3,004 research outputs found
A Note on the Secrecy Capacity of the Multi-antenna Wiretap Channel
Recently, the secrecy capacity of the multi-antenna wiretap channel was
characterized by Khisti and Wornell [1] using a Sato-like argument. This note
presents an alternative characterization using a channel enhancement argument.
This characterization relies on an extremal entropy inequality recently proved
in the context of multi-antenna broadcast channels, and is directly built on
the physical intuition regarding to the optimal transmission strategy in this
communication scenario.Comment: 10 pages, 0 figure
Uniform Infall toward the Cometary H II Region in the G34.26+0.15 Complex?
Gas accretion is a key process in star formation. However, the gas infall
detections in high-mass star forming regions with high-spatial resolution
observations are rare. Here we report the detection of gas infall towards a
cometary ultracompact H{\sc ii} region "C" in G34.26+0.15 complex. The hot core
associated with "C" has a mass of 76 M_{\sun} and a volume density of
1.1 cm. The HCN (3--2), HCO (1--0) lines observed by
single-dishes and the CN (2--1) lines observed by the SMA show redshifted
absorption features, indicating gas infall. We found a linear relationship
between the line width and optical depth of the CN (2--1) lines. Those
transitions with larger optical depth and line width have larger absorption
area. However, the infall velocities measured from different lines seem to be
constant, indicating the gas infall is uniform. We also investigated the
evolution of gas infall in high-mass star forming regions. At stages prior to
hot core phase, the typical infall velocity and mass infall rate are 1
km s and M_{\sun}\cdotyr, respectively. While in
more evolved regions, the infall velocity and mass infall rates can reach as
high as serval km s and M_{\sun}\cdotyr,
respectively. Accelerated infall has been detected towards some hypercompact
H{\sc ii} and ultracompact H{\sc ii} regions. However, the acceleration
phenomenon becomes inapparent in more evolved ultracompact H{\sc ii} regions
(e.g. G34.26+0.15)
Molecular environments of 51 Planck cold clumps in Orion complex
A mapping survey towards 51 Planck cold clumps projected on Orion complex was
performed with J=1-0 lines of CO and CO at the 13.7 m telescope
of Purple Mountain Observatory. The mean column densities of the Planck gas
clumps range from 0.5 to 9.5 cm, with an average value of
(2.91.9) cm. While the mean excitation temperatures
of these clumps range from 7.4 to 21.1 K, with an average value of 12.13.0
K. The averaged three-dimensional velocity dispersion in these
molecular clumps is 0.660.24 km s. Most of the clumps have
larger than or comparable with . The H
column density of the molecular clumps calculated from molecular lines
correlates with the aperture flux at 857 GHz of the dust emission. Through
analyzing the distributions of the physical parameters, we suggest turbulent
flows can shape the clump structure and dominate their density distribution in
large scale, but not affect in small scale due to the local fluctuations.
Eighty two dense cores are identified in the molecular clumps. The dense cores
have an averaged radius and LTE mass of 0.340.14 pc and 38
M_{\sun}, respectively. And structures of low column density cores are more
affected by turbulence, while those of high column density cores are more
concerned by other factors, especially by gravity. The correlation of the
velocity dispersion versus core size is very weak for the dense cores. The
dense cores are found most likely gravitationally bounded rather than pressure
confined. The relationship between and can be well fitted
with a power law. The core mass function here is much more flatten than the
stellar initial mass function. The lognormal behavior of the core mass
distribution is most likely determined by the internal turbulence.Comment: Accepted to The Astrophysical Journal Supplement Series (ApJS
Molecular gas and triggered star formation surrounding Wolf-Rayet stars
The environments surrounding nine Wolf-Rayet stars were studied in molecular
emission. Expanding shells were detected surrounding these WR stars (see left
panels of Figure 1). The average masses and radii of the molecular cores
surrounding these WR stars anti-correlate with the WR stellar wind velocities
(middle panels of Figure 1), indicating the WR stars has great impact on their
environments. The number density of Young Stellar Objects (YSOs) is enhanced in
the molecular shells at 5 arcmin from the central WR star (lower-right
panel of Figure 1). Through detailed studies of the molecular shells and YSOs,
we find strong evidences of triggered star formation in the fragmented
molecular shells (\cite[Liu et al. 2010]{liu_etal12}Comment: 1 page, IAUS29
Generalized Cut-Set Bounds for Broadcast Networks
A broadcast network is a classical network with all source messages
collocated at a single source node. For broadcast networks, the standard
cut-set bounds, which are known to be loose in general, are closely related to
union as a specific set operation to combine the basic cuts of the network.
This paper provides a new set of network coding bounds for general broadcast
networks. These bounds combine the basic cuts of the network via a variety of
set operations (not just the union) and are established via only the
submodularity of Shannon entropy. The tightness of these bounds are
demonstrated via applications to combination networks.Comment: 30 pages, 4 figures, submitted to the IEEE Transaction on Information
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