270 research outputs found
Structure and Stability of Phase Transition Layers in the Interstellar Medium
We analyze the structure and stability of the transition layer (or front)
that connects the cold neutral medium and warm neutral medium in the
plane-parallel geometry. Such fronts appear in recent numerical simulations of
a thermally bistable interstellar medium. The front becomes an evaporation or
condensation front depending on the surrounding pressure. The stability
analysis is performed in both long- and short-wavelength approximations. We
find that the plane-parallel evaporation front is unstable under corrugational
deformations, whereas the condensation front seems to be stable. The
instability is analogous to the Darrieus-Landau instability in combustion
front. The growth rate of the instability is proportional to the speed of the
evaporation flow and the corrugation wavenumber for modes with wavelength much
longer than the thickness of the front, and it is suppressed at scales
approximately equal to the thickness of the front. The timescale of the
instability is smaller than the cooling timescale of the warm neutral medium
( Myr), and can be as small as the cooling timescale of the cold
neutral medium ( Myr). Thus, this instability should be one of
the processes for driving the interstellar turbulence.Comment: 19 pages, 9 figures, accepted for publication in the Astrophysical
Journa
Synthetic Observations of Carbon Lines of Turbulent Flows in Diffuse Multiphase Interstellar Medium
We examine observational characteristics of multi-phase turbulent flows in
the diffuse interstellar medium (ISM) using a synthetic radiation field of
atomic and molecular lines. We consider the multi-phase ISM which is formed by
thermal instability under the irradiation of UV photons with moderate visual
extinction . Radiation field maps of C, C, and CO line
emissions were generated by calculating the non-local thermodynamic equilibrium
(nonLTE) level populations from the results of high resolution hydrodynamic
simulations of diffuse ISM models. By analyzing synthetic radiation field of
carbon lines of [\ion{C}{2}] 158 m, [\ion{C}{1}] (809 GHz),
(492 GHz), and CO rotational transitions, we found a high ratio
between the lines of high- and low-excitation energies in the diffuse
multi-phase interstellar medium. This shows that simultaneous observations of
the lines of warm- and cold-gas tracers will be useful in examining the thermal
structure, and hence the origin of diffuse interstellar clouds.Comment: 16 pages, 10 figures : accepted for publication in ApJ. PDF version
with high resolution figures is available
(http://yso.mtk.nao.ac.jp/~ymasako/paper/ms_hires.pdf
Angular Momentum Transport by MHD Turbulence in Accretion Disks: Gas Pressure Dependence of the Saturation Level of the Magnetorotational Instability
The saturation level of the magnetorotational instability (MRI) is
investigated using three-dimensional MHD simulations. The shearing box
approximation is adopted and the vertical component of gravity is ignored, so
that the evolution of the MRI is followed in a small local part of the disk. We
focus on the dependence of the saturation level of the stress on the gas
pressure, which is a key assumption in the standard alpha disk model. From our
numerical experiments it is found that there is a weak power-law relation
between the saturation level of the Maxwell stress and the gas pressure in the
nonlinear regime; the higher the gas pressure, the larger the stress. Although
the power-law index depends slightly on the initial field geometry, the
relationship between stress and gas pressure is independent of the initial
field strength, and is unaffected by Ohmic dissipation if the magnetic Reynolds
number is at least 10. The relationship is the same in adiabatic calculations,
where pressure increases over time, and nearly-isothermal calculations, where
pressure varies little with time. Our numerical results are qualitatively
consistent with an idea that the saturation level of the MRI is determined by a
balance between the growth of the MRI and the dissipation of the field through
reconnection. The quantitative interpretation of the pressure-stress relation,
however, may require advances in the theoretical understanding of non-steady
magnetic reconnection.Comment: 45 pages, 5 tables, 17 figures, accepted for publication in Ap
Making the corona and the fast solar wind: a self-consistent simulation for the low-frequency Alfven waves from photosphere to 0.3AU
We show that the coronal heating and the fast solar wind acceleration in the
coronal holes are natural consequence of the footpoint fluctuations of the
magnetic fields at the photosphere, by performing one-dimensional
magnetohydrodynamical simulation with radiative cooling and thermal conduction.
We initially set up a static open flux tube with temperature 10^4K rooted at
the photosphere. We impose transverse photospheric motions corresponding to the
granulations with velocity = 0.7km/s and period between 20 seconds and 30
minutes, which generate outgoing Alfven waves. We self-consistently treat these
waves and the plasma heating. After attenuation in the chromosphere by ~85% of
the initial energy flux, the outgoing Alfven waves enter the corona and
contribute to the heating and acceleration of the plasma mainly by the
nonlinear generation of the compressive waves and shocks. Our result clearly
shows that the initial cool and static atmosphere is naturally heated up to
10^6K and accelerated to 800km/s.Comment: 4 pages, 3 figures, ApJL, 632, L49, corrections of mistypes in
eqs.(3) & (5), Mpeg movie for fig.1 (simulation result) is available at
http://www-tap.scphys.kyoto-u.ac.jp/~stakeru/research/suzuki_200506.mp
Magneto-Hydrodynamics of Population III Star Formation
Jet driving and fragmentation process in collapsing primordial cloud are
studied using three-dimensional MHD nested grid simulations. Starting from a
rotating magnetized spherical cloud with the number density of n=10^3 cm^-3, we
follow the evolution of the cloud up to the stellar density n=10^22 cm^-3. We
calculate 36 models parameterizing the initial magnetic and rotational energies
(\gamma_0, \beta_0). In the collapsing primordial clouds, the cloud evolutions
are characterized by the ratio of the initial rotational to magnetic energy,
\gamma_0/\beta_0. The Lorentz force significantly affects the cloud evolution
when \gamma_0 > \beta_0, while the centrifugal force is more dominant than the
Lorentz force when \beta_0 > \gamma_0. When the cloud rotates rapidly with
angular velocity of \Omega_0 > 10^-17 (n/10^3 cm^-3)^2/3 s^-1 and \beta_0 >
\gamma_0, fragmentation occurs before the protostar is formed, but no jet
appears after the protostar formation. On the other hand, a strong jet appears
after the protostar formation without fragmentation when the initial cloud has
the magnetic field of B_0 > 10^-9 (n/10^3 cm^-3)^2/3 G and \gamma_0 > \beta_0.
Our results indicate that proto-Population III stars frequently show
fragmentation and protostellar jet. Population III stars are therefore born as
binary or multiple stellar systems, and they can drive strong jets, which
disturb the interstellar medium significantly, as well as in the present-day
star formation, and thus they may induce the formation of next generation
stars.Comment: 37 pages, 10 figures, Submitted to ApJ, For high resolution figures,
see http://astro3.sci.hokudai.ac.jp/~machida/astro-ph.pd
Unifying low and high mass star formation through density amplified hubs of filaments
Context: Star formation takes place in giant molecular clouds, resulting in
mass-segregated young stellar clusters composed of Sun-like stars, brown
dwarves, and massive O-type(50-100\msun) stars. Aims: To identify candidate
hub-filament systems (HFS) in the Milky-Way and examine their role in the
formation of the highest mass stars and star clusters. Methods: Filaments
around ~35000 HiGAL clumps that are detected using the DisPerSE algorithm. Hub
is defined as a junction of three or more filaments. Column density maps were
masked by the filament skeletons and averaged for HFS and non-HFS samples to
compute the radial profile along the filaments into the clumps. Results:
~3700~(11\%) are candidate HFS of which, ~2150~(60\%) are pre-stellar,
~1400~(40\%) are proto-stellar. All clumps with L>10^4 Lsun and L>10^5 Lsun at
distances respectively within 2kpc and 5kpc are located in the hubs of HFS. The
column-densities of hubs are found to be enhanced by a factor of ~2
(pre-stellar sources) up to ~10 (proto-stellar sources). Conclusions: All
high-mass stars preferentially form in the density enhanced hubs of HFS. This
amplification can drive the observed longitudinal flows along filaments
providing further mass accretion. Radiation pressure and feedback can escape
into the inter-filamentary voids. We propose a "filaments to clusters" unified
paradigm for star formation, with the following salient features: a)
low-intermediate mass stars form in the filaments slowly (10^6yr) and massive
stars quickly (10^5yr) in the hub, b) the initial mass function is the sum of
stars continuously created in the HFS with all massive stars formed in the hub,
c) Feedback dissiption and mass segregation arise naturally due to HFS
properties, and c) explain age spreads within bound clusters and formation of
isolated OB associations.Comment: 20 pages, 17 figures, Accepted by Astronomy and Astrophysic
On the evolution of anomalous X-ray pulsars and soft gamma ray repeaters with fallback disks
We show that the period clustering of anomalous X-ray pulsars (AXPs) and soft
gamma-ray repeaters (SGRs), their X-ray luminosities, ages and statistics can
be explained with fallback disks with large initial specific angular momentum.
The disk evolution models are developed by comparison to self-similar
analytical models. The initial disk mass and angular momentum set the viscous
timescale. An efficient torque, with (1 - w^2) dependence on the fastness
parameter w leads to period clustering in the observed AXP-SGR period range
under a wide range of initial conditions. The timescale t_0 for the early
evolution of the fallback disk, and the final stages of fallback disk
evolution, when the disk becomes passive, are the crucial determinants of the
evolution. The disk becomes passive at temperatures around 100 K, which
provides a natural cutoff for the X-ray luminosity and defines the end of
evolution in the observable AXP and SGR phase. This low value for the minimum
temperature for active disk turbulence indicates that the fallback disks are
active up to a large radius greater than ~10^{12} cm. We find that transient
AXPs and SGRs are likely to be older than their persistent cousins. A fallback
disk with mass transfer rates corresponding to the low quiescent X-ray
luminosities of the transient sources in early evolutionary phases would have a
relatively lower initial mass, such that the mass-flow rate in the disk is not
sufficient for the inner disk to penetrate into the light cylinder of the young
neutron star, making mass accretion onto the neutron star impossible. The
transient AXP phase therefore must start later. The model results imply that
the transient AXP/SGRs, although older, are likely to be similar in number to
persistent sources (abridged).Comment: 42 pages, 22 figures. Accepted for publication in the Astrophysical
Journa
- âŠ