1 (112)

.

t

I

Chapter 5

Source(s) of Short-lived Nuclides in theEarly Solar System and Time Scales

0

The implications of the results obtained from the K-Ca and Mg-Al isotopic analysesof individual refractory phases and CAIs from the primitive meteorites, Murchison,

Efremovka and Allende are discussed in the present chapter. The two most importantresults obtained in this study are:

(1) Excess 41K was found in all the CAIs from the Efremovka meteorite analyzed ithis work and also in several samples from Allende and MurchisonThe observed excess in 41K also correlates with the measured

nalyzed phases and clearly demonstrates the one time presence ofm the analyzed phases. The initial value for 41Ca/40Ca at the time of theirformation was estimated to be (1.38 ± 0.13) x 10~8 (Fig. 5.1).

in

meteorites.

Ca/39K ratio40

in the a 41Ca

(2) Observation of correlated presence/absence of the two short-lived nuclides,26A1 and 41Ca, in CAIs as well as individual refractory phase (hibonite) fromEfremovka, Allende and Murchison meteorites. They are either present with

1.4 x 10-8 for 41Ca/40Ca, and%

very low abundances (Fig. 5.2). This result,

their characteristic initial values of

for 26A1/27A1 or are absent/occur i

5 x 10"5rsj

m

98

5

1 (113)

0.14

Murchison A

Allende

Efremovka

Terrestrial O0.12 i

T- (41Ca/40Ca)s(1.38 ± 0.13) X1O'8 T -1

*G)

CO\ HB

0.10* 1

*

{

*0.08

Vf-••••••» > ••a

O".•4

H

a0.06 1

1 2 3 4

(40Ca / 39K) x 106

4

Figure 5.1: K-Ca isotopic systematics in refractory phases from the meteorites, Efre¬

movka, Allende and Murchison.

99

*

1 (114)

obtained for the first time in this study, suggests that 11Ca and 26A1 are coupled

either in their original production site(s) or were thoroughly mixed in some

parcels of proto-solar cloud before they were incorporated into the refractory

phases.

In the following, we will discuss these observations to infer the most plausi¬

ble source(s) of these two short-lived nuclides present in the early solar system from

among the various sources proposed so far and discussed briefly in chapter 1. We

show that the correlated presence/absence of the two short-lived nuclides in indi¬

vidual refractory phases helps us to better constrain the source(s) of the short-lived

nuclides. Before discussing this aspect, we briefly consider the implications of the

absence of signatures of these radionuclides in certain refractory phases analyzed i

this work.

in

5.1 Distribution of 1lCa and 26A1 in the Early Solar Sys¬tem

While the presence of excess 4IK and 26Mg in refractory phases is attributed to the

presence of the radionuclides, 41Ca and 26Al, respectively, at the time of formation of

these phases, the absence of such an excess in some of the refractory phases analyzed

in this work can be due to several causes. These include: (i) a heterogeneity of these

radionuclides in the solar nebula (due to variable mixing of the source material with

the nebular material), (ii) late formation of these phases, and (iii) reprocessing of

material and/or redistribution of Mg and K isotopes as a result of multiple secondaryevents affecting the refractory phases in the early solar syst

Based on detailed petrographic and magnesium isotopic studies of refractory

inclusions from Allende (Podosek et al., 1991), Vigarano (MacPherson and Davis,1993)

em.

100

1 (115)

HibonitesMurchison A A

AilendeEfremovka •Terrestrial

0.148

o> AV2 \08 0.144 p

O)

28

0.140$v

50 100 150

27Al / 24Mg

0.120.75

41Ca/40Ca= 1.4x 10'8i 0.50*CDMM- w

0.09*

/

0.25AH—A

-ft ~ ------L5J.1......£*7ÿ _- < ••••* ' ••••

f

••••• ••••

0.060.002 4

1 2 3

(ÿCa /MKX 106) (ÿCa / mK x 107)

top panel) also do not have 41K excess (lower panel).excess (open symbols;

101

1 (116)

and Leoville (Caillet et al., 1993) meteorites, a correlation was observed between the

presence of disturbed Mg-Al isotopic systematics and secondary alteration effects.

The observed secondary alterations must have taken place within the first few million

years after the formation of CAIs, that resulted in Mg-isotopic redistribution between

the adjoining mineral phases leading to either complete erasure of the signatures of

Mg excess or reducing the initial 26A1/2TA1 value to less than the canonical value

of 5 x 10“5. The relatively low value of (8-5 ± 1.6) x 10~5 for 41Ca/4GCa in the case

of Allende EGG3 inclusion (based on combined data obtained bv us and Hutcheon

et al., 1984), as compared to the value of ~ 1.4 x 10“' seen in Efremovka CAIs may

be attributed to possible disturbances in K-Ca isotopic systematics in this CAI. This

is consistent with the petrographic studies of EGG3 inclusion which also suggest

presence of secondary alteration effect in this CAI (Meeker et al., 1983). The Mg-

Al isotopic systematics of the inclusion was however not affected significantly

indicated by the inferred initial value of 5

1984).

26

as

10 for 27Al26Al Armstrong et aAX •

In the case of HAL hibonite, eventhough, its petrographic studies reveal

ondary alteration presumably taking place in a nebular environment (Allen et al.,

1980), the absence of 41Ca and low abundance of (5.4 - 1.3) >

(Fahey et al., 1987) would be difficult to explain on its basis as it would

rather large time interval of

The preservation of nuclear isotopic anomalies in calcium

sec-

: 10-* for 26A1/27A1

require a

7 Ma for the closure of the Mg-Al isotopic system.t

and titanium found in this

CAI (Lee et al., 1979, 1980) over such a time scale seems to be very difficult. Thus

heterogeneous distribution of 26A1 and in the solar nebula appears to be a

plausible reason for the absence of these radionuclides in HAL hibonite as well as

of the Murchison hibonites analyzed in this work.

more

some Isotopic heterogeneity in

the solar nebula has also been proposed earlier to explain the absence of J Mg excess

in CM hibonites (MacPherson et al., 1995). The magnesiumin FUN inclusions and-

102

1 (117)

and oxygen isotopic analyses of corundum grains from Murchison meteorite by Vi-

rag et al. (1991) also suggest the presence of more than one 2<>A1 component in the

solar nebula. Although the absence of 20Al and 41Ca in some of the analyzed phases

in the present study could represent an inhomogeneity in the distribution of these

two nuclides in the solar nebula, the fact remains that they are both present in their

canonical abundances in all the petrographically unaltered Efremovka and Allende

CAIs. Further, as the petrographic context of the Murchison hibonites are unknown,

we shall assume that the initial 26A1/27A1 and 41Ca/40Ca were uniform in the solar

nebula, atleast in the region of CAI formation, with an initial value of 5 x 10" 5 and

1.4 x 10~8, respectively, in the remainder of our discussion.

5.2 Source(s) of the Short-lived Nuclides, 2 Al and ; Cain the Early Solar System

The two proposals made to explain the presence of the short-lived26Al in the early solar system phases are:

nuclides, Ca and1

(i) presence (or production) of these short-lived nuclides in the early solar svstem* w

that were incorporated live' into the CAIs during their formation, followed by

in situ decay within these objects,

(ii) contribution from variable amounts of "fossil" 41K and 26Mg of radiogenic origin

locked in refractory stardust that are part of the initial components of the solar

nebula from which the CAIs were formed.

Apart from these, an alternate mechanism for producing 41Ca could be

mogenic production by secondary neutrons during recent cosmic ray exposure of the

meteorites [ via (n/y) reaction with 40Ca present in the CAIs of refractory phases]

cos-

103

1 (118)

and its subsequent decay. In order to explain the presence of nCa with the observed

initial 41Ca/40Ca ratio of ~ 1.4 x 10-8 in the various analyzed refractory phases, a

neutron fluence of ~ 3 x 1016 cm-2 is necessary. The expected secondary neutron

fluence in a meteorite depends mainly on three parameters: the cosmic ray exposure

duration, the preatmospheric size and the chemical composition of the meteorite. The

preatmospheric size is an important parameter as the production of secondary neu¬

trons within a meteorite increases with shielding depth and reaches a maximum at a

shielding depth of ~ 100-150 gm cm-2. The Efremovka meteorite has a cosmic ray

exposure age of ~ llMa (Murty et al., 1995,1996; Mazor et al., 1970), whereas, Allende

and Murchison meteorites have relatively lo

and

w cosmic ray exposure ages of ~ 5.2 Ma

2 Ma, respectively (Fireman and Gobel, 1970; Hohenberg et al., 1990). As the

estimated neutron fluence in Allende is rather low, < 1015cm'2 (Gobel et al., 1982),

the presence of 41Ca in the Allende refractory phases due to production by secondary

neutrons during cosmic ray exposure of the meteorite is not possible. In the case of

Efremovka, its preatmospheric size is expected to be much smaller than Allende for

which the recovered mass exceed 2 tons compared to 21 kg for Efremovka. Thus,

the neutron fluence experienced by Efremovka is not expected to be much higherthat of Allende. In fact, recent noble gas data for Kr and Xe and for cosmogenic

36C1 in bulk samples of Efremovka (Murty et al., 1995, 1996) suggest that the neutron

fluence experienced by Efremovka is almost a factor of ten lower than the value forAllende. A large preatmospheric size for the Efremovka meteorite is also ruled out

by preliminary data for cosmic ray produced nuclear tracks in Efremovka as wellthe measured activities of 36C1,

as26Al and 10Be in this meteorite and the cosmogenic

Ne/21Ne ratio of 1.11 obtained from noble gas data (Murty et al., 1996). Thus, the22

possibility of cosmogenic production of 41Ca in Efremovka can be ruled out.

An additional argument against the cosmogenic production of 4ICa by

ondary neutrons is the absence of 41K excess in HAL hibonite from Allende

sec-

meteorite

104

1 (119)

and some individual hibonite grains from Murchison meteorite. It would be difficult

to explain the presence as well as absence of 11K excess in different phases of the

same meteorite if the production took place during the recent cosmic ray exposures of

these meteorites in interplanetary space. Therefore, the possibility that the observed

K excess in refractory phases and CAIs of these meteorites is due to production by

secondary neutrons during their cosmic ray exposures in the interplanetary space can

be ruled out.

'ii

5.2.1 Fossil origin of the short-lived nuclides in the early solarsystem

The possibility that the short-lived nuclides found i early solar system solids (CAIs)in

could be of "fossil" origin was proposed by Clayton (1977,1982, 1986). In this modelthe decay of the short-lived nuclides took place within interstellar grains that hadpreferentially incorporated such nuclides in their stellar formation sites. The casefor the possible presence of excess 41K of "fossil" origin in refractory inclusions ofprimitive meteorites, which would enhance the 41K/39K ratio i these objects abovemthe normal solar system value, was specifically noted by Clayton (1977). The pposed scenario involves the formation of refractory condensates (stardust)environment (e.g., supernova envelope) that are enriched i

ro-

in stellar

their refractory elementconcentrations (e.g., Ca) compared to the volatile (e.g., K) and as such theyhigh Ca/K ratio and also excess 41 K from 41

m

will haveCa decay. Since these stellar condensates

are expected to be an important component of the solar nebula they will find theirway into the CAIs that can inherit excess «K from the stellar condensates.this scenario has i

(Amari etal.,

41K and 40Ca/39K i

of mixing between

Whileits own appeal and excess «K has been detected in interstellar dust

1995), it does not readily explain the observed correlation betweenone makes the ad hoc assumption

. However, petrographic

excessthe refractory phases unlessin

components of arbitrary compositions

105

1 (120)

studies of the analyzed Efremovka CAIs clearly show that they were formed via crys¬

tallization of refractory melts and one would expect complete K isotopic equilibration

during their formation which rules out preservation of signals resulting from varying9

degrees of mixing of distinct components within individual CAIs. Thus, although the

presence of a very small amount of "fossil" 41 K and 2<iMg in the CAIs cannot be ruled

out unequivocally, we do not consider this to be the primary source for the observed

excess of 41 K and 26Mg in the analyzed CAIs and refractory phases.

The 41K excess in the analyzed refractory phases and CAIs, and its correlation

with 40Ca/39K can therefore be best explained by considering the presence of live

41Ca in the early solar system at the time of CAI formation. The presence of live 41Ca

and 26A1 in the early solar system may be due to any one or more of the following

processes:

(i) introduction of freshly synthesized 41Ca and 26A1 from

the solar nebula,

specific stellar site(s) to

(ii) production of 41Ca and 26A1 in early solar system matter by nuclear reactions

initiated by energetic particles from an active (T Tauri) early Sun,

(iii) irradiation of the proto-solar cloud by

cloud complex.

energetic particles within a molecular

If the first alternative is true then the presence of the short-lived

and 2bAl in CAIs puts an extremely stringent constraint on the value of A, the time

interval between the last nucleosynthetic input to the solar nebula and the formation

of some of the first solar system solids (CAIs). We shall first discuss the other two

possibilities before discussing this aspect in detail.

nuclides, 41Ca

106

1 (121)

5.2.2 Production of 11Ca and 2(3A1 by energetic particleirradiation

Irradiation by an active early Sun

As discussed in the introductory chapter, the possibility of production of nuclides

(radiogenic as well as non-radiogenic) due to energetic particles irradiation in the solar

nebula has been suggested by several groups. In this scenario, the short-lived nuclides

produced due to interaction of energetic particles from an active early Sun with

solar system matter. One can consider in situ production of nuclides in CAIs as well

as their production in nebular gas and dust prior to the formation of the CAIs. Several

proposals were made to explain the enhanced abundances of both radiogenic and non-

radiogenic nuclides in meteoritic matter by considering an enhanced flux of energetic

particles from the early Sun (Heymann and Dziczkaniec, 1976; Heymann et al., 1978;

Clayton et al., 1977; Lee, 1978; Kaiser and Wasserburg, 1983; Clayton and Jin, 1995;

Shu et al., 1996). Evidence for an active early Sun, with a time averaged proton flux

are

of 100 to 1000 times more than the contemporary average (based on lunar sample

data), has been obtained recently from measurements of cosmogenic neon isotopes in

solar flare irradiated olivine grains from several carbonaceous chondrites (Hohenberg

et al., 1990; see also Caffee et al., 1991). However, there are several problems with the

solar particle irradiation scenario. These are; (i) use of arbitrary spectral parameters

for the solar energetic particles to avoid co-production of nuclides for which

anomalous abundances have been found in meteoritic phases, (ii) requirement of

very high energetic particle fluence, and (iii) mismatch with observational data for

the short-lived isotopes (e.g., 26A1 and 53Mn) by solar energetic particles. Inspite of

the attempts made so far, a rigorous calculation of cosmogenic production of all the

short-lived nuclides, including llCa and the recently discovered 3t!Cl (Murty et al.,

1997) with appropriate cross section data is still lacking. In the present study, we ha

no

ve

107

1 (122)

carried out detailed calculations for the production of the short-lived nuclides, 26Al,

Ca, 36C1 and 53Mn in CAI precursor material in the solar nebula by solar energetic

particles to further investigate this problem. We have included in our calculations all

the possible low-energy (< 100 MeV/amu) solar proton and alpha particle induced

reactions in different target elements that can produce these radionuclides. The alphato proton abundance ratio in solar energetic particles was taken as 0.1. The relevant

reactions along with their reaction cross sections are shown in Fig. 5.3. The reaction

cross sections for the production of 26Al, 4lCa and 03Mn were taken from Ramaty et

al. (1996). In the absence of measured reaction cross sections for ,6C1 productionhave estimated the

, we

by considering equivalent reactions, based on compilationof reaction cross sections for Z = 20-40 (Lorenzen and Brune, 1974) and reaction

same

section data for production of 26Al, 41Ca and 53Mn (Ramaty et al., 1996). In- cross

particular, the reaction cross section for 33S(a,p)36Cl was based on compilation of crosssections from Lorenzen and Brune (1974); for the 34S(a,pn)36Clreactions, 39K(a,pn)41Ca and 24Mg(a,pn)26Al were used and the 41K(p,n)41Ca reactioncross section was used for 36S(p,n)36Cl reaction. The equivalent reactions consideredfor 37Cl(p,pnm are 5<Fe(p,pn)«Fe, "Ca(p,pn)<'Ca and 27Al(p,pn)“Al, and for thereaction 39K(p,3pn)36Cl, the intermediate values of

reaction, equivalent

cross section from the reactions44Ca(p,p3n)41Ca and 28Si(p,2pn)2tiAl were used. For 35Cl(Q,3He)36Cl, we used thereaction cross sections of 40Ca(Q/3He)4ICa. However, as the threshold for the formerreaction is 2.5 MeV/amu as compared to ~ 7 MeV/amu for the later, thecross section for 36C1 production were taken by scaling the energy from E to E/2.3.

reaction

The calculations for cosmogenic production of radionuclides were carried outby considering different spectral parameters ior the solar energetic particlesenergetic particles that are associated with solar-flare events havekinetic energy of 1-100 MeV/amu with their flux decreasingenergy (Caffee et al., 1988). The solar particle energy spectrum i$ expressed either

. These

a typical range in

rapidly with increasing

as

108

1 (123)

1000:

23 38Na(a,n) Ar(a,n) 41K(p,n)27 r

Al(p,pn)39K(a,pn)

10028 A

/ i

r /Si(a,apn) 42/ Ca(p,pn)\i

U \I \

i03 \ \\100 h \e I \27Al(a,an) /

D \\ \

B/y \ I

28 ic Si(p,2pn) \l / K \\

44/ Ca(p,p3n)ar IN >10 18 *\

\\10 hu V

26 rMg(p,n)

\r \

Mg(a,pn)

26A1 41

Ca40Ca(a,3He)1 li I 1* A

1 l I

1 *10 100 1000 1 10 100 10fi~V w

1000 r1000*

:»

J4yFe( a,an-ÿCr(p,nV33 \ V

S(a,p) rl / X°S(p,n)V

t

*0j

r(a,p2\ r\1* /

I \\\/ / 100\l EiV\\ \ \/\

N

K At 4,

Fa n pnr't\ 37 V •.

JO 100 - Cl(p,pn)i \i/ r\ v \\\ r / VA

X v1 I\ \ \/ \\o \

// \ /\ I\\e \\ v

10 \r \o

I\v

57n ,re(D

f

’,an

39K(p,3pn)'I/

/V

56„W 10 *

Fe(p,p3n Fe4 L\ or\\ V

1 r\V

Mn(n n 'A\ : n) Mn\

\\ 34 5635 3 S(a,pn) Fe(p,a)CI(a /He)

V

36Cl S3

Mn0.1 h:1 ii i i 4 A A A A > 1E A A. 1 ‘III

1 10 100 1 10 100 1000Energy (MeV/amu) Energy (MeV/amu)

4

Figure5.3: Nuclear reaction cross sections for p41Ca, 36C1 and 53Mn from various target nuclides.

roduction of short-lived nuclides, *>bAl,%

109*

1 (124)

a power law in kinetic energy, E, or as an exponential in rigidity R (momentum per

unit charge):

dN(5.1)= constant x

dE

ordN

(5.2)= constant x edR

where 7 is the power law exponent, and Ro, the characteristic rigidity, defining

the spectral hardness. Typical values for 7 and Ro in contemparary solar flares range

from 2 to 4, and 50 to 200 MV, respectively (Reedy and Arnold, 1972; Lai, 1972;

Goswami et al., 1988). We have performed calculations for the above range in •7 and

Ro-

CAI-precursor grains present in the solar nebula were considered as the target

material and we have assumed them to be of chondritic (Cl) composition (Anders

and Ebihara, 1982). A grain size in the range of ten microns to cm, following a

number-size distribution of the type, dn/dr -0a r", was chosen for the production

calculations. Only ionization energy loss process has been considered for the low

energy solar particles and appropriate range-energy relation was used to obtain the

modified differential energy spectra at different depths within a grain. The production

rates at various depths within grains of different sizes were calculated following the

approach described by Lai (1972).

The production rates of the radionuclides, 26Al, 41Ca, and 53Mn at different4

depths within grains of three different sizes are shown in Figures 5.4 to 5.7. The

equilibrium production rates are expressed in terms of production per kilogram of

material per minute (dpm/kg). Except for grains of radii less than 0.1 gm/cm2, the

production rates fall steeply with depth. Production rates for grain with radius 0.034

110

1 (125)

104Energy spectradN/dE -E*T

Rigidity spectradN/dR «- e

1 -4103 r T -3 Si

"\R0- 200 s\ X_

Os\102 \\ t -2

V\ s 102 Vsrs\ \\\ \\\\ R0 *100 \\

\ \Ro* 50 10' \\ r

\\ sRadius = 3.4 gm/cm* Radius = 3.4 gm/cm2 S

101 LUJL 10° jj

10"* 10*3 10-2 10*' 10° 10' 10*4 1Q-3 10-2 10*' 10° 10*

104—I5X102U)

i -4103f :

__S. 3 Ss---5ÿÿ200<

R >r -2102o t

© sfflk. 2X102 Rg » 100co‘5 Ro-50 10' ri Radius x 0.5 gnVcm2 Radius = 0.5 grrVcm1£

io° L-L....i10210*3 10*2 10°10*4 10-'10-2 10°10-2 10-'10-1

104

5X102 JL:4103 -

__2

R*-50Y -2

102Rÿ - 200

R© * 100

10' r

Radius x 0.034 gm/cm2 Radius = 0.034 gm/cm2

2X102 10° . i ,,iXX

10-'10-*10 310-4 10"* 103 10* 10->

Radius (gm/cm2)Radius (gm/cm2)

Figure 5.4: Production rate (dpm/kg) of the radionuclide, 26A1 as function of depthfor grains of three different sizes. Energy and rigidity spectra with different 7 and Rovalues, respectively, were used for the calculations.

Ill

_

1 (126)

10210lEnergy spectradN/dE •E

Rigidity spectradN/dR « e‘™

5-50R0- 100-4

x

Ro - 200 V -3101 \10°\\\x \ \

\ \ V\w\$\

\ 10°10-’\

1 -2\\\v\

Radius = 3.4 gm/cm*Radius = 3.4 gm/cm* \ \...i...I10-'10-* ...I J xuL ...i

1011n®I V10-4 103 10*2 10*110-4 10*3 102 10-1 10° 101

101 102

C5

I Ro-50-4Rÿ- 100 'V.

X -3 \101 r__ •CD S

ORo-200 \

\ \*5 10°\\

X * 2V-

10°c \o\ \15 \

\

Radius « 0.5 gm/cm* * Radius a 0.5 gnVcm*

101 lt| 1 ***** 10 1

104 103 102 10*’ 10° 10*4 10*3 10-2 10-1 10°

101 10*

1 -4

Ro-50X -3

10'

R0- 100X -2

10°Ro - 200

Radius a 0.034 gm/cm* Radius = 0.034 gm/cm2

10° 10-’10-4 10-> 10-* 10-1 10-* 10 s 10-* 10-'

Radius (gm/cm*) Radius (gm/cmJ)

Figure 5.5: Production rate (dpm/kg) of the radionuclide, 41Ca as function of depthfor grains of three different sizes. Energy and rigidity spectra with different 7 and Rovalues, respectively, were used for the calculations.

112

1 (127)

103101Energy spectradN/dE - E'T

Rigidity spectradN/dR -

102Rÿ-50 .4

N\Rÿ-200

t - 3 \\10110° \X \-\\s\ V\1 -2

\ N\\

10°\ R,- 100\ \x\ \Radius = 3.4 gnVcm2Radius* 3.4 gnVcm2 \\

10-' o-L 44J l44 J10-'AAJ aaA

10110-4 10*3 io* 1a1 io°10*4 10* 10'2 10*1 10° 10’

101 104

03

i 103

%0 102* -4Ro- 100 R0-50o

X -3\

%k.

Fn 101 SR0 - 200 Xc

oT -2*5 w v:—10°\ [ \

\ P Radius = 0.5 gm/cm1

IQ-1 Li-k.

10-4

Radius = 0.5 gm/cm2 \V .10° -uiL A

xVj

10-4 10'3 10*2 10*1 10° 10* 10*2 10*’ 10°

10* r—101

Ro-50 102 "l -4Ro- 100

•--'IJtl__

-210*Rg » 200

10° r

Radius z 0.034 gm/cm2 Radius a 0.034 gm/cm2

10° 1a1t t il

10410*4 10*1a3 10*10-1 10* 10*>

Radius (gm/cm2) Radius (gm/cm2)

Figure 5.6: Production rate (dpm/kg) of the radionuclide, 36C1 as function of depthfor grains of three different sizes. Energy and rigidity spectra with different 7 and Rovalues, respectively, were used for the calculations.

113

1 (128)

104Energy spectraRigidity spec

dN/dR «

tradN/dE •E T

102103

\

\ 1 -«\\ R0 - 200

102: * -2\ X101 \ \

\ WRg- 100 \ <

10'R,-50 \

. Radius * 3.4 grrVcm*Radius x 3.4 gm/cm* N.

X

10° jftj J J 1 10° M. . _

I A Ill a *a » > it I A a « a i AA i Ai a a a a a a a aA A A a

10-* 103 10* 10-' 10° 10’4

10"* 10-* 10* 4

05

i4>

102

\\c2

\0 \ X R0- 200 102 \X

o \ \ -V

\\v

X

« \\ -- 4

X a

\ \ S.

Ro-50o \\ -\s 4 x

\

i \R0- 100\CL\ Radius x 0.5 gnv cm2Radius x 0.5 gm/cm1 \

XA 101a l i i i ml10’ mi a i i i nil A A tataaia i i-L i i aaA i 1

10-* 10-* 10* 10-' 10° 10’ io-* 41 u 4 n

10s3x10* 1

*

2x10* R.-50

Ro-joo__Rÿ-200

1 4

X -3

't -2

Radius x 0.034 gnVcm2Radius s 0.034 gm/cm2102 l -> A A I A a I

102 A L A A A * A l 1 A iA A AAAAA A

10* m-2I v 110* 10-110*10*4 I V

Radius (gnvcm2)Radius (gm/cm2)

Figure 5.7: Production rate (dpm/kg) of the.radionuclide, 53Mn as function of depthfor grains of three different sizes. Energy and rigidity spectra with different and Rovalues, respectively, were used for the calculations.

%

114

a

1 (129)

gm/cm2 (radius ~ 100pm, density = 3.4 gm/cm3) are almost uniform throughout the

grain and are higher than those for larger grains. This is because of "effective" 47r

irradiation in case of smaller grains as compared to

larger grains. The maximum contribution to the average production

comes from the smaller grains due to the power law distribution in grain size used

by us.

effective” 2nirradiation for the//

rate basicallyJ

= 3 andThe average production rates for single grains ot

Ro = 100 MV are shown in Fig. 5.8a. These values were nnaiiv

the average production rate for an ensemble

grain size distribution, dn/dr oc r-J; with

over grain sizes in the range of 10

four radionuclides for = 3, Ro = 100 MV, and J =

\r\fhV 1IX 1 lf difftterent sizes w

determine

iiowing the

sed1 t1 to1 1

1 -«x •,,,,prpnf fOLill1 Cl 11 31 CJ 1v/ot grains or i r r

n 4- TIIP mfpcu i. i lie li lie v was carneaa flOhL U 111-s a nvll l & L

\f frw1

,/rn to 1 IIIIX4 X11 IVJ X vil l* r7K f PC1 Cx LC enp 7K\11te a v d VA V. Illi « «CTPei tie: L ii ix X Iv

*1X «4

KhI-* i4 arp sx ui e jum\

r 4 KwA X 1

The energetic particle flux required for production

Cl and 53Mn at the required level

meteorite phases was estimated using the a\

in grain ensemble of 10 /im - 1cm. The meteoritic ab”"'-’

the observed initial ratios of 5 x 10-5 for 26A1/27A1, 1.4

for 36C1/35C1, respectively. In case of 53Mn, two initial v

Allegre, 1985) and 3.6 x 10-b (Lugmair et al., 1995) were

The energetic particle flux needed for production of the radionuclides

the contemporary long-term, time averaged

solar energetic proton flux of ~ 100 protons/cm2/sec (Reedy and Marti, 1991) for E

> 10 MeV/amu obtained from lunar sample data. The flux enhancement factors

shown in Fig. 5.9 as function of duration of irradiation by solar energetic particles.

« 11 1r- *-

X vp i n PC« x

v viniirX l LX l» xxvxv4 X IV XXV

26Al, 41Ca, 36 j-x lU 111U Ivi i

» * x 1

i ii ill nrpeLxl ILXCxX ILL3f n PI vLX IL XX SH [ V H

r-\ >11 1y>4

r XX 1v

x 111 p I

LX LXL l X ly X lr> t* X/> r>

X CX 1CJ of fnPQP niUi U ICoC ill1 Hrprap-p nrfv X wA Vw X V es

X

1

pÿfim il XX X l CX IL LX tro™es wereIXX IVXIXX L v. XXL

-i r\ — H forxux 4XXX l LX X V

allies, 4.4 > — / T1• 1f birck ai l LX

x

tor initial 03Mn ,DMn.11cpnLXOCLX

s are expressedA

in term of flux enhancement factor over

are

The important results obtained from these calculations, that can be clearly seen

from Fig. 5.9 are:%

115

*

1 (130)

103*AI .(E)

"Al (R)“Mn (E)

MMn (R)

•••••••••

: d102D) •c

EQ.

X3 *C\ (E)

4,Ca (E)**"*CI (R)

<1Ca (R)®—T

CD

CO 10'

o

I\

10° "XL.Q_

C

Y =3

R0= 100 MV

'vV.

(a)10-' lit

1 i i 1 i J-*-

10*2 10*1 1 no 1n1v

Grain radius, r (gm/cm2

26Al (E

26MJ

MMn (E) £“Mn (R)

io>102

EQ.

"O

CDOJ

O

'€ 36Ol\*/\

1013

4,Ca (E) »-CL

“a (R)a<1Ca (R)

0m*

(b)10°

3 4

3 (Grain-size distribution parameter)

-;3M-r Srains °f different sÿTndR refer"toaÿ

XeSCdrar-T *"<"*ÿ“* Values of S™"»diLbuHon

6 •

116

1 (131)

108 108Rigidity spectra

R0 = 100Energy spectra

Y =3107 g; 107 K: \ ; \K\ \\ 26AI106 K-. 106\ \

26A I\ \\\ \105 - 105\ \

\ \\ 36CI\53Mn-H41Ca - 'X104 E- 104\

36CI —\

o103 =- 53Mn-H103O

dn/dr « r'3co : dn/dr « r'3 53Mn-L “ Mn-L102 l I » 1 1 1 n i i l i mi 1 1 t I 1 Ml I I I I I III 1021 I I I I 111 1111 Mili ill mil tini i iiii

10'3 10-2 10'1 10° 101 102<D 1n21 mU1 noi u10'3 102 10'1 V

E0 108108o

Energy spectra

Y =3Rigidity spectra

FL = 100CO107 K107 V

\ \CD \

106 K.\X 106 \K*. \

\ 26A!LL 26AI ’ 41Ca105105 \

\ \ \\ \\41Ca JbCl\ \

\ 104104 \ 36CI53103 Mn-H103 53Mn-H s dn/dr « r'4F dn/dr <* r’4 5 Mn-L"53Mn-L

102 i linni i i 1 1 m l i i mu1 1 1 11 i 1 111iii i i Minii i i I i in102 i i mmi i M i mi i i nni

103 10-2 10‘1 10° 101 1 n210° 101 102-110-3 10-2 10 I

Irradiation Time (Ma)

Figure 5.9: Flux enhancement factor required to produce the radionuclides to match

their observed initial abundances in meteoritic phases.

117

1 (132)

(1) Coproduction of 26A1 along with any of the other radionuclides is not possible

for any enhancement factor irradiation time combination. Production of 2'A1

requires at least one order of magnitude higher flux compared to the other

nuclides.

(2) Coproduction of the nuclides, 41Ca, 36Cl and !Mn requires a flux enhancement

104 over the contemporary average flux even for irradiation times longer

than a million year if we consider the higher initial o3Mn

of

Mn value./ JJ

(3) For the lower value of 3.6 x 10-6 for initial 53M /o5Mn (Lugmair et al., 1995),

production of o3Mn is possible even with a moderate flux enhancement of

n/

1000 for an irradiation time scale of ~ 1 Ma.

The above results coupled with our observati

of 41Ca and 26Al in individual refractory phases

orites, show that we can rule out the cosmogenic production of these radionuclides by

solar energetic particles. The production of 26Al to match meteoritic observation will

overproduce ‘,1Ca. In addition, the inferred flux enhancement factor for production

of 26Al is orders of magnitude higher than that inferred by Hohenberg et al. (1990)

from studies of solar flare irradiated olivine g

indirectly from studies of T Tauri stars (Gahm, 1989) that also suggest flux enhance¬

ment factors of the order of 100-1000. Although it is difficult to rule out the possibility

of a higher flux of solar energetic particles at the time of formation of CAI from their

precursor components than at the time of irradiation of the olivine grains in CM

chondrite, the problem of co-production o( 41Ca and 26Al remains. We therefore do

not favour the proposal that nuclear interactions by energetic particles from an active

early Sun could be the cause for the observed excess of 41K and 2DMg in the refractory

phases and inclusions found in the primitive meteorites. The possibility that 53Mn

present in the early solar system may be produced by solar energetic particles remain

on of correlated presence/absence

and inclusions from different mete-

rarns in CM chondrites and estimated

118

1 (133)

a viable suggestion if the lower value for initial 53Mn/55Mn is proven to be correct by

future experiments.

Irradiation in the proto-solar cloud

The recent discovery

Orion nebula (Bloemen et ai, 1994) provide

through nuclear processes in

of enhanced -f ray flux from OB star formation region in the

s a direct measure of energy generation

The measured flux of the 7star formation regions.

rays from nuclear deexcitation of 12C and ieO is h H_ hundred fold higher than previously

and suggests enhanced abundances of low enerrrwpredicted (Ramaty et al., 1979) 6)

cosmic ray carb'on, oxygen and other heavier ions in these regions. The sources of

these particles inside the molecul

could be T Tauri stars and mass

ar cloud are not unambi uouslv known but they

;ecta from stars or exploding supernovae.9 X O 1

proto-solar cloud, within such a

molecular cloud complex, by reactions induced by an enhanced flux of e

Theo

possibility of production of different nuclides in the

nergetic 10ns

oy several authors (Clayton, 1994; Marti

Ramaty et al., 1996). The detailed

analysis carried out by Ramaty et al.(1996), who have considered production of both

light isotopes of Li and B as well as the extinct nuclides, 26Al,

that it may be possible to produce the required amount of 41Ca to explain the initial

41Ca/40Ca in meteoritic phases, while 26Al will be under-produced as in the case

of solar particle irradiation, discussed in the last section. Unfortunately, Ramaty et

al.(1996) assumed the initial ratios measured in the CAIs as representative values for

and secondary neutrons has been proposed

and Lingenfelter, 1995; Clayton and Jin, 19y

4:Ca and 53Mn, indicate

the proto-solar nebula which are not correct (particularly in the case of 41Ca having a4

short mean life), unless one assumes that production continued throughout the period

of proto-solar cloud collapse and close up to the time of formation of the CAIs. The

inability to coproduce the two short-lived nuclides, 41Ca and 20Al in their required

119

1 (134)

amount also makes this model less attractive compared to the proposal of injection of

material from stellar sources in which some of the extinct nuclides could be assumed

to be cogenetic. Thus, injection of freshly synthesized material from specific stellar

site(s) into the proto-solar clouds remain the most viable process for explaining the

presence of the short-lived nuclides, and particularly 41Ca and 2<>A1, in the early solar

system. In the following, we consider the scenario of injection of freshly synthesized

41Ca and 26A1 to the solar nebula from specific stellar sources, and the constraint

time scales of processes that can be put from our observations.

on

5.3 Extinct Radionuclides and Time Scales of Early SolarSystem Processes

*

The presence of 41Ca, 26A1 and several other short-lived now-extinct nuclides in the

early solar system has been established from isotopic studies of suitable phases from

primitive meteorites. These radionuclides can serve as useful chronometers of early

solar system processes. In particular, if we attribute the presence of these nuclide to

injection of freshly synthesized material from specific stellar source(s), wecan estimate

the value of A, the time interval between the injection of the short-lived nuclides to

the solar nebula and the formation of some of the first solar system solids (CAIs) in

which their decay products have been observed. Obviously the radionuclide with

the shortest meanlife will provide the most stringent constraint on the value of A.

Prior to our work (Srinivasan et al., 1994, 1996), 2t)Al (r ~ IMa) was the shortest lived

radionuclide whose presence in the early solar system was conclusively established.

We have now shown that 41Ca, that has a much shorter meanlife than 26Al, was also

present in the early solar system.

4

Several stellar sources like novae, supernovae, Wolf-Rayet and asymptotic

120

1 (135)

(i) explosive nucleosynthesis

and Weaver, 1980, 1995),

f T A

in supernovae; P~4xl0 ' to ~ 2 x evf •A

0

(ii) hydrostatic carbon burning < / A_

Am\ JL JLA A Iin massive stars; P ~ 1 IP i i

(iii) high temperature H-burning

0.1 to 20 (Hillebrandt and Theileman, 1982, Cameron, 1985,

Wasserburg et al., 1994),

in novae and asvmn tOy

p n ri CL1 11L/ 4

L kr ii

*Ar f »

(iv) Wolf-Rayet star (core H-burning): P (ejecta

Walter and Maeder, 1989).

LJ XA

I 7

While the production ratio of (26A1/27A1) vari

the situation in the case of 41Ca, is much better

have been considered for production of 41Ca and corre<=

j varies ov<*« rA

Vw rA k.

t

m th (l v A l VA A A k V.

*

«

"1 i rr4

(i) explosive oxygen burning; P 1.5 x 5 (Wiry10 1 A3ÿr V b—»w •r

(ii) explosive silicon burning; P 10 3 (Bodanskv « /Pf P l 7hn )

A x vy / •WA A •#\ /

(iii) explosivesupernovae nucleosynthesis; P~ (0.6-2.8) x 1C

1993; Woosley and Weaver, 1995)

VAV V

44 A

? f9f

(iv) nucleosynthesis in TP-AGB stars; P 1.0 x 10-2 (Wasserburss et at., iLb

(v) Wolf-Rayet stars (core He-burning); P(ejecta) ~ 3 x 10-3

1988).

(Dearborn and r> iDldN

4

The production ratio for 41Ca is thus more tightly constrained than that of 26A1 and

10~3 to IQ"2.ranges from

\

122

%

1 (136)

It is possible to obtain the value of A based on the theoretically estimated

production ratio and measured initial ratio for

have a priori knowledge of the dilution factor, i

any one pair of these isotopes, if we

, i.e., the amount of dilution of the freshly

injected material containing the radionuclide ('"Ca or 26Al) and its stable counterpart

( Ca or Al), with the pre-existing material in the proto-solar cloud containing only

the stable nuclides and devoid of the short-lived species. Unfortunatly, there is no

simple way of rigorously estimating this dilution factor and one can only obtain

approximate values using meteorite data for the somewhat longer-lived isotopes

107Pd (r ~ lOMa) and 129I ( 23Ma) (see, e.g., Cameron, 1985; Cameron et al., 1993;

Wasserburg et al., 1994), and assuming that they were also introduced into the solar

nebula by the same event.

r ~

We can circumvent the difficulty of estimating the dilution factor and derive a

self consistent value of A by combining data for two short-lived nuclides that

present in the early solar system and were injected to the solar nebula from the

stellar source. The correlated observation of excess 41K and 26Mg in refractory ph

and CAIs makes 41Ca and 20Al a suitable pair for such an analysis and we can consider

several sources (supernova, TP-AGB star and Wolf-Rayet star) as being responsible

for injecting freshly synthesized 2(jAl and 41Ca to the solar nebula. If we assume that

the dilution factor is similar for both the nuclides,

were

same

ases

we get the following expression for

A:

Mi1 Pi•InA = - (5.3)A,' A j Pi Mj j

where P, and P )refer to stellar production ratios, M, and Mj refer to measured ratios

in CAIs, and A, and Aj refer to the decay constants of the two short-lived nuclides.

Obviously, one can use the value of A, obtained from a given (P„ P,) combination,

specific to a particular stellar source, to infer a self consistent value for the dilution

123

1 (137)

factor. Using the measured initial ratios (M) of 5 x 10-5 and 1.4 x 10~8 for (26A1/27A1)

and ( Ca/ Ca), respectively, and the production ratios (P) noted earlier, we obtain

the following solution for different stellar sources:

(i) Supernova: A ~ 0.8 to 1.4 Ma; D (Dilution factor) 180 to 20,

(ii) TP-AGB star: A ~ 0.6 Ma; D ~ 10,000, and

(iii) Wolf Rayet star: A 1 Ma; D ~ 400.

It should be noted that both in the case of TP-AGB and Wolf-Rayet star the

production of 26A1 and 41Ca takes place in different regions of the evolving star and

over extended periods. 26A1 is synthesized during H-shell burning (TP-AGB)/

H-burning(WR), and 11Ca is synthesized during He-shell burning(TP-AGB)/core He

core

burning(WR). It is therefore necessary to take into account the ratio of nucleosynthetic

products from these two source regions that finally reach the outer envelope and get

ejected while evaluating the value of A. This ratio is close to unity (0.7 to 2) for

TP-AGB star (Wasserburg et al., 1994). In the case of Wolf-Rayet stars we have used

the time averaged values for the abundance ratios of these nuclides in the ejected

material (Dearborn and Blake, 1985,1988). As already noted, the effective production

of 41Ca in TP-AGB star would also be somewhat lower than the values noted above

as its production persists over a period, comparable to its mean life, and one has to

take into account the decay of 41Ca during this duration (Wasserburg et al., 1995).

Although the A values deduced above cluster around a narrow range (0.6 to

1.4Ma), irrespective of the stellar source, the corresponding dilution factor varies over

an extremely wide range (20 to 10,000). For example, if we consider a TP-AGB star as

the source for 41Ca and 26Al in the early solar system, this event need to supply only

0.01 percent of the stable nuclide component in the solar system, which appears

quite reasonable. Such a star can contaminate about 100 M0 of the molecular cloud

124

1 (138)

with its ejecta in the form of AGB wind /planetary nebula during the final stages of

its evolution. Although the probability of such an event taking place during the life

time of a molecular cloud is rather low because of the longer time scale for evolution

of AGB stars (few billion years) compared to the typical life time of molecular clouds(few tens of million year), the possibility of an AGB association with a molecular

cloud cannot be ruled out. Based on observational data, the probability of such an

association has been estimated to be ~1 %in a million year (Kastner and Myers,1994);

and over the life time of a molecular cloud, this probability could be much higher. A

close encounter between AGB star and the proto-solar cloud could have resulted

in injection of the radionuclides, 41Ca and 26A1 into the proto-solar cloud and mighthave also triggered the collapse of the proto-solar cloud (see e.g., Cameron, 1993).

an

On the other hand, if we take supernova as a plausible source, the same event

need to supply more than one percent of the stable nuclides in the solar system, and

in particular those of Mg, Al, K and Ca. Cameron et al.(1995) have recently suggested

that freshly synthesized supernova material will get diluted to different extent duringthe post-explosion fall back event, leading to an effectively higher production of 26Al

compared to the other short-lived nuclides. In fact they concluded that supernovaassociated with a massive star, initially going through a Wolf-Rayet stage, could bea plausible source for several of the observed short-lived nuclides, including 41Caand 2(>A1, in the early solar system. In addition, such

a distance of 2-10 parsec from the proto-solar cloud could i

event taking place even atan

principle trigger them

collapse of the proto-solar cloud. An added advantage of considering a supernova

was alsopresent in the early solar system (Birck and‘Allegre, 1985; Lugmair et al., 1995) andwhich cannot be synthesized in TP-AGB stars.

as a source is its capability to synthesize the short-lived nuclide 53Mn that

Irrespective of the problem of identification of the exact stellar source of the

125

1 (139)

freshly synthesized 2bAl and 41Ca, the presence of uCa in the early solar system, and

refractory phases and CAIs have tightly

constrained the time interval A between the last injection of freshly synthesized matter

to the solar nebula and the formation of the first solar system solids (CAIs) to < IMa

the observed correlation of 2(3A1 and 41Ca in

with a plausible value of ~ 0.6Ma. The relatively small value of A obtained by

~ 2.2Ma) in

us

is not inconsistent with the observation of the extinct nuclide 60Fe (

differentiated meteorites (Shukolyukov and Lugmair, 1993a, b; Lugmair et al., 1995)

interval between the isolation of the solar nebula and thewhich suggest that the time

formation of large (ÿkm-sized) objects and their subsequent heating, melting and

6 Ma (Lugmair et al., 1995). It should be noted that the smallrecrystallization is <

value of A also places a very strong constraint on the dynamical evolution of the solar

the time scale for the collapse of thesystem, as it also provides the upper limit

proto-solar cloud to form the Sun

the deduced value of < 1 Ma for A suggests a rather dense proto¬

on

If we assume a time scale for collapse,

solar cloud with n//

which the Sun and the solar system have evolved.~ 104 cm-3, from

126