Division of Theoretical Astronomy, National Astronomical Observatory of Japan

Research Highlights

Polarization Structure of Magnetically Supported Molecular Filaments


Observations of thermal dust emissions with Herschel satellite have revealed that molecular clouds consist of many filaments (Menshchikov et al. 2010). That is, the molecular filaments are the building blocks of interstellar clouds. On the other hand, near IR interstellar polarization indicates the filaments are extending in the perpendicular direction to the interstellar magnetic field (Sugitani et al. 2011).
Magnetohydrostatic equilibrium solutions of isothermal clouds, in which the gravity is balanced with the Lorentz force, thermal pressure and the external pressure, are obtained with a self-consistent field method. We obtained an empirical relation between the maximum supported mass against the self-gravity, λmax(Tomisaka 2014). Here, we studied polarization structures of the thermal dust emissions expected for these filaments.
The polarization of the thermal dust emissions comes from dusts which are aligned to the interstellar magnetic field. In the figure, color and bars represent, respectively, the polarization degree and the direction of polarization (B-vector of the electromagnetic wave). We chose a line-of-sight of θ=80deg and φ=90 deg in the left panel. That is, a filament extending in the z-direction is assumed to be observed from the lateral direction nearly parallel to the large-scale magnetic field. The middle panel corresponds to a low-density filament (center-to-surface density ratio F = 10) and the right one is a high-density filament with F=300.
When we observe the filament from direction of the interstellar magnetic field, we expect extremely low polarization degree since dusts have no special alignment in the plane of sky (middle panel). On the contrary, high-density filaments indicate relatively large polarization degree whose directions are perpendicular to the filament (right panel). This may explain the reason why the perpendicular configuration is often found between the interstellar magnetic field and the filaments.
Polarization Structure of Filamentary Clouds, by Tomisaka, Kohji, 2015, ApJ, in press
[ADS] [arXiv]
Kohji Tomisaka (private website)

Great Progress towards the Origin of R-process

(Left) "Universality" of the elemental-abundance pattern as a function of atomic number Z between the solar-system and the metal-poor halo stars.
(Right) Illustration of r-process nucleosynthesis in core-collapse supernova explosion.
Credit: Akihiro Ikeshita / CG: Masayo Mikami (NAOJ Astronomy Information Center)

Using RIKEN’s Radioactive Isotope Beam Factory, an international research team succeeded in measuring beta-decay half-lives of 110 neutron-rich nuclei, 40 of which were measured for the first time in this measurement. Collaborating with S. Shibagaki and T. Kajino (University of Tokyo/National Astronomical Observatory of Japan), they carried out theoretical calculations of r-process nucleosynthesis with the newly measured half-lives. Their results expected from core-collapse supernova scenario are consistent with observed elemental abundances in the solar system and metal-poor stars.

Please visit following websites for more detail.
RIKEN press release
β-Decay Half-lives of 110 Neutron-Rich Nuclei across the N = 82 Shell Gap: Implications for the Mechanism and Universality of the Astrophysical r-process
G. Lorusso, et al., Phys. Rev. Lett. 114.192501
Shota Shibagaki and Toshitaka Kajino (personal website)

Loading relativistic velocity distributions in particle simulations

In particle-in-cell (PIC) simulations and Monte-Carlo simulations, particle velocities are initialized by using random variables. However, algorithms to generate relativistically-moving distributions are not well established. This article proposes rejection methods for full Lorentz transformation of arbitrary distribution functions.
Seiji Zenitani, Phys. Plasmas 22, 042116 (2015).
[Journal] [arXiv]
Seiji Zenitani (personal website)

The initial mass function of star clusters

Star clusters form in molecular clouds, and their mass distributes from several tens to a few tens of thousands solar masses. The observed mass function of star clusters is known to have a power-law slope of -2, but the origin of the power-law index is still unclear. In turbulent molecular clouds, stars and star clusters form along the filamentary structures. We developed a new scheme, in which the hydrodynamical and N-body simulations are divided, in order to simulate the formation of a bunch of star clusters. In our simulation a few tens of star clusters formed from an each molecular cloud, and we found that the cluster mass function that originates from an individual molecular cloud is written by a Schechter function with a power-law slope of -1.73 at 2 Myr and -1.67 at 10 Myr, which fits to observed cluster mass function of the Carina region. The superposition of mass functions have a power-law slope of around -2, which fits the observed mass function of star clusters in the Milky Way, M31 and M83.
Michiko Fujii and Simon Portegies Zwart, Monthly Notices of the Royal Astronomical Society, 449, 726-740 [ADS]
Michiko Fujii [personal webpage]


3rd DTA Symposium:
The Origins of Planetary Systems: from the Current View to New Horizons

2015 June 1 (Mon) - June 4 (Thu)
NAOJ Mitaka, Large seminar room
webpage: http://th.nao.ac.jp/meeting/dta2015a_planet/