分类： 天文学 >> 天文学 提交时间： 2023-02-19
Jianghui Ji
Haitao Li
Junbo Zhang
Liang Fang
Dong Li
Su Wang
Yang Cao
Lei Deng
Baoquan Li
Hao Xian
Xiaodong Gao
Ang Zhang
Fei Li
Jiacheng Liu
Zhaoxiang Qi
Sheng Jin
Yaning Liu
Guo Chen
Mingtao Li
Yao Dong
摘要： The Closeby Habitable Exoplanet Survey (CHES) mission is proposed to discover habitable-zone Earth-like planets of the nearby solar-type stars ($\sim 10~\mathrm{pc}$ away from our solar system) via micro-arcsecond relative astrometry. The major scientific objectives of CHES are: to search for Earth Twins or terrestrial planets in habitable zones orbiting 100 FGK nearby stars; further to conduct a comprehensive survey and extensively characterize the nearby planetary systems. The primary payload is a high-quality, low-distortion, high-stability telescope. The optical subsystem is a coaxial three-mirror anastigmat (TMA) with a $1.2 \mathrm{~m}$-aperture, $0.44^{\circ} \times 0.44^{\circ}$ field of view and $500 \mathrm{~nm}-900 \mathrm{~nm}$ working waveband. The camera focal plane is composed of 81 MOSAIC scientific CMOS detectors each with $4 \mathrm{~K} \times 4 \mathrm{~K}$ pixels. The heterodyne laser interferometric calibration technology is employed to ensure micro-arcsecond level (1 $\mu$as) relative astrometry precision to meet the requirements for detection of Earth-like planets. CHES satellite operates at the Sun-Earth L2 point and observes the entire target stars for 5 years. CHES will offer the first direct measurements of true masses and inclinations of Earth Twins and super-Earths orbiting our neighbor stars based on micro-arcsecond astrometry from space. This will definitely enhance our understanding of the formation of diverse nearby planetary systems and the emergence of other worlds for solar-type stars, and finally to reflect the evolution of our own solar system.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： Pulsars are special objects whose positions can be determined independently from timing, radio interferometric, and Gaia astrometry at sub-milliarcsecond (mas) precision; thus, they provide a unique way to monitor the link between dynamical and kinematic reference frames. We aimed to assess the orientation consistency between the dynamical reference frame represented by the planetary ephemeris and the kinematic reference frames constructed by Gaia and VLBI through pulsar positions. We identified 49 pulsars in Gaia Data Release 3 and 62 pulsars with very long baseline interferometry (VLBI) positions from the PSR$\pi$ and MSPSR$\pi$ projects and searched for the published timing solutions of these pulsars. We then compared pulsar positions measured by timing, VLBI, and Gaia to estimate the orientation offsets of the ephemeris frames with respect to the Gaia and VLBI reference frames by iterative fitting. We found orientation offsets of $\sim$10 mas in the DE200 frame with respect to the Gaia and VLBI frame. Our results depend strongly on the subset used in the comparison and could be biased by underestimated errors in the archival timing data, reflecting the limitation of using the literature timing solutions to determine the frame rotation.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
Niu Liu
Sebastien Lambert
Patrick Charlot
Zi Zhu
Jia-Cheng Liu
Nan Jiang
Xiao-Sheng Wan
Cheng-Yu Ding
摘要： We extend previous works by considering two additional radio frequencies (K band and X/Ka band) with the aim to study the frequency dependence of the source positions and its potential connection with the physical properties of the underlying AGN. We compared the absolute source positions measured at four different wavelengths, that is, the optical position from the Gaia Early Data Release 3 (EDR3) and the radio positions at the dual S/X, X/Ka combinations and at K band, as available from the third realization of the International Celestial Reference Frame (ICRF3), for 512 common sources. We first aligned the three ICRF3 individual catalogs onto the Gaia EDR3 frame and compare the optical-to-radio offsets before and after the alignment. Then we studied the correlation of optical-to-radio offsets with the observing (radio) frequency, source morphology, magnitude, redshift, and source type. The deviation among optical-to-radio offsets determined in the different radio bands is less than 0.5 mas, but there is statistical evidence that the optical-to-radio offset is smaller at K band compared to S/X band for sources showing extended structures. The optical-to-radio offset was found to statistically correlate with the structure index. Large optical-to-radio offsets appear to favor faint sources but are well explained by positional uncertainty, which is also larger for these sources. We did not detect any statistically significant correlation between the optical-to-radio offset and the redshift. The radio source structure might also be a major cause for the radio-to-optical offset. For the alignment of with the Gaia celestial reference frame, the S/X band frame remains the preferred choice at present.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： We present an updated study on assessing the axes stability of the third generation of the International Celestial Reference Frame (ICRF3) in terms of linear drift and scatter based on the extragalactic source position time series from analyses of archival very long baseline interferometry observations. Our results show that the axes of the ICRF3 are stable at a level of 10 to 20 microseconds of arc, and it does not degrade after the adoption of the ICRF3 when observations from new networks are included. We also show that the commonly used method of deriving the position time series (four-step solution) is robust.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： The celestial reference frame is realized by absolute positions of extragalactic sources that are assumed to be fixed in the space. The fixing of the axes is one of the crucial points for the International Celestial Reference System (ICRS) concept. However, due to various effects such as its intrinsic activity, the apparent position of the extragalactic sources may vary with time, resulting in a time-dependent deviation of the frame axes that are defined by the positions of these sources. We aim to evaluate the axis stability of the third realization of the International Celestial Reference Frame (ICRF3). We first derive the extragalactic source position time series from observations of very long baseline interferometry (VLBI) at the dual $S/X$-band (2.3/8.4~GHz) between August 1979 and December 2020. We measured the stability of the ICRF3 axes in terms of the drift and scatter around the mean: (i) we estimate the global spin of the ICRF3 axes based on the apparent proper motion (slope of the position time series) of the ICRF3 defining sources; (ii) we also construct the yearly representations of the ICRF3 through annually averaged positions of the ICRF3 defining sources and estimate the dispersion in the axis orientation of these yearly frames. The global spin is no higher than $\mathrm{0.8\,\mu as\,yr^{-1}}$ for each ICRF3 axis with an uncertainty of $\mathrm{0.3\,\mu as\,yr^{-1}}$, corresponding to an accumulated deformation smaller than $\mathrm{30\,\mu as}$ for the celestial frame axes during 1979.6--2021.0. The axis orientation of the yearly celestial frame becomes more stable as time elapses, with a standard deviation of 10--20$\mathrm{\,\mu as}$ for each axis. The axes of the ICRF3 are stable at approximately 10--20~$\mathrm{\mu as}$ from 1979.6--2021.0 and the axis stability does not degrade after the adoption of the ICRF3.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： Pulsars are special objects whose positions can be determined independently from timing, radio interferometric, and Gaia astrometry at sub-milliarcsecond (mas) precision; thus, they provide a unique way to monitor the link between dynamical and kinematic reference frames. We aimed to assess the orientation consistency between the dynamical reference frame represented by the planetary ephemeris and the kinematic reference frames constructed by Gaia and VLBI through pulsar positions. We identified 49 pulsars in Gaia Data Release 3 and 62 pulsars with very long baseline interferometry (VLBI) positions from the PSR$\pi$ and MSPSR$\pi$ projects and searched for the published timing solutions of these pulsars. We then compared pulsar positions measured by timing, VLBI, and Gaia to estimate the orientation offsets of the ephemeris frames with respect to the Gaia and VLBI reference frames by iterative fitting. We found orientation offsets of $\sim$10 mas in the DE200 frame with respect to the Gaia and VLBI frame. Our results depend strongly on the subset used in the comparison and could be biased by underestimated errors in the archival timing data, reflecting the limitation of using the literature timing solutions to determine the frame rotation.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： We examine the vertical structure of Galactic disk kinematics over a
Galactocentric radial distance range of $R=5-15$ $\rm{kpc}$ and up to $3$
$\rm{kpc}$ away from the Galactic plane, using the K-type giants surveyed by
LAMOST. Based on robust measurements of three-dimensional velocity moments, a
wobbly disk is detected in a phenomenological sense. An outflow dominates the
radial motion of the inner disk, while in the outer disk there exist alternate
outward and inward flows. The vertical bulk velocities is a combination of
breathing and bending modes. A contraction-like breathing mode with amplitudes
increasing with the distance to the plane and an upward bending mode dominate
the vertical motion outside $R_0$, and there are reversed breathing mode and
bending mode at $R
分类： 天文学 >> 天文学 提交时间： 2023-02-19
Di-Chang Chen
Ji Wei Xie
Ji-Lin. Zhou
Jia-Yi Yang
Subo Dong
Zi Zhu
Zheng Zheng
Chao Liu
Weikai Zong
Ali Luo
摘要： The radius valley, a dip in the radius distribution of exoplanets at ~1.9 Earth radii separates compact rocky Super-Earths and Sub-Neptunes with lower density. Various hypotheses have been put forward to explain the radius valley. Characterizing the radius valley morphology and its correlation to stellar properties will provide crucial observation constraints on its origin mechanism and deepen the understanding of planet formation and evolution. In this paper, the third part of the Planets Across the Space and Time (PAST) series, using the LAMOST-Gaia-Kepler catalog, we perform a systematical investigation into how the radius valley morphology varies in the Galactic context, i.e., thin/thick galactic disks, stellar age and metallicity abundance ([Fe/H] and [alpha/Fe]). We find that (1) The valley becomes more prominent with the increase of both age and [Fe/H]. (2) The number ratio of super-Earths to sub-Neptunes monotonically increases with age but decreases with [Fe/H] and [alpha/Fe]. (3) The average radius of planets above the valley (2.1-6 Earth radii) decreases with age but increases with [Fe/H]. (4) In contrast, the average radius of planets below the valley (R < 1.7 Earth radii) is broadly independent on age and metallicity. Our results demonstrate that the valley morphology as well as the whole planetary radius distribution evolves on a long timescale of giga-years, and metallicities (not only Fe but also other metal elements, e.g., Mg, Si, Ca, Ti) play important roles in planet formation and in the long term planetary evolution.
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