記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Oxford University Press  

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 M⊙ protostar 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 W m-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, 107 such protostars are required to reach the maximum luminosity simultaneously in a pregalactic cloud. It is improbable that this condition is satisfied in a realistic scenario of early structure formation.

DOI： 10.1093/pasj/56.3.487

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記述言語：英語   掲載種別：研究論文（学術雑誌）  

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Progress of Theoretical Physics  

We investigate the star formation regulation by UV radiation and supernova explosion.The self-regulation via the photodissociation of hydrogen molecule is very effective for low mass clouds. One O5 star can photodissociate H_2 in the whole of the star formation hostcloud. Thus, the metallicity is lower, subsequent star formation cannot occur because of the lack of the coolant. The critical metallicity is around 10^<-2～3> of the solar metallicity.The regulation via supernova explosion is also important for the low mass cloud, since the gravitational binding energy is lower than the typical supernova explosion energy. The survival condition is estimated using simple model. Finally, the condition to evolve into luminous objects is discussed, qualitatively.

DOI： 10.1143/PTPS.147.181

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その他リンク： https://projects.repo.nii.ac.jp/?action=repository_uri&item_id=240452

記述言語：英語   掲載種別：研究論文（学術雑誌）  

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：UNIV CHICAGO PRESS  

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 similar to6 times per night and using Difference Image Analysis (DIA). We detect 28 microlensing candidates in 12 GB fields corresponding to 16 deg(2). 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 t(E,out) in our simulations. They are associated with blending and unresolved sources, and are allowed for in our measurements. We compute an optical depth tau = 2.59(-064)(+0.84) x 10(-6) toward the GB for events with timescales 0.3 &lt; t(E) &lt; 200 days. We consider disk- disk lensing, and obtain an optical depth tau(bulge) = 3.36(-0.81)(+1/11) x 10(-6)[0.77/(1 - f(disk))] for the bulge component assuming a 23% stellar contribution from disk stars. These observed optical depths are consistent with previous measurements by the MACHO and OGLE groups, and still higher than those predicted by existing Galactic models. We present the timescale distribution of the observed events, and find there are no significant short events of a few days, in spite of our high detection efficiency for short-timescale events down to t(E) similar to 0.3 days. We find that half of all our detected events have high magnification (&gt;10). These events are useful for studies of extrasolar planets.

DOI： 10.1086/375212

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Institute of Physics Publishing  

We investigate the detailed processes at work in the drift of magnetic fields in molecular clouds. To the frictional force, whereby the magnetic force is transmitted to neutral molecules, ions contribute more than half only at cloud densities nH ≲ 104 cm-3, and charged grains contribute more than about 90% at nH gsim
10 6 cm-3. Thus, grains play a decisive role in the process of magnetic flux loss. Approximating the flux loss time tB by a power law tB ∝ B-γ, where B is the mean field strength in the cloud, we find γ ≈ 2, characteristic of ambipolar diffusion, only at nH ≲ 107 cm-3, at which ions and the smallest grains are pretty well frozen to the magnetic fields. At nH &gt
107 cm-3, γ decreases steeply with nH, and finally at nH ≈ ndec ≈ a few × 1011 cm-3, at which the magnetic fields effectively decouple from the gas, γ ≫ 1 is attained, reminiscent of Ohmic dissipation, although flux loss occurs about 10 times faster than by pure Ohmic dissipation. Because even ions are not very well frozen at nH &gt
107 cm-3, ions and grains drift slower than the magnetic fields. This insufficient freezing makes t B more and more insensitive to B as nH increases. Ohmic dissipation is dominant only at nH ≳ 1 × 1012 cm-3. While ions and electrons drift in the direction of the magnetic force at all densities, grains of opposite charges drift in opposite directions at high densities, at which grains are major contributors to the frictional force. Although magnetic flux loss occurs significantly faster than by Ohmic dissipation even at very high densities, such as nH ≈ ndec, the process going on at high densities is quite different from ambipolar diffusion, in which particles of opposite charges are supposed to drift as one unit.

DOI： 10.1086/340587

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