1. Theoretical Astrophysics Group
Masa-aki HASHIMOTO, Professor
Hitoshi YAMAOKA, Assistant Professor
Mami MACHIDA, Assistant Professor
Masaomi ONO, Assistant Professor
graduate students: 5 undergraduate students: 5
URL:
Theoretical Astrophysical group belongs to particle physics group of Kyushu University. We have been carrying astrophysical research since We conduct research in a wide range of topics including stellar evolution, supernova explosions, origin of elements, big bang nucleosynthesis, compact objects, accretion disks, astrophysical jets.
You can find more information about our research, teaching and outreach activities, visiting our home page. Seminars and Colloquium are scheduled here and all you are welcome to join with us. We highly appreciate various opinions and participation.
1. Nucleosynthesis in the Universe
Our group investigate the nucleosynthesis in massive stars [1], supernova explosions [2, 3], X-ray bursts on accreting neutron stars [4], and the Big-bang [5]. Here, we introduce one of our recent researches.
There are uncertain nuclear reaction rates that could affect the evolution of massive stars and supernova yields. We investigate [1] the effects of triple-α and 12C(α, γ)16O reaction rates on the production of supernova yields for massive stars using our stellar evolution code coupled with nuclear reaction networks and Lagrange hydrodynamic code. First, we examine the evolution of massive stars for different combinations of the two reaction rates. We find that 20 M8 (M8: solar mass) stars proceed significantly different evolutionary paths for some combinations (see Figure 1). Second, we perform calculations of supernova explosions in spherical symmetry and the nucleosynthesis. The results show that the conventional rate is appropriate for the triple-α reaction rate and rather higher value of the reaction rate within the uncertainties is favorable for a 12C(α, γ )16O rate.
Fig. 1 Evolutionary paths on the central density and central temperature plane for two combinations of the reaction rates [1].
2. Matter mixing in core-collapse supernova explosions
Observations of SN1987A have indicated some mixing during the supernova shock wave propagation to explain the observational features. However, the mechanism of the mixing is still a topic of debate. We perform [6] two dimensional hydrodynamic simulations of matter mixing in aspherical core-collapse supernova explosions (see Figure 2) using an adaptive mesh refinement hydrodynamic code coupled with a small nuclear reaction network. The simulations have been carried out with use of a super computer. Recently, we also test the possibility of large density perturbations in the progenitor star [7].
Fig. 2 Density (left) and 56Ni mass fraction (right) color maps just before the supernova shock reaches the surface of the star [6].

2. 3. 3D numerical modeling of supernova remnants
Supernova remnants (SNRs) are observed in wide range of wave lengths and, we can obtain clues of the mechanism of the supernova explosions. Additionally, it is considered that SNRs are natural accelerators of high energy comic rays up to 1015 eV. However, most theoretical modeling of SNRs is limited in spherical symmetry. We try to make three dimensional (3D) model by 3D hydro and magnetohydrodynamic (MHD) simulations with several physical processes, non-equilibrium ionization and so on [8].
Fig. 3 Volume rendered image of density distribution of a SNR [8].
4. Structure of compact objects
We have been investigated structures of compact objects which are composed of nuclear matter whose compositions are nuclear matter, hyperons, quarks. We examine cooling curves and X-ray bursts.
Fig. 4 Time evolution of the azimuthal component of magnetic field averaged in the azimuthal direction [9].
Fig. 5 Map of polarized intensity at 8GHz. Color shows the intensity, and black lines show magnetic vector [10].
5. Origin of galactic magnetic field and its observational visualization
Milky way is one of a spiral galaxy. From the radio observations, the typical magnetic field strength is a few μG. The origin and nature of magnetic fields, however, still remain various questions. Therefore, we carried out global 3D MHD simulations of dynamo activities in galactic gaseous disks [9]. Numerical results indicate the growth of azimuthal magnetic fields non-symmetric to the equatorial plane. As the magneto-rotational instability (MRI) grows, the mean strength of magnetic fields is amplified until the magnetic pressure becomes as large as 10% of the gas pressure. When the local plasma (pgas/pmag) becomes less than 5 near the disk surface, magnetic flux escapes from the disk by the Parker instability within one rotation period of the disk. Figure 4 shows the time evolution of the azimuthal field. The buoyant escape of coherent magnetic fields drives dynamo activities by generating disk magnetic fields with opposite polarity to satisfy the magnetic flux conservation. We also calculates the radio observables from numerical simulation data [10]. Therefore, we show that the magnetic vector observed at centimeter wavelengths traces the global magnetic field inside the disk, while the polarized intensity of the foreground halo is predominant at meter wavelengths.
References
[1] Kikuchi, Y. et al. 2015, PTEP, 2015, 063E01
[2] Ono, M. et al. 2012, PTP, 128, 741
[3] Saruwatari, M. et al. 2013, J. Astrophys., 2013,
[4] Hashimoto, M. et al. 2014, J. Astrophys., 2014,
[5] Ichimasa et al. 2014, Phys. Rev. D, 90,
[6] Ono, M. et al. 2013, ApJ., 773, 16
[7] Mao, J. et al. 2015, ApJ., 808, 164
[8] Ono et al. 2016, in prep.
[9] Machida, M. et al. 2013, ApJ., 764, 81
[10] Morita, Y. et al. submitted to PASJ