Cosmic‐Ray Production of⁶Li by Structure Formation Shocks in the Early Milky Way: A Fossil Record of Dissipative Processes during Galaxy Formation

Abstract

While the abundances of Be and B observed in metal-poor halo stars are well explained as resulting from spallation of CNO-enriched cosmic rays (CRs) accelerated by supernova shocks, accounting for the observed 6 Li in such stars with supernova CRs is more problematic. Here we propose that gravitational shocks induced by infalling and merging subgalactic clumps during hierarchical structure formation of the Galaxy should dissipate enough energy at early epochs so that the CRs accelerated by such shocks can provide a natural explanation of the observed 6 Li. In clear contrast to supernovae, structure formation shocks do not eject freshly synthesized CNO nor Fe, so that -fusion, which is the only effective production channel at low metallicity, is capable of generating sufficient 6 Li with no accompanying Be or B and no direct correspondence with Fe. Correlations between the 6 Li abundance and the kinematic properties of the halo stars may also be expected in this scenario. Further, more extensive observations of 6 Li in metal-poor halo stars, e.g., by the Subaru High Dispersion Spectrograph or the Very Large Telescope UV-Visual Echelle Spectrograph, may offer us an invaluable fossil record of dissipative dynamical processes that occurred during the formation of our Galaxy.

Cosmic Ray Production of Beryllium and Boron at High Redshift

“…This is the first theoretical indication of a plateau for these elements which does not resort to exotic models of BBN. This results is in contrast to that found in the structure formation scenario (Suzuki & Inoue 2002), where the medium responsible for cosmic ray acceleration contains very few if any metals, and therefore the CR nucleosynthetic contribution to Be and B in the IGM should be minimal. The latest observations of Be in low-metallicity halo stars, which indicate a hint of a plateau , would then favor a CCR scenario.…”

“…GCRN builds on the BBN value, yielding a 6 Li abundance which would be proportional to metallicity. Thus, an additional source of 6 Li is required at low metallicity (½Fe/H < À2), and different scenarios have been discussed which include the production of 6 Li during the epoch of structure formation (Suzuki & Inoue 2002;Nakamura et al 2006;Tatischeff & Thibaud 2007) or through the decay of relic particles during the epoch of big bang nucleosynthesis (e.g., Kawasaki et al 2005;Jedamzik et al 2006;Kusakabe et al 2006;Pospelov 2006;Cyburt et al 2006).…”

Cosmic rays from dark matter annihilation and big-bang nucleosynthesis

“…(1), the amount of energetic particles produced by the annihilation process is proportional to the annihilation cross section, and hence the effects on the light element abundances become more enhanced as the cross section becomes larger. Since the light 2 For references, see also [48] for possible astrophysical scenarios to enhance 6 Li abundance, and [49] for its observational uncertainties. 3 The depletion factor D 7 is defined as D 7 = ∆ log 10 ( 7 Li/H).…”

Production and evolution of Li, Be, and B isotopes in the Galaxy

“…Kusakabe et al 2008;Jedamzik & Pospelov 2009 and references therein). Observations of 6 Li/H constrain then the masses/cross-sections/densities of the massive particle; 2) Pre-galactic, by fusion reactions of 4 He nuclei, accelerated by the energy released by massive stars (Reeves 2005) or by shocks induced during structure formation (Suzuki & Inoue 2002;Rollinde et al 2005Rollinde et al , 2006Evoli et al 2008); 3) In situ production by stellar flares, through 3 He+ 4 He reactions (Deliyannis & Malaney 1995) involving large amounts of accelerated 3 He (Tatischeff & Thibaud 2007). Prantzos (2006b) showed that the energetics of 6 Li production by accelerated particles severely constrain any scenario proposed in category (2) above, including jets accelerated by massive black holes; this holds also for the "stellar flare" scenario (3), the parameters of which have to be pushed to their extreme values to obtain the "upper envelope" of the Asplund et al (2006) observations.…”