Inoue, S

Nakamura, K

Shigeyama, T

Wanajo, S

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PoS(NIC-IX)148PoS(NIC-IX)148Light Element Production in the CircumstellarMatter of Type Ic Supernovae at Low MetallicityKo Nakamura∗Department of Astronomy, Graduate School of Science, University of TokyoResearch Center for the Early Universe, Graduate School of Science, University of TokyoE-mail: nakamura@resceu.s.u-tokyo.ac.jpSusumu Inoue†Shinya Wanajo‡Toshikazu Shigeyama§We investigate energetic Type Ic supernovae as production sites for 6Li and Be in the earlystages of the Milky Way. Recent observations have revealed that some very metal-poor starswith [Fe/H]< −2.5 possess unexpectedly high abundances of 6Li. Some also exhibit enhancedabundances of Be as well as N. From a theoretical point of view, recent studies of the evolution ofmetal-poor massive stars show that rotation-induced mixing can enrich the outer H and He layerswith C, N, and O (CNO) elements, particularly N, and at the same time cause the intense massloss of these layers. Here we consider energetic supernova explosions occurring after the progen-itor star has lost all but a small fraction of the He layer. The fastest portion of the supernova ejectaand the circumstellar matter (CSM), both of which are composed of He and CNO, can interactdirectly and induce light-element production through spallation and He-He fusion reactions. TheCSM should be sufficiently thick to energetic particles so that the interactions terminate within itsinnermost regions. We calculate the resulting 6Li/O and 9Be/O ratios in the ejecta+CSM materialout of which the very metal-poor stars may form. We find that they are consistent with the ob-served values if the mass of the He layer remaining on the pre-explosion core is ∼0.01−0.1 Mand if the mass fraction of N mixed in the He layer is∼0.01. Further observations of 6Li, Be, andN at low metallicity should provide us with critical tests of this production scenario.International Symposium on Nuclear Astrophysics - Nuclei in the Cosmos - IX25-30 June 2006CERN∗Speaker.†National Astronomical Observatory of Japan‡Department of Astronomy, Graduate School of Science, University of Tokyo§Research Center for the Early Universe, Graduate School of Science, University of Tokyoc© Copyright owned by the author(s) under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike Licence. http://pos.sissa.it/PoS(NIC-IX)148PoS(NIC-IX)148Light Element Production in the Circumstellar Matter of Type Ic Supernovae at Low MetallicityKo Nakamura1. IntroductionAmong the light elements Li, Be, and B (LiBeB), 7Li is thought to arise from a variety ofprocesses, including big bang nucleosynthesis [1] and the ν-process in Type II supernovae [2]. Onthe other hand, the main production channel for the rest, in particular for 6Li and Be, is believed tobe cosmic-ray-induced nuclear reactions. The most widely discussed models of LiBeB productionare based on cosmic rays accelerated in supernova shocks [3, 4]. Observations of metal-poor stars inthe Galactic halo show a primary relation between [Fe/H] and [Be/H] or [B/H], which is consistentwith spallation by cosmic rays enriched with C, N, and O (CNO) from fresh supernova ejectaimpinging on interstellar H or He [5].An alternative possibility was proposed by [6, 7], who considered explosions of Type Ic super-novae (SNe Ic) as a site for primary LiBeB production. Since it is expected that a fraction of C andO in the surface layers of the ejecta is accelerated to energies above the threshold for spallation re-actions when the shock passes through the stellar surface, LiBeB production can occur through thedirect interaction of the ejecta with the ambient material, without the need for shock acceleration ofcosmic rays. This was explored in greater depth by Nakamura & Shigeyama (2004)[8], who usedmore realistic stellar models and equations of state, together with a one-dimensional relativistic hy-drodynamic code to investigate in detail how much of the ejecta mass acquires sufficient energiesfor the LiBeB production. All of these studies used stellar models that completely lost their H andHe envelopes and assumed the target to be interstellar matter (ISM) consisting of 90% H and 10%He, ignoring any circumstellar matter (CSM). Therefore, the only reactions under considerationwere C,O+H,He→ LiBeB.Recent observations [9, 10] have revealed that some very metal-poor stars possess surprisinglyhigh abundances of 6Li, much higher than expected from standard supernova cosmic ray models.The measured values are also higher than can be accommodated in the SN Ic production scenariodiscussed above, even in the case of an energetic explosion similar to SN 1998bw, as can be seenfrom the results of Nakamura & Shigeyama (2004) [8]. However, besides spallation of C andO, the fusion reaction He+He → 6Li may also be potentially important. This reaction will besignificant in the conceivable case that a small fraction of He is still remaining in the surface layerof the core at the time of the explosion, while most of the He has been transported to the CSMthrough mass loss. The evolution of metal-poor massive stars has recently been calculated [11, 12]by taking into account the effects of rotation-induced mixing, and it has been shown that extensivemass loss occurs even in extremely metal-poor cases, in addition to significant enrichment of theN abundance near the surface. If this N is accelerated at the shock breakout of the SN explosion,a significant amount of Be can be produced through the reaction N + He → 9Be, because of itslow threshold Eth (∼ 6MeVA−1) and high cross section at peak σp (∼ 24 mbarns) compared withother 9Be-producing reactions (e.g., Eth ∼ 8MeVA−1 and σp ∼ 8 mbarns for O + He → 9Be).Indeed, recent observations indicate that the abundances of both Be and N may be enhanced insome metal-poor stars in which 6Li is detected [13, 14, 15]. Here, we focus on the interactionsHe,CNO+He,N→ LiBeB occurring between the ejecta accelerated by the energetic SN explosionand the CSM, in order to account for the abundances in such very metal-poor stars.2PoS(NIC-IX)148PoS(NIC-IX)148Light Element Production in the Circumstellar Matter of Type Ic Supernovae at Low MetallicityKo Nakamura2. Summary of ObservationsObservational determination of the abundances of 6Li in metal-poor stars is extremely chal-lenging due to its deficiency and to the proximity of its absorption lines to the much strongerones of the 7Li isotope. Owing to the advent of large telescopes and the improved capabilitiesof observational instruments, reliable measurements have become possible only in the last sev-eral years. 24 metal-poor halo stars were observed [9] with VLT/UVES and 6Li was positivelydetected in nine of them at the ≥ 2σ significance level. The subdwarf LP 815-43 is the mostmetal-poor object ([Fe/H] = −2.74) with 6Li in their sample. In the even more metal-poor starG 64-12 ([Fe/H]= −3.17) a tentative detection with Subaru/HDS was reported [10], although adetailed analysis is still ongoing (W. Aoki et al. 2006, in preparation). With VLT/UVES, the Beabundances of these two stars have been measured [13, 14] to be logε(Be) =−1.09±0.20 for LP815-43 and −1.10± 0.15 for G 64-12. The Be abundance in G 64-12 is considerably higher thanthat expected from previous measurements of the Be-Fe relation in stars with similar metallicities,and this may be the case for LP 815-43 as well. Nitrogen abundances in 31 metal-poor stars wereanalyzed [15] and it was found that both LP 815-43 and G 64-12 are more N rich than average.3. Calculations and ResultsMeynet et al. (2006) [12] and R. Hirschi (2006, in preparation) simulated the evolution oflow-metal massive stars with rapid rotation and found that N is enhanced in the He layers andthat they have lost a significant amount of their mass in spite of their initial low metallicities. Ifthere is a considerable amount of N in the He layers, this N is accelerated after the shock breakoutof SN explosion and a significant amount of Be will be produced through a reaction N + He →Be because of its low threshold energy and high cross section at peak. Referring to their results,we consider stellar models which have lost completely their H-rich layers and partly their He/Nlayers as progenitor stars. Here we calculate an explosion of a ∼15M star, originated from main-sequence star with mass ∼40M, with an explosion energy of 3× 1052 ergs corresponding to SN1998bw [16]. Accelerated ejecta consist of He, C, N, and O will collide with circumstellar He andN stripped from a progenitor star and produce the light elements through spallation reactions andHe-He fusion. We use "thick target" approximation to estimate the energy loss of accelerated ejectaassuming that circumstellar He/N is so thick that production of the light elements terminate withinthe cicumstellar matter and ejecta lose energy mainly by Coulomb collisions with free electronsso that other processes like escaping from a given system can be ignored. Detail is discussed inNakamura et al. (2006) [17]. The cross sections of the reactions are given by Read & Viola (1984)[18] and Mercer et al. (2001) [19].As a result of the localized production of the light elements, their abundance ratios with respectto heavy elements averaged over the CSM and SN ejecta are likely to be inherited by stars ofthe next generation, as pointed out by Shigeyama & Tsujimoto (1998) [20]. Thus, we comparethe abundance ratios calculated from the above model with the metal-poor stars discussed in thepreceding section.We focus on the abundances of the very metal-poor star LP 815-43 from which 6Li, Be,and N have been detected, all at apparently enhanced levels. As seen in the literatures (§2),3PoS(NIC-IX)148PoS(NIC-IX)148Light Element Production in the Circumstellar Matter of Type Ic Supernovae at Low MetallicityKo Nakamura                                    	                                                	                 Figure 1: Mass ratios of 6Li (top panel) and 9Be (bottom panel) to O as functions of MHe,N. Shown arethe cases with mass fractions of N, XN = 0.005 (solid line), XN = 0.01 (dotted line), XN = 0.02 (dash-dotted line), and without N, XN = 0 (dashed line). Shaded regions represent the observed ratios for the verymetal-poor star LP 815-43 [9, 13], including the error.X(6Li)/X(O) ∼ 6.88+3.08−3.22 × 10−7 and X(9Be)/X(O) ∼ 1.32+0.77−0.49 × 10−8 for this star. The massMHe,N of the He/N layer on the preexplosion core and its mass fraction XN of N are the main pa-rameters. Figure 1 shows our results. Both X(6Li)/X(O) and X(9Be)/X(O) show good agreementwith the observational data of LP 815-43 when MHe,N ∼ 0.01− 0.1M and XN ∼ 0.005− 0.01.These are consistent with recent simulations of the evolution of metal-poor massive stars with ro-tation ([12]; R. Hirschi 2006, in preparation). The observed X(9Be)/X(O) for G64-12 can also bereproduced with parameters analogous to those for LP 815-43.4. Conclusions and DiscussionWe have proposed a new mechanism to produce the light elements, especially 6Li and 9Berecently observed in metal-poor stars. This is based on recent theoretical findings that rotating,metal-poor massive stars can lose substantial amounts of their envelopes and end up with SNe Icwith a small amount of He and N in the outermost layers. The outer layers composed of He andCNO are accelerated at the shock breakout and undergo spallation and He-He fusion reactions toproduce mainly 6Li and 9Be. Our calculations show that if ∼ 0.01− 0.1M of the He layer isremaining on the core in a SN explosion of a massive star with Mms > 30M, then the observedabundance ratios 6Li/O can be reproduced. If the He layer contains a small amount (∼ 0.5%−1%)of N, we can also reproduce 9Be/O.Compared with the standard picture of cosmic-ray shock acceleration by normal supernovae,our high yield of 6Li results from three crucial differences. First, we consider a large explosionenergy, of order 1052 ergs, as observed in some SNe such as SN 1998bw. Second, our mechanismconsiders the direct interaction of fast ejecta with the CSM, and it does not involve an efficiencyfactor for cosmic ray acceleration and is not affected by losses or escape during ISM propagation.Third, the energy distribution of the accelerated particles is a very steeply falling power law, with4PoS(NIC-IX)148PoS(NIC-IX)148Light Element Production in the Circumstellar Matter of Type Ic Supernovae at Low MetallicityKo Nakamuraspectral index ∼ 4.6 as opposed to 2 for shock-accelerated particles. This means that most of theenergy is contained in the lowest energy portions, where the cross sections for both He+He→ 6Liand N+He→ 9Be peak.Both the degree of mass loss and the amount of N enrichment in the progenitor star are ex-pected to be sensitive to its initial rotation speed [11, 12]. The actual rotation speed is presumablydistributed over a wide range, as is the explosion energy and the extent of mass loss at the timeof the explosion. Therefore, dispersions in the 6Li and Be abundances are expected, and currentobservations suggest that this may indeed be the case for 6Li [9, 10, 21] as well as Be [22]. Ourscenario predicts a close relation between Be and N, since Be arises directly as a consequence ofN spallation. A looser correlation between 6Li and N is also expected, as effective 6Li productionrequires sufficient mass loss, which in turn implies significant N enrichment. Further observationsof 6Li, Be, and N for a large sample of metal-poor stars should provide us with definitive tests.We are grateful to Georges Meynet, Raphael Hirschi, Takeru K. Suzuki, and Sean Ryan forvaluable discussions. This work has been partially supported by the grants in aid (16540213,17740108) of the Ministry of Education, Science, Culture, and Sports in Japan.References[1] Spite, F., & Spite, M. 1982, A & A, 115, 357[2] Woosley, S. E., Hartmann, D. H., Hoffman, R. D., & Haxton, W. 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Light Element Production in the Circumstellar Matter of Type Ic Supernovae at Low Metallicity

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