Ryoichi Nishi

Microlensing Optical Depth toward the Galactic Bulge from Microlensing Observations in Astrophysics Group Observations during 2000 with Difference Image Analysis

We analyze the data of the gravitational microlensing survey carried out by the Microlensing Observations in Astrophysics (MOA) group during 2000 toward the Galactic bulge (GB). Our observations are designed to detect efficient high-magnification events with faint source stars and short-timescale events, by increasing the sampling rate up to ~6 times per night and using Difference Image Analysis (DIA). We detect 28 microlensing candidates in 12 GB fields corresponding to 16 deg2. We use Monte Carlo simulations to estimate our microlensing event detection efficiency, where we construct the I-band extinction map of our GB fields in order to find dereddened magnitudes. We find a systematic bias and large uncertainty in the measured value of the timescale tE,out in our simulations. They are associated with blending and unresolved sources, and are allowed for in our measurements. We compute an optical depth τ = 2.59 × 10-6 toward the GB for events with timescales 0.3 10). These events are useful for studies of extrasolar planets.

THE MOLECULAR OUTFLOWS IN THE {rho} OPHIUCHI MAIN CLOUD: IMPLICATIONS FOR TURBULENCE GENERATION

We present the results of CO (J = 3 – 2) and CO (J = 1 – 0) mapping observations toward the active cluster-forming clump, L1688, in the ρ Ophiuchi molecular cloud. From the CO (J = 3 – 2) and CO (J = 1 – 0) data cubes, we identify five outflows, whose driving sources are VLA 1623, EL 32, LFAM 26, EL 29, and IRS 44. Among the identified outflows, the most luminous outflow is the one from the prototypical Class 0 source, VLA 1623. We also discover that the EL 32 outflow located in the Oph B2 region has very extended blueshifted and redshifted lobes with wide opening angles. This outflow is most massive and has the largest momentum among the identified outflows in the CO (J = 1 – 0) map. We estimate the total energy injection rate due to the molecular outflows identified by the present and previous studies to be about 0.2 L ☉, larger than or at least comparable to the turbulence dissipation rate [≈(0.03 – 0.1)L ☉]. Therefore, we conclude that the protostellar outflows are likely to play a significant role in replenishing the supersonic turbulence in this clump.

Primordial Molecular Emission in Population III Galaxies

We study formation of molecules in primordial prestellar clumps and evaluate the line luminosities to assess detectability by next-generation facilities. If the initial H_2 fraction is sufficiently high, HD becomes an important coolant in the clumps. The luminosity from such HD cooling clumps is lower than that from H_2 cooling ones because of lower temperature ( Li + H_2. LiH does not become an important coolant in any density range. The luminous emission lines from the prestellar cores include H_2 rovibrational lines: 1-0 Q(1), 1-0 O(3), 1-0 O(5), and pure rotational lines: 0-0 S(3), 0-0 S(4), 0-0 S(5). The next-generation facilities SPICA and JWST are able to detect H_2 emission in a large pre-galactic cloud that forms metal-free stars at a high rate of \sim 10^3 M_s/yr at redshift z<10. We also derive an analytical expression for the luminosity that reproduces the numerical results.

H2 Line Emission Associated with the Formation of the First Stars

Molecular hydrogen (H2) line radiation emitted in the formation events of first-generation stars is evaluated in a discussion of its detectability by future observational facilities. H2 luminosity evolution from the onset of prestellar collapse until the formation of a ∼ 100 Mprotostar is followed. Calculations are extended not only to the early phase of the runaway collapse, but also to the later phase of accretion, whose observational features have not been studied before. Contrary to the runaway collapse phase, where the pure-rotational lines are always dominant, in the accretion phase rovibrational line emission becomes prominent. The maximum luminosity is also attained in the accretion phase for strong emission lines. The peak intensity of the strongest rovibrational line reaches ∼ 10 −29 Wm −2 , corresponding to the flux density of 10 −5 µJy, for a source at the typical redshift of first-generation star formation, 1 + z = 20. Although the redshifted rovibrational H2 emission from such an epoch falls in the wavelength range of the next-generation infrared satellite, Space Infrared Telescope for Cosmology and Astrophysics, for exceeding the detection threshold, 10 7 such protostars are required to reach the maximum

Star Formation in the Primordial Gas : Overview

Recently, the study for the star formation process in the primordial gas progresses very much. We show the outline of the thermal and dynamical evolution of primordial gas clouds in the universe after decoupling. The thermal process of the primordial gas is much different from the thermal process of the present-day interstellar gas. Especially, hydrogen molecule is the most important coolant at the low temperature gas (∼ 10 3 K). Considering the formation of H 2 and the cooling by H 2 line emission, we can study the thermal evolution of the primordial gas and investigate the star formation process. We propose the following scenario to form primordial stars efficiently. First, by pancake collapse of the overdensity regions in the expanding universe. disk like clouds form A disk like cloud has a tendency to fragment into filamentary clouds. Filamentary clouds are formed and they collapse dynamically. After collapse is almost halted by pressure force, the filament fragments into cloud cores. Finally, a primordial star forms in a cloud core.

Scientific goals of Nano-JASMINE

Nano-JASMINE is an ultrasmall Japanese satellite (with a weight of 35 kg), designed to carry out an astrometric mission. The target accuracy is 3 milliarcseconds (mas) for stars brighter than magnitude 7.5 at zw-band wavelengths of 0.6–1.0 μm. The observational strategy is the same as that of Gaia and Hipparcos. The time span of 20 years since the Hipparcos mission will enable us to update the proper motion data obtained at that time. With the help of these updated measurements, we expect that some stars will be resolved into multiple stars. In addition, taking advantage of the small primary mirror (with a diameter of 5 cm), we can measure bright stars which cannot be observed with Gaia because of saturation limits. The core data reduction for the Nano-JASMINE mission will use Gaia’s Astrometric Global Iterative Solution (agis). A collaboration between the Gaia agis and Nano-JASMINE teams was initiated in 2007.

On the fragment mass scale of primordial gas clouds: Effect of dark matter component

We discuss the evolution and the fragmentation of cylindrical gas clouds with primordial composition, using the virial equation. Comparing the timescale of collapse with that of fragmentation, we obtain the fragment mass scale for the various initial density. We estimate the minimum mass of a fragment that is formed from the primordial gas cloud. The mass scale is essentially determined by the Chandrasekhar mass. The effect of dark matter is also investigated. If the filamentary cloud consists not only of ordinary gas but also some dark matter, the fragmentation of the cylindrical cloud occurs at the lower density than the pure gas case. As a result, the fragment mass scale hardly becomes Chandrasekhar mass scale. In this case, massive star formation is strongly expected.