Division of Theoretical Astronomy, National Astronomical Observatory of Japan

Research Highlights

New constraints on primordial magnetic fields from the CMB

Dai G. Yamazaki (Division of Theoretical Astronomy, National Astronomical Observatory of Japan) investigates the effects of the background PMF on the CMB. The sound speed of the tightly coupled photon-baryon fluid is increased by the background PMF. The increased sound speed causes the odd peaks of the CMB temperature fluctuations to be suppressed, and the CMB peak positions to be shifted to larger scale. The background PMF causes the stronger decaying potential, and increases the amplitude of the CMB. These two effects of the background PMF on a smaller scale cancel out, and the overall effects of the background PMF are suppression of the CMB around the first peak and shifting peaks to large scale. We also discuss obtaining information about the PMF generation mechanisms, and examine the non-linear evolution of the PMF by the constraint on the maximum scale for the PMF distributions. Finally, he discusses degeneracies between the PMF parameters and the standard cosmological parameters.

*This work was accepted for publication as a Regular Article in Physical Review D on March 31 2014.
CMB with the background primordial magnetic field
Dai G. Yamazaki, Accepted 31 March 2014
Please visit here for more detail. Dai G. Yamazaki (private website)

Some remarks on the diffusion regions in magnetic reconnection

   The so-called "diffusion region" near the magnetic reconnection point is very important in reconnection physics. However, the structure of the diffusion regions is very puzzling in a kinetic plasma.
   In this article, we study the structure of the diffusion region in kinetic reconnection, by reconsidering the notion of magnetic diffusion. We interpret that the the magnetic diffusion is a relaxation process to the frozen-in state, and then we propose the field-aligned component of the frozen-in condition to evaluate a diffusion-like process (See Figure). By using a Vlasov simulation, we confirm diffusion signatures near the reconnection site.
   This research extends the notion of the magnetic diffusion to a kinetic plasma. It also provides a hint to another important problem --- the relevance between the magnetic diffusion and the disconnection of magnetic field lines, i.e., magnetic reconnection.
Seiji Zenitani and Takayuki Umeda, Phys. Plasmas, 21, 034503
[Journal] [arXiv]
Seiji Zenitani (personal website)

Structure of Magnetized Molecular Filaments


Observations of thermal dust emissions with Herschel satellite have revealed that molecular clouds consist of many filaments. 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.
In this article, equilibrium solutions of isothermal clouds are obtained with a self-consistent field method. The figure shows a typical solution of cross section of these filaments. Dashed and closed solid curves represent, respectively, magnetic field lines and isodensity contour lines.
From calculated 118 models, we obtained an empirical formula expressing the maximum supported mass against the self-gravity (critical mass). It is shown that the critical mass is proportional to the magnetic flux threading the filament.
The importance of this article is that structure and the maximum mass of the building block of molecular clouds are first determined theoretically.
Magnetohydrostatic Equilibrium Structure and Mass of Filamentary Isothermal Cloud Threaded by Lateral Magnetic Field, by Tomisaka, Kohji, 2014, ApJ, 785, 24 (12pp)
[ADS] [arXiv]
Kohji Tomisaka (personal website)

The moment of core collapse in star clusters with a mass function

Core-collapse time is an important timescale to understand the dynamical evolution of star clusters. While the evolution of single-component models is theoretically understood well, more realistic models with stellar mass functions is not yet. We performed a series of N-body simulations of star clusters with mass functions, and we numerically and analytically showed that the dynamical evolution of star clusters with mass functions is driven by the most massive stars and as a result the core-collapse time is anti-correlated with the mass of the most massive stars in the cluster. Clusters with mass functions effectively behave as small-N systems composed of massive stars. When the mass of the most massive star in a cluster is comparable to the core mass, the cluster does not show significant density increase. This type of evolution would be common for young clusters harboring massive stars.

Movie: Evolution of a star cluster with a mass function
Red: 100 times more massive than the mean mass, orange: 10-100 times, yellow: 2-10 times, green: 1-2 times, blue less massive than the mean mass. While massive stars sink to the cluster center due to the dynamical friction, light stars move to the outskirts. A binary from at T=10, and then it kicks the surrounding stars out of the cluster.
Michiko Fujii and Simon Portegies Zwart, Monthly Notices of the Royal Astronomical Society, 2013, 439, 1003 [ADS]
Michiko Fujii [personal webpage]