282 research outputs found
The Formation and Destruction of Molecular Clouds and Galactic Star Formation
We describe an overall picture of galactic-scale star formation. Recent
high-resolution magneto-hydrodynamical simulations of two-fluid dynamics with
cooling/heating and thermal conduction have shown that the formation of
molecular clouds requires multiple episodes of supersonic compression. This
finding enables us to create a scenario in which molecular clouds form in
interacting shells or bubbles on a galactic scale. First we estimate the
ensemble-averaged growth rate of molecular clouds over a timescale larger than
a million years. Next we perform radiation hydrodynamics simulations to
evaluate the destruction rate of magnetized molecular clouds by the stellar FUV
radiation. We also investigate the resultant star formation efficiency within a
cloud which amounts to a low value (a few percent) if we adopt the power-law
exponent -2.5 for the mass distribution of stars in the cloud. We finally
describe the time evolution of the mass function of molecular clouds over a
long timescale (>1Myr) and discuss the steady state exponent of the power-law
slope in various environments.Comment: 7 pages, 3 figures. Accepted for publication in Astronomy and
Astrophysic
A Constraint on the Amount of Hydrogen from the CO Chemistry in Debris Disks
The faint CO gases in debris disks are easily dissolved into C by UV
irradiation, while CO can be reformed via reactions with hydrogen. The
abundance ratio of C/CO could thus be a probe of the amount of hydrogen in the
debris disks. We conduct radiative transfer calculations with chemical
reactions for debris disks. For a typical dust-to-gas mass ratio of debris
disks, CO formation proceeds without the involvement of H because a small
amount of dust grains makes H formation inefficient. We find that the CO to
C number density ratio depends on a combination of
, where is the hydrogen nucleus
number density, is the metallicity, and is the FUV flux normalized
by the Habing flux. Using an analytic formula for the CO number density, we
give constraints on the amount of hydrogen and metallicity for debris disks. CO
formation is accelerated by excited H either when the dust-to-gas mass
ratio is increased or the energy barrier of chemisorption of hydrogen on the
dust surface is decreased. This acceleration of CO formation occurs only when
the shielding effects of CO are insignificant. In shielded regions, the CO
fractions are almost independent of the parameters of dust grains.Comment: 29pages, 13figures, accepted for Ap
Metallicity Dependence of Molecular Cloud Hierarchical Structure at Early Evolutionary Stages
The formation of molecular clouds out of HI gas is the first step toward star
formation. Its metallicity dependence plays a key role to determine star
formation through the cosmic history. Previous theoretical studies with
detailed chemical networks calculate thermal equilibrium states and/or thermal
evolution under one-zone collapsing background. The molecular cloud formation
in reality, however, involves supersonic flows, and thus resolving the cloud
internal turbulence/density structure in three dimension is still essential. We
here perform magnetohydrodynamics simulations of 20 km s^-1 converging flows of
Warm Neutral Medium (WNM) with 1 uG mean magnetic field in the metallicity
range from the Solar (1.0 Zsun) to 0.2 Zsun environment. The Cold Neutral
Medium (CNM) clumps form faster with higher metallicity due to more efficient
cooling. Meanwhile, their mass functions commonly follow dn/dm proportional to
m^-1.7 at three cooling times regardless of the metallicity. Their total
turbulence power also commonly shows the Kolmogorov spectrum with its 80
percent in the solenoidal mode, while the CNM volume alone indicates the
transition towards the Larson's law. These similarities measured at the same
time in the unit of the cooling time suggest that the molecular cloud formation
directly from the WNM alone requires a longer physical time in a lower
metallicity environment in the 1.0-0.2 Zsun range. To explain the rapid
formation of molecular clouds and subsequent massive star formation possibly
within 10 Myr as observed in the Large/Small Magellanic Clouds (LMC/SMC), the
HI gas already contains CNM volume instead of pure WNM.Comment: 23 pages, 11 figures. Accepted for publication in Ap
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