65,549 research outputs found
Planetary companions orbiting M giants HD 208527 and HD 220074
Aims. The purpose of the present study is to research the origin of planetary
companions by using a precise radial velocity (RV) survey.
Methods. The high-resolution spectroscopy of the fiber-fed Bohyunsan
Observatory Echelle Spectrograph (BOES) at Bohyunsan Optical Astronomy
Observatory (BOAO) is used from September 2008 to June 2012.
Results. We report the detection of two exoplanets in orbit around HD 208527
and HD 220074 exhibiting periodic variations in RV of 875.5 +/- 5.8 and 672.1
+/- 3.7 days. The RV variations are not apparently related to the surface
inhomogeneities and a Keplerian motion of the planetary companion is the most
likely explanation. Assuming possible stellar masses of 1.6 +/- 0.4 and 1.2 +/-
0.3 M_Sun, we obtain the minimum masses for the exoplanets of 9.9 +/- 1.7 and
11.1 +/- 1.8 M_Jup around HD 208527 and HD 220074 with an orbital semi-major
axis of 2.1 +/- 0.2 and 1.6 +/- 0.1 AU and an eccentricity of 0.08 and 0.14,
respectively. We also find that the previously known spectral classification of
HD 208527 and HD 220074 was in error: Our new estimation of stellar parameters
suggest that both HD 208527 and HD 220074 are M giants. Therefore, HD 208527
and HD 220074 are so far the first candidate M giants to harbor a planetary
companion.Comment: 7 pages, 9 figures, 4 tables, accepted for publisation in Astronomy &
Astrophysic
Mass retention efficiencies of He accretion onto carbon-oxygen white dwarfs and type Ia supernovae
Type Ia supernovae (SNe Ia) play a crucial role in studying cosmology and
galactic chemical evolution. They are thought to be thermonuclear explosions of
carbon-oxygen white dwarfs (CO WDs) when their masses reach the Chandrasekar
mass limit in binaries. Previous studies have suggested that He novae may be
progenitor candidates of SNe Ia. However, the mass retention efficiencies
during He nova outbursts are still uncertain. In this article, we aim to study
the mass retention efficiencies of He nova outbursts and to investigate whether
SNe Ia can be produced through He nova outbursts. Using the stellar evolution
code Modules for Experiments in Stellar Astrophysics, we simulated a series of
multicycle He-layer flashes, in which the initial WD masses range from 0.7 to
1.35 Msun with various accretion rates. We obtained the mass retention
efficiencies of He nova outbursts for various initial WD masses, which can be
used in the binary population synthesis studies. In our simulations, He nova
outbursts can increase the mass of the WD to the Chandrasekar mass limit and
the explosive carbon burning can be triggered in the center of the WD; this
suggests that He nova outbursts can produce SNe Ia. Meanwhile, the mass
retention efficiencies in the present work are lower than those of previous
studies, which leads to a lower birthrates of SNe Ia through the WD + He star
channel. Furthermore, we obtained the elemental abundances distribution at the
moment of explosive carbon burning, which can be used as the initial input
parameters in studying explosion models of SNe Ia.Comment: 8 pages, 12 figures, 2 tables, published in Astronomy & Astrophysics
(A&A 604, A31, 2017
An integrated wind risk warning model for urban rail transport in Shanghai, China
The integrated wind risk warning model for rail transport presented has four elements:
Background wind data, a wind field model, a vulnerability model, and a risk model. Background
wind data uses observations in this study. Using the wind field model with effective surface
roughness lengths, the background wind data are interpolated to a 30-m resolution grid. In the
vulnerability model, the aerodynamic characteristics of railway vehicles are analyzed with CFD
(Computational Fluid Dynamics) modelling. In the risk model, the maximum value of three
aerodynamic forces is used as the criteria to evaluate rail safety and to quantify the risk level under
extremely windy weather. The full model is tested for the Shanghai Metro Line 16 using wind
conditions during Typhoon Chan-hom. The proposed approach enables quick quantification of real-
time safety risk levels during typhoon landfall, providing sophisticated warning information for
rail vehicle operation safety
Optimization in the design of a 12 gigahertz low cost ground receiving system for broadcast satellites. Volume 1: System design, performance, and cost analysis
The technical and economical feasibility of using the 12 GHz band for broadcasting from satellites were examined. Among the assigned frequency bands for broadcast satellites, the 12 GHz band system offers the most channels. It also has the least interference on and from the terrestrial communication links. The system design and analysis are carried out on the basis of a decision analysis model. Technical difficulties in achieving low-cost 12 GHz ground receivers are solved by making use of a die cast aluminum packaging, a hybrid integrated circuit mixer, a cavity stabilized Gunn oscillator and other state-of-the-art microwave technologies for the receiver front-end. A working model was designed and tested, which used frequency modulation. A final design for the 2.6 GHz system ground receiver is also presented. The cost of the ground-terminal was analyzed and minimized for a given figure-of-merit (a ratio of receiving antenna gain to receiver system noise temperature). The results were used to analyze the performance and cost of the whole satellite system
Optimization in the design of a 12 gigahertz low cost ground receiving system for broadcast satellites. Volume 2: Antenna system and interference
The antenna characteristics are analyzed of a low cost mass-producible ground station to be used in broadcast satellite systems. It is found that a prime focus antenna is sufficient for a low-cost but not a low noise system. For the antenna feed waveguide systems are the best choice for the 12 GHz band, while printed-element systems are recommended for the 2.6 GHz band. Zoned reflectors are analyzed and appear to be attractive from the standpoint of cost. However, these reflectors suffer a gain reduction of about one db and a possible increase in sidelobe levels. The off-axis gain of a non-auto-tracking station can be optimized by establishing a special illumination function at the reflector aperture. A step-feed tracking system is proposed to provide automatic procedures for searching for peak signal from a geostationary satellite. This system uses integrated circuitry and therefore results in cost saving under mass production. It is estimated that a complete step-track system would cost only $512 for a production quantity of 1000 units per year
Scheduling of Multicast and Unicast Services under Limited Feedback by using Rateless Codes
Many opportunistic scheduling techniques are impractical because they require
accurate channel state information (CSI) at the transmitter. In this paper, we
investigate the scheduling of unicast and multicast services in a downlink
network with a very limited amount of feedback information. Specifically,
unicast users send imperfect (or no) CSI and infrequent acknowledgements (ACKs)
to a base station, and multicast users only report infrequent ACKs to avoid
feedback implosion. We consider the use of physical-layer rateless codes, which
not only combats channel uncertainty, but also reduces the overhead of ACK
feedback. A joint scheduling and power allocation scheme is developed to
realize multiuser diversity gain for unicast service and multicast gain for
multicast service. We prove that our scheme achieves a near-optimal throughput
region. Our simulation results show that our scheme significantly improves the
network throughput over schemes employing fixed-rate codes or using only
unicast communications
Helium star evolutionary channel to super-Chandrasekhar mass type Ia supernovae
Recent discovery of several overluminous type Ia supernovae (SNe Ia)
indicates that the explosive masses of white dwarfs may significantly exceed
the canonical Chandrasekhar mass limit. Rapid differential rotation may support
these massive white dwarfs. Based on the single-degenerate scenario, and
assuming that the white dwarfs would differentially rotate when the accretion
rate , employing Eggleton's
stellar evolution code we have performed the numerical calculations for
1000 binary systems consisting of a He star and a CO white dwarf (WD). We
present the initial parameters in the orbital period - helium star mass plane
(for WD masses of and , respectively), which
lead to super-Chandrasekhar mass SNe Ia. Our results indicate that, for an
initial massive WD of , a large number of SNe Ia may result from
super-Chandrasekhar mass WDs, and the highest mass of the WD at the moment of
SNe Ia explosion is 1.81 , but very massive () WDs
cannot be formed. However, when the initial mass of WDs is , the
explosive masses of SNe Ia are nearly uniform, which is consistent with the
rareness of super-Chandrasekhar mass SNe Ia in observations.Comment: 6 pages, 7 figures, accepted for publication in Astronomy and
Astrophysic
An easy-to-use diagnostic system development shell
The Diagnostic System Development Shell (DSDS), an expert system development shell for diagnostic systems, is described. The major objective of building the DSDS is to create a very easy to use and friendly environment for knowledge engineers and end-users. The DSDS is written in OPS5 and CommonLisp. It runs on a VAX/VMS system. A set of domain independent, generalized rules is built in the DSDS, so the users need not be concerned about building the rules. The facts are explicitly represented in a unified format. A powerful check facility which helps the user to check the errors in the created knowledge bases is provided. A judgement facility and other useful facilities are also available. A diagnostic system based on the DSDS system is question driven and can call or be called by other knowledge based systems written in OPS5 and CommonLisp. A prototype diagnostic system for diagnosing a Philips constant potential X-ray system has been built using the DSDS
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