214 research outputs found
Massive core/star formation triggered by cloud-cloud collision: Effect of magnetic field
We study effect of magnetic field on massive dense core formation in
colliding unequal molecular clouds by performing magnetohydrodynamic
simulations with sub-parsec resolution (0.015 pc) that can resolve the
molecular cores. Initial clouds with the typical gas density of the molecular
clouds are immersed in various uniform magnetic fields. The turbulent magnetic
fields in the clouds consistent with the observation by Crutcher et al. (2010)
are generated by the internal turbulent gas motion before the collision, if the
uniform magnetic field strength is 4.0 G. The collision speed of 10 km
s is adopted, which is much larger than the sound speeds and the
Alfv\'{e}n speeds of the clouds. We identify gas clumps with gas densities
greater than 5 10 g cm as the dense cores and trace
them throughout the simulations to investigate their mass evolution and
gravitational boundness. We show that a greater number of massive,
gravitationally bound cores are formed in the strong magnetic field (4.0
G) models than the weak magnetic field (0.1 G) models. This is partly
because the strong magnetic field suppresses the spatial shifts of the shocked
layer that should be caused by the nonlinear thin shell instability. The
spatial shifts promote formation of low-mass dense cores in the weak magnetic
field models. The strong magnetic fields also support low-mass dense cores
against gravitational collapse. We show that the numbers of massive,
gravitationally bound cores formed in the strong magnetic field models are much
larger than the isolated, non-colliding cloud models, which are simulated for
comparison. We discuss the implications of our numerical results on massive
star formation.Comment: Published in PASJ (for special issue on cloud-cloud collision). This
arXiv version is similar to the published paper (
https://academic.oup.com/pasj/article/doi/10.1093/pasj/psaa059/5867032
Mass Supply to Galactic Center due to Nested Bars in the Galaxy
We investigate rapid mass supply process by nested bars in the Galaxy by
numerical simulation. We simulate gas flow in the whole galaxy disk with nested
bars, which are the outer bar and the inner bar, especially with highly spatial
resolution in the galactic central region. We assume two cases of inner bar
size which are a smaller one and a larger one than the radius of the 200 pc gas
ring which is corresponds to the Central Molecular Zone. From our numerical
results, in the large size bar cases, the inner bars with large elongation
induce sufficient mass inflow and destroy the 200 pc gas ring. On the other
hand, in the small size bar cases, the inner bars with large elongation induce
large mass inflow and do not destroy the 200 pc gas ring. This mass inflow is
caused by straight shocks excited by the inner bar. In this case, nuclear gas
disks of ~ 15 pc radius are formed. The nuclear gas disks are
self-gravitationally unstable and we expect formation of compact star clusters
under strong tidal force in the nuclear gas disks. We discuss evolution of the
nuclear gas disk.Comment: 36 pages, 11 figures. Submitted to Ap
Effects of a Supermassive Black Hole Binary on a Nuclear Gas Disk
We study influence of a galactic central supermassive black hole (SMBH)
binary on gas dynamics and star formation activity in a nuclear gas disk by
making three-dimensional Tree+SPH simulations. Due to orbital motions of SMBHs,
there are various resonances between gas motion and the SMBH binary motion. We
have shown that these resonances create some characteristic structures of gas
in the nuclear gas disk, for examples, gas elongated or filament structures,
formation of gaseous spiral arms, and small gas disks around SMBHs. In these
gaseous dense regions, active star formations are induced. As the result, many
star burst regions are formed in the nuclear region.Comment: 19 pages, 11 figures, accepted for publication in Ap
Tidal disruption of dark matter halos around proto-globular clusters
Tidal disruption of dark matter halos around proto-globular clusters in a
halo of a small galaxy is studied in the context of the hierarchical clustering
scenario by using semi-cosmological N-body/SPH simulations assuming the
standard cold dark matter model (). Our analysis on formation and
evolution of the galaxy and its substructures archives until . In such
a high-redshift universe, the Einstein-de Sitter universe is still a good
approximation for a recently favored -dominated universe, and then our
results does not depend on the choice of cosmology. In order to resolve small
gravitationally-bound clumps around galaxies and consider radiative cooling
below , we adopt a fine mass resolution (m_{\rm SPH} = 1.12 \times
10^3 \Msun). Because of the cooling, each clump immediately forms a
`core-halo' structure which consists of a baryonic core and a dark matter halo.
The tidal force from the host galaxy mainly strips the dark matter halo from
clumps and, as a result, theses clumps get dominated by baryons. Once a clump
is captured by the host halo, its mass drastically decreases each pericenter
passage. At , more than half of the clumps become baryon dominated
systems (baryon mass/total mass ). Our results support the tidal
evolution scenario of the formation of globular clusters and baryon dominated
dwarf galaxies in the context of the cold dark matter universe.Comment: 9page, 13 figures. Accepted for publication in ApJ. A high-resolution
PDF of the paper can be obtained from http://th.nao.ac.jp/~takayuki/ApJ05
Magnetohydrodynamics of Cloud Collisions in a Multi-phase Interstellar Medium
We extend previous studies of the physics of interstellar cloud collisions by
beginning investigation of the role of magnetic fields through 2D
magnetohydrodynamic (MHD) numerical simulations. We study head-on collisions
between equal mass, mildly supersonic diffuse clouds. We include a moderate
magnetic field and two limiting field geometries, with the field lines parallel
(aligned) and perpendicular (transverse) to the colliding cloud motion. We
explore both adiabatic and radiative cases, as well as symmetric and asymmetric
ones. We also compute collisions between clouds evolved through prior motion in
the intercloud medium and compare with unevolved cases.
We find that: In the (i) aligned case, adiabatic collisions, like their HD
counterparts, are very disruptive, independent of the cloud symmetry. However,
when radiative processes are taken into account, partial coalescence takes
place even in the asymmetric case, unlike the HD calculations. In the (ii)
transverse case, collisions between initially adjacent unevolved clouds are
almost unaffected by magnetic fields. However, the interaction with the
magnetized intercloud gas during the pre-collision evolution produces a region
of very high magnetic energy in front of the cloud. In collisions between
evolved clouds with transverse field geometry, this region acts like a
``bumper'', preventing direct contact between the clouds, and eventually
reverses their motion. The ``elasticity'', defined as the ratio of the final to
the initial kinetic energy of each cloud, is about 0.5-0.6 in the cases we
considered. This behavior is found both in adiabatic and radiative cases.Comment: 40 pages in AAS LaTeX v4.0, 13 figures (in degraded jpeg format).
Full resolution images as well as mpeg animations are available at
http://www.msi.umn.edu:80/Projects/twj/mhd-cc/ . Accepted for publication in
The Astrophysical Journa
How do different spiral arm models impact the ISM and GMC population?
The nature of galactic spiral arms in disc galaxies remains elusive.
Regardless of the spiral model, arms are expected to play a role in sculpting
the star-forming interstellar medium. As such, different arm models may result
in differences in the structure of the interstellar medium and molecular cloud
properties. In this study we present simulations of galactic discs subject to
spiral arm perturbations of different natures. We find very little difference
in how the cloud population or gas kinematics vary between the different
grand-design spirals, indicting that the interstellar medium on cloud scales
cares little about where spiral arms come from. We do, however, see a
difference in the interarm/arm mass spectra, {and minor differences in tails of
the distributions of cloud properties} (as well as radial variations in the
stellar/gaseous velocity dispersions). These features can be attributed to
differences in the radial dependence of the pattern speeds between the
different spiral models, and could act as a metric of the nature of spiral
structure in observational studies.Comment: 18 pages, 17 figures. Accepted for publication in MNRA
Remarks on flavor-neutrino propagators and oscillation formulae
We examine the general structure of the formulae of neutrino oscillations
proposed by Blasone and Vitiello(BV). Reconstructing their formulae with the
retarded propagators of the flavor neutrino fields for the case of many
flavors, we can get easily the formulae which satisfy the suitable boundary
conditions and are independent of arbitrary mass parameters ,
as is obtained by BV for the case of two flavors. In this two flavor case, our
formulae reduce to those obtained by BV under -invariance condition.
Furthermore, the reconstructed probabilities are shown to coincide with those
derived with recourse to the mass Hilbert space which is
unitarily inequivalent to the flavor Hilbert space . Such a
situation is not found in the corresponding construction a la BV. Then the new
factors in the BV's formulae, which modify the usual oscill ation formulae, are
not the trace of the flavor Hilbert space construction, but come from
Bogolyubov transformation among the operators of spin-1/2 ne utrino with
different masses.Comment: revtex, 16 page
Chemical Evolution of the Galaxy Based on the Oscillatory Star Formation History
We model the star formation history (SFH) and the chemical evolution of the
Galactic disk by combining an infall model and a limit-cycle model of the
interstellar medium (ISM). Recent observations have shown that the SFH of the
Galactic disk violently variates or oscillates. We model the oscillatory SFH
based on the limit-cycle behavior of the fractional masses of three components
of the ISM. The observed period of the oscillation ( Gyr) is reproduced
within the natural parameter range. This means that we can interpret the
oscillatory SFH as the limit-cycle behavior of the ISM. We then test the
chemical evolution of stars and gas in the framework of the limit-cycle model,
since the oscillatory behavior of the SFH may cause an oscillatory evolution of
the metallicity. We find however that the oscillatory behavior of metallicity
is not prominent because the metallicity reflects the past integrated SFH. This
indicates that the metallicity cannot be used to distinguish an oscillatory SFH
from one without oscillations.Comment: 21 pages LaTeX, to appear in Ap
Dynamics and Excitation of Radio Galaxy Emission-Line Regions - I. PKS 2356-61
Results are presented from a programme of detailed longslit spectroscopic
observations of the extended emission-line region (EELR) associated with the
powerful radio galaxy PKS 2356-61. The observations have been used to construct
spectroscopic datacubes, which yield detailed information on the spatial
variations of emission-line ratios across the EELR, together with its kinematic
structure. We present an extensive comparison between the data and results
obtained from the MAPPINGS II shock ionization code, and show that the physical
properties of the line-emitting gas, including its ionization, excitation,
dynamics and overall energy budget, are entirely consistent with a scenario
involving auto-ionizing shocks as the dominant ionization mechanism. This has
the advantage of accounting for the observed EELR properties by means of a
single physical process, thereby requiring less free parameters than the
alternative scheme involving photoionization by radiation from the active
nucleus. Finally, possible mechanisms of shock formation are considered in the
context of the dynamics and origin of the gas, specifically scenarios involving
infall or accretion of gas during an interaction between the host radio galaxy
and a companion galaxy.Comment: 35 pages, LaTeX, uses aas2pp4.sty file, includes 9 PostScript
figures. Two additional colour plates are available from the authors upon
request. Accepted for publication in the Astrophysical Journa
Cercarial Density in the River of an Endemic Area of Schistosomiasis Haematobia in Kenya
The cercarial density in natural water and number of infected Bulinus globosus were monitored over a one-year period to identify the transmission foci in an endemic area of schistosomiasis haematobia in Kenya. Overall prevalence and intensity of infection of the study community were 59.2% and 10.9 eggs/10 ml of urine. Cercariometry was carried out on 456 occasions at 20 study sites while snail sampling was done on 465 occasions at the same sites over a one-year period. Cercariometry was exclusively done at flowing water habitats. The results showed the focality and seasonality of transmission. Cercariae were detected on 44 occasions at 11 sites. The detections were made on seven occasions at two study sites, six occasions at one site, four occasions at four sites, three occasions at one site, two occasions at two sites, and one occasion at one site. Densities of 1?4 cercariae/100 liters of water were found on 31 occasions. Five to nine cercariae/100 liters of water were found on seven occasions, 10?19 cercariae/100 liters of water were found on two occasions, and high cercarial densities greater than 20 cercariae/100 liters of water were found on four occasions. The highest count was 52 cercariae/100 liters of water. The presence of cercariae in natural water was shown to depend on the water temperature, but the intensity and duration of sunlight did not affect the presence of cercariae in water. The monthly variability of cercarial density was proportional to the number of infected snails. Cercarial density was highest in March and April, in the middle of the rainy season, whereas no cercariae were detected in cool dry season. The snail population peaked late in March, the beginning of the long rainy season, remained high for two months, and decreased rapidly late in May when heavy rain occurred. The overall infection rate of snails was 7.3% and the majority of infected snails were collected from March to May. There was no definite correlation between the presence or absence of cercariae and infected snails. Cercariae were frequently found where infected snails were absent and cercariae were sometimes absent where infected snails were present. Cercariometry and snail sampling remain quite complementary in identifying the transmission foci of schistosomiasis
- …