Rapporteur paper on nucleosynthesis and cosmic rays

4ll—416, 1984 Adv. Space Res.Vol.4, No.2—3, pp. Printed in Great Britain. All rights reserved.

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RAPPORTEUR PAPER ON NUCLEOSYNTHESIS AND COSMIC RAYS M. Cassé CEN-Saclay, Service d’Astrophysique, 91191 Gif-sur-Yvette Cedex, France

1. INTRODUCTION Nucleosynthesis and cosmic ray astrophysics have a mutual interest, because of the possibility that part of the isotopic anomalies inferred at the cosmic—ray sources — those that can be considered as true nuclear anomalies — may witness specific stages of nucleosynthesis (e.g. He burning). But it would be unrealistic to consider that all the pecularities of the cosmic ray source (CR5) abundances can be explained in purely nuclear terms. Besides the difficulty of this task (see Arnould, this volume), many recent reviews and general articles (see 1—4 and references therein) have emphasized that the CBS abundances look for the most part normal, once atomic selection effects are washed out (see Koch—Miramond and Meyer, this issue and the thorough discussion of Meyer (1)). It would be at least as naive to ignore discriminating physico—chemical effects operating in the CR reservoir than to neglect fractionation effects in meteorites (5). Our goal in the present symposium was to arrive, if possible, at a clear separation between atomic and nuclear effects influencing the CRS abundances, but in passing other subjects had inevitably to be touched. It is reasonable to think that isotopic fractionation is much less severe than elemental fractionation as is generally the case in cosmochemistry. Therefore cosmic ray physicists rely heavily on isotopic measurements to identify the thermonuclear processes responsible for the synthesis of CR nuclei. The experimental background has been extensively discussed at this meeting by Wiedenbeck (see also recent reviews (6—~). A salient feature of the isotopi~ 0sourcecomposition is a conspicuous excess of Ne (by a factor 3 to 4 relativ~5t~6 Ne, a~9c~paredto solar flare neon). There may also be a slight excess of ‘ Mg and ‘ Si by factors of.~.l.7relative to the T~jorisotopes. In addition, it is worth mentioning in this context that carbon (mainly C) is about twice as h~h in galactic CR sources relative to solar energetic particles and that oxygen (mainly 0) is also possibly high by a factor of—l.5 (1). The challenge is to correlate the maximum number of anomalies in the framework of a consistent model, as simple as possible, and to explain quantitatively their respective magnitudes. 2. NUCLEOSYNTHESIS AND COSMIC RAYS 2.1. Bulk composition The stage has been set by Thielemann, who has reviewed the nucleosynthesis of type I and type II supernovae in the framework of existing models. The heart of the subject has been tackled by Arnould and Maeder, and interesting complements have been given by Blake and Dearborn and by Prantzos in the poster session. Arnould has shown in a systematic way that among the various kinds of supernovae explored so far, none presents a global yield that could account for the detailed structure of the elemental CBS composition (at least from H to Zn), confirming earlier conclusions (1,2). This failure is not a surprise (see above). It1~sto be1gemarked moreover that the recent revision of the rate of the key reaction C(.c,T) 0 (10), while attenuating the deviations between the calculated composition of the ejects of 20 to 25 N0 model stars (11,12) and the local galactic composition, does not help in explaining the CBS composition (1). 2.2. Isotopic anomalies Arnould has scrutinized the various explanations put f~wardand exposed their relative merits. None can simultaneously account for the large Ne overabundance and the 411

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M. Cass~

moderate enhancement of the neutron rich isotopes of Si. Convinced of this fact, Maeder proposes a compromise between the Wolf—Rayet (WR) model (see 1—4,13) and the metal—rich supernova (MRSN) model (14). The price to pay is to accept that most of the cosmic ray arriving at Earth are of non—local origin (at variance with Lund, see below). In the hybrid image suggested by Maeder, the birthplace of the observed cosmic radiation is located in the inner galaxy, 2~kpc aw~ from us. In this region the metallicity (and therefore the production of Si and Si) and the stellar activity (and therefore the number of MRSN arid WR) are higher than here. This interesting suggestion, based on nucleosynthetic arguments only, needs now to be integrated in a physical propagation model, since it is rather demanding from this point of view. Two posters have been devoted to WR nucleosynthesis: Prantzos, extending previous work (15—17), presented detailed predictions concerning a2~ossible excess of neutron rich—isotopes of medium—heavy elements correlated to the Ne excess. In agrg~ment with Prantzos, Blake and Dearborn emphasized that a sizeable enhancement of Fe in CR sources would be a clear signature of WR nucleosynthesis (see table 1). Enhancement factors after dilution with normal CR’s (13) Isotopic ratios

Prantzos (with convective overshooting) 3.9

22Ne/20Ne 25Me/24Mg 26 24 Mg! Mg 29 28 Si! Si

1.5

30Si/28S1

1.0

Blake and Dearborn (without convective over— shooting)

1.5 1.0

1.5 40K/39K

3

57Fe/56Fe

1.2

1.2

58Fe/56Fe 61 60 Ni! Ni

1.6

2.4

1.2

Table 1

—

Correlated nuclear anomalies predicted by models of Wolf—Rayet stars

The agreement is remarkable considering the underlying model uncertainties and among other things the di~erent prescriptions used to describe internal mixing. Since a definite excess of Fe is predicted,a precise isotopic analysis of iron would be greatly appreciated. Now the ball is rolling towards the observers’ side. The same authors have ~gdependently announced a very important result fort —ray line astronomy, concerning Al, the cherished nucleus nuclear astrophysicists (see e.g. (18,19)). It has been recently realized that Al could be synthesized and ejected during the early 9glution of massive stars, provided that these stars are i) massive enough to produce Al through hydrogen burning at high temperature (i.e. through the Ne—Na and Mg—Al cycles), and ii) peeled off sufficiently by mass loss to reveal their convective core (via stellar winds and Roche lobe overflow when they are in close binary systems). It was thought initially that Only very massive objects, above say 100 M0 were suitable for this purpose (20), but recent revisions in influent~l reaction rates (21,22) allows less massive stars to be quite prolific sources O~5 Al. So prolific indeed that according to Dearborn and Blake (DB)(23) the decay of Al expelled from WR stars could entirely explain the ‘( —ray line emission at 1.8 MeV recorded by the HEAO 3 satellite in the direc~ionof2the1galac~ic center (24,25) an~’ confirmed by the SMM satellite (3.3 ÷0.7 10 ~ cm a rad ). This flux seems far too large to be produced by supernova nucleosynthesis (19). Surprisingly enough the results of the two WR models are in close agreement in spite of their differences in stellar modelization, especially as concerns internal mixing (see table 2). More recent work (26,27) leads to slightly different results.

Nucleosynthesis and Cosmic Rays

413

Mass fraction of 26 Al ejected in the WR stage

Initial mass

Blake and Dearborn

25

Prantzos

1.5 l08

40

3. l0~

50

3. l0~

80

6.5 l0~

100

2. 106 2. ia_6

120

Table 2

26Al ejected in the WR stage —

After this long di~ession, let us come back to our CR preoccupations: it is to be stressed that any Al excess at the CR sources, due to the operation of WR stars (and/or Of stars, i.e. those stars which are intermediate between 0 and WR) will be certainly overwhelmed by the secondary production of this nucleus “en route” (see (28)). 3. THE NITROGEN PUZZLE A net difference persists between the CRS abundance of N derived from low energy (60 — 500 MeV/n) and higher energy (0.8 — 25 GeV,/n) observations (see Wiedenbeck, this conference, and references therein). Since this problem is unsettled I shall limit myself to a few comments dictated by common sense (hopefully). The interpretation of data, especially at low energy, depends critically upon the nuc}~arcro~ sectio~ ( a- ‘s) adopted, and above all on those concerning the production of N and l~fj.om 0, as clearly shown by Guzik and Wefel at this conference. The reaction C _.~‘ B is also important t 14a~gessthe grammage traversed by the particles. Only two measurements of the 0 ‘s for ‘ N production are available at 460 and 2000 MeV,’n (29). Guzik and Wefel therefore try to e~ima~5~e shape of the excitation functions by analogy with the better measured C —s ‘ B reactions. More measurements of the kind of those performed by Webber et al.(29) are required, especially between 200 MeV,’n and 1 GeV/n. In the meantime we must content ourselves with conjectures. If N proves to be definitely low (N/’04 6%), a~4originof most CR nu~ei in supernova ejects may be the only solution, simply because N is converted into Ne in the advanced stages of stellar evolution. However, Arnould does not consider this assertion as certain. A possible way out could be that CR nuclei are boosted at higher energy after fragmentation (30). If however N/O proves to be above 6%, the stellar—injection models aimed at explaining the bulk of CR’s (1—3) will be safe. But let us come back to the present situation. ~gnsf~ering only results consistent with both the CR elemental N/a ratio and the isotopic N! N observed ratio, we are left with the following N/a source ratios: Author “Low E”

Guzik (1981) (31) Guzik and Wefel (this conference)

“High E”

N/O at sources 7% 3 to 6%

Goret et al.(32)

5 to 9%

Ferrando and Goret (this conference)

5 to 15%

Now, the local galactic N/a ratio after Meyer (1) is 6.5 to 15% (note the large uncertainty). These numbers bracket the value of 12% recommended by Cameron (1982) (33). Therefore the situation is unconclusive.

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M. Cass~

4. THE HYDROGEN AND HELIUM PUZZLES This topic has been discussed along the same lines by Engelmann et al. in the poster session (paper submitted to Astronomy and Astrophysics), by Koch—Miramond and Meyer and by Lund. The exceptional quality of’ the HEAO III—C2 data (see Koch—Miramond (34) and references therein) allows a precise analysis of the energy spectra of a variety of elements from C to Fe between 0.7 and 25 GeV/n. The high—precision observed spectra have been extrapolated, point by point, back to the sources, through an energy dependent leaky box model. The energy dependent escape length has been adjusted to fit the B/C and Sc—Cr/Fe abund~ngeratios at different energies. The best fit of’ the data is obtained with a R dependence above a rigidity of 5.5 GV, and the source spectra of C, 0, Ne, Mg, Si, Ca and Fe are derived. They are found to be remarkably similar and can be well fitted by a simple analytical form inspired by the shock wave accele~6 ration theory dJ/dE = KP , withY= 2.41 + 0.02. Under the assumption that the R dependence of the escape length continues up to very high (TeV) energies, the same propagation model applied to the C + N + 0 combined spectrum released by JACEE (35) yields Y~.2.l in the TeV/n region. In contrast, the H and He source spectra, obtained in the same way, maintain approximately the same index over the whole range covered so far i.e. 1 GeV to 5. 10 GeV, remaining close to 2.15. Although the assumption that the same leaky box model applies equally to all nuclei from H to Fe and to all energies from the GeV to the TeV regions is not guaranteed (see e.g. (36,37)), this dichotomy is thought provoking. Lund proposes that we are bathed in the H and He flux produced in a large number of sources scattered in the galaxy, whereas higher Z CR’s emanate from a unique nearby source. As far as the CRS composition is concerned a unique source in our vicinity would explain the general similarity to the solar system material (see e.g.(38)). However this idea is in conflict with the Maeder’s proposal aimed at explaining the isotopic pecularities of the source composition. The suggestion of a unique local source of Z>2 elements cannot be ruled out bu~ 2seemsquite delicate since one would have to imagine a mechanism that produces the Ne excess and the otherwise normal heavy CR component in one and the same source. Such a special source is not an absolute necessity since injection by widespread flaring stars followed by reacceleration by interstellar shock waves (Montmerle, this issue and (1—3)) would yield identical spectra for all species. 5. THE ABUNDANCE STANDARD To explain the low abundance of the low FIP—volatile elements, like Ge and Pb, Binns et al. question the solar system abundance values for these elements obtained from CI carbonaceous chondrites. While it would seem quite implausible that CII carbonaceous chondrites could be a better guide (39), it is still true that the elements for which the CI values are indeed the most questionable are the volatile ones, for which the CII values ‘LIfer from the CI. In our view, a definite conclusion can come only from improved photospheric determinations. We feel that the low Ge and Pb abundances relative to CI carbonaceous chondrites will ultimately be interpreted in terms of either 1) these elements being slightly overabundant in CI relative to photosphere or ii) volatility being the parameter governing the CRS abundances, in which case CR’s should primarily be grain destruction products (40—43). At this point one cannot choose between the two possibilities. 6. THE COSMIC RAY PATHLENGTH DISTRIBUTION The energy dependence of the secondary to primary ratios is traditionally considered as a good indicator of the amount of’ matter traversed by CR particles (grasmage). Guzik and Wefel have analyzed the B/C and Sc—Mn/Fe ratios in the energy range 0.05 to 50 GeV/n in the framework of a sophisticated propagation model (see also (44)). The results indicate that a suppression of short pathlengths seems required to match the variations of the Sc—Mn/Fe ratio at low E, i.e. that the lower energy praticles are trapped more efficiently in the source region and therefore traverse more material. An energy dependent nested leaky box model (45) is suggested. The authors make the interesting remark that shock acceleration would lead to the opposite, the higher energy particles being those that spend the longest time in the shuck region, bouncing back and forth. However if the density in the shock region is negligibly small, as it must be for the mechanism to work, the argument does not apply. Ramadurai et ml. use the Ca—Mn/Fe ratio from (46) to define the pathlength distribution at low energy. They arrive essentially at the same conclusion, but in addition they try to correlate the overabundance of Fe—secondaries at low energy with the anomalous abundance of antiprotons through the model proposed by Eichler (47) and extended by Mauger et Stephens (48). Note that it is preferable to take the Sc—Mn/Fe ratio as indicative of the grammage t~versedrather than the Ca—Mn/Fe ratio since the primary component of Ca (mainly Ca) is substantial. We shall keep in mind, from these studies, that the predictions of the simple leaky box model are in conflict with observations of the Sc—Mn/Fe ratio in the low energy regime.

Nucleosynthesis and Cosmic Rays

415

As a working hypothesis Ramadurai adopts a smoothly truncated exponential pathlength distribution to derive the source abundances of’ ultraheavies. This particular distribution supplemented by nuclear cross sections from Letaw et al.1983 (49) leads Ramadurai to conclude to a general—40% enrichment in r—process elements of the sources (see however Binns et ml., this issue, and references therein). Such a moderate enhancement cannot be ruled out easily. However the conclusion must be taken with caution since according to the author “at present it is not possible to arrive to a unique propagation model”. The abundances obtained are similar to the ones obtained by Letaw et al.(1984) (50) in the framework of quite different hypotheses. Since the solution is not unique, the situation remains ambiguous. 7. PREGALACTIC COSMIC RAYS AND PREGALACTIC NUCLEOSYNTHESIS Hayakawa speculates that pregalactic (pop III) massive stars sight have triggered shock waves in the intergalactic medium through their supersonic stellar winds and their explosions. At a redshift z = 100 the intergalactic density is comparable to the mean interstellar density at present time. Therefore supersonic mass outflows are expected to have generated intergalactic shock waves similar to the present interstellar ones. Pursuing the analogy, it is natural to suppose that these intergalactic shocks must have built up a population of’ pregalactic cosmic rays. However quantitative modeling is hampered by many unknowns as for instance the mass distribution of pregalactic stars, their mass loss rates, etc.. Hayakawa estimates that in the pregalactic times, stellar winds are more efficient accelerators than stellar explosions, though only— 1% of the wind mechanical energy is transfered to the particles. The spectrum of the accelerated cosmic rays is predicted to be ve 5y poor in high energ~ particles. Such a soft spectrum is not suitable to produce D and He by spallation of He, but an interesting am~untof i may be produced by the o~+ ~ reaction with moreover the right proportions of Li and Li, at variance to a spectrum similar to that of galactic cosmic rays (51). According to certain authors (51,52,53), nuclear reactions in a cosmological context is of interest because it offers an alternative to the Big—Bang nucleosynthesis as it may produce very light elements. Kuznetzova and Lavrukhina consider the explosion of massive stars of the first generation. According to these authors a shock wave propagating in the ~xtended envelope of an exploding star could give rise to an interesting amount of D and He via the destruction of helium into free neutrons and protons, and their recombina— tion. The agent of destruction is, in this model, an intense flux of protons generated by the ~s~age of the shock wave. The cnoice of a pr~on energy s~ctrumof the form N(E)dE = KE with an integral flux of protons of 5 10 prot~nscm above 25 MeV allows the calculation to reproduce the cosmic abundances of D and He. The idea is interesting (in the same spirit see Audouze and Silk (53)). However a physical discussion of the mechanism that generates the energetic protons with the good energy spectrum is lacking. 8. CONCLUSION Simplicity and unity have flown away. In the field of cosmic rays we are far from unification, grand or small.

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