A model of cosmic-ray origin is suggested which aims to account for some salient features of the composition. Relative to solar abundances, the Galactic cosmic rays (GCR) are deficient in hydrogen and helim (H and He) by an order of magnitude when the two compositions are normalized at iron. Our conjectural model implicates supernovae of Type I (SN-I) as sources of some of the GCR. SN-I occur approximately as often as SN-II, through their genesis is thought to be different. Recent studies of nucleosynthesis in SN-I based on accreting white dwarfs, find that the elements from Si to Fe are produced copiously. On the other hand, SN-I are virtually devoid of hydrogen, and upper limits deduced for He are low. If SN-I contribute significantly to the pool of GCR by injecting energetic particles into the interstellar medium (ISM), then this could explain why the resulting GCR is relatively deficient in H and He. A test of the model is proposed, and difficulties are discussed

Shapiro, M. M.

English

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DOSUPERNOVAEOF TYPE I PLAY A ROLE
IN COSMIC-RAYPRODUCTION?
Maurice M. Shapiro
Max Planck Institut fur Astrophysik*
8046 Garching, West Germany
ABSTRACT
A model of cosmic-ray origin is suggested which
aims to account for some salient features of the
composition. Relative to solar abundances, the _
Galactic cosmic rays (GCR) are deficient in hydro-
gen and helim (H and He) by an order of magnitude
when the two compositions are normalized at iron.
Our conjectural model implicates supernovae of
Type I (SN-I) as sources of some of the GCR.
SN-I occur approximately as often as SN-II,
though their genesis is thought to be different.
Recent studies of nucleosynthesis in SN-I based
on accreting white dwarfs, find that the elements
from Si to Fe are produced capiou_ly(]:)'.,On the
other hand,iSN-I are virtually devoid of hydrogen,
and upper limits deduced for He are low. If SN-I
contribute significantly to the pool of GCR by
injecting energetic particles into the interstellar
medium (ISM), then this could explain why the
resulting GCR is relatively deficient in H and He.
A test of the model is proposed, and difficulties
are discussed.
I. Introduction. A puzzling feature of cosmic-ray composition is the
enhancement of many heavy primary nuclei (HPN) among the particles
arriving in the vicinity of the earth (2). Compared to solar composi-
tion or the local galactic abundances, this ten-fold enrichment in
peak elements like Mg, Si, and Fe can also be described as a relative
deficiency of H and He. However it may be characterized, this order-
of-magnitude discrepancy, unlike other anomalies in composition, can-
not be explained by fragmentation in the interstellarmedium or by '
selection effects that depend on the first ionization potential. Any
viable theory of cosmic-ray genesis must account for this anomaly.
It is noteworthy that the source energy spectra of H and He
also differ from those of the HPN; the latter are somewhat steeper (3).
Taken together, the dearth of H and He, and the difference in their
energy spectra from those of the HPN, raise the question whether these
two components have had different histories. On the other hand, the
fact that their production spectra are no.tvery different suggests that
a common acceleration mechanism may be working for both.
2. Supernovae and Cosmic Rays. Some three decades ago, supernovae
and their remnants were invoked as self-contained sites of cosmic-ray
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origin, i.e,, as energy sources, material sources, and regions of
acceleration. In trying to explain the source composition_ the mas_
sive precursors of Type II supernovae (SN-_I)were favored as sites of
nucleosynthesis for the material destined to become cosmic rays, Recent
calculations of SN-II explosions show, however, that it would be dlff_-.
cult to eject sufficient Fe, since the iron is left in the core of the
residual neutron star. Moreover, it is doubtful that a sufficient
flux of heavy nuclei could escape from the SN_I'Iexplosion through the
surrounding red-giant envelope,
3. Supernovae of Type I as Injectors of HPN, We propose a conjectural
model for the genesis of GCR in which the pool of cosmic_ray nuclei is
a mixture of two components:
(A) the bulk of cosmic-ray hydrogen and helium, and
a minor portion of HPN, with an overall composition
resembling that of the solar system or the ISM; and
(B) the bulk of the HPN (_some90 per cent?) originating
predominantly in supernova outbursts of Type I,
With this model we hope to account for the relative deficit of
H and He in the cosmic rays.
First we recall some distinguishing features of SN-.Iand SN_!I.
While the two types of explosions occur with comparable frequency, their
precursors are quite different according to currently favored theories,
A Type II supernova results from th_ evolution of a single massive star;
an SN-I, on the other hand, evolves from an accreting white dwarf in a
binary system. Among the observational_ differences are the following,
An SN-II has a thick hydrogen envelope, whereas in SN-I light spectra,
the lack of H lines implies the absence of a substantialhydrogen enve-
lope. The same SN-I spectra have been used to deduce very low upper
limits to the He abundance, The precursors of SN_I are old stars of
low mass. The two types of supernovae have different light curves. SN_I
are considered to have steep density gradients and to generate strong
shocks.
Referring to the two components (A and B) of the GCR postulated
above, we envisage the following scenarios for their injection and
acceleration: (A) The arriving ("primary") H and He nuclei, and a small
fraction of the HPN a_e injected, e,g., from flares in main-sequence F_
to-M stars; other source-injectorsare not excluded, They.are then
accelerated at expanding shock fronts in the ISM, energized mainly by
supernova outbursts. (B) Nearly all of the HPN are injected....with
modest energies-- from SN-I explosions, and then accelerated in the
ISM by the same shock waves as component CA).
4. Nucleosynthesis in SN-I. Since the SN-II seem incapable, by them-
selves, of yielding a solar-type composition, attention has turned to
SN-I models that might produce a plausible mix of metals, especially to
carbon-deflagrationmodels (l, 4, 5). These involve a rapidly accreting
white dwarf in a close binary system, The evolution of the progenitor
system is calculated from the start of accretion to the complete _srup-
tion of the white dwarf. From this outburst one gets _0.5 M of _Ni
which decays to cobalt, then to iron. The radioactivities generate the
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observed light curves. From a sequence of nuclear burnings in the white
dwarf, one obtains a copious supply of the intermediate elements, from
Co to Ca. Based on this calculated output, it has been possible to conj
struct synthetic profiles of line spectra; these are consistent with
the spectra observed near maximum brightness (6), Certain of the carbon
deflagration models seem to work well, There is a whole class of such
models which depend on the choice of certain parameters, The products
of these modeled SN-I events are complementary to those of nucleosynthe_
sis in SN-II, giving an ISM composition in reasonable accord with obser-
vations.
5. Test of the Cosmic-Ray Model, Our prescription for a critical test,
dealing mainly with component (B), starts with the calculated output of
nuclides from SN-I using a promising model of nucleosynthesis, e,g.,
one like "W7" of Nomoto, Thielemann and Yokoi, A fraction of the ejected
material is assumed to be boosted in energy to, say, tens of MeV or
higher, and injected into the ISM. The initial ensemble of sub-relativi-
stic nuclei is then "propagated" through the ISM with an appropriate
distribution of path lengths. This can be done with available computa-
tional programs utilizing the latest cross sections for fragmentation.
The calculated residual distribution of nuclides arriving locally can
be compared with observations of metal abundances in the solar system
(taking account of solar modulation at sub-GeV energies), Finally, one
tries to construct a plausible mix of components (A) and (B) so as to
get a reasonable fit to the observed, arriving composition.
6. Discussion. It would be fortuitous if the initial attempts at fitt-
ing were to succeed in accounting for the cosmic-ray composition as well
as the composition of (thermal) solar material. In the SN-I models, the
main uncertainties at the burning fronts reside in the hydrodynamics
rather than in the nuclear physics (7). The quasi,convectivemixing
length is unknown, and must be selected rather arbitrarily (18), Yet the
final deflagration yields depend decisively on this choice.
Apart from uncertainties in models of nucleosynthesis,one may
ask whether particle injection by SN-I will work, A conceivable obstacle
is the energy loss during the adiabatic expansion. It seems, however,
that while such loss may prevent acceleration at or near the source up
to relztivistic energies, it would not preclude injection into the ISM
at modest energies. The steep density gradients and strong shocks
occurring in SN-I could provide the limited boost in energy that is
required.
, It should be e_hasized that we are concerned here mainly with
particle energies _I0 _ eV/amu, a domain comprising, by far, most of
the cosmic rays, and for which the composition is rather well known,
A different explanation of HPN enhancement has been proposed
by D. Eichler et al. (9,10). They argue that shocks in the ISM prefer-
_ntially accelerate partially ionized heavy elements over protons, Their
model may provide an attractive alternative to the one presented here,
7. Conclusions. If SN-I contribute significantly to the Galactic pool
of cosmic rays by injecting enough HPN into the ISM, then this would
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solve a major cosmic-ray problem--we would understand why the GCR is
relatively deficient in H and He, From our model one could expect some
difference in the energy spectra of components (A_ and (B). On the
other hand, because the main acceleration of the HPN is accomplished
by the same mechanism that energizes the H and He, it is understandable
that the spectral difference between the two components is not great.
8. Acknowledgments. Invaluable discussions with W. Hillebrandt, K.
Nomoto, F. Thielemann, and E. MUller are gratefully acknowledged. I
am also indebted to Professors R. Kippenhahn,W. Hillebrandt and their
colleagues for the hospitality and stimulating atmosphere of the Max
Planck Institut fCr Astrophysik in Garching. I thank Beth Kessler for
her timely assistance.
References
I. Nomoto, K., F.K. Thielemann, and K. Yokoi (1984), Ap.J. 286, 644.
2. Shapiro, M. and R.Silberberg (1975), Phil Trans. Royal Soc.London,
A 277, 319.
3. Engelmann, J. J. et al. (1981), 17th ICRC, Paris, 9, 97.
4. Woosley, S. et al. (1984), in Proc. Erice Workshop on Stellar
Nucleosynthesis, Reidel Publ. Co., Dordrecht.
5. Nomoto, K.,F.K. Thielemann, and J. C. Wheeler (1984), Ap.J. 279:L 23
6. Branch, D., et al. (1982), Astroph. J. 252, L61.
7. Arnett, D.,(i-985-)-,remarks at Moriond Symposium on Nucleosynthesis,
Les Arcs, France.
8. Thielemann, F. K. and Nomoto, K., private communications.
9. Eichler, D. and K. Hainebach, (1981), Phys. Rev. Ltrs. 47, 1560.
lO. Ellison, D.C., F.C. Jones, and D. Eichler (1981), J. Ge-ophys.50,110.


Do supernovae of type 1 paly a role in cosmic-ray production?

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