W 1996/9 FEVRIER 1996

Transcription

1 W 1996/9 FEVRIER 1996., DAPNIA EtVANGIONI-FLAM, i LIGHT ELEMENT PRODUCTION BY LOW ENERGY NUCLEI FROM MASSIVE STARS M,CASSE, RiRAMATY,, J,,

2 Le DAPNIA (Dtipartement d Astrophysique, de physique des Particles, de physique Nuc16aire et de l Instrumentation Associ6e) regroupe Ies activites du Service d Astrophysique (SAp), du Department de Physique des Particles E16mentaires (DPhPE) et du D6partement de Physique Nuc16aire (DPhN). Adresse : DAPNIA, Biitiment 141 CEA Saclay F Gif-sur-Yvette Cedex

3 LIGHT ELEMENT PRODUCTION BY LOW ENERGY NUCLEI FROM MASSIVE STARS 1 Elisabeth Vangioni-Flaml, Michel Cass@, Reuven Ramaty3 1. Institut d Astrophysique de Paris, CNRS, 98 bis Bd Arago, Paris, France 2. Service d Astrophysique, DSM/DAPNIA, Saclay, Gif sur Yvette, France 3. Goddard Space Flight Center, NASA, Greenbelt, MD 20771, USA Abstract The detection of gamma rays from the Orion complex, attributed to deexcitation of fast carbon and oxygen nuclei injected in the cloud, has important implications for the production and evolution of light isotopes in the Galaxy. Massive stars, through their explosion and winds, seem to be prolific sources of C-O rich low energy nuclei. This low energy nuclear component plays a larger role than the usual Galactic Cosmic Rays in shaping the evolution of 6Li, 9Be, 10B and above all 1lB, especially in the early galactic evolution. The 1lB/ IOB meteoritic ratio finds with this mechanism a clear astrophysical explanation. The typical particle energies of this low energy component are around 30 MeV/ nucleon, and the enhancement of 1lB over 10B is predominantly due to the interaction of C nuclei with ambient H. Subject headings: stars : abundances, supernovae, ISM : cosmic rays, Galaxy : evolution,

4 1. Introduction Up to the nineties, the spallative LiBeB origin was one of the more assured of all astrophysical hypotheses. Notwithstanding 7Li, whose main origin was assumed to be primordial and possibly stellar, the only problem lay in the meteoritic llb/ 10B, predicted to be -2.5 in GCR spallation, and observed to be -4 (Shima and Honda 1962, Chaussidon and Robert 1995). A high flux of low energy cosmaic rays was taylored to cure this weakness (Meneguzzi and Reeves 1975, Walker, Mathews and Viola 1985). Its composition was assumed to be similar to that of usual GCR, i.e. proton and alpha rich, but this component was purely hypothetical. Now the discovery of a strong gamma-ray line emission of C and O from the Orion complex (Bloemen et al 1994) has prompted cosmic ray physicists to reassess the problem of the production and evolution of lithium, beryllium and boron, traditionally attributed to galactic cosmic rays (GCR) (Vangioni-Flam et al 1995, Casse et al 1995b, Ramaty et al 1995 b,1996). Indeed, this new component, though of low energy, has a different composition from the one proposed by Meneguzzi and Reeves (1975), and has the adventage of being supported by observations. The data indicate that C and O are copiously accelerated by an unknown mechanism within Orion, and in all likelihood, in other active star forming regions. What is the source of these fast C and O nuclei in Orion and, more generally, what is the contribution of their interaction with ambient interstellar helium and hydrogen to the production of light elements in the Galaxy? Both gamma ray line emission and LiBeB production depend on the composition and energy spectrum of the accelerated beam, and less sensitively on the target composition. WC stars as astrophysical sources of

5 fast nuclei have various due to the absence of H, the moderate He content, 3 and the richness of C and O relative to heavier nuclei; these lead to a relatively low energy demand to produce the gamma ray lines and avoid the overproduction of gamma rays in the undesired (l-3 MeV) spectral band (Ramaty et al 1995a). However, the WC stage cannot be isolated from the preceding and following stages of evolution, potentially favorable to the injection and acceleration of nuclei, i.e. the O, Of and WR episodes, on one hand, and the SN explosion, on the other hand. Maeder (1992) offers all the more importantly, the effect of metallicity on mass loss can be identified on the same basis. In this paper, we focus on the galactic evolution of LiBeB produced by low energy cosmic rays. The adopted WC composition is rather close to that of the ejecta of massive stars subject to mass loss (Cass6 et al 1995 b). Different source compositions from the solar system to supernovae, and Wolf-Rayet stars have been tested using a simple injection spectrum of the form N(E)dE=cst up to a critical energy Ec and E-n with n > 4 at higher energies. This spectrum (Meneguzzi and Reeves 1975, Ramaty et al 1979) will serve provisionally as a standard until a more physical spectrum is found. Detailed calculations of light isotope production by low energy cosmic rays with various abundances have been carried out by Ramaty et al (1996) who showed that the energetic constraints indeed favor a proton poor beam and a rather high Ec, of the order of 30 MeV/n, still consistent with the width of the C and O gamma ray lines. However, the galactic evolution of LiBeB was not treated in that paper. Z. Source properties and isotope production.

6 [n previous works, various source compositions have been 4 analyzed systematically (Vangioni-Flam et al 1995, Rarnaty et al 1995a,b, 1996). Here we discuss the most favorable cases. Different source compositions, including the pre and post WC stages both using strong (s) and reduced (r) mass losses are compared in table 1 to that of the pure WC case, taken as reference. The SS and SN60 (Casse et al 1995b) cases are given for illustrative purposes. Also shown is the composition of a SN ejecta (E60) resulting from the explosion of a 60M0 star not affected by mass loss since its metallicity (Z= O.001) is too low to trigger an efficient stellar wind. The production ratios and the Be production rate, normalized to the observed C and O gamma-ray lines are presented in table 2. Recall that the adopted Ec and n are 30 MeV/n and 10 respectively. Casse et al (1995) have choosen to enhance the 1lB production at the expense of a lower Ec (9 MeV/ n). However, Ramaty et al (1996) have shown that it is energetically much more favorable to produce the light isotopes at higher energies. Indeed, by combining the data in the last two lines of table 2 for SN60, 9 MeV/n and WC, 30 MeV/n, we see that the Be production requires about 200 times less energy at 30 MeV/n than at 9 MeV/n. Thus, in the following, except if explicitly noted, all the calculations are made at 30 MeV/n. The shape of the spectrum is justified theoretically by Bykov (1995), who has developed a model of non thermal particule acceleration in Orion. A beam of purely SS composition impinging on a medium of similar composition would produce much more Li than the WR and SN cases selected due to the operation of the a + a reaction. Taking E&30 MeV/n, as seen in table 2, the production ratios and rates are rather similar. In other words, taking into account the pre and post WC stages does not affect the main conclusions of Ramaty et al (1995a). Note however that the energetic is slightly less favorable (about 7 against

7 14 in table 2). The results seem robust since they are not very sensitive to the adopted mass loss rates (strong or reduced) nor to the metallicity influencing the mass loss rates. The main effect of changing the break energy, EG is to increase the energy demand. (compare column 3 Assuming that the winds and explosions of massive stars accelerate their own material, leading to a spectrum of the required kind (Bykov 1995) which remains to be demonstrated, the same yields, at first order can be taken all along the galactic lifetime. There is however a slight difference in the llb/ 10B ratio between WC and E60 (table 3). The solar system ratio is reproduced since in the galactic disk, the metallicity is high enough to induce WC with more favorable llb/ 1 B ratio. Strickly speaking, metallicity dependent yields should be considered. But quite rapidly mass loss becomes efficient, WC are operative quite early and it is a good approximation to adopt their yields over the gaiactic lifetime. Assuming a steady state, each rate (expressed in nuclei of the desired species per second) translates into a yield (total number of nuclei produced, or more conveniently amount in solar masses per Orion like region) once multiplied by the duration of the process, i.e. the irradiation time ~, taken as a free parameter. Now we are in position to test the WC solution against galactic evolutionary models of the kind applied recently to the SN case. 3. Galactic evolution of light elements. On a basis of the local conditions derived from the Orion region, we enlarge the perspective to the whole gaiaxy. Assuming that the most massive stars (> 60 Mo on the main sequence) explode in their parental giant molecular cloud and thereby induce the Orion process, we integrate this process in a standard evolutionary model of galactic evolution (Cass6 et

8 al 1995a, b). As usual, the star formation rate is taken proportional to the gas mass fraction. The initial mass function (IMF), is of the Salpeter type with x = 1.7. Note that we use a normal IMF and Hot a biased one favoring massive stars. This galactic evolutionary model is used to follow the behaviour of LiBe B isotopes, including stellar destruction of these fragile species. Besides the classical observable of galactic evolution, the model is constrained to reproduce: i) the observed log(be/ H) and 1og(B/H) vs [Fe/H]. ii) the l*b/ IOB ratio observed in the solar system, amounting to 4. iii) the Be/ H and B/H values oberved in the present and local galactic environment (Be/H = (within a factor of 1.5), B/H= (2+1) J Arnould and Forestini 1%0) iv) the observed log(li/ H) vs [Fe/H], avoiding overproduction at The nucleosynthetic yields of heavy elements are taken from early Be and B evolution. As said previously, the yieds of LiBeB have been calculated using the WC source composition of Ramaty et al 1996, interacting with a zero metallicity interstellar medium (He/ H= O.07) to mimic the very early galactic evolution Lithium As shown in fig. 1, overproduction of Li is avoided at halo metallicities, and thus the Spite plateau (Spite and Spite 1993) is preserved. The relative contributions of each physical source of lithium is displayed. The primordial lithium component has been supressed to reveal the GCR and the low energy component (LEC). Above [Fe/H] = -1 corresponding to

9 the beginning of the disk evolution, spallation, both at low and high energy, 7 falls short to fit the solar abundances. Thus, a pure 7Li component of stellar origin is required to fill the gap and to reach the solar system Li isotopic ratio. Low mass AGB stars would be good candidate to produce the remaining lithium. Moreover, due to their relatively long lifetime, they would eject their lithium with the required delay in the galactic evolution and thus the Spite plateau would be once again respected. On the other hand, with a reasonable 7Li yield of 1. to Mo (e.g. Abia et al 1993) we reproduce the observed meteoritic ratio, 7Li/ 6Li = 12.5, at solar birth Beryllium and Boron The low energy component (LEC) dominates the early evolution of Be and B, whereas GCR contribute but only marginally afterwards, as clearly shown in figures 3a, 3b. The 1lB/ IOB ratio, originally equal to 4.9 is diluted by the GCR (2.5) component to the observed value of 4. It is worth mentioning that the boron meteoritic value is more consistent with our evolutionary scheme than the photospheric value (fig. 2. b). To test the sensitivity of the model to the irradiation time, z, we have varied it between and 10 5 years. As shown in fig. 2 a, b, the best time is the shorter one. The B/Be ratio evolves from 26 at the beginning of the galactic evolution to 18.5 at present, which is well in the observed range. The uncertainty on the Fe yield of massive stars, fortunately, has little influence on the results due to the steep IMF, as checked numerically. We also have tested the sensitivity of the results to the slope of the IMF. Adopting x = 2 instead of 1.7 leads to a good fit with ~ = 10 5 years. 4. Conclusion.

10 The material expelled by very massive stars, of typically 60 Mo on 8 the Zero Age Main Sequence, seems to have the right composition to explain both the gamma-ray line emission of Orion and the evolution of Be and B in the early Galaxy. The next task will be to derive the detailed energy spectrum of the nuclei accelerated in winds and SN ejecta in a cloud context. It is expected that it will not be too different from the simple one adopted in this article. The early galactic evolution of Be and B is clearly governed Iy low energy nuclei of the kind that produce gamma-ray line in Orion. The contribution of this component to the subsequent (disk) evolution is far from being negligible. The uncertainties on the solar abundance of Be and B preclude definite conclusion, but it seems that GCR play a marginal role in the overall LiBeB story. Spallation is not sufficient to explain the whole Li behaviour. The present 7Li abundance can be recovered assuming that low mass (1 to 2 Me), low metallicity stars produce about 10-8 Mo of this isotope. This yield settles an important constraint on the evolution of low mass stars, especially complex. The 1lB problem is alleviated if not solved. Yet, a slight contribution of neutrino spallation in SNII explosions cannot be excluded (Olive et al 1994, Vangioni-Flam et al 1996) but it is severely restricted. More general estimates, including binary Wolf-Rayet stars, neutrino spallation of Carbon in SNII explosions, refined cosmic ray spallation, and metallicity dependent yields will be presented in forthcoming papers (Lemoine et al 1996). AknowIedgements.

11 We thank Roland Lehoucq for numerical expertise and Yvette Oberto for helping us in the preparation of the manuscript. The work of EVF was supported by PICS 114, origin and evolution of the light elements, CNRS.

12 10 References Abia, C., 1sern & Canal, R. 1993, A&A. 275,96 Anders, E. & Grevesse, N. 1989, Geochimica et Cosmochimica Acta 53, 197 Arnould, M. & Forestini, M. 1989, Nuclear astrophysics: Proceedings of the third international summer school, Larabida, Ed.: M. Lozano, M.L Gallardo & J.M. Arias, p. 48 Boesgaard, A. & King, J.R. 1993, A.J. 106,2309 Bloemen, H. et al. 1994, A&A 281, L5 Bykov, A.M. 1995, Space Sci. Rev. 1995, 74,(3/4), to appear Casse, M., Vangioni-Flam, E., Lehoucq, R. & Oberto, Y. 1995a, in Nuclei in the Cosmos, Assergi, Italy eds: M. Busso, R. Gallino & C. Raiteri, p.539 Casse, M., Lehoucq R. & Vangioni-Flam, E. 1995b, Nature, 373, 318 Chaussidon,M. and Robert, F. 1995, Nature, 374, 337 Duncan, D., Lambert,D. & Lemke, M. 1992, ApJ. 401,584 Gilmore, G., Gustafsson B., Edvardsson, B. & Nissen, P.E. 1992, Nature 357, 379 Kiselman, D. and Carlsson, M. A&A, 1995, in the light element abundances ESO/ EIPC Workshop, ed. P. Crane, Springer, p372 Kohl., J. L., Parkinson W.H. & Withbroe.N.,1977, ApJ. 212, L101 Maeder, A. 1992, A&A. 264, Maeder, A. & Meynet, G. 1987, A&A. 182, 243 Meneguzzi, M. & Reeves, H. 1975, A&A. 40,99 Prantzos, N., Casse, M. & Vangioni-Flam E. 1993, ApJ. 403,630 Olive, K. A., Prantzos, N., Scully S. & Vangioni-Flam E. 1994, ApJ. 424, 666 Ramaty, R., Kozlovsky, B. & Lingenfelter, R.E. 1975, Space Sci. Rev. 18, 341 Ramaty, R., Kozlovsky, B. & Lingenfelte,R.E. 1979, ApJS. 40,487 Ramaty, R., Kozlovsky,. & Lingenfelter R.E. 1995a, ApJ. 438, L 21

13 11 Ramaty, R., Kozlovsky, B. & Lingenfelter, R.E. 1995b, Proceedings of the 17th Texas Symposium, New York Acad. Sci., in press Ramaty, R., Kozlovsky,B. & Lingenfelter, R. E., 1996, ApJ, in press Read S.M. & Viola V.E. 1984, Atomic Data and Nuclear Data Tables, 31, 359 Rebolo, R., Molaro, P. & Beckman, J. E., 1988, A&A, 192, 192 Ryan S., Norris 1., Bessel M. & Deliyannis C. 1994, ApJ. 388, 184 Shima, M. and Honda, M. 1962, J. Geophys. Res., 68, 2849 Spite F. & Spite M. 1993, in Origin and Evolution of the Elements, ed. N. Prantzos, E. Vangioni-Flam & M. Casse, Cambridge University Press, p. 201 Vangioni-Flam, E., Lehoucq, R. & Cass6, M. 1995, in The light element abundances ESO/EIPC Workshop Isola d Elba, Italy, ed: P. Crane, p. 389 Vangioni-Flam, E., Casse, M., Olive, K., Fields, B., 1996, ApJ, submitted Walker, T. P., Mathews, G.J. and Viola, V.E. 1985, ApJ, 229,745 Weaver T. M. & Woosley S.E. 1993, Phys. Rep. 227,65

14 12 SS: Solar System composition from Anders and Grevesse (1989) WC: Maeder (1987) adopted by Ramaty et al (1995) W(s) : O + WR wind composition from Maeder (1993) resulting from a high mass loss rate (strong wind) W(r): idem with a reduced mass loss rate W(s) + E: supernova ejecta added to the wind. W(r) + E : idem E60 (Z = 0.001) : Ejecta only, since mass loss is insignificant at such a low metallicity. The E60 case, at Z=ZO, is given for a sake of comparison.

15 Table 2: Production ratios and rates w(s) W(r) SN60 Wc s s E60 Ec=30 Ec=30 Ec =9 Ec=30 Ec=30 (Z=O.001) Ec=30 7Li/6Li llb/10b Li/Be B/Be dbe/dt (10%-1) y/erg These results have been obtained with a solar system target composition W (s) : strong wind, Maeder (1993) W(r): reduced wind, Maeder (1993) SN60: supernova ejecta, Weaver and Woosley (1993) WC: Maeder (1987). SS: Solar System, Anders and Grevesse (1989) 60 Mo (Z =0.001), Maeder (1993) W(S) + E and W(r) + E give results close to W(s) and W(r) respectively.

16 table 3. Production ratios and rate due to spallation of WC material on zero metallicity ISM 7~/6Ll llb/10b Li/& B/Be dbe/dt (s-q

17 15 Figure captions. Fig. 1 Lithium evolution, Data points from Spite and Spite 1993 (plateau), Rebolo, Molaro and (primordial+ LEC+GCR+stars). Dotted line: GCR component only. Dashed line: GCR+LEC components. Fig. 2. a. Berv ilium evolution. Data points from Ryan et al 1990, Gilmore et al 1992, Ryan et al 1994, and Boesgaard and King (1993). Full line: LEC+GCR components with Fig. 2. b. Boron evolution, Same as figure 2a. Data points from Duncan et al 1992 and Kiselman et al The three values at solar [Fe/H], from bottom to top correspond to solar photospheric (Khol et al 1977), present galactic average (Arnould and Forestini 1989) and meteoritic (Anders and Grevesse, 1989). Fig. 3a. Production of Bervllium bv LEC and GCR. Contribution to the beryllium evolution of LEC+GCR (a) and LEC Fig. 3. b. Production of Boron bv LEC and GCR. Contribution to the boron evolution of LEC+GCR (a) and LEC

18 I I I I I I I I 1 1 I [ I I I I I I [ I I \ s -9.+ J + w o -10 r t. i b++- T I [F+H] \ I,,

19 II i I I I I 1 4 E z \ & z IBe ri F T I-?=A I Jr L,., 0,

20 E -10 a 5 o [ Fe/H ]

21 5 % \ & z -lo I1,! I I 1 I I I 1 I I I I I I I I I I I [ F+H ] o

22 --J I I I I -B I i i I I I I I I I I I I I I I I I I E -10 z \ 6 E Qo o / r,