Division of Theoretical Astronomy, National Astronomical Observatory of Japan

Research Highlights

Cosmological solutions to the Lithium problem:
Big-bang nucleosynthesis with photon cooling,
X-particle decay and a primordial magnetic field

The 7Li abundance calculated in BBN with the baryon-to-photon ratio fixed from fits to the CMB power spectrum is inconsistent with the observed lithium abundances on the surface of metal-poor halo stars. Previous cosmological solutions proposed to resolve this 7Li problem include photon cooling (possibly via the Bose-Einstein condensation of a scalar particle) or the decay of a long-lived X-particle (possibly the next-to-lightest supersymmetric particle).
In this paper, A theory group of Dr. Yamazaki, Dr. Kusakabe, Prof. Kajino, Prof. Mathwes, and Prof. Cheoun reanalyze these solutions, both separately and in concert. They also introduce the possibility of a primordial magnetic field (PMF) into these models. They constrain the X-particles and the PMF parameters by the observed light element abundances using a likelihood analysis to show that the inclusion of all three possibilities leads to an optimum solution to the lithium problem. They deduce allowed ranges for the X-particle parameters and energy density in the PMF that can solve 7Li problem.

Cosmological solutions to the Lithium problem: Big-bang nucleosynthesis with photon cooling, X-particle decay and a primordial magnetic field
Phys. Rev. D 90, 023001 [ADS] [arXiv]
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Dai G. Yamazaki (private website)

Formation of Carbon Grains in Very Massive Primordial Stars

Since the beginning of this century, the presense of huge amounts of dust grains has been confirmed at the times around one tenth of the present cosmic age. Since then, the origin of cosmic dust in the early universe has been hotly debated.
We investigate the formation of dust in a stellar wind during the red-supergiant phase of a very massive first star with the zero-age main sequence mass of 500 times the solar mass. We find that, such a very massive star can produce, at most, one solar mass (300000 times the earth mass) of carbon grains in total during its lifetime. This indicates that, if the first stars are very massive as suggested by many studies, then they can be the first important sources of small solid particles in the universe.
The figure plots the total mass of carbon grains formed during the evolution of a very massive first star whose initial mass is 500 times solar mass, as a function of mass ejection rate. The figure shows that, for the mass ejection rate of 0.003 solar mass per year, 1.7 times solar mass of carbon grains can form in total with the average radius around 0.02 um.

Dust Production Factories in the Early Universe: Formation of Carbon Grains in Red-Supergiant Winds of Very Massive Population III Stars, by Takaya Nozawa, et al., 2014, ApJ Letters, 787, L17 (5pp)
[ADS] [arXiv]
Takaya Nozawa

A New Production Mechanism for Ultra-High-Eneregy Cosmic Ray (UHECR) Neutrinos

T. Kajino, A. Tiokuhisa at NAOJ and The University of Tokyo and their international collaboration team have recently proposed a new theory for UHECR tau-neutrinos that could keep ultra-high energy > 10^15 eV. Although two neutrino events, whose energies are higher than 10^15 eV, were detected in IceCube Neutrino Observatory last year, origin of the UHECR is still an unanswered biggest mystery of the century. Although Ginzburg & Syrovatskii (1965) and Waxman & Bahcall (1997) theoretically proposed a model to produce neutral mesons pi, D_s, J/ψ as the source of high-energy neutrinos, they easily lose energy during the passage in space for relatively longer life, and neutrino energy cannot be as high as 10^15 eV. Kajino and his collaborators discovered that the strong interaction between UHECR hadrons like protons or irons can copiously produce heavy neutral mesons like upsilons in synchrotron emission near the strongly magnetized neutron star (i.e. magnetar) or active galactic nuclei (AGN), which quickly decay to produce tau-lepton pairs and create eventually UHECR tau-neutrinos.
T. Kajino, A. Tokuhisa, G. J. Mathews, T. Yoshida & M. A. Famiano., ApJ. 782 (2014), 70.
T.Kajino(personal website)