Chinese Astronomy and Astrophysics最新文献

Currently, more than 5 000 exoplanets have been detected, and exoplanetary science is evolving from a focus on exoplanet detection to a focus on comprehensive exoplanetary characterization. Over the past 20 years, through the atmospheric characterization of about 100 exoplanets, a basic framework has been established for atmospheric detection methods, a series of atmospheric spectral forward modeling and retrieval methods, and atmospheric theory for transiting and directly imaged planets. The James Webb Space Telescope (JWST) has unprecedented detection capabilities in the near to mid-infrared spectra, and high-quality data will drive the development of atmospheric theory and models. The early released scientific results of Cycle-1 have shown the ability of JWST to characterize the atmospheres of transiting and directly imaged exoplanets, as well as the initial constraints on the atmospheres of potentially habitable planets around nearby low-mass star. The pursuit of finely detailed exoplanet atmospheric characterization in the era of JWST has already begun, and in conjunction with future missions with atmospheric survey capabilities, such as ARIEL (Atmospheric Remote-sensing Infrared Exoplanet Large-survey) and large-aperture adaptive-optics ground-based telescopes in the next 5 years, will reveal the diversity of exoplanet atmospheres at a much deeper level.

The characteristics of Low Surface Brightness Galaxies (LSBGs) are very important for understanding the overall characteristics of galaxies. It is of great significance to search and expand the samples of Low Surface Brightness Galaxies by modern machine learning, especially deep learning algorithm. LSBGs are difficult to discern automatically and accurately with traditional methods because of their obscure features. However, deep learning does have the advantage of automatically identifying complex and effective features. To solve this problem, an algorithm named You Only Look Once version X-CS (YOLOX-CS) is proposed to search LSBG in large sample sky survey. Firstly, five classical target detection algorithms are compared through experiments and the optimal YOLOX algorithm is selected as the basic algorithm. Then, the YOLOX-CS framework is constructed by combining different attention mechanisms and different optimizers. The data set uses images from the Sloan Digital Sky Survey (SDSS), labelled from LSBG in the $\alpha .40$-SDSS DR7 (the cross coverage area of 40% HI Arecibo Legacy Fast ALFA Survey and SDSS Data Release7) survey. Due to the small number of samples in this data set, Deep Convolutional Generative Adversarial Networks (DCGAN) model is used to expand the experimental test data. After comparing with a series of target detection algorithms, YOLOX-CS has a good test result in searching LSBG recall rate and Average Precision (AP) value in two data sets before and after expansion. The recall rate and AP value in the test set without expansion data set reach 97.75% and 97.83%, respectively. In the expanded data set of DCGAN model, under the same test set, the recall rate reaches 99.10% and the AP value reaches 98.94%, which proves that the algorithm has excellent performance in LSBG search. Finally, the algorithm is applied to SDSS photometric data, and 765 LSBG candidates are obtained.

The corona is a key region of solar atmospheric activity, and the source of solar-terrestrial space weather. Limited by observation, research on the plasma structure and magnetic field state of the lower coronal atmosphere is still very lacking, and there are few international studies on the brightness stratification of the lower coronal atmosphere in the visible light band. In this paper, the coronal green line (${\text{Fe}}_{\text{XIV}}$5303 Å) observation data of Lijiang coronagraph YOGIS (Yunnan Observatories Green-line Imaging System) is used to analyze the bright structure and mid-coronal loops in the inner coronal region (1.03$R_{\odot }$–1.25$R_{\odot }$, where $R_{\odot }$ represents the solar radius) effectively. By fitting the exponential decay of the intensity of the bright structures at the radial height of the sun and comparing these fitting results, it is found that the decay index of the static inner coronal loops obtained is around a fixed value. Then, the more obvious coronal loops are extracted, and by performing the same exponential fitting on the intensities of different heights, the obtained decay index is also relatively similar to that of the bright structure, which provides a reference for further study of the evolution of physical parameters in the corona.

Light quark matter (i.e., udQM) comprised of up and down quarks might be stable with respect to nuclear matter and strange quark matter (SQM). In this work we investigate the stability of nuggets made of such types of matter, which shows that udQM nuggets at certain sizes are more stable than others if a large symmetry energy is adopted. In such cases, there may be compact dwarfs comprised entirely of udQM nuggets and electrons, i.e., udQM dwarfs. We then examine the structure of such udQM dwarfs and consider their possibility of being covered by normal matter crusts. It is found that udQM dwarfs typically have smaller radii, and those covered by normal matter can explain the recently observed white dwarfs with unusually small radii.

The Imaging Science Subsystem (ISS) mounted on Cassini spacecraft took images of Saturn’s inner satellites between 2004 and 2017. In some of these images, the satellites are so close to Saturn rings that their images are affected by the scattered light from Saturn rings, which results in poor even impossible measurements of targets. A background removal algorithm is proposed to measure such images. A total of 70 ISS images of seven inner satellites near Saturn rings (Janus, Epimetheus, Atlas, Prometheus, Pandora, Methone, and Anthe) have been reduced by using the proposed method. Compared with the results from the method without removing the background affected by scattered light, the proposed method demonstrates an improvement in accuracy of at least 43%. The final results show that the residuals of these measurements relative to the JPL (Jet Propulsion Laboratory) ephemeris SAT415 have means of about 0.72 km and 2.26 km in right ascension and declination, with standard deviations of 10.99 km and 11.36 km, respectively.

Hydrogen masters have high short-term stability, using them as master clocks can generate high-precision local standard time in a short period of time. However, the hydrogen maser has a frequency drift phenomenon, which leads to its poor long-term stability, thus affecting the accuracy of the local time. The rotation of the millisecond pulsar is highly stable. With the help of its high long-term stability, it can regularly control the frequency of the hydrogen atomic clock, thus control the real-time signal. The stability of four millisecond pulsars in the second data set released by the International Pulsar Timing Array (IPTA) is analyzed. At the same time, the frequency stability of a hydrogen maser of the National Time Service Center of the Chinese Academy of Sciences (NTSC) is analyzed by Hadamard variance. Finally, a strategy for steering the frequency of hydrogen master using pulsars is given.

The charged components of cosmic ray secondary particles are deflected by thunderstorm electric fields as they pass through kilometer-scale thunderclouds. As a result, the information on location of secondary particles reaching the observation level will be affected, and the lateral distribution will also be changed. In this paper, the Monte Carlo method is used to simulate the effects of near-earth thunderstorm electric field on the lateral distribution of secondary particles at LHAASO. A vertical and uniform atmospheric electric field model is used in our simulations. The results show that during thunderstorm the lateral distribution of secondary particles widens, and the variation amplitude is not only associated with the strength of electric field, but also dependent upon the primary energy and zenith angle of cosmic rays. In an electric field of - 1000 V$·$cm${}^{-1}$ (below the threshold of the Relatively Runaway Electron Avalanche, RREA), the variation amplitude of the lateral distribution of secondary particles is about 0.7% for $\theta =$ 0${}^{\circ }$, and the variation amplitude reaches 4.7% for $\theta =$ 50${}^{\circ }$. The primary energy of cosmic rays is about 180 GeV, the increasing amplitude is about 0.6%. When the primary energy is about 560 TeV, the variation can be up to 20.1%. In an electric field of - 1700 V$·$cm${}^{-1}$ (above the threshold of the RREA process), the increasing amplitude of the lateral distribution is greater than that in an electric field of - 1000 V$·$ cm${}^{-1}$. And the variation amplitude is 3.8% for $\theta =$ 0${}^{\circ }$ and 34% for $\theta =$ 50${}^{\circ }$, respectively. For the primary energy of about 180 GeV, the increasing amplitude of secondary particles is 9.9%. For the primary energy of about 560 TeV, the variation can be as high as 119%. Our simulation results are helpful to understand the deflection mechanisms of cosmic ray secondary particles generated by the near-earth thunderstorm electric field, as well as the variation of LHAASO data during thunderstorms.

The slant-axis telescope has a novel structure, and its unique structural design is more suitable for extreme environment such as the South Pole. However, there is a lack of research on the dynamic modeling and the controller design of slant-axis telescopes at home and abroad. This paper proposes a dynamic modeling and robust control method for slant-axis telescope. Firstly, the dynamical analysis of the slant-axis telescope is carried out. The 2-degree-of-freedom rigid body model of the telescope is established by the Lagrange method. Next, combining the flexibility of the driver system and disturbance, a mathematical model of the rigid-flexible coupling system of the slant-axis telescope is completed. Then, according to the established mathematical model, a sliding mode controller based on the disturbance observer is designed to suppress the disturbance, and realize the robust control of the slant-axis telescope. Finally, the simulation results show that the disturbance observer based sliding mode controller gets better dynamic performance and anti-disturbance characteristics than the traditional Proportion-Integration-Differentiation (PID) controller in the case of considering nonlinear external interference of the model.

Red dwarfs are one of the smallest and dimmest main-sequence stars. With the improvement of observation technology, a rich population of planets have been discovered around them, which exhibits distinct distribution characteristics from the planets around Solar-like stars. Earth-like planets have a higher occurrence rate around red dwarfs, and the planetary systems are tightly compact. The closer habitable zone makes it easier to search for habitable planets. The distribution of the radius valley between super-Earths and sub-Neptunes also differs from that around solar-like stars. Moreover, the discovery of giant planets around these low-mass stars can be hard to explain by current leading theories of planet formation. As the mass of solid material in the protoplanetary disk decreases with decreasing stellar masses, the formation of giant planets remains challenging. The observations from telescopes such as Transiting Exoplanet Survey Satellite (TESS), James Webb, and Atacama Large Millimeter/submillimeter Array (ALMA) have provided invaluable insights and opportunities for the study of planetary formation. In this paper, we review the observations of different populations of planets and summarize the up-to-date understanding of planetary formation around red dwarfs.

With the increase of antenna aperture and frequency, the influence of solar heat on its performance becomes more and more serious. In this paper, the thermal model of the 110 m aperture radio telescope to be built in Xinjiang is established to study its temperature field characteristics on the summer solstice. The results are as follows: during the day, the highest temperature of the main reflector can reach 42.86${}^{\circ }$C, which appears at 14 o’clock, and the temperature of the legs also reaches the peak of 41.74${}^{\circ }$C at the same time. The horizontal temperature difference of the back frame will exceed 1${}^{\circ }$C at 5, 18, and 19.5 o’clock, the antenna pointing performance will be greatly affected. The temperature field at night is also not uniform, and the temperature difference of the pitching structure is significantly higher than that of other components, with the maximum temperature difference of 6.42${}^{\circ }$C. Through the method of numerical simulation and test, it is proved that the wall thickness difference of components is the main reason for the large temperature difference at night.