271 research outputs found
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
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
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
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
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