A selective injection mechanism of cosmic ray seed nuclei due to nuclear decay effects is proposed. It is shown that 22NA ejected by novae explosion can explain the excess 22Ne in the galactic cosmic ray source by invoking energization and ionization during its beta-decay process in interstellar space

Yanagita, S.

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17S
OG 8.2-12
INTERSTELLAR 2ZNa AS A POSSIBLE SOURCE OF
THE EXCESS 22Ne IN THE GALACTIC COSMIC RAY
S. Yanagita
Department of Earth Sciences
Ibaraki University
Bunkyo 2-chome, Mito 310, Japan
ABSTRACT
We propose a selective injection mechanism of
cosmic ray seed nuclei due to nuclear decay
effects. It is shown that 22Na ejected by novae
explosion can explain the excess 22Ne in the
galactic cosmic ray source by invoking
energization and ionization during its beta-
decay process in interstellar space.
I. Introduction. Recent investigations on the cosmic ray
charge and isotopic composition have revealed many remarkable
features to understand the origin of the galactic cosmic ray
(GCR). Among of these are;the organization by the first
ionization potential(FIP) of the elemental abundance in GCR
source relative to the local galactic abundance(l,2); the
excess abundance of several neutron rich isotopes at GCR
source(l,3). The most significant and well-established fact
is that 22Ne is overabundant in GCR source by a factor of_3.5
with respect to the solar system value(1).
In this contribution, we want to propose a new mechanism
to explain this peculiar imprint bearing the nature of GCR
source, and also want to discuss the origin of the source
material relating with the recent observational result of
galactic gamma-ray lines(4,5).
2. Mechanism producing the excess 22Ne in GCR source.
It has been suggested that the elemental abundance pattern of
GCR source relative to the local galactic abundance is well
organized by FIP and that this pattern is reproduced by an
exponential distribution of temperature at GCR source, with
an average temperature T0_7000 K(6). Accordingly, a dramatic
increase in the relative abundance might be expected for
species which attain a significantly higher speed than that
of thermal particles, if the particles are injected and
accelerated from the tail of a Maxwellian distribution. An
example of this kind of preferential heating is proposed to
explain 3He-rich solar flare events(7).
Along the similar lines of thought, we want to propose
here a new mechanism of selective heating and subsequent
preferential injection due to nuclear decay effects which is
applicable in principle to any isotopes which have radio-
active progenitors. Specificall, we _consider here the case
of 22Na. This isotope decays to 22Ne via beta-decay with a
half life of _2.6 yr and with a maximum beta-decay energy of
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OG 8.2-12
_0.55 MeV. The 22Ne produced by this decay will attain an
energy E _I0 eV owing to the nuclear recoil effect. The
r
temperature corresponding to this energy is _1.2x10SK and is
significantly higher than the average temperature at GCR
source, _7000 K(6). In addition to this heating, 22Ne will be
ionized by the "shakeoff" process during the beta-decay(8) r
even if the parent 22Na is neutral. This ionization effect
might be significant, because neon is virtually neutral at a
temperature of _7000 K. Because of these energization and
ionization effects, it is expected that the 22Ne population
originated from 2eNa may be selectively picked up as cosmic
ray seeds and be subjected to subsequent acceleration
processes. Certainly, to show this mechanism actually does
work, we must investigate if the thermalization and
recombination timescales of recoiled 22Ne are long enough so
that the nucleus has a chance to be accelerated. But, here,
we want to restrict our discussion only to pointing out the
possibility of the preferential injection mechanism due to
the nuclear decay effects as shown above. Instead, in
following, we want to compare quantitatively the rate of
2eNa supply into interstellar medium(ISM) with the producticn
rate of 22Ne in GCRs.
3. Estimation of production rates. The excess isotopic
abundance of ZZNe in GCR source to be explained is
_n2e=(2eNe/2°Ne)GC R- 1.6x(22Ne/2°Ne)_ 0.2
where (22Ne/20Ne)GC R and (22Ne/2°Ne) are the neon isotopic
ratios in GCR source (_0.4) and in the solar system (_0.12),
respectively(1,3).Here we assume the excess Z2Ne up to _1.6
times of the solar system value is explained by the
super metalicity hypothesis(9) (although our conclusion is
not affected whether the factor of 1.6 is taken into account
or not). Then the production rate of the excess Z2Ne in the
whole galaxy is estimated as follows,
• _" _ 38 .N22_N_x(n20/n_)x_n22--3x10 partlcles/sec
where Nc_,_I042 pa_icles/sec, is the production rate of
GCR in _He whole galaxy and is estimated from the localcosmic
ray density _10-1°/cm 3, the volume of the galaxy _I067cm3 and
the life time of GCRs, _I07yr(I0). (n20/ncp) is the relative
abundance of 2°Ne in the cosmic ray, ,_5xI0"4(2). From our
point of view, it is necessary that sufficient amount of 22Na
exists in interstellar space to maintain 22Ne abundance in
GCR. In other words, the rate of 22Na supply should be higher
than the production rate of the excess 22Ne in GCR. Explosive
nucleosynthesis in novae is expected to produce 22Na and 26AI
(11). The gamma-ray spectroscopy experiment has detected the
line with an energy of 1.8 MeV, consistent with the decay of
_3 M_ of 28AI in the interstellar space(5). The source of
the observed 26AI has been attributed to galactic novae, not
supernovae(5). We can estimate the total amount of 22Na of
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OG 8.2-12
novae origin, M2z, from the observed amount of 26AI, M26_
3 Me, by using the simple model for the chemical evolution
of the galaxy(12) as following,
M22=M26 x (X22/X26)
_M26x (P22/P26) x (T22/Tz6)
where,(X22/X2_) is the abundance ratio of 22Na and 26AI in
the present interstellar space; (P22/P26) is the average
isotopic production ratio of 22Na and 26AI for explosive
process in novae and is _I0-3 (13) ;(T22/T26) is the ratio of
half lives,_3.6 x 10 -6 From this analysis the amount of
22Na in the interstellar space is estimated as_2.2x102Sg or
5.9x1047atoms. If the decay and production are in equilibrium
, the rate of supply of 22Na is estimated as_5x1039atoms/sec
(approximately the same value is also obtained from the
occurance of novae in the galaxy _40/yr (14), the average
mass of matter ejected by a nova explosion _40-_Me and the
concentration of 22Na in the ejecta (2410) x 10-Tg/g (13)).
This value of the production rate(or the decay rate) of 22Na
, Q22 ,is not inconsistent with the upper limit _4.4x1042
atoms/sec obtained by HEAO3 observation of the diffuse
galactic gamma-ray lines(4) and is higher than nN22_3x1038
atoms/sec, the production rate of the excess 22Ne in GCR
source. The ratio of d N22 to Q22 is _6%. This means that
the probability of 22Na ejected by novae to become a cosmic
ray 22Ne is 6%. Therefore, novae can supply the seeds of
excess 22Ne in GCRs, in principle. 22Na ejected by novae
explosion might be preferentially injected as cosmic ray
seeds owing to the effects shown above.
4. Discussion and conclusion. Although the amount of 22Na
ejected by novae explosion is enough to account the excess
22Ne, the probability of 6% seems to be rather high. So it
is interesting here to estimate the _robability for the
bulk GCRs and enhancement factor of eNe to be realized by
our selective injection model. It has been suggested by the
shock acceleration model for cosmic ray production that
energetic particles are drawn directly from a thermal pool
(15). If this view is correct, we can estimate the average
efficiency of interstellar medium(ISM) to become cosmic rays.
The ISM is undergoing continual change on a time scale of
_Gyr (16). If our galaxy is in a steady state (or if the
amount of ISM is constant) during the last several Gyr, the
rate of supply of ISM from stars is estimated as _3xI0 sl
atoms/sec by dividing the amount of ISM with ]09yr. So the
efficiency of _SM is N_R/(3x10Sl)=q0-9. Consequently, the
efficiency of 2Na is _x107 times higher than that of the
general ISM. On the other hand, the enhancement factor of
exp(E /kTo)=2.8x107 is expected by our selective injection
22Nemechanlsm for due to the nuclear decay effects if the
average temperature of the thermal pool of GCR source is
_7000 K as suggested by the ionic model for GCR scurce(6).
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OG 8.2-12
This value is not different so much to the required
enhancement factor _6x107. Furthermore, the requirement may
be weekened if there is any source of 22Na other than novae.
Finally, we should comment on a question, that is, why
the excess is seen only for 22Ne. The answer is that the
selective injection mechanism due to the nuclear decay
effects does not work fully for isotopes other than 2_Na,
because they are either too .short-lived or have been locked
up into grains. Only 22Na has an appropriate half life and
remains in gas phase because Na is expected to behave as a
volatile in space.
In conclusion, it may bepossible to explain the origin
of the observed 22Ne excess in GCR source as due to
interstellar 2ZNa selectively injected by the nuclear recoil
and shakeoff effects.
References
I. Simpson, J.A., 1983, Ann. Rev. Nucl. Part. Sci., 33, 323.
2. Meyer, J.P., 1985, Ap. J. Suppl., 57, 151.
3. Koch-Miramond, L., 1981, Proc. 17th Int. Cosmic Ray Conf.
Paris, 12, 21.F
4. Mahoney, W.A. et al., 1982, Ap. J., __262, 742.
5. Mahoney, W.A. et al., 1984, Ap. J., 286, 578.
6. Arnaud, M. and Casse, M., 1985, Astr. Ap., 144, 64.
7. Fisk, L.A., 1978, Ap. J., 224, 1048.
8. Freedman, M.S., 1974, Ann.-_v. Nucl. Sci., 24, 209.
9. Woosley, S.E. and Weaver, T.A., 1981, Ap. J., 243, 651.
10. Garcia-Munoz, M. et al., 1981, Proc. 17th Int. Cosmic
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11. Clayton, D.D. and Hoyle, F., 1974, Ap. J., 187, LI01.
12. Clayton, D.D., 1984, Ap. J., 280, 144.
13. Hillebrandt, W. and Thielmann, F.K., 1982, Ap. J., 255,
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14. Allen, C.W., 1976, "Astrophysical Quantities" (London;
Athlone).
15. Eichler, D., 1979, Ap. J., 22__9,419.
16. Salpeter, E.E., 1977, Ann. Rev. Astr. Ap., 15, 267.


Interstellar (Na-22) as a possible source of the excess (Ne-22) in the galactic cosmic ray

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