arXiv subjects

Ryoichi Nishi

Publications and source records attributed to Ryoichi Nishi.

23 records · Page 2Linked to original sources

arXiv1998

The Thermal Evolution of the Postshock Layer in Pregalactic Clouds

We re-examine the thermal evolution of the postshock layer in primordial gas clouds. Comparing the time scales, we find that the evolutionary paths of postshock regions in primordial gas clouds can be basically understood in terms of the diagram drawn in the ionization degree vs temperature plane. The results obtained from the diagram are independent of the density in the case that we do not include photodissociation and photoionization. We also argue that the diagram is not only relevant to the case of the steady postshock flow, but also to the isochorically cooling gas.

astro-ph↗

arXiv1998

Expected EAGLE event rate towards the Magellanic Clouds

We propose to search for MACHOs by observing EAGLE (Extremely Amplified Gravitational LEnsing) events of a majority of dim stars. This search is independent of the usual one. For the detection limit of EAGLE (about 20 mag), about 100 f (τ^{LMC} / 3 \times 10^{-7}) (100 days / ) EAGLE events/y are expected to result from all the dim stars in LMC. Here τ^{LMC} and are the optical depth and the average duration of microlensing events, respectively, while f (0 < f < 1) is a parameter depending on the unknown stellar luminosity function. The observed mean duration of EAGLE events also depends on the luminosity function and is (0.01 - 0.4) times the usual duration of microlensing events, which corresponds to (1 - 30) days. The follow-up observation using larger telescopes may enable us to determine the impact parameter and the true duration of the event. If f is determined by another independent method, we can also determine τ^{LMC}. Even if f is undetermined, the detection of EAGLE events strongly suggest that MACHOs are not due to variable source stars, since EAGLE events are due to the dim main-sequence stars. Although for the SMC, the event rate is smaller by a factor of about 7, it is still a substantial number (about 13 f (τ^{SMC} / 3 \times 10^{-7} (100 days / ) events/y).

astro-ph↗

arXiv1996

Fragmentation of the Primordial Gas Clouds and the Lower Limit on the Mass of the First Stars

We discuss the fragmentation of primordial gas clouds in the universe after decoupling. Comparing the time scale of collapse with that of fragmentation, we obtain the typical mass of a fragment both numerically and analytically. It is shown that the estimated mass gives the minimum mass of a fragment which is formed from the primordial gas cloud and is essentially determined by the Chandrasekhar mass.

astro-ph↗

arXiv1996

The fragment mass scale of the primordial gas clouds I

We investigated the thermal evolution of non-spherical primordial gas clouds of mass $ M=10^6M_\odot $. We studied two cases: 1) prolate and oblate clouds without angular momentum, 2) initially spherical, oblate and prolate clouds with angular momentum. In the spherical case, the magnitude of the angular momentum is the key quantity which determines the fragment mass. The fragment mass is found to be $M \gsim 100 M_\odot$ for $10^{-2} \lsim λ\lsim 1$, where $λ$ is the cosmological spin parameter. For the oblate shape of initial gas clouds, the angular momentum is almost always not important, In the case of prolate clouds with some angular momentum, collapse proceeds in the same way as spherical case, and the bounce occurs to form a disk due to the angular momentum. In any case, the estimated ``fragment mass'', must be above $100 M_\odot$ for the primordial gas clouds with typical angular momentum and typical oblateness or prolateness.

astro-ph↗

arXiv1995

The Microlensing and Halo Models of the Galaxy

We investigated the dependence of the optical depth $τ$ of the microlensing events on model parameters of the Galactic halo. We only consider Galactic mass models in which the rotation curve inside the Sun is compatible with the observation and LMC is bound to the Galaxy. It is found that $τ$ varies up to a factor 2.5 from the standard spherical and flat rotation halo model. This implies that only the most centrally concentrated halo model can be consistent with the observation if the halo consists of only MACHOs. We also calculate the power $x$ of IMF of MACHO consistent with Tyson's CCD survey as well as Bahcall \etal's observation by {\em HST}. It is found that $x$ is greater than 5.

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