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 ($L_{2D}$) is sensitive to the cloud thickness. When the cloud is thin, the index of CVD-$L_{2D}$ relation ($\gamma$ in the relation CVD$\sim L_{2D}^{\gamma}$) reflects the underlying energy spectrum ($E(k)\sim k^{-\beta}$) in that $\gamma\sim(\beta-1)/2$. The CVD-$L_{2D}$ relation becomes flatter ($\gamma\to 0$) 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 $\sim 0.7$ 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 H0H_0 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 $w_x$, and a singularity arises at `$w_x = -1$'. 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 $w_x$. 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 $w_0$, 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 $H_0$ is clearly released. As a final remark, we mention that according to the Bayesian analysis, $\Lambda$-cold dark matter ($\Lambda$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 A$\rm_v$$\sim$0.05-2, roughly corresponding to the self-shielding threshold of H$_2$ and $^{13}$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{H$_2$O}} 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{H$_2$O}} sources in the southern sky observed by the Submillimeter Wave Astronomy Satellite (ortho-{\rm{H$_2$O}} 1$_{10}$ -- 1$_{01}$ line, 556.936~GHz). The typical molecular clumps in our sample have H$_2$ column densities of $10 ^{22}$ to $10 ^{23}${\,}cm$^{-2}$ and ortho-{\rm{H$_2$O}} abundances of 10$^{-10}$. Compared with previous studies, the ortho-{\rm{H$_2$O}} abundances are at a low level, which can be caused by the low temperatures of these clumps. To estimate the ortho-{\rm{H$_2$O}} abundances, we used the CS $J = 2 \to 1$ line (97.98095~GHz) and CS $J = 5 \to 4$ (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$_2$ column densities. The observations of {\rm{N$_2$H$^+$}} ($J = 1 \to 0$) for these clumps were also acquired by SEST and the corresponding abundances were estimated. The {\rm{N$_2$H$^+$}} abundance in each clump shows a common decreasing trend toward the center and {a} typical abundance range from 10$^{-11}$ to 10$^{-9}$.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 $z_p$ 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 $z_p$ 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 $w_0$. For the case where $z_p$ is fixed to specific values, and for the full data combination, we find that with increasing $z_p$ the mean value of $w_0$ slowly moves into the phantom regime, however the cosmological constant is always allowed within 1$\sigma$ confidence-level. However, the significant effect is that with increasing $z_p$ the correlations between $w_0$ and $w_a$ change from negative to positive, with the case $z_p =0.35$ 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 $(0.45\pm 0.05)\times 10^{33} {\rm erg\ s^{-1}}$ 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 $w_x =-1$ 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 $\Omega_ch^2$, $\theta_{MC}$, $\xi$, and the derived parameters $\Omega_{m0}$, $\sigma_8$ and $H_0$.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 H0H_0 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 $H_0$ values in perfect agreement with its direct measurements and therefore they offer an alleviation to the $H_0$-tension. Finally, performing a Bayesian analysis we show that although $\Lambda$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