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Theoretical Astrophysics

  • Masa-aki Hashimoto, Professor
  • Mami Machida, Assistant Professor
Theoretical Astrophysics group is a part of the department of physics, particle physics group of Kyushu University. We have been carrying astrophysics research since 1998. We conduct research in a wide range of topics including stellar evolution, supernova explosions, nucleo-synthesis, cosmology, accretion disks, astrophysical jets etc.... 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 your comments and participation.

1. Nucleosynthesis in stellar interiors, astrophysical phenomena

The origin of elements is one of important enigmas in the universe. Nucleosynthesis in stellar interiors and high energy astrophysical phenomena such as supernova explosions is the key to elucidate the origin. Our group investigate the nucleosynthesis in massive stars [1], supernova explosions [2, 3], X-ray bursts in an accreting neutron star [4], and the Big-bang [5]. Here, we introduce one of our recent works.

There are some uncertain nuclear reaction rates that could significantly 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 a stellar evolution code coupled with a nuclear reaction network and a 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 a combination (see Figure 1). Second, we perform calculations of supernova explosions in spherical symmetry and the nucleosynthesis. The results show that a conventional rate is adequate for a triple-α reaction rate and rather higher value of the reaction rate within the limitation of the experimental uncertainties is favorable for a 12C(α, γ)16O rate.

Figure 1. Evolutionary paths in 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 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 carried out with use of a super computer. Continued on the following page.

One of the observational features of SN 1987A is that 56Ni synthesized by the explosion is mixed into fast moving outer layers of the star. In order to clarify the key conditions for reproducing such high velocity of 56Ni, we examine the matter mixing in aspherical core-collapse supernovae. To see the effects of Rayleigh–Taylor (RT) instability, perturbations of a small amplitude are also tested. We find that both aspherical explosions with clumpy structures and perturbations of pre-supernova origins may be necessary to reproduce the observed high velocity of 56Ni. Recently, we also test the possibility of large density perturbations in the progenitor star [7].

Figure 2. Density (left) and 56Ni mass fraction (right) color maps just before the supernova shock reaches the surface of the star [6].

3. 3D numerical modeling of supernova remnants

Supernova remnants (SNRs) are observed in wide range of wave lengths and from the observations, 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 (CRs) up to 1015 eV. However, most theoretical modeling of SNRs is limited in spherical symmetry. We try to make a three dimensional (3D) model by 3D hydro and magnetohydrodynamic (MHD) simulations with several physical processes, non-equilibrium ionization, Coulomb interaction between ions end electrons, the feedback of accelerated CRs and so on [8].

Figure 3. Volume rendered image of density distribution of a 3D SNR model [8].

Origin of galactic magnetic field and its observational visualization

Spiral galaxy such as Milky way is one of a spiral galaxy. From the radio observations, the typical magnetic field strength is a few μG. The origin of and nature of magnetic fields, however, still has 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 β (=p gas/p mag) 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.

Figure 4. Time evolution of the azimuthal component of magnetic field averaged in the azimuthal direction [9].

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 (see figure 5).

Figure 5. Map of polarized intensity at 8GHz. Color shows the intensity, and black lines show magnetic vector [10].


[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, 506146
[4] Hashimoto, M. et al. 2014, J. Astrophys., 2014, 817986
[5] Ichimasa et al. 2014, Phys. Rev. D, 90, 023527
[6] Ono, M. et al. 2013, ApJ., 773, 16
[7] Mao, J. et al. 2015, ApJ., 808, 164 [8] Ono, M. et al. 2016, in prep.
[9] Machida, M. et al. 2013, ApJ., 764, 81
[10] Morita, Y. et al. submitted to PASJ