71 research outputs found
The Microlensing and Halo Models of the Galaxy
We investigated the dependence of the optical depth 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
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 of IMF of MACHO consistent with Tyson's CCD survey as
well as Bahcall \etal's observation by {\em HST}. It is found that is
greater than 5.Comment: 17 pages (including 8 figures), uuencoded gz-compressed postscrip
Evolution of Primordial Protostellar Clouds --- Quasi-Static Analysis ---
The contraction processes of metal-free molecular clouds of starlike mass (or
cloud cores) are investigated. We calculate radiative transfer of the H_2 lines
and examine quasi-static contraction with radiative cooling. Comparing two
time-scales, the free-fall time t_ff and the time-scale of quasi-static
contraction t_qsc (nearly equal to t_cool, the cooling time) of these cores, we
find that the ratio of the two time-scales t_ff/t_qsc, i.e., the efficiency of
cooling, becomes larger with contraction even under the existence of cold and
opaque envelopes. In particular, for fragments of primordial filamentary
clouds, for which t_ff is nearly equal to t_qsc at the fragmentation epoch,
they collapse dynamically in the free-fall time-scale. This efficiency of
cooling is unique to line cooling.Comment: 13 pages, 8 Postscript figures, uses ptpte
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.Comment: 15pages, tar gzipped, 5 figures, PTP TeX (PTP style files are in
http://www2.yukawa.kyoto-u.ac.jp/~ptpwww/ptptex-eng.ptp.html). Progress of
Theoretical Physics, in pres
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 (\tau^{LMC} / 3 \times 10^{-7}) (100 days / ) EAGLE events/y are
expected to result from all the dim stars in LMC. Here \tau^{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 \tau^{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 (\tau^{SMC} / 3
\times 10^{-7} (100 days / ) events/y).Comment: 8 pages, Prog. Theor. Phys. in pres
H_2 Line Emission Associated with the Formation of the First Stars
Molecular hydrogen line radiation emitted in formation events of
first-generation stars are evaluated in a discussion of its detectability by
future observational facilities. H_2 luminosity evolution from the onset of
prestellar collapse until the formation of a \sim 100 M_{\odot} 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 \sim 10^{-29} (W/m^2), corresponding to the flux
density of 10^{-5} (\mu Jy), for a source at the typical redshift of
first-generation star formation, 1+z=20. Although the redshifted rovibrational
H_2 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 luminosity simultaneously in a pregalactic cloud.
It is improbable that this condition is satisfied in a realistic scenario of
early structure formation.Comment: 16 pages, 5 figures, accepted by PAS
Formation and Disruption of Cosmological Low Mass Objects
We investigate the evolution of cosmological low mass (low virial
temperature) objects and the formation of the first luminous objects. First,
the `cooling diagram' for low mass objects is shown. We assess the cooling rate
taking into account the contribution of H_2, which is not in chemical
equilibrium generally, with a simple argument of time scales. The reaction
rates and the cooling rate of H_2 are taken from the recent results by Galli &
Palla (1998). Using this cooling diagram, we also estimate the formation
condition of luminous objects taking into account the supernova (SN) disruption
of virialized clouds. We find that the mass of the first luminous object is
several times 10^7 solar mass, because smaller objects may be disrupted by the
SNe before they become luminous. Metal pollution of low mass (Ly-alpha) clouds
also discussed. The resultant metallicity of the clouds is about 1/1000 of the
solar metallicity.Comment: 11 pages, 2 figures, To appear in ApJ
Back Reaction to the Spectrum of Magnetic Field in the Kinetic Dynamo Theory --- Modified Kulsrud and Anderson Equation ---
We take account of the lowest order back reaction on the fluid and modify the
Kulsrud and Anderson equation
obtained in the kinetic dynamo theory, where is the energy density
of the magnetic field. Furthermore, we apply our present result to some
astrophysical stages where the magnetic field is expected to be amplified by
the dynamo mechanism.Comment: 9 pages, LaTex, to appear in Prog. Theor. Phy
The Minimum Total Mass of MACHOs and Halo Models of the Galaxy
If the density distribution \rho (r) of MACHOs is spherically symmetric with
respect to the Galactic center, it is shown that the minimal total mass
M_{min}^{{ MACHO}} of the MACHOs is 1.7\times 10^{10}\sol \tau_{-6.7}^{{ LMC}}
where \tau_{-6.7}^{{ LMC}} is the optical depth (\tau^{{ LMC}}) toward the
Large Magellanic Cloud (LMC) in the unit of 2\times 10^{-7}. If \rho (r) is a
decreasing function of r, it is proved that M_{min}^{{ MACHO}} is 5.6\times
10^{10}\sol \tau_{-6.7}^{{ LMC}}. Several spherical and axially symmetric halo
models of the Galaxy with a few free parameters are also considered. It is
found that M_{min}^{{ MACHO}} ranges from 5.6\times 10^{10}\sol \tau_{-6.7}^{{
LMC}} to \sim 3 \times 10^{11}\sol \tau_{-6.7}^{{ LMC}}. For general case, the
minimal column density \Sigma_{min}^{{ MACHO}} of MACHOs is obtained as
\Sigma_{min}^{{ MACHO}} =25 \sol { pc}^{-2}\tau_{-6.7}^{{ LMC}}. If the clump
of MACHOs exist only halfway between LMC and the sun, M_{min}^{{ MACHO}} is
1.5\times 10^9\sol. This shows that the total mass of MACHOs is smaller than 5
\times 10^{10}\sol , i.e. \sim 10\% of the mass of the halo inside LMC, either
if the density distribution of MACHOs is unusual or \tau^{{ LMC}}\ll 2\times
10^{-7}.Comment: 5 pages, 5 Encapsulated PostScript figures, Latex, Accepted for
publication in Apj. Letter
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