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

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

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

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

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

The spatial distribution of neutral hydrogen as traced by low HI mass galaxies

The formation and evolution of galaxies with low neutral atomic hydrogen (HI) masses, M$_{\rm HI}$$<$10$^{8}h^{-2}$M$_{\odot}$, are affected by host dark matter halo mass and photoionisation feedback from the UV background after the end of reionization. We study how the physical processes governing the formation of galaxies with low HI mass are imprinted on the distribution of neutral hydrogen in the Universe using the hierarchical galaxy formation model, GALFORM. We calculate the effect on the correlation function of changing the HI mass detection threshold at redshifts $0 \le z \le 0.5$. We parameterize the clustering as $\xi(r)=(r/r_{0})^{-\gamma}$ and we find that including galaxies with M$_{\rm HI}$$<$10$^{8}h^{-2}$M$_{\odot}$ increases the clustering amplitude $r_{0}$ and slope $\gamma$ compared to samples of higher HI masses. This is due to these galaxies with low HI masses typically being hosted by haloes with masses greater than 10$^{12}{h}^{-1}$M$_{\odot}$, and is in contrast to optically selected surveys for which the inclusion of faint, blue galaxies lowers the clustering amplitude. We show the HI mass function for different host dark matter halo masses and galaxy types (central or satellite) to interpret the values of $r_{0}$ and $\gamma$ of the clustering of HI-selected galaxies. We also predict the contribution of low HI mass galaxies to the 21cm intensity mapping signal. We calculate that a dark matter halo mass resolution better than $\sim$10$^{10}{h}^{-1}$M$_{\odot}$ at redshifts higher than 0.5 is required in order to predict converged 21cm brightness temperature fluctuations.Comment: 14 pages, 10 figures, accepted for pubication in MNRA

Birthrates and delay times of Type Ia supernovae

Type Ia supernovae (SNe Ia) play an important role in diverse areas of astrophysics, from the chemical evolution of galaxies to observational cosmology. However, the nature of the progenitors of SNe Ia is still unclear. In this paper, according to a detailed binary population synthesis study, we obtained SN Ia birthrates and delay times from different progenitor models, and compared them with observations. We find that the Galactic SN Ia birthrate from the double-degenerate (DD) model is close to those inferred from observations, while the birthrate from the single-degenerate (SD) model accounts for only about 1/2-2/3 of the observations. If a single starburst is assumed, the distribution of the delay times of SNe Ia from the SD model is a weak bimodality, where the WD + He channel contributes to the SNe Ia with delay times shorter than 100Myr, and the WD + MS and WD + RG channels to those with age longer than 1Gyr.Comment: 11 pages, 2 figures, accepted by Science in China Series G (Dec.30, 2009

Low-amplitude and long-period radial velocity variations in giants HD 3574, 63 Cygni, and HD 216946 (Research Note)

Aims. We study the low-amplitude and long-period variations in evolved stars using precise radial velocity measurements. Methods. The high-resolution, fiber-fed Bohyunsan Observatory Echelle Spectrograph (BOES) was used from September 2004 to May 2014 as part of the exoplanet search program at the Bohyunsan Optical Astronomy Observatory (BOAO). Results. We report the detection of low-amplitude and long-period orbital radial velocity variations in three evolved stars, HD 3574, 63 Cyg, and HD 216946. They have periods of 1061, 982, and 1382 days and semi-amplitudes of 376, 742, and 699 m/s, respectively.Comment: 6 pages, 7 figures, 4 tables, accepted for publisation in Astronomy & Astrophysic