7,708 research outputs found
Large-Scale Gravitational Instability and Star Formation in the Large Magellanic Cloud
Large-scale star formation in disk galaxies is hypothesized to be driven by
global gravitational instability. The observed gas surface density is commonly
used to compute the strength of gravitational instability, but according to
this criterion star formation often appears to occur in gravitationally stable
regions. One possible reason is that the stellar contribution to the
instability has been neglected. We have examined the gravitational instability
of the Large Magellanic Cloud (LMC) considering the gas alone, and considering
the combination of collisional gas and collisionless stars. We compare the
gravitationally unstable regions with the on-going star formation revealed by
Spitzer observations of young stellar objects. Although only 62% of the massive
young stellar object candidates are in regions where the gas alone is unstable,
some 85% lie in regions unstable due to the combination of gas and stars. The
combined stability analysis better describes where star formation occurs. In
agreement with other observations and numerical models, a small fraction of the
star formation occurs in regions with gravitational stability parameter Q > 1.
We further measure the dependence of the star formation timescale on the
strength of gravitational instability, and quantitatively compare it to the
exponential dependence expected from numerical simulations.Comment: Accepted for publication in ApJ, 10 pages, 5 figure
Interaction of massless Dirac field with a Poincar\'e gauge field
In this paper we consider a model of Poincar\'e gauge theory (PGT) in which a
translational gauge field and a Lorentz gauge field are actually identified
with the Einstein's gravitational field and a pair of ``Yang-Mills'' field and
its partner, respectively.In this model we re-derive some special solutions and
take up one of them. The solution represents a ``Yang-Mills'' field without its
partner field and the Reissner-Nordstr\"om type spacetime, which are generated
by a PGT-gauge charge and its mass.It is main purpose of this paper to
investigate the interaction of massless Dirac fields with those fields. As a
result, we find an interesting fact that the left-handed massless Dirac fields
behave in the different manner from the right-handed ones. This can be
explained as to be caused by the direct interaction of Dirac fields with the
``Yang-Mills'' field. Accordingly, the phenomenon can not happen in the
behavior of the neutrino waves in ordinary Reissner-Nordstr\"om geometry. The
difference between left- and right-handed effects is calculated quantitatively,
considering the scattering problems of the massless Dirac fields by our
Reissner-Nordstr\"om type black-hole.Comment: 10pages, RevTeX3.
Topological meaning of Z numbers in time reversal invariant systems
We show that the Z invariant, which classifies the topological properties
of time reversal invariant insulators, has deep relationship with the global
anomaly. Although the second Chern number is the basic topological invariant
characterizing time reversal systems, we show that the relative phase between
the Kramers doublet reduces the topological quantum number Z to Z.Comment: 4 pages, typos correcte
Discovery of very high energy gamma-ray emission in the W 28 (G6.4-0.1) region, and multiwavelength comparisons
H.E.S.S. observations of the old-age (>10^4yr; ~0.5deg diameter) composite
supernova remnant (SNR) W 28 reveal very high energy (VHE) gamma-ray emission
situated at its northeastern and southern boundaries. The northeastern VHE
source (HESS J1801-233) is in an area where W 28 is interacting with a dense
molecular cloud, containing OH masers, local radio and X-ray peaks. The
southern VHE sources (HESS J1800-240 with components labelled A, B and C) are
found in a region occupied by several HII regions, including the ultracompact
HII region W 28A2. Our analysis of NANTEN CO data reveals a dense molecular
cloud enveloping this southern region, and our reanalysis of EGRET data reveals
MeV/GeV emission centred on HESS J1801-233 and the northeastern interaction
region.Comment: 4 pages, 3 figures, proceedings of the 30th ICRC, Merida, Mexico,
200
Star Forming Dense Cloud Cores in the TeV {\gamma}-ray SNR RX J1713.7-3946
RX J1713.7-3946 is one of the TeV {\gamma}-ray supernova remnants (SNRs)
emitting synchrotron X rays. The SNR is associated with molecular gas located
at ~1 kpc. We made new molecular observations toward the dense cloud cores,
peaks A, C and D, in the SNR in the 12CO(J=2-1) and 13CO(J=2-1) transitions at
angular resolution of 90". The most intense core in 13CO, peak C, was also
mapped in the 12CO(J=4-3) transition at angular resolution of 38". Peak C shows
strong signs of active star formation including bipolar outflow and a
far-infrared protostellar source and has a steep gradient with a
r^{-2.20.4} variation in the average density within radius r. Peak C and
the other dense cloud cores are rim-brightened in synchrotron X rays,
suggesting that the dense cloud cores are embedded within or on the outer
boundary of the SNR shell. This confirms the earlier suggestion that the X rays
are physically associated with the molecular gas (Fukui et al. 2003). We
present a scenario where the densest molecular core, peak C, survived against
the blast wave and is now embedded within the SNR. Numerical simulations of the
shock-cloud interaction indicate that a dense clump can indeed survive shock
erosion, since shock propagation speed is stalled in the dense clump.
Additionally, the shock-cloud interaction induces turbulence and magnetic field
amplification around the dense clump that may facilitate particle acceleration
in the lower-density inter-clump space leading to the enhanced synchrotron X
rays around dense cores.Comment: 22 pages, 7 figures, to accepted in The Astrophysical Journal. A full
color version with higher resolution figures is available at
http://www.a.phys.nagoya-u.ac.jp/~sano/ApJ10/ms_sano.pd
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