Chinese Astronomy and Astrophysics最新文献

The solar active regions are the regions where various active phenomena occur in the solar atmosphere. Accurate detection and identification of the solar active regions are of great scientific significance to understand the formation mechanism of the solar magnetic field. In this paper, we propose an automatic detection and recognition technology for solar active regions based on the advantages of Scale Invariant Feature Transform (SIFT) and Clustering by Fast Search and Find of Density Peaks (DPC). Firstly, enhance the contrast of longitudinal magnetic image of Helioseismic and Magnetic Imager (HMI) of Solar Dynamics Observatory (SDO). Then extract the feature points by SIFT. Finally, cluster the feature points by fast search and find of density peaks so as to automatically detect and identify the solar active regions. The results show that the combination of SIFT and DPC can accurately detect the solar active region without human-computer interaction.

Recent gravitational wave detection efforts have yielded a variety of unexpected binary merger event types, revealing our ignorance regarding one of the more fundamental pillars of astrophysics–the binary evolution process. To make up for this shortfall, we have to not only rely on such rare transient events as binary mergers, but also other independent, more persistently observable inspirations. We introduce one such candidate in this review, namely the Thorne-Żytkow Objects (TŻO), that are hypothetical celestial bodies born when neutron stars sink into the centers of red giants and/or red supergiants, whose properties and inferred formation history would undoubtedly clue us in on the vitals of the progenitor binary. First, focus on the theoretical overview of the structure and evolution of massive TŻO; then focus on the observational findings of massive TŻO, especially HV2112 in the Small Magellanic Cloud; finally, summarize current related work.

In view of the scarcity of terahertz astronomical point source targets, this paper studies the method of using a small optical telescope coaxial with terahertz antenna to assist the pointing measurement of terahertz telescope and establish the pointing error correction model. Based on the 1.2 m oblique axis terahertz antenna of Purple Mountain Observatory, the experiment of optical aided pointing measurement is carried out. A 100 mm aperture refractive optical telescope mounted on the antenna back frame is used to obtain the pointing measurement accuracy better than 2${}^{″}$. In addition, by analyzing the structure of the slant axis antenna and the error sources of atmospheric refraction and local star time deviation, a slant axis optical pointing correction model with 23 error terms is established, and the fitting accuracy of about 3${}^{″}$ is achieved. Finally, with the help of high-precision digital photogrammetry, the photoelectric axis consistency is calibrated, and its influence on the pointing model accuracy is discussed. The research results of this paper will provide technical reference for the 5 m Dome A Terahertz Explorer (DATE5) and other terahertz telescopes in pointing measurement and correction.

Lithium (Li) is one of the most important light elements that was primordially synthesized in the Big Bang Nucleosynthesis (BBN). It is also an element that confused astrophysicists for decades, as its observed abundance often contradicts with the theoretical prediction in many different types of celestial objects. Li-rich giant stars are such objects. Their atmospheres contain anomaly high Li abundance than that expected by the standard stellar evolution model. Although the first Li-rich giant star was discovered almost 40 years ago, their origin is still being debated. With the launch of massive spectroscopic survey program such as the Large Sky Area Multi-Object Fiber Spectroscopy Telescope (LAMOST) survey, the extending of available asteroseismology data from space satellites (such as Kepler), and the developments of data-driven techniques, breakthroughs have been archived in the field of Li-rich studies. In this paper, we review the progress that was made during the past four decades, and present our up-to-date understanding to Li-rich giant stars.

Fermi satellite released fairly accurate measurement of extragalactic gamma-ray background (EGB) from 100 MeV to 820 GeV, which dramatically improves our understanding of the high energy gamma-ray background. But for TeV band, space detectors are useless, and we must rely on ground based gamma-ray telescopes such as imaging atmospheric Cherenkov telescopes (IACTs). Until now, the extragalactic TeV gamma-ray background has not been well investigated. In this paper, we calculate the lower bound of extragalactic TeV gamma-ray background radiation by using the energy spectrum of low state of 61 TeV sources, including 53 blazars, 6 radio galaxies, and 2 star-burst galaxies, respectively. Our results show that blazars are the main contributor of the extragalactic TeV gamma-ray background, especially for Mrk 501 and Mrk 421, whose contribution at 0.5-4.5 TeV is about 58$\%$. And the contribution above 4.5 TeV are mainly from the sources H 1426+428, 1ES 1959+650, and 1ES 0229+200, their energy spectrum distribution has been extended to more than 10 TeV. Finally, we calculate the contribution of blazars, radio galaxies, and star-burst galaxies, respectively. Similar calculation is used to subclass of blazars and sources with different redshifts.

Geosynchronous satellite with big inclination is not completely stationary relative to the ground. Its sub-satellite point moves periodically in the north-south direction with ground track of shape like “8”. The larger the inclination angle is, the larger the range of motion is. Such effect makes the effective exposure time of Geosynchronous satellite limited when observe it by optical telescope with stare mode, and the SNR (Signal Noise Ratio) of Geosynchronous satellite image could not be increased with long exposure time. In the monitoring of Geosynchronous orbit without changing hardware, a method called sub-pixel translating superposition of successive frames of images is proposed. By translating and aligning images in the sub-pixel scale according to the moving speed of the object and the time interval of the adjacent frame images, the positions of the Geosynchronous satellite in the image sequences are coincident, and then by superposition of these translational images, the SNR of moving Geosynchronous objects and the detection capability of the whole system can be improved. Results of superposition of real observed images show that the method can remarkably improve the SNR of moving. When five images are superimposed, compared with the SNR of the original image, the SNR of the superimposed image by integer pixels is increased by around 1.7 times, and the SNR of the superimposed image by sub-pixel is increased by about 2 times.

The SDR-TWSTFT (Two-Way Satellite Time and Frequency Transfer based on Software Defined Receiver) link collects measurement data per second and then fits the raw data into 300 seconds by mathematical model. The time transfer results of the link are therefore affected by short-term measurement noise and non-model errors, and exhibit certain characteristics of random noise. A frequency domain amplitude analysis method is proposed to determine the filtering factor and construct a low-pass Vondrak filter to meet the requirements. By analyzing the measured data of the SDR-TWSTFT link between the NTSC (National Time Service Center) of Chinese Academy of Sciences and the PTB (Physikalisch-Technische Bundesanstalt) of Germany, it is found that the method is effective in filtering the high frequency noise in the average time of the link, can improve the confidence of the link time transfer results, and also the short-term frequency and time stability of the filtered link is significantly improved.

Computational efficiency of the recursion of eccentricity functions is investigated, and a kind of batch recursion method is given. Its computational efficiency is significantly superior to the direct calculation method. Moreover, this kind of batch recursion is forward so that the magnitudes of eccentricity functions experience from small to large change in the recursive process. Hence in this way the high accuracy of the recursion of eccentricity functions can be guaranteed.

In order to clarify the early warning and response requirements of near-Earth asteroids impact, the concept of “short-term hazardous asteroids” is proposed, that is, near-Earth asteroids with an equivalent diameter greater than 10 meters that may pose a threat to the Earth in the next 100 years. Based on the 756 short-term hazardous asteroids that have been discovered so far, the orbital distribution characteristics of short-term hazardous asteroids are analyzed, and the study shows that their orbital distribution differs from that of general near-Earth asteroids: the semi-major axis of the short-term hazardous asteroids is more concentrated in 1 au, and the orbital plane is more concentrated in the ecliptic plane. On the basis of the quantity model of near-Earth asteroids, a preliminary estimation model for the quantity of short-term hazardous asteroids is established, and the population of short-term hazardous asteroids with potential threat in the next 100 years is predicted. Some specific research on short-term hazardous asteroids is of great significance to the development of near-Earth asteroid search and monitoring strategies.

In order to investigate redshift space distortion effect on voids, VIDE (Void Identification and Examination toolkit) algorithm is used to find cosmic voids in real space and redshift space based on a mock galaxy catalog produced by the semi-analytical galaxy formation model. The voids can be divided into “collapsed” type and “expanded” type, according to the galaxy velocity on the void wall. The results show that the fraction of the “collapsed” voids decrease as the voids’ size grows, while “expanded” voids are contrary. The effective radius of two type voids differs by 20% in real space, and the mean radial density profile of “collapsed” voids is significant higher than that of “expanded” voids. Using the member galaxies to match the voids in two spaces, the comparison of the voids’ number distributions of two spaces shows that the difference of voids’ number between them is related to the voids’ size, and half of the voids in redshift space can not matched to the voids in real space. For the matched voids, the redshift space distortion has stronger effect on the density profile of the “collapsed” voids. For the unmatched ones, their density profile is clearly different, and the infall movement of galaxies on their shell is more obvious in real space.