Observations from the Cosmic Ray Isotope Spectrometer (CRIS) aboard NASA’s Advanced Composition Explorer (ACE) have shown that all relevant galactic cosmic-ray isotopic ratios measured are consistent with an OB-association origin of galactic cosmic rays (GCRs). Additionally

CRIS measurements of the isotopic abundances of ^(59)Ni and ^(59)Co have shown that the ^(59)Ni has completely decayed into ^(59)Co, indicating a delay of >105 years between nucleosynthesis and acceleration. However, it has been suggested that shocks generated from high-velocity Wolf-Rayet winds in the OB-association environment must accelerate nuclei synthesized in nearby core-collapse supernovae on a time scale short compared to the ^(59)Ni half-life of 7.6x10^4 years. If this were the case, it would

imply that OB associations could not be the source of most galactic cosmic rays. In this paper, we describe

the OB-association history and environment and show that the time scales for acceleration are such that most ^(59)Ni should be expected to decay naturally in that setting, strengthening the argument that OB associations are the likely source of a substantial fraction of galactic cosmic rays

Binns, W. R.

Cummings, A. C.

de Nolfo, G. A.

Israel, M. H.

Leske, R. A.

Mewaldt, R. A.

Stone, E. C.

von Rosenvinge, T. T.

Wiedenbeck, M. E.

Observations from the Cosmic Ray Isotope Spectrometer (CRIS) aboard NASA’s Advanced Composition Explorer (ACE) have shown that all relevant galactic cosmic-ray isotopic ratios measured are consistent with an OB-association origin of galactic cosmic rays (GCRs). Additionally

CRIS measurements of the isotopic abundances of ^(59)Ni and ^(59)Co have shown that the 59Ni has completely decayed into ^(59)Co, indicating a delay of >10^5 years between nucleosynthesis and acceleration. However, it has been suggested that shocks generated from high-velocity Wolf-Rayet winds in the OB-association environment must accelerate nuclei synthesized in nearby core-collapse supernovae on a time scale short compared to the ^(59)Ni half-life of 7.6x10^4 years. If this were the case, it would

imply that OB associations could not be the source of most galactic cosmic rays. In this paper, we describe the OB-association history and environment and show that the time scales for acceleration are such that most ^(59)Ni should be expected to decay naturally in that setting,  strengthening the argument that OB associations are the likely source of a substantial fraction of galactic cosmic rays

Caltech Authors - Main

30TH INTERNATIONAL COSMIC RAY CONFERENCE  
 
 
 
Can 59Ni Synthesized in OB Associations Decay to 59Co Before Being Accelerated to 
Cosmic-ray Energies? 
W.R. BINNS 1, A.C. CUMMINGS 2, G.A. DE NOLFO 3, M.H. ISRAEL 1, R.A. LESKE 2, R.A. MEWALDT 2,        
T.T. VON ROSENVINGE4, E.C. STONE 2, M.E. WIEDENBECK5 
1 Washington University, St. Louis, MO 63130 USA 
2 California Institute of Technology, Pasadena, CA 91125 USA 
3 CRESST and Astroparticle Physics Laboratory NASA/GSFC, Greenbelt, MD 20771, USA 
4 NASA/Goddard Space Flight Center, Greenbelt, MD 20771 USA 
5 Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109 USA  
 wrb@wuphys.wustl.edu 
Abstract: Observations from the Cosmic Ray Isotope Spectrometer (CRIS) aboard NASA’s Ad-
vanced Composition Explorer (ACE) have shown that all relevant galactic cosmic-ray isotopic ratios 
measured are consistent with an OB-association origin of galactic cosmic rays (GCRs). Additionally 
CRIS measurements of the isotopic abundances of 59Ni and 59Co have shown that the 59Ni has com-
pletely decayed into 59Co, indicating a delay of >105 years between nucleosynthesis and acceleration. 
However, it has been suggested that shocks generated from high-velocity Wolf-Rayet winds in the 
OB-association environment must accelerate nuclei synthesized in nearby core-collapse supernovae 
on a time scale short compared to the 59Ni half-life of 7.6x104 years. If this were the case, it would 
imply that OB associations could not be the source of most galactic cosmic rays. In this paper, we de-
scribe the OB-association history and environment and show that the time scales for acceleration are 
such that most 59Ni should be expected to decay naturally in that setting, strengthening the argument 
that OB associations are the likely source of a substantial fraction of galactic cosmic rays. 
Introduction 
Cassé et al. [1] first suggested that ejecta from 
Wolf-Rayet stars, mixed with material of solar 
energetic particle composition, which was taken 
to be similar to solar system composition, could 
explain the large 22Ne/20Ne ratio measured in the 
galactic cosmic rays (GCRs) by several experi-
ments referenced in [2]. Subsequently, Higdon et 
al. [3] have shown that the 22Ne/20Ne ratio meas-
ured by the Cosmic Ray Isotope Spectrometer 
(CRIS) aboard NASA’s Advanced Composition 
Explorer (ACE) is consistent with a superbubble 
origin of GCRs.  In their model ejecta from Wolf-
Rayet (WR) stars and supernovae (SNe) from 
massive precursor stars (Types Ibc, II) in super-
bubbles are mixed with interstellar medium mate-
rial (ISM) of solar wind (SW) composition. They 
find that a mixture of 18 5% of WR plus SN 
ejecta with the remainder being ISM material can 
account for the 22Ne/20Ne ratio measured by ACE-
CRIS. They conclude that the large neon ratio is a 
“natural consequence of the superbubble origin of 
GCRs”. 
Binns et al. [2] compared a number of other iso-
topic ratios, in addition to 22Ne/20Ne, with results 
from a two-component model in which WR ejecta 
are mixed with ISM material (see [4,5,6] for 
model details). They find good agreement for all 
relevant isotopes with the models if ~20% of WR 
material is mixed with ISM, consistent with the 
fraction obtained by Higdon et al. [3]. (Note that 
the fraction of WR material in both [3] and [2] 
refers to the total amount of mass ejected from 
the star from birth until the end of the WR phase.) 
Binns et al. [2] concluded that that OB associa-
tions within superbubbles are the likely source of 
at least a substantial fraction of GCRs. 
Another important constraint for the origin of 
cosmic rays, also obtained from ACE-CRIS 
measurements, is the requirement that nuclei 
synthesized and accelerated by SNe must be ac-
59NI DECAY IN OB ASSOCIATIONS 
celerated at least 105 yr after synthesis. Wieden-
beck, et al. [7] showed that 59Ni, which decays 
only by electron-capture, has completely decayed, 
within the measurement uncertainties, to 59Co. 
The 59Ni can decay if, prior to its acceleration as 
cosmic rays, it resides for >105 years in dust 
grains or in a plasma environment. In the super-
bubble environment, the mean time between SNe 
is ~3-35 105 years, depending upon the number 
of stars in the OB association [8]. If SN shocks 
are the accelerators of the superbubble material, 
then this gives sufficient time for 59Ni, synthe-
sized in previous SNe, to decay. 
However, it has been suggested that the high-
velocity WR winds, which contain a similar ki-
netic energy to that contained in SNRs, should 
accelerate 59Ni on time scales shorter than its 
half-life, thus not allowing the 59Ni to decay [9]. 
If this were the case, it would be a strong argu-
ment against the OB-association origin of GCRs. 
In the above scenario, is there a mechanism that 
allows the 59Ni to decay? Binns et al. [10, 11] 
note that, although the kinetic energy in WR 
winds and SNe are similar, the power in WR 
winds is approximately one-tenth that in SNe. In 
addition, 59Ni can still decay if it is accelerated to 
energies <150 MeV/nuc since the nucleus will not 
always be fully stripped of orbital electrons [12]. 
Adiabatic deceleration may also be significant 
within the superbubble. However, there is another 
mechanism that should allow most 59Ni synthe-
sized in superbubbles to decay. 
OB Association Timeline 
OB associations are stellar clusters containing 
massive stars with initial mass >8M . The most 
massive stars have short lifetimes before core-
collapse (a few million years) while an 8 M  star 
(the lightest star that can undergo core-collapse 
[5, 6]), has a life time of ~40 MY. In Fig. 1 we 
show a schematic OB-association timeline for an 
association in which the stars are coeval. The OB-
association lifetime begins with the condensation 
of molecular cloud material into massive stars at 
T=0 and ends when the least massive star that can 
undergo core-collapse (~8 M ) ends its life as a 
supernova, ~40 MY later. Shortly after the stars 
are formed, the most massive stars evolve into the 
Wolf-Rayet phase as shown in Fig. 1. Their high-
velocity winds (~2,000-3,000 km/s) produce large 
low-density bubbles in the molecular cloud. Ex-
panding shocks from subsequent SN explosions 
then coalesce, producing a superbubble.  
  
Figure 1: Schematic OB-association timeline 
Fig. 1 shows the time interval that the most mas-
sive stars spend in the WR phase, and the epoch 
for which that occurs in the OB association, for 
rotating stars with initial masses ranging from 40 
to 120 M . The most massive star modeled enters 
the WR phase ~2 MY after association birth, and 
the least massive star that can evolve into a WR 
star exits that phase roughly 4 MY later. The low-
initial-mass cutoff for entering the WR phase is 
model dependent and is believed to be between 
25 M  and 40 M  [4, 5, 6]. The end of the WR 
phase is followed by core collapse. So there is, at 
most, an approximate 4 MY period in the life of a 
coeval OB association for which acceleration of 
superbubble material by WR winds could occur. 
The most massive stars are very rare, so except 
for very large associations, this period will be 
significantly less than 4 MY. The initial mass 
function for OB associations is often taken to go 
approximately as dN/dM  M-2.35 [13, 14].  
Let us suppose, as is argued by Heger et al. [15], 
that most stars with initial mass  40 M  and 
metallicity roughly solar or less do not undergo a 
SN explosion after core collapse, but instead 
“directly” collapse to form a black hole. Stars 
with initial mass 25-40 M  and metallicity 
roughly solar or less undergo core-collapse to 
30TH INTERNATIONAL COSMIC RAY CONFERENCE 2007 
form a black hole by “fallback”, resulting in a 
very weak SN shock with little ejecta.  
Daflon et al. [16] have measured the metallicity 
of young OB stars in associations as a function of 
galacto-centric radius. Their observations show 
that for OB associations within 1kpc of the Sun, 
most have metallicity that is solar or less. (Note 
that the metallicity of the stars, which condense 
out of old giant-molecular-cloud material, is dis-
tinct from that of the superbubble medium, which 
has a high metallicity resulting from fresh ejecta 
from recent SNe in the association). In the Heger 
et al. picture [15], stars with metallicity higher 
than ~solar result in SNe of type SNIb,c. These 
SNe are believed to result from WR stars. Addi-
tionally, there are massive stars that core-collapse 
into “hypernova”, which are poorly understood, 
and estimated to occur in ~1-10% of the massive 
core-collapse events [17]. If this picture is correct 
we see that a substantial fraction of core-collapse 
events during the WR epoch will not eject large 
amounts of newly synthesized material, including 
59Ni, into the superbubble. So the predominant 
material available for acceleration by the WR 
winds appears to be wind material ejected from 
the association stars since their birth, plus any 
normal ISM that is in the vicinity. 
Fig. 1 also shows that stars with mass low enough 
so that they do not enter the WR phase (~8 M   
M  25 M ) undergo core-collapse as SNe in 
which 59Ni is synthesized and injected into the 
superbubble. The most massive stars will undergo 
SN explosions first with later SNe accelerating 
previously injected material in the superbubble. 
In this simple picture it appears that the injection 
of the 22Ne-rich wind material from WR stars and 
the injection of 59Ni from the SNe of stars with 
initial mass 8 M   M  25 M  into the super-
bubble are largely separated in time. Thus the 
appropriate time scale for acceleration of most 
SN ejecta, including the 59Ni, would be the time 
between SN shocks after the WR epoch in super-
bubbles, not the shorter time scales associated 
with WR shocks in the WR epoch. Since the time 
between SNe is typically ~3  105 years for a 
large association [9], and the 59Ni half-life for 
decay is 7.6 x 104 years, in this picture, there is 
sufficient time for it to decay to 59Co.  
However, many OB associations are composed of 
“subgroups” with different ages [18]. For these 
associations, this simple picture needs to be modi-
fied since the WR winds from younger subgroups 
occur during the time when substantial 59Ni is 
being ejected by SNe in older subgroups. To 
quantitatively estimate the fraction of OB-
association lifetime for which WR winds are 
active, we have performed a Monte Carlo calcula-
tion. In this calculation, we randomly generated 
~6 104 stars with mass >8M  using the initial-
mass distribution dN/dM  M-2.35.  The stars were 
then grouped into subgroups containing, e.g., 10 
stars each (top panel in Fig. 2), and four sub-
groups were grouped to form associations. Thus, 
about 1,500 associations with four subgroups of 
10 stars each were generated. The time that each 
star in an association was in the WR phase was 
then obtained from [5, 6] and the sum of the time 
with one or more stars in the WR phase was cal-
culated (times appropriate for rotating stars were 
used). It was assumed that there is a 4 MY 
0
50
100
10 stars/subgroup  mean=2.8 MY
0
10
20
30
30 stars/subgroup
 
 
mean=6.8 MY
0
5
10
15
80 stars/subgroup
 
 
N
um
be
r o
f O
B
 A
ss
oc
ia
tio
ns
mean=11.8 MY
0.0 4.0x106 8.0x106 1.2x107 1.6x107 2.0x107
0
5
10
15
100 stars/subgroup
 
 
Time in WR Phase (years)
mean=12.7 MY
Figure 2: Monte Carlo calculation of the total 
time that at least one star in an OB association is 
in the WR phase.  
time interval between subgroup formation time 
[18]. Fig. 2 shows the results of this calculation 
for subgroup sizes of 10, 30, 80, and 100 stars.  
Note that for four subgroups spaced in time inter-
vals of 4MY, the association lifetime is ~52 MY. 
In Fig. 3 we show a plot of these results as a func-
tion of subgroup size for both rotating and non-
59NI DECAY IN OB ASSOCIATIONS 
rotating WR stars [5, 6]. We see that for associa-
tions with four subgroups, with each subgroup 
having ~20 OB stars, the time with one or more 
stars in the WR phase is ~5 MY, or ~10% of the 
0 20 40 60 80 100
0
2
4
6
8
10
12
14
 Number of OB Stars in Sub-group
 T
im
e 
in
 W
R
 P
ha
se
 (M
Y)
With Rotation
Without Rotation
 
Figure 3: Plot of Monte Carlo results—Total time 
in WR phase vs. number of OB stars/subgroup. 
association lifetime, for rotating star models, and 
is about half that for non-rotating stars. For large 
subgroups WR winds are blowing for as much as 
~25% of the time for rotating star models.  
In Fig. 4, we show a plot of the radial distance 
from the Sun of OB associations in the solar 
neighborhood. The vertical and horizontal bar 
lengths represent the number of OB stars and the 
age respectively (see figure legend), for those 
associations for which that information is avail-
able. SCO-OB2 subgroups 2, 3, and 4, are large, 
with ~100 stars/subgroup while the Orion-OB1 
subgroups have 15-20 stars each.  
 
-800 -400 0 400 800
-800
-400
0
400
800
CMA OB1
TR 27
SCO OB4
MON OB1
CAM OB1
CAS OB14
CEP OB4
CYG OB3
CEP OB2
CYG OB4
CYG OB7
VUL OB4
LAC OB1
ORI OB1c
ORI OB1d
ORI OB1b
ORI OB1a
COL 140
M6
TRU 10
CEP OB6
COLL 357
PER OB2
PER OB3
SCO OB2_3
SCO OB2_1
SCO OB2_5
SCO OB2_2
SCO OB2_4
Col 121
Y-
co
or
d 
(p
c)
X-coord (pc)
l=0
Vertical Bar length= 100 OB Stars
VELA OB2
S
Horizontal Bar length=10 MY age
 
Figure 4: Plot of OB associations near the Sun 
Summary 
Thus we see that even if WR winds do accelerate 
particles to cosmic ray energies, the WR epoch is 
confined in time and could result in the accelera-
tion of only a relatively small fraction of 59Ni 
synthesized in OB associations. For this reason 
and for the other reasons cited in the introduction, 
it appears that the observation by Wiedenbeck et 
al. [7] that all or most of the 59Ni in GCRs has 
decayed to 59Co is likely consistent with the OB-
association origin of galactic cosmic rays. Fur-
thermore, the nearby associations shown in Fig. 4 
are the likely sources of GCRs observed at Earth. 
Acknowledgements 
This research was supported by NASA at Caltech, 
WU, the JPL, and GSFC (under Grants NAG5-
6912 and NAG5-12929). 
References 
[1] M. Cassé, et al., ApJ, 258, 860, 1982. 
[2] W.R. Binns, et al., ApJ, 634, 351, 2005. 
[3] J.C. Higdon, et al., ApJ, 590, 822, 2003. 
[4] M. Arnould, et al., A&A, 453, 653, 2006. 
[5] G. Meynet, et al. A&A, 404, 975, 2003. 
[6] G. Meynet, et al., A&A, 429, 518, 2005. 
[7] M.E. Wiedenbeck, et al., ApJ, 523,L61, 1999. 
[8] J.C. Higdon, et al., ApJ, 509, L33, 1998. 
[9] N. Prantzos, 2005, Private communication. 
[10] W.R. Binns, et al., New Astron. Revs, 50, 
No. 7-8, 516, 2006. 
[11] W.R. Binns, et al., Accepted for publication 
in Space Sci. Revs., Vol 27, 2007. 
[12] M.E. Wiedenbeck et al., AIP Conference 
Proc,. 528, 363, 2000. 
[13] E.E. Salpeter, ApJ, 121, 161, 1955. 
[14] J.C. Higdon, et al., ApJ, 628, 738, 2005. 
[15] A. Heger, et al., ApJ, 591, 288, 2003. 
[16] S. Daflon, et al., ApJ, 617, 1115, 2004. 
[17] C.L. Fryer, et al., ApJ, 650, 1028, 2006. 
[18] A.I. Sargent, ApJ, 233, 163, 197. 


Can ^(59)Ni Synthesized in OB Associations Decay to ^(59)Co Before Being Accelerated to Cosmic-ray Energies?

30TH INTERNATIONAL COSMIC RAY CONFERENCE  
 
 
 
Can 59Ni Synthesized in OB Associations Decay to 59Co Before Being Accelerated to 
Cosmic-ray Energies? 
W.R. BINNS 1, A.C. CUMMINGS 2, G.A. DE NOLFO 3, M.H. ISRAEL 1, R.A. LESKE 2, R.A. MEWALDT 2,        
T.T. VON ROSENVINGE4, E.C. STONE 2, M.E. WIEDENBECK5 
1 Washington University, St. Louis, MO 63130 USA 
2 California Institute of Technology, Pasadena, CA 91125 USA 
3 CRESST and Astroparticle Physics Laboratory NASA/GSFC, Greenbelt, MD 20771, USA 
4 NASA/Goddard Space Flight Center, Greenbelt, MD 20771 USA 
5 Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109 USA  
 wrb@wuphys.wustl.edu 
Abstract: Observations from the Cosmic Ray Isotope Spectrometer (CRIS) aboard NASA’s Ad-
vanced Composition Explorer (ACE) have shown that all relevant galactic cosmic-ray isotopic ratios 
measured are consistent with an OB-association origin of galactic cosmic rays (GCRs). Additionally 
CRIS measurements of the isotopic abundances of 59Ni and 59Co have shown that the 59Ni has com-
pletely decayed into 59Co, indicating a delay of >105 years between nucleosynthesis and acceleration. 
However, it has been suggested that shocks generated from high-velocity Wolf-Rayet winds in the 
OB-association environment must accelerate nuclei synthesized in nearby core-collapse supernovae 
on a time scale short compared to the 59Ni half-life of 7.6x104 years. If this were the case, it would 
imply that OB associations could not be the source of most galactic cosmic rays. In this paper, we de-
scribe the OB-association history and environment and show that the time scales for acceleration are 
such that most 59Ni should be expected to decay naturally in that setting, strengthening the argument 
that OB associations are the likely source of a substantial fraction of galactic cosmic rays. 
Introduction 
Cassé et al. [1] first suggested that ejecta from 
Wolf-Rayet stars, mixed with material of solar 
energetic particle composition, which was taken 
to be similar to solar system composition, could 
explain the large 22Ne/20Ne ratio measured in the 
galactic cosmic rays (GCRs) by several experi-
ments referenced in [2]. Subsequently, Higdon et 
al. [3] have shown that the 22Ne/20Ne ratio meas-
ured by the Cosmic Ray Isotope Spectrometer 
(CRIS) aboard NASA’s Advanced Composition 
Explorer (ACE) is consistent with a superbubble 
origin of GCRs.  In their model ejecta from Wolf-
Rayet (WR) stars and supernovae (SNe) from 
massive precursor stars (Types Ibc, II) in super-
bubbles are mixed with interstellar medium mate-
rial (ISM) of solar wind (SW) composition. They 
find that a mixture of 18 5% of WR plus SN 
ejecta with the remainder being ISM material can 
account for the 22Ne/20Ne ratio measured by ACE-
CRIS. They conclude that the large neon ratio is a 
“natural consequence of the superbubble origin of 
GCRs”. 
Binns et al. [2] compared a number of other iso-
topic ratios, in addition to 22Ne/20Ne, with results 
from a two-component model in which WR ejecta 
are mixed with ISM material (see [4,5,6] for 
model details). They find good agreement for all 
relevant isotopes with the models if ~20% of WR 
material is mixed with ISM, consistent with the 
fraction obtained by Higdon et al. [3]. (Note that 
the fraction of WR material in both [3] and [2] 
refers to the total amount of mass ejected from 
the star from birth until the end of the WR phase.) 
Binns et al. [2] concluded that that OB associa-
tions within superbubbles are the likely source of 
at least a substantial fraction of GCRs. 
Another important constraint for the origin of 
cosmic rays, also obtained from ACE-CRIS 
measurements, is the requirement that nuclei 
synthesized and accelerated by SNe must be ac-
59NI DECAY IN OB ASSOCIATIONS 
celerated at least 105 yr after synthesis. Wieden-
beck, et al. [7] showed that 59Ni, which decays 
only by electron-capture, has completely decayed, 
within the measurement uncertainties, to 59Co. 
The 59Ni can decay if, prior to its acceleration as 
cosmic rays, it resides for >105 years in dust 
grains or in a plasma environment. In the super-
bubble environment, the mean time between SNe 
is ~3-35 105 years, depending upon the number 
of stars in the OB association [8]. If SN shocks 
are the accelerators of the superbubble material, 
then this gives sufficient time for 59Ni, synthe-
sized in previous SNe, to decay. 
However, it has been suggested that the high-
velocity WR winds, which contain a similar ki-
netic energy to that contained in SNRs, should 
accelerate 59Ni on time scales shorter than its 
half-life, thus not allowing the 59Ni to decay [9]. 
If this were the case, it would be a strong argu-
ment against the OB-association origin of GCRs. 
In the above scenario, is there a mechanism that 
allows the 59Ni to decay? Binns et al. [10, 11] 
note that, although the kinetic energy in WR 
winds and SNe are similar, the power in WR 
winds is approximately one-tenth that in SNe. In 
addition, 59Ni can still decay if it is accelerated to 
energies <150 MeV/nuc since the nucleus will not 
always be fully stripped of orbital electrons [12]. 
Adiabatic deceleration may also be significant 
within the superbubble. However, there is another 
mechanism that should allow most 59Ni synthe-
sized in superbubbles to decay. 
OB Association Timeline 
OB associations are stellar clusters containing 
massive stars with initial mass >8M . The most 
massive stars have short lifetimes before core-
collapse (a few million years) while an 8 M  star 
(the lightest star that can undergo core-collapse 
[5, 6]), has a life time of ~40 MY. In Fig. 1 we 
show a schematic OB-association timeline for an 
association in which the stars are coeval. The OB-
association lifetime begins with the condensation 
of molecular cloud material into massive stars at 
T=0 and ends when the least massive star that can 
undergo core-collapse (~8 M ) ends its life as a 
supernova, ~40 MY later. Shortly after the stars 
are formed, the most massive stars evolve into the 
Wolf-Rayet phase as shown in Fig. 1. Their high-
velocity winds (~2,000-3,000 km/s) produce large 
low-density bubbles in the molecular cloud. Ex-
panding shocks from subsequent SN explosions 
then coalesce, producing a superbubble.  
  
Figure 1: Schematic OB-association timeline 
Fig. 1 shows the time interval that the most mas-
sive stars spend in the WR phase, and the epoch 
for which that occurs in the OB association, for 
rotating stars with initial masses ranging from 40 
to 120 M . The most massive star modeled enters 
the WR phase ~2 MY after association birth, and 
the least massive star that can evolve into a WR 
star exits that phase roughly 4 MY later. The low-
initial-mass cutoff for entering the WR phase is 
model dependent and is believed to be between 
25 M  and 40 M  [4, 5, 6]. The end of the WR 
phase is followed by core collapse. So there is, at 
most, an approximate 4 MY period in the life of a 
coeval OB association for which acceleration of 
superbubble material by WR winds could occur. 
The most massive stars are very rare, so except 
for very large associations, this period will be 
significantly less than 4 MY. The initial mass 
function for OB associations is often taken to go 
approximately as dN/dM  M-2.35 [13, 14].  
Let us suppose, as is argued by Heger et al. [15], 
that most stars with initial mass  40 M  and 
metallicity roughly solar or less do not undergo a 
SN explosion after core collapse, but instead 
“directly” collapse to form a black hole. Stars 
with initial mass 25-40 M  and metallicity 
roughly solar or less undergo core-collapse to 
30TH INTERNATIONAL COSMIC RAY CONFERENCE 2007 
form a black hole by “fallback”, resulting in a 
very weak SN shock with little ejecta.  
Daflon et al. [16] have measured the metallicity 
of young OB stars in associations as a function of 
galacto-centric radius. Their observations show 
that for OB associations within 1kpc of the Sun, 
most have metallicity that is solar or less. (Note 
that the metallicity of the stars, which condense 
out of old giant-molecular-cloud material, is dis-
tinct from that of the superbubble medium, which 
has a high metallicity resulting from fresh ejecta 
from recent SNe in the association). In the Heger 
et al. picture [15], stars with metallicity higher 
than ~solar result in SNe of type SNIb,c. These 
SNe are believed to result from WR stars. Addi-
tionally, there are massive stars that core-collapse 
into “hypernova”, which are poorly understood, 
and estimated to occur in ~1-10% of the massive 
core-collapse events [17]. If this picture is correct 
we see that a substantial fraction of core-collapse 
events during the WR epoch will not eject large 
amounts of newly synthesized material, including 
59Ni, into the superbubble. So the predominant 
material available for acceleration by the WR 
winds appears to be wind material ejected from 
the association stars since their birth, plus any 
normal ISM that is in the vicinity. 
Fig. 1 also shows that stars with mass low enough 
so that they do not enter the WR phase (~8 M   
M  25 M ) undergo core-collapse as SNe in 
which 59Ni is synthesized and injected into the 
superbubble. The most massive stars will undergo 
SN explosions first with later SNe accelerating 
previously injected material in the superbubble. 
In this simple picture it appears that the injection 
of the 22Ne-rich wind material from WR stars and 
the injection of 59Ni from the SNe of stars with 
initial mass 8 M   M  25 M  into the super-
bubble are largely separated in time. Thus the 
appropriate time scale for acceleration of most 
SN ejecta, including the 59Ni, would be the time 
between SN shocks after the WR epoch in super-
bubbles, not the shorter time scales associated 
with WR shocks in the WR epoch. Since the time 
between SNe is typically ~3  105 years for a 
large association [9], and the 59Ni half-life for 
decay is 7.6 x 104 years, in this picture, there is 
sufficient time for it to decay to 59Co.  
However, many OB associations are composed of 
“subgroups” with different ages [18]. For these 
associations, this simple picture needs to be modi-
fied since the WR winds from younger subgroups 
occur during the time when substantial 59Ni is 
being ejected by SNe in older subgroups. To 
quantitatively estimate the fraction of OB-
association lifetime for which WR winds are 
active, we have performed a Monte Carlo calcula-
tion. In this calculation, we randomly generated 
~6 104 stars with mass >8M  using the initial-
mass distribution dN/dM  M-2.35.  The stars were 
then grouped into subgroups containing, e.g., 10 
stars each (top panel in Fig. 2), and four sub-
groups were grouped to form associations. Thus, 
about 1,500 associations with four subgroups of 
10 stars each were generated. The time that each 
star in an association was in the WR phase was 
then obtained from [5, 6] and the sum of the time 
with one or more stars in the WR phase was cal-
culated (times appropriate for rotating stars were 
used). It was assumed that there is a 4 MY 
0
50
100
10 stars/subgroup
 mean=2.8 MY
0
10
20
30
30 stars/subgroup
 
 
mean=6.8 MY
0
5
10
15
80 stars/subgroup
 
 
N
um
be
r 
of
 O
B
 A
ss
o
ci
at
io
n
s
mean=11.8 MY
0.0 4.0x106 8.0x106 1.2x107 1.6x107 2.0x107
0
5
10
15
100 stars/subgroup
 
 
Time in WR Phase (years)
mean=12.7 MY
Figure 2: Monte Carlo calculation of the total 
time that at least one star in an OB association is 
in the WR phase.  
time interval between subgroup formation time 
[18]. Fig. 2 shows the results of this calculation 
for subgroup sizes of 10, 30, 80, and 100 stars.  
Note that for four subgroups spaced in time inter-
vals of 4MY, the association lifetime is ~52 MY. 
In Fig. 3 we show a plot of these results as a func-
tion of subgroup size for both rotating and non-
59NI DECAY IN OB ASSOCIATIONS 
rotating WR stars [5, 6]. We see that for associa-
tions with four subgroups, with each subgroup 
having ~20 OB stars, the time with one or more 
stars in the WR phase is ~5 MY, or ~10% of the 
0 20 40 60 80 100
0
2
4
6
8
10
12
14
 Number of OB Stars in Sub-group
 T
im
e
 in
 W
R
 P
h
as
e
 (
M
Y
)
With Rotation
Without Rotation
 
Figure 3: Plot of Monte Carlo results—Total time 
in WR phase vs. number of OB stars/subgroup. 
association lifetime, for rotating star models, and 
is about half that for non-rotating stars. For large 
subgroups WR winds are blowing for as much as 
~25% of the time for rotating star models.  
In Fig. 4, we show a plot of the radial distance 
from the Sun of OB associations in the solar 
neighborhood. The vertical and horizontal bar 
lengths represent the number of OB stars and the 
age respectively (see figure legend), for those 
associations for which that information is avail-
able. SCO-OB2 subgroups 2, 3, and 4, are large, 
with ~100 stars/subgroup while the Orion-OB1 
subgroups have 15-20 stars each.  
 
-800 -400 0 400 800
-800
-400
0
400
800
CMA OB1
TR 27
SCO OB4
MON OB1
CAM OB1
CAS OB14
CEP OB4
CYG OB3
CEP OB2
CYG OB4
CYG OB7
VUL OB4
LAC OB1
ORI OB1c
ORI OB1d
ORI OB1b
ORI OB1a
COL 140
M6
TRU 10
CEP OB6
COLL 357
PER OB2
PER OB3
SCO OB2_3
SCO OB2_1
SCO OB2_5
SCO OB2_2
SCO OB2_4
Col 121
Y
-c
oo
rd
 (
pc
)
X-coord (pc)
l=0
Vertical Bar length= 100 OB Stars
VELA OB2
S
Horizontal Bar length=10 MY age
 
Figure 4: Plot of OB associations near the Sun 
Summary 
Thus we see that even if WR winds do accelerate 
particles to cosmic ray energies, the WR epoch is 
confined in time and could result in the accelera-
tion of only a relatively small fraction of 59Ni 
synthesized in OB associations. For this reason 
and for the other reasons cited in the introduction, 
it appears that the observation by Wiedenbeck et 
al. [7] that all or most of the 59Ni in GCRs has 
decayed to 59Co is likely consistent with the OB-
association origin of galactic cosmic rays. Fur-
thermore, the nearby associations shown in Fig. 4 
are the likely sources of GCRs observed at Earth. 
Acknowledgements 
This research was supported by NASA at Caltech, 
WU, the JPL, and GSFC (under Grants NAG5-
6912 and NAG5-12929). 
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Can ^(59)Ni Synthesized in OB Associations Decay to ^(59)Co Before Being Accelerated to Cosmic-ray Energies

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