Division of Theoretical Astronomy, National Astronomical Observatory of Japan

Research Highlights

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/


Research Highlights

Quantum theoretic approach to in-medium effects on the neutrino scattering

Neutrino scattering provides a unique signal for understanding the weak interaction processes in an extremely high-temperature and high-density matter inside the proto-neutron stars born in core-collapse supernova explosions. COSNAP* international collaboration group conducted by Prof. M.-K. Cheoun (Soongsil Univ./NAOJ) and Prof. T. Kajino (NAOJ/Univ. of Tokyo) carried out a quantum mechanical calculation of the neutrino scattering cross sections on nucleons and nuclei at E < 100 GeV in the quark meson coupling (QMC) model and compared the theoretical result with neutrino data detected at high-energy particle accelerators MiniBooNE and NOMAD. It is found that below 1 GeV at energies relevant for applications to the neutrino processes in the supernova matter, one needs to include nucleonic form factor and nuclear structure effects as well as the Fermi motion of nucleons.
The paper is selected as a highlight of Journal of Physics G and has been taken in labtalk-article (http://iopscience.iop.org/0954-3899/labtalk-article/59987) entitled "When and how do we include the in-medium effects in neutrino scattering off target nuclei?". In this article both laboratories, Prof. Cheoun's Labo and Prof. Kajino's COSNAP* Labo, are introduced as active theory groups that continuously produce high-quality science papers.
This shows a schematic image that the high-energy neutrino beams from accelerators like Fermilab MiniBooNE are unique experimental tools to study the neutrino processes inside the proto-neutron stars born in core-collapse supernova explosions. The inset Figure shows that the degrees of freedom of strangeness make the important effects on the neutrino scattering cross sections. (Photograph are taken from http://www-boone.fnal.gov/virtual_tour/.)

M.-K. Cheoun, K.-S. Kim, H.-C. Kim, W.-Y. So, T. Maruyama and T. Kajino,
J. Phys. G42 (2015) 045102.
*COSNAP: COSmology and Nuclear AstroPhysics theory group
T. Kajino (http://th.nao.ac.jp/MEMBER/kajino/)

Shocking structure in a supersonic reconnection jet

   Shock diamonds appear in a jet, when the jet supersonically emanates from the nozzle. They are one of the most beautiful examples of high-speed fluid dynamics. A cyclic structure within a cosmic jet is also attributed to shock-diamonds.
   Magnetic reconnection releases a fast outflow jet from the reconnection site. When the jet speed exceeds that of the sound, shock-diamonds similarly appear. Our high-resolution magnetohydrodynamic (MHD) simulation reveals multiple shock-diamonds in a reconnection system. In the figure, the triangles indicate the plasma vertical motion associated with shock-diamonds. We recognize under-expanded shock-diamonds inside the reconnection jet (the left circle) and over-expanded shock-diamonds ahead of the magnetic island (the right circle).
   High-speed fluid dynamics (also known as compressible fluid dynamics) will be important in our future research, because astrophysical MHD systems often involve violent supersonic flows.
Seiji Zenitani, Phys. Plasmas, 22, 032114
[Journal] [arXiv]
Seiji Zenitani (personal website)