14,422 research outputs found
The Five-hundred-meter Aperture Spherical Radio Telescope Project and its Early Science Opportunities
The National Astronomical Observatories, Chinese Academy of Science (NAOC),
has started building the largest antenna in the world. Known as FAST, the
Five-hundred-meter Aperture Spherical radio Telescope is a Chinese mega-science
project funded by the National Development and Reform Commission (NDRC). FAST
also represents part of Chinese contribution to the international efforts to
build the square kilometer array (SKA). Upon its finishing around September of
2016, FAST will be the most sensitive single-dish radio telescope in the low
frequency radio bands between 70 MHz and 3 GHz. The design specifications of
FAST, its expected capabilities, and its main scientific aspirations were
described in an overview paper by Nan et al. (2011). In this paper, we briefly
review the design and the key science goals of FAST, speculate the likely
limitations at the initial stages of FAST operation, and discuss the
opportunities for astronomical discoveries in the so-called early science
phase.Comment: Proceedings of IAUS 291 "Neutron Stars and Pulsars: Challenges and
Opportunities after 80 years", J. van Leeuwen (ed.); 6 pages, 2 figure
A New Method for Constraining Molecular Cloud Thickness: A study of Taurus, Perseus and Ophiuchus
The core velocity dispersion (CVD) is a potentially useful tool for studying
the turbulent velocity field of molecular clouds. CVD is based on centroid
velocities of dense gas clumps, thus is less prone to density fluctuation and
reflects more directly the cloud velocity field. Prior work demonstrated that
the Taurus molecular cloud CVD resembles the well-known Larson's linewidth-size
relation of molecular clouds. In this work, we studied the dependence of the
CVD on the line-of-sight thickness of molecular clouds, a quantity which cannot
be measured by direct means. We produced a simple statistical model of cores
within clouds and analyzed the CVD of a variety of hydrodynamical simulations.
We show that the relation between the CVD and the 2D projected separation of
cores () is sensitive to the cloud thickness. When the cloud is thin,
the index of CVD- relation ( in the relation CVD) reflects the underlying energy spectrum () in that . The CVD- relation becomes
flatter () for thicker clouds. We used this result to constrain
the thicknesses of Taurus, Perseus, and Ophiuchus. We conclude that Taurus has
a ratio of cloud depth to cloud length smaller than about 1/10-1/8, i.e. it is
a sheet. A simple geometric model fit to the linewidth-size relation indicates
that the Taurus cloud has a pc line-of-sight dimension. In contrast,
Perseus and Ophiuchus are thicker and have ratios of cloud depth to cloud
length larger than about 1/10-1/8.Comment: ApJ accepted, with 14 pages, 16 figure
Interacting dark energy with time varying equation of state and the tension
Almost in all interacting dark energy models present in the literature, the
stability of the model becomes potentially sensitive to the dark energy
equation of state parameter , and a singularity arises at `'.
Thus, it becomes mandatory to test the stability of the model into two separate
regions, namely, for quintessence and phantom. This essentially brings in a
discontinuity into the parameters space for . Such discontinuity can be
removed with some specific choices of the interaction or coupling function. In
the present work we choose one particular coupling between dark matter and dark
energy which can successfully remove such instability and we allow a dynamical
dark energy equation of state parameter instead of the constant one. In
particular, considering a dynamical dark energy equation of state with only one
free parameter , representing the current value of the dark energy
equation of state, we confront the interacting scenario with several
observational datasets. The results show that the present cosmological data
allow an interaction in the dark sector, in agreement with some latest claims
by several authors, and additionally, a phantom behaviour in the dark energy
equation of state is suggested at present. Moreover, for this case the tension
on is clearly released. As a final remark, we mention that according to
the Bayesian analysis, -cold dark matter (CDM) is always
favored over this interacting dark energy model.Comment: 20 pages, 14 figures, 8 tables; version published in PR
Quantifying Dark Gas
A growing body of evidence has been supporting the existence of so-called
"dark molecular gas" (DMG), which is invisible in the most common tracer of
molecular gas, i.e., CO rotational emission. DMG is believed to be the main gas
component of the intermediate extinction region between A0.05-2,
roughly corresponding to the self-shielding threshold of H and CO.
To quantify DMG relative to HI and CO, we are pursuing three observational
techniques, namely, HI self-absorption, OH absorption, and TeraHz C
emission. In this paper, we focus on preliminary results from a CO and OH
absorption survey of DMG candidates. Our analysis show that the OH excitation
temperature is close to that of the Galactic continuum background and that OH
is a good DMG tracer co-existing with molecular hydrogen in regions without CO.
Through systematic "absorption mapping" by Square Kilometer Array (SKA) and
ALMA, we will have unprecedented, comprehensive knowledge of the ISM components
including DMG in terms of their temperature and density, which will impact our
understanding of galaxy evolution and star formation profoundly.Comment: 4 pages, 5 figures, Proceedings Asia-Pacific Regional IAU Meeting
(APRIM) 201
Water abundance in four of the brightest water sources in the southern sky
We estimated the ortho-{\rm{HO}} abundances of G267.9--1.1, G268.4--0.9,
G333.1--0.4 and G336.5--1.5, four of the brightest ortho-{\rm{HO}} sources
in the southern sky observed by the Submillimeter Wave Astronomy Satellite
(ortho-{\rm{HO}} 1 -- 1 line, 556.936~GHz). The typical
molecular clumps in our sample have H column densities of to {\,}cm and ortho-{\rm{HO}} abundances of 10. Compared
with previous studies, the ortho-{\rm{HO}} abundances are at a low level,
which can be caused by the low temperatures of these clumps. To estimate the
ortho-{\rm{HO}} abundances, we used the CS line
(97.98095~GHz) and CS (244.93556~GHz) line observed by{ the}
Swedish-ESO 15\,m Submillimeter Telescope (SEST) to calculate the temperatures
of the clumps and the 350~\upmum dust continuum observed by{ the} Caltech
Submillimeter Observatory (CSO) telescope to estimate the H column
densities. The observations of {\rm{NH}} () for these
clumps were also acquired by SEST and the corresponding abundances were
estimated. The {\rm{NH}} abundance in each clump shows a common
decreasing trend toward the center and {a} typical abundance range from
10 to 10.Comment: Published in RAA. 11 figure
Observational constraints on dynamical dark energy with pivoting redshift
We investigate the generalized Chevallier-Polarski-Linder (CPL)
parametrization, which contains the pivoting redshift as an extra free
parameter. We use various data combinations from cosmic microwave background
(CMB), baryon acoustic oscillations (BAO), redshift space distortion (RSD),
weak lensing (WL), joint light curve analysis (JLA), cosmic chronometers (CC),
and we include a Gaussian prior on the Hubble constant value, in order to
extract the observational constraints on various quantities. For the case of
free we find that for all data combinations it always remains
unconstrained, and there is a degeneracy with the current value of the dark
energy equation of state . For the case where is fixed to specific
values, and for the full data combination, we find that with increasing
the mean value of slowly moves into the phantom regime, however the
cosmological constant is always allowed within 1 confidence-level.
However, the significant effect is that with increasing the correlations
between and change from negative to positive, with the case corresponding to no correlation. This feature indeed justifies why a
non-zero pivoting redshift should be taken into account.Comment: 19 pages, 10 tables, 11 figures; comments are welcome
Studies of Turbulence Dissipation in Taurus Molecular Cloud with Core Velocity Dispersion (CVD)
Turbulence dissipation is an important process affecting the energy balance
in molecular clouds, the birth place of stars. Previously, the rate of
turbulence dissipation is often estimated with semi-analytic formulae from
simulation. Recently we developed a data analysis technique called
core-velocity-dispersion (CVD), which, for the first time, provides direct
measurements of the turbulence dissipation rate in Taurus, a star forming
cloud. The thus measured dissipation rate of is similar to those from dimensional analysis and also consistent
with the previous energy injection rate based on molecular outflows and
bubbles.Comment: Accepted by Ap
Forecasting Interacting Vacuum-Energy Models using Gravitational Waves
The physics of the dark sector has remained one of the controversial areas of
modern cosmology at present and hence it naturally attracts massive attention
to the scientific community. With the developments of the astronomical data,
the physics of the dark sector is becoming much more transparent than it was
some twenty years back. The detection of gravitational waves (GWs) has now
opened a cluster of possibilities in the cosmological regime. Being motivated
by the detection of GWs and its possible impact on the physics of dark matter
and dark energy, in this work we focus on the interacting dark energy models.
Assuming the simplest possibility in which the vacuum energy with
equation-of-state is allowed to interact with the pressureless dark
matter, we have extracted the constraints of the cosmological parameters. We
find that the addition of the GWs data to the CMB measurements significantly
improves up to a factor 4 of the parameters space, and up to a factor 2 for the
full combination of current cosmological datasets, namely
CMB+BAO+Pantheon+RSD+R16+CC+WL. The most affected parameters by the inclusion
of the GWs are , , , and the derived parameters
, and .Comment: 19 pages, 9 figures, and 5 tables; version accepted for publication
by JCA
Distributed Estimation of Oscillations in Power Systems: an Extended Kalman Filtering Approach
Online estimation of electromechanical oscillation parameters provides
essential information to prevent system instability and blackout and helps to
identify event categories and locations. We formulate the problem as a state
space model and employ the extended Kalman filter to estimate oscillation
frequencies and damping factors directly based on data from phasor measurement
units. Due to considerations of communication burdens and privacy concerns, a
fully distributed algorithm is proposed using diffusion extended Kalman filter.
The effectiveness of proposed algorithms is confirmed by both simulated and
real data collected during events in State Grid Jiangsu Electric Power Company
Observational constraints on one-parameter dynamical dark-energy parametrizations and the tension
The phenomenological parametrizations of dark-energy (DE) equation of state
can be very helpful, since they allow for the investigation of its cosmological
behavior despite the fact that its underlying theory is unknown. However,
although there has been a large amount of research on DE parametrizations which
involve two or more free parameters, the one-parameter parametrizations seem to
be underestimated. We perform a detailed observational confrontation of five
one-parameter DE models, with observational data from cosmic microwave
background (CMB), Joint light-curve analysis sample from Supernovae Type Ia
observations (JLA), baryon acoustic oscillations (BAO) distance measurements,
and cosmic chronometers (CC). We find that all models favor a phantom DE
equation of state at present time, while they lead to values in perfect
agreement with its direct measurements and therefore they offer an alleviation
to the -tension. Finally, performing a Bayesian analysis we show that
although CDM cosmology is still favored, one-parameter DE models have
similar or better efficiency in fitting the data comparing to two-parameter DE
parametrizations, and thus they deserve a thorough investigation.Comment: 15 pages, 6 captioned figures, many tables; version published in
Physical Review
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