دانلود رایگان مقاله تولید اشعه کیهانی کهکشانی القا شده لیتیوم در ابر ماژلانی کوچک

abstract

Recently, the first lithium detection outside of the Milky Way was made in low-metallicity gas of the Small Magellanic Cloud, which was at the level of the expected primordial value. Part of the observed lithium in any environment has primordial origin, but there is always some post-BBN (Big Bang Nucleosynthesis) contamination, since lithium can also be produced in cosmic-ray interactions with the interstellar medium. Using the fact that processes involving cosmic rays produce lithium, but also gamma rays through neutral pion decay, we use the Small Magellanic Cloud gamma-ray observations by FermiLAT to make predictions on the amount of lithium in this galaxy that was produced by galactic cosmic rays accelerated in supernova remnants. By including both fusion processes, as well as spallation of heavier nuclei, we find that galactic cosmic rays could produce a very small amount of lithium. In the case of 6Li isotope (which should only be produced by cosmic rays) we can only explain 0.16% of the measured abundance. If these cosmic rays are indeed responsible for such small lithium production, observed abundances could be the result of some other sources, which are discussed in the paper.

3. Discussion

Using the Fermi-LAT gamma-ray detection of the SMC we have estimated the amount of lithium that can be produced by GCRs which we assume to be the cosmic-ray population producing observed gamma-ray flux. Even though, GCRs are expected to be the dominant CR population, our calculation shows that GCR-produced 6Li is less then 1% of the observed abundance. We have included αα interactions, as well as spallation processes. In case of SMCs metallicity and CR spectrum, spallation is a subdominant production channel and most of the CR produced lithium in case of this galaxy is produced via fusion processes. As the most extreme assumption, we can say that the Fermi-LAT diffuse gamma-ray background [2] is entirely produced by gamma rays from unresolved SMC-like galaxies. If we go through the same procedure as before, with this extreme assumption, we can explain 55% of the observed 6Li abundance. On the other hand this extreme assumption can produce only 12% of the observed 7Li abundance (and 0.04% without this extreme assumption), which is consistent with the fact that this lithium isotope, is in big part produced in the BBN. Observed abundance of this isotope in the SMC is consistent with the expected primordial abundance, so after removing GCR-produced 7Li that we get, we still won’t deviate much from the expected primordial abundance. Also, in [20] it was found that the observed lithium isotopic ratio of 0.13 implies that CRs in general could have produced 19% of the observed 7Li abundance in the SMC. All of this, leaves room for some additional CR component next to the GCRs.