174 research outputs found
Stochastic Acceleration of Cosmic Rays in the Central Molecular Zone of the Galaxy
Particle acceleration in the inner ~ 200 pc of the Galaxy is discussed, where
diffuse TeV gamma-rays have been detected by the High Energy Stereoscopic
System (HESS) observation. The diffuse gamma-ray emission has a strong
correlation with molecular clouds with large velocity dispersion, indicating
the presence of turbulence. It is argued that the turbulence may contribute to
the acceleration of cosmic rays via stochastic acceleration. The stochastic
acceleration may energize cosmic-ray protons up to ~ 100 TeV and electrons to ~
1 TeV in a relatively tenuous medium. The difference in the efficiency between
protons and electrons supports the hadronic scenario of the diffuse TeV
gamma-ray emission.Comment: 4 pages, 1 figure, accepted by PASJ Letter
Molecular and Atomic Gas toward HESS J1745-303 in the Galactic Center: Further Support for the Hadronic Scenario
We have compared the TeV gamma-rays with the new 12CO J=2-1 data toward HESS
J1745-303 in the Galactic center and confirmed that the molecular gas
MG358.9-0.5 toward (l, b)=(358.9, -0.5 at VLSR=-100-0 km s-1 shows a reasonable
positional agreement with the primary peak (northern part) of the gamma-ray
source. For the southern part of HESS J1745-303, we see no CO counterpart,
whereas the HI gas in the Parkes 21 cm HI dataset shows a possible counterpart
to the gamma-ray source. This HI gas may be optically thick as supported by the
HI line shape similar to the optically thick 12CO. We estimate the total mass
of interstellar protons including both the molecular and atomic gas to be
2x10^6 Mo and the cosmic-ray proton energy to be 6x10^{48} ergs in the hadronic
scenario. We discuss possible origins of the cosmic-ray protons including the
nearby SNR G359.1-0.5. The SNR may be able to explain the northern gamma-ray
source but the southern source seems to be too far to be energized by the SNR.
As an alternative, we argue that the second-order Fermi acceleration in the
inter-clump space surrounded by randomly moving high-velocity clumps may offer
a possible mechanism to accelerate protons. The large turbulent motion with
velocity dispersion of ~15 km s-1 has energy density two orders of magnitude
higher than in the solar vicinity and is viable as the energy source.Comment: To be published in PASJ Vol. 64 No.1 (February 2012
Properties of star formation of the Large Magellanic Cloud as probed by young stellar objects
We perform a systematic study of evolutionary stages and stellar masses of
young stellar objects (YSOs) in the Large Magellanic Cloud (LMC) to investigate
properties of star formation of the galaxy. There are 4825 sources in our YSO
sample, which are constructed by combining the previous studies identifying
YSOs in the LMC. Spectral energy distributions of the YSOs from optical to
infrared wavelengths were fitted with a model consisting of stellar, polycyclic
aromatic hydrocarbon and dust emissions. We utilize the stellar-to-dust
luminosity ratios thus derived to study the evolutionary stages of the sources;
younger YSOs are expected to show lower stellar-to-dust luminosity ratios. We
find that most of the YSOs are associated with the interstellar gas across the
galaxy, which are younger with more gases, suggesting that more recent star
formation is associated with larger amounts of the interstellar medium (ISM).
N157 shows a hint of higher stellar-to-dust luminosity ratios between active
star-forming regions in the LMC, suggesting that recent star formation in N157
is possibly in later evolutionary stages. We also find that the stellar mass
function tends to be bottom-heavy in supergiant shells (SGSs), indicating that
gas compression by SGSs may be ineffective in compressing the ISM enough to
trigger massive star formation. There is no significant difference in the
stellar mass function between YSOs likely associated with the interface between
colliding SGSs and those with a single SGS, suggesting that gas compression by
collisions between SGSs may also be ineffective for massive star formation.Comment: 26 pages, 16 figures, accepted for publication in Ap
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