分类： 天文学 >> 天文学 提交时间： 2024-06-03
摘要： The planetary obliquity plays a significant role in determining physical properties of planetary surfaces and climate. As direct detection is constrained due to the present observation accuracy, kinetic theories are helpful to predict the evolution of the planetary obliquity. Here the coupling effect between the eccentric Kozai-Lidov (EKL) effect and the equilibrium tide is extensively investigated, the planetary obliquity performs to follow two kinds of secular evolution paths, based on the conservation of total angular momentum. The equilibrium timescale of the planetary obliquity $t_{ mathrm{eq}}$ varies along with $r_{t}$, which is defined as the initial timescale ratio of the tidal dissipation and secular perturbation. We numerically derive the linear relationship between $t_{ mathrm{eq}}$ and $r_{t}$ with the maximum likelihood method. The spin-axis orientation of S-type terrestrials orbiting M-dwarfs reverses over $90^ circ$ when $r_{t} > 100$, then enter the quasi-equilibrium state between $40^ circ$ and $60^ circ$, while the maximum obliquity can reach $130^ circ$ when $r_{t} > 10^4 $. Numerical simulations show that the maximum obliquity increases with the semi-major axis ratio $a_1$/$a_2$, but is not so sensitive to the eccentricity $e_2$. The likelihood of obliquity flip for S-type terrestrials in general systems with $a_2 < 45$ AU is closely related to $m_1$. The observed potential oblique S-type planets HD 42936 b, GJ 86 Ab and $ tau$ Boot Ab are explored to have a great possibility to be head-down over the secular evolution of spin.
分类： 地球科学 >> 空间物理学 分类： 其他 分类： 其他 分类： 其他 提交时间： 2024-06-03
摘要： The Closeby Habitable Exoplanet Survey (CHES) is dedicated to the astrometric exploration for habitable-zone Earth-like planets orbiting solar-type stars in close proximity, achieving unprecedented micro-arcsecond precision. Given the elevated precision, thorough consideration of photocenter jitters induced by stellar activity becomes imperative. This study endeavors to model the stellar activity of solar-type stars, compute astrometric noise, and delineate the detection limits of habitable planets within the astrometric domain. Simulations were conducted for identified primary targets of CHES, involving the generation of simulated observed data for astrometry and photometry, accounting for the impact of stellar activity. Estimation of activity levels in our samples was achieved through chromospheric activity indices, revealing that over 90% of stars exhibited photocenter jitters below 1 $ mu mathrm{as}$. Notably, certain proximate stars, such as $ alpha$ Cen A and B, displayed more discernible noise arising from stellar activity. Subsequent tests were performed to evaluate detection performance, unveiling that stellar activity tends to have a less pronounced impact on planetary detectability for the majority of stars. Approximately 95% of targets demonstrated a detection efficiency exceeding 80%. However, for several cold stars, e.g., HD 32450 and HD 21531, with the habitable zones close to the stars, a reduction in detection efficiency was observed. These findings offer invaluable insights into the intricate interplay between stellar activity and astrometric precision, significantly advancing our understanding in the search for habitable planets.
分类： 核科学技术 >> 辐射物理与技术 提交时间： 2024-05-26
摘要： The Very Large Area gamma-ray Space Telescope (VLAST) is a mission concept proposed to detect gamma#2;ray photons through both Compton scattering and electron-positron pair production mechanisms, thus enablingthe detection of photons with energies ranging from MeV to TeV. This project aims to conduct a comprehensivesurvey of the gamma-ray sky from a low-Earth orbit using an anti-coincidence detector, a tracker detectorthat also serves as a low-energy calorimeter, and a high-energy imaging calorimeter. We developed a MonteCarlo simulation application of the detector using the GEANT4 toolkit to evaluate the instrument performance,including the effective area, angular resolution, and energy resolution, and explored specific optimizations ofthe detector configuration. Our simulation-based analysis indicates that the current design of the VLAST isphysically feasible, with an acceptance above 10 m2 sr which is four times larger than that of the Fermi-LAT,an energy resolution better than 2% at 10 GeV, and an angular resolution better than 0.2 ◦ at 10 GeV. TheVLAST project promises to make significant contributions to the field of gamma ray astronomy and enhanceour understanding of the cosmos.
分类： 天文学 >> 天文学 提交时间： 2023-12-29
Pei Wang
Jian Li
Long Ji
Xian Hou
Erbil Gugercinoglu
Di Li
Diego F. Torres
Yutong Chen
Jiarui Niu
Weiwei Zhu
Bing Zhang
En-wei Liang
Li Zhang
Mingyu Ge
Zigao Dai
Lin Lin
Jinlin Han
Yi Feng
Chenhui Niu
Yongkun Zhang
摘要： Magnetars are neutron stars with extremely strong magnetic fields, frequently powering high-energy activity in X-rays. Pulsed radio emission following some X-ray outbursts have been detected, albeit its physical origin is unclear. It has long been speculated that the origin of magnetars' radio signals is different from those from canonical pulsars, although convincing evidence is still lacking. Five months after magnetar SGR 1935+2154's X-ray outburst and its associated Fast Radio Burst (FRB) 20200428, a radio pulsar phase was discovered. Here we report the discovery of X-ray spectral hardening associated with the emergence of periodic radio pulsations from SGR 1935+2154 and a detailed analysis of the properties of the radio pulses. The complex radio pulse morphology, which contains both narrow-band emission and frequency drifts, has not been seen before in other magnetars, but is similar to those of repeating FRBs - even though the luminosities are many orders of magnitude different. The observations suggest that radio emission originates from the outer magnetosphere of the magnetar, and the surface heating due to the bombardment of inward-going particles from the radio emission region is responsible for the observed X-ray spectral hardening.
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