be/10Be 100 could have
been produced.
6. CONCLUSIONS AND OUTLOOK
We present new model calculations for the production of
the relatively short-lived radionuclides 7Be, 10Be, 14C, 22Na,
36Cl, 44Ti, 41Ca, 53Mn, 60Fe, 63Ni, 92Nb, and 93Nb by spallation reactions in the early solar system. By assuming that
the particle spectra produced by YSOs were similar to those
emitted from the present Sun, we can for the first time simultaneously model the solar system initial ratios 10Be/9Be,
26Al/27Al, 41Ca/40Ca, 53Mn/55Mn, and 92Nb/93Nb.
The concordance of measured and modeled solar system
initial ratios provides stringent constraints on possible CAI
production mechanisms in the framework of a local production scenario. A major challenge for any such scenario is to
explain the quite similar 26Al/27Al and 10Be/9Be ratios in
many different CAIs. This experimental finding requires
that the material now forming most of the CAIs has experienced surprisingly homogeneous particle fluences, despite
probable variable flux densities, irradiation times, and
shielding conditions. By combining the requirement for a
homogeneous irradiation with a major result obtained by
our model calculations, that the target must have solar composition throughout the irradiation, we present a possible
CAI production scenario. In this model a gaseous target
with solar chemical composition is irradiated. Convection
and/or turbulent mixing in the target results in a homogeneous irradiation of the material. After an exposure time of up
to 1 Myr for most of the material, the CAIs crystallized with
10Be, 26Al, 41Ca, 53Mn, and 92Nb abundances close to the
values inferred for the early solar system. However, the
7Be/10Be ratios in those CAIs would be too low by about 7
orders of magnitude compared to meteorite data. Therefore, we have to assume that some material must have been
irradiated with flux densities about 105 times higher than the
early solar system average on timescales of less than about 1
yr. Finally, the flux densities needed for modeling are in
both cases within the physically reasonable ranges of YSOs.
Besides the established short-lived radionuclides 10Be,
26Al, 41Ca, 53Mn, and 92Nb, we also present initial abundances for 14C, 22Na, 44Ti, 36Cl, 54Mn, 63Ni, and 91Nb. While the
modeled solar system initial 14C/12C would lead to permil
effects in nitrogen in early solar system carbon-rich condensates, the isotopic anomalies resulting from the decay of 44Ti
are much smaller than present-day analytical precision. The
modeled solar system initial 22Na/23Na and 36Cl/35Cl
would result in easily measurable 22Ne and 36Ar concentrations. Since so far such large amounts of pure 22Ne and 36Ar
have not been observed in CAIs, most of the gas produced
by early spallation must have been lost by diffusion, if ever
present. Unfortunately, the modeled solar system initial
91Nb/93Nb is too low to produce detectable isotopic
anomalies in 91Zr.
The modeled 60Fe/56Fe falls short by at least 3 orders of
magnitude and, according to very recent data, by even
about 6 orders of magnitude. From this one can conclude
that an external source, e.g., a supernova, is necessary for
some of the neutron-rich short-lived radionuclides, e.g.,
60Fe and 182Hf. Even more importantly, if a supernova is the
616
LEYA, HALLIDAY, & WIELER
source for the 60Fe, the injected material would have a
26Al/60Fe ratio close to the range now inferred for the early
solar system (Tachibana & Huss 2003; Mostefaoui et al.
2003). Therefore, if the new value for the solar system initial
60Fe/56Fe is correct, this supports a triggered supernova
source for both isotopes. Consequently, a reevaluation of
the origin of the other relatively short-lived radionuclides,
in particular 26Al, would then be necessary.
Some major discrepancies exist for 54Mn and 63Ni. For
54Mn the model predicts (54Cr)/(53Cr) ratios of about
unity, which is 4 times lower than some published values.
Furthermore, for 63Ni the model gives a solar system initial
abundance about 103 times lower than recently inferred
from Cu isotope data. It is not clear if these discrepancies
indicate that the local production scenario is not an appropriate and/or sole scenario for the origin of the relatively
short-lived radionuclides in the early solar system. This
deserves further study.
This work was supported by the Swiss National Science
Foundation. Numerous discussions with M. Chaussidon
and G. Lugmair are appreciated.
REFERENCES
Bischoff, A. 1976, Ph.D. thesis, Univ. Münster
Miller, J. M. 1998, Space Sci. Rev., 86, 79
Bodemann, R. 1993, Ph.D. thesis, Univ. Hannover
Mostefaoui, S., et al. 2003, Lunar Planet. Sci. Conf., 34, 1585
Cameron, A. G. W. 2001, Lunar Planet. Sci. Conf., 32, 1035
Münker, C., et al. 2000, Science, 289, 1538
Murty, S. V. S., Goswami, J. N., & Shukolyukov, Yu. A. 1997, ApJ, 475,
———. 2003, Lunar Planet. Sci. Conf., 34, 1083
L65
Chaussidon, M., Robert, F., & McKeegan, K. D. 2002, Lunar Planet. Sci.
Neumann, S. 1999, Ph.D. thesis, Univ. Hannover
Conf., 33, 1563
Nichols, R. H., Jr., et al. 2002, Geochim. Cosmochim. Acta, submitted
Chaussidon, M., Robert, F., McKeegan, K. D., & Krot, A. N. 2001, Lunar
Ostriker, E. C., & Shu, F. A. 1995, ApJ, 447, 813
Planet. Sci. Conf., 32, 1862
Feigelson, E. D., Garmier, G. P., & Pravdo, S. H. 2002, ApJ, 572, 335
Phinney, D., Whitehead, B., & Anderson, D. 1979, Lunar Planet. Sci.
Gosling, J. T. 1993, J. Geophys. Res., 98, 18937
Conf., 10, 885
Goswami, A. N., Marhas, K. K., & Sahijpal, S. 2001, ApJ, 549, 1151
Rao, M. N., et al. 1994, Geochim. Cosmochim. Acta, 58, 4231
Gounelle, M., et al. 2001, ApJ, 548, 1051
Reames, D. V., et al. 1997, ApJ, 483, 515
Grossmann, L., & Ganaphaty, R. 1976, Geochim. Cosmochim. Acta, 40,
Reedy, R. C. 1977, Lunar Planet. Sci. Conf., 8, 825
967
———. 2002, Lunar Planet. Sci. Conf., 33, 1938
Hashizume, K., Marty, B., & Wieler, R. 2002, Earth Planet. Sci. Lett., 202,
Reynolds, J. H. 1960, Phys. Rev. Lett., 4, 8
201
Russell, S. S., Gounelle, M., & Hutchison, R. 2001, Philos. Trans. R. Soc.
Kornacki, A. S., & Fegley, B., Jr. 1986, Earth Planet. Sci. Lett., 79, 217
London, A359, 1991
Lee, T., Papanastassiou, D. A., & Wasserburg, G. J. 1977, ApJ, 211, L107
Russell, S. S., et al. 1998, Meteoritics Planet. Sci., 32, A132
Lee, T., et al. 1998, ApJ, 506, 898
Schönbächler, M., et al. 2002, Science, 295, 1705
Leya, I., et al. 2000, Meteoritics Planet. Sci., 35, 259
Shu, F. H., Shang, H., & Lee, T. 1996, Science, 271, 1545
———. 2001, Meteoritics Planet. Sci., 36, 1547
Shukolyukov, A., & Lugmair, G. W. 1993, Science, 259, 1138
Luck, J. M., et al. 2003, Geochim. Cosmochim. Acta, 67, 143
———. 2001, Meteoritics Planet. Sci., 36, A188
Lugmair, G., & Shukolyukov, A. 1998, Geochim. Cosmochim. Acta, 62,
Shukolyukov, Yu. A., et al. 2001, Geochim. Int. 39 (Suppl. 1), 110
2863
Simon, S. B., et al. 2002, Meteoritics Planet. Sci., 37, 533
MacPherson, G. J., Davis, A. M., & Zinner, E. K. 1995, Meteoritics Planet.
Srinivasan, G. 2002, Meteoritics Planet. Sci., 37, A135
Sci., 30, 365
Srinivasan, G., Huss, G. R., & Wasserburg, G. J. 2000, Meteoritics Planet.
MacPherson, G. J., & Huss, G. R. 2001, Lunar Planet. Sci. Conf., 32, 1882
Sci., 35, 1333
Mason, G. M., Dwyler, J. R., & Mazur, J. E. 2000, ApJ, 545, L157
Srinivasan, G., Ulyanov, A. A., & Goswami, J. N. 1994, ApJ, 431, L67
Mason, G. M., Mazur, J. E., & Dwyler, J. R. 1999, ApJ, 525, L133
Sugiura, N., Shuzou, Y., & Ulyanov, A. 2001, Meteoritics Planet. Sci., 36,
McKeegan, K. D., Chaussidon, M., & Robert, F. 2000, Science, 289, 1334
1397
Meyer, B. S., & Clayton, D. D. 2000, Space Sci. Rev., 92, 133
Tachibana, S., & Huss, G. R. 2003, Lunar Planet. Sci. Conf., 34, 1737
Michel, R., & Nagel, P. 1997, International Codes and Model Comparison
Vogel, N., et al. 2003, Lunar Planet. Sci. Conf., 34, 1873
Wasserburg, G. J., Gallino, R., & Busso, M. 1998, ApJ, 500, L189
for Intermediate Energy Activation Yields (NSC/DOC[97]-1; Paris:
Young, P. G., Arthur, E. D., & Chadwick, M. B. 1998, GNASH-FKK
NEA)
(computer code system) (PSR-0125/07; Paris: NEA)
Michel, R., & Neumann, S. 1998, in Int. Conf. Isotopes in the Solar System,
ed. J. N. Goswami & S. Krishnaswami (Proc. Indian Acad. Sci. 107;
Bangalore: Indian Acad. Sci.), 441