294 research outputs found
Identifying Anticyclonic Vortex Features Produced by the Rossby Wave Instability in Protoplanetary Disks
Several nearby protoplanetary disks have been observed to display large scale
crescents in the (sub)millimeter dust continuum emission. One interpretation is
that these structures correspond to anticyclonic vortices generated by the
Rossby wave instability within the gaseous disk. Such vortices have local gas
over-densities and are expected to concentrate dust particles with Stokes
number around unity. This process might catalyze the formation of
planetesimals. Whereas recent observations showed that dust crescent are indeed
regions where millimeter-size particles have abnormally high concentration
relative to the gas and smaller grains, no observations have yet shown that the
gas within the crescent region counter-rotates with respect to the
protoplanetary disk. Here we investigate the detectability of anticyclonic
features through measurement of the line-of-sight component of the gas velocity
obtained with ALMA. We carry out 2D hydrodynamic simulations and 3D radiative
transfer calculation of a protoplanetary disk characterized by a vortex created
by the tidal interaction with a massive planet. As a case study, the disk
parameters are chosen to mimic the IRS 48 system, which has the most prominent
crescent observed to date. We generate synthetic ALMA observations of both the
dust continuum and 12CO emission around the frequency of 345 GHz. We find that
the anticyclonic features of vortex are weak but can be detected if both the
source and the observational setup are properly chosen. We provide a recipe for
maximizing the probability to detect such vortex features and present an
analysis procedure to infer their kinematic properties.Comment: 14 pages, 8 figures, Accepted for publication in Astrophysical
Journa
Efficient use of space-time clustering for underwater acoustic communications
Underwater acoustic (UWA) communication channels are characterized by the spreading of received signals in space (direction of arrival) and in time (delay). The spread is often limited to a small number of space-time clusters. In this paper, the spacetime clustering is exploited in a proposed receiver designed for guard-free orthogonal frequency-division multiplexing (OFDM) with superimposed data and pilot signals. For separation of space clusters, the receiver utilizes a vertical linear array (VLA) of hydrophones, whereas for combining delay-spread signals within a space cluster, a time-domain equalizer is used. We compare a number of space-time processing techniques, including a proposed reduced-complexity spatial filter, and show that techniques exploiting the space-time clustering demonstrate an improved detection performance. The comparison is done using signals transmitted by a moving transducer, and recorded on a 14-element non-uniform VLA in sea trials at distances of 46 km and 105 km
The Librating Companions in HD 37124, HD 12661, HD 82943, 47 Uma and GJ 876: Alignment or Antialignment?
We investigated the apsidal motion for the multi-planet systems. In the
simulations, we found that the two planets of HD 37124, HD 12661, 47 Uma and HD
82943 separately undergo apsidal alignment or antialignment. But the companions
of GJ 876 and And are only in apsidal lock about .
Moreover, we obtained the criteria with Laplace-Lagrange secular theory to
discern whether a pair of planets for a certain system are in libration or
circulation.Comment: 13 Pages, 3 figures, 2 tables, Published by ApJ Letters, 591, July 1,
2003 (Figures now included to match the publication
Utilisation of Health Services in Poor Rural China: An Analysis Using a Logistic Regression Model
The aim of this article is to explore the determinants of utilisation of health services in poor rural areas of China. Such a systematic identification of the factors which influence the demand for, and use of, health care services may provide information that policy makers can use to target services to those in greatest need and assist in the design of risk-sharing schemes to protect the poor from the adverse impact of health expenditures
Soft-Decision-Driven Sparse Channel Estimation and Turbo Equalization for MIMO Underwater Acoustic Communications
Multi-input multi-output (MIMO) detection based on turbo principle has been shown to provide a great enhancement in the throughput and reliability of underwater acoustic (UWA) communication systems. Benefits of the iterative detection in MIMO systems, however, can be obtained only when a high quality channel estimation is ensured. In this paper, we develop a new soft-decision-driven sparse channel estimation and turbo equalization scheme in the triply selective MIMO UWA. First, the Homotopy recursive least square dichotomous coordinate descent (Homotopy RLS-DCD) adaptive algorithm, recently proposed for sparse single-input single-output system identification, is extended to adaptively estimate rapid time-varying MIMO sparse channels. Next, the more reliable a posteriori soft-decision symbols, instead of the hard decision symbols or the a priori soft-decision symbols, at the equalizer output, are not only feedback to the Homotopy RLS-DCD-based channel estimator but also to the minimum mean-square-error (MMSE) equalizer. As the turbo iterations progress, the accuracy of channel estimation and the quality of the MMSE equalizer are improved gradually, leading to the enhancement in the turbo equalization performance. This also allows the reduction in pilot overhead. The proposed receiver has been tested by using the data collected from the SHLake2013 experiment. The performance of the receiver is evaluated for various modulation schemes, channel estimators, and MIMO sizes. Experimental results demonstrate that the proposed a posteriori soft-decision-driven sparse channel estimation based on the Homotopy RLS-DCD algorithm and turbo equalization offer considerable improvement in system performance over other turbo equalization schemes
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