125 research outputs found
Do the environmental conditions affect the dust-induced fragmentation in low-metallicity clouds ?: Effect of pre-ionization and far-ultraviolet/cosmic-ray fields
We study effects of the fully ionized initial state, or pre-ionization, on
the subsequent thermal evolution of low-metallicity clouds under various
intensities of the external far-ultraviolet(FUV) and cosmic-ray(CR) fields. The
pre-ionization significantly affects the thermal and dynamical evolution of
metal-free clouds without FUV/CRs by way of efficient HD formation. On the
other hand, the pre-ionization effect on the thermal evolution is limited in
very low-density regime for more metal-enriched clouds ([Z/H] >~ -4) or those
under modest FUV (>10^{-3}) or CR field (>0.1 of the present-day Galactic disk
levels). In any case, for >10^8 cm^{-3}, neither the initial ionization state
nor the irradiating FUV strength affect the thermal evolution. The dust cooling
is an important mechanism for making sub-solar mass fragments in
low-metallicity gas. Since this fragmentation occurs at the temperature minimum
by the dust cooling at >10^{10} cm^{-3}, this process is not vulnerable either
to initial ionization state or external radiation.Comment: 11 pages, 5 figures, PASJ accepte
The critical radiation intensity for direct collapse black hole formation: dependence on the radiation spectral shape
It has been proposed that supermassive black holes (SMBHs) are originated
from direct-collapse black holes (DCBHs) that are formed at z gtrsim 10 in the
primordial gas in the case that H2 cooling is suppressed by strong external
radiation. In this work, we study the critical specific intensity J^crit
required for DCBH formation for various radiation spectral shapes by a series
of one-zone calculations of a collapsing primordial- gas cloud. We calculate
the critical specific intensity at the Lyman-Werner (LW) bands J^crit_LW,21 (in
units of 10^-21 erg s^-1 Hz^-1 sr^-1 cm^-2) for realistic spectra of metal-poor
galaxies. We find J^crit is not sensitive to the age or metallicity for the
constant star formation galaxies with J^crit_LW,21 = 1300-1400, while J^crit
decreases as galaxies become older or more metal-enriched for the instantaneous
starburst galaxies. However, such dependence for the instantaneous starburst
galaxies is weak for the young or extremely metal-poor galaxies: J^crit_LW,21 =
1000-1400 for the young galaxies and J^crit_LW,21 approx 1400 for the extremely
metal-poor galaxies. The typical value of J^crit for the realistic spectra is
higher than those expected in the literature, which affects the estimated DCBH
number density n_DCBH. By extrapolating the result of Dijkstra, Ferrara and
Mesinger, we obtain n_DCBH sim 10^-10 cMpc^-3 at z = 10, although there is
still large uncertainty in this estimation.Comment: 11 pages, 6 figures, submitted to MNRA
Condition for low-mass star formation in shock-compressed metal-poor clouds
Shocks may have been prevalent in the early Universe, associated with
virialization and supernova explosions, etc. Here, we study thermal evolution
and fragmentation of shock-compressed clouds, by using a one-zone model with
detailed thermal and chemical processes. We explore a large range of initial
density (1-1e5 /cm^3), metallicity (0-1e-2 Z_sun), UV strength (0-500 times
Galactic value), and cosmic microwave background temperature (10 and 30 K).
Shock-compressed clouds contract isobarically via atomic and molecular line
cooling, until self-gravitating clumps are formed by fragmentation. If the
metals are only in the gas-phase, the clump mass is higher than ~ 3 M_sun in
any conditions we studied. Although in some cases with a metallicity higher
than ~ 1e-3 Z_sun, re-fragmentation of a clump is caused by metal-line cooling,
this fragment mass is higher than ~ 30 M_sun. On the other hand, if about half
the mass of metals is condensed in dust grains, as in the Galactic interstellar
medium, dust cooling triggers re-fragmentation of a clump into sub-solar mass
pieces, for metallicities higher than ~ 1e-5 Z_sun. Therefore, the presence of
dust is essential in low-mass (< M_sun) star formation from a shock-compressed
cloud.Comment: 15 pages, 8 figures, accepted for publication in MNRA
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