Keywords

The next generation of very long baseline interferometry (VLBI) is stepping into the era of microarcsecond (μas) astronomy, and pushing astronomy, especially astrometry, to new heights. VLBI with the Square Kilometre Array (SKA), SKA-VLBI, will increase current sensitivity by an order of magnitude, and reach astrometric precision routinely below 10 μas, even challenging 1 μas. This advancement allows precise parallax and proper motion measurements of various celestial objects. Such improvements can be used to study objects (including isolated objects, and binary or multiple systems) in different stellar stages (such as star formation, main-sequence stars, asymptotic giant branch stars, pulsars, black holes, white dwarfs, etc.), unveil the structure and evolution of complex systems (such as the Milky Way), benchmark the international celestial reference frame, and reveal cosmic expansion. Furthermore, the theory of general relativity can also be tested with SKA-VLBI using precise measurements of light deflection under the gravitational fields of different solar system objects and the perihelion precession of solar system objects.

In this work, we consider a conventional test of gravitational wave (GW) propagation which is based on the phenomenological parameterized dispersion relation to describe potential departures from General Relativity (GR) along the propagation of GWs. But different from tests conventionally performed previously, we vary multiple deformation coefficients simultaneously and employ the principal component analysis (PCA) method to remedy the strong degeneracy among deformation coefficients and obtain informative posteriors. The dominant PCA components can be better measured and constrained, and thus are expected to be more sensitive to potential departures from the waveform model. Using this method we analyze ten selected events and get the result that the combined posteriors of the dominant PCA parameters are consistent with GR within 99.7% credible intervals. The standard deviation of the first dominant PCA parameter is three times smaller than that of the original dispersion parameter of the leading order. However, the multi-parameter test with PCA is more sensitive to not only potential deviations from GR but also systematic errors of waveform models. The difference in results obtained by using different waveform templates hints that the demands of waveform accuracy are higher to perform the multi-parameter test with PCA. Whereas, it cannot be strictly proven that the deviation is indeed and only induced by systematic errors. It requires more thorough research in the future to exclude other possible reasons in parameter estimation and data processing.

Tianwen-1 is China's first independent interplanetary exploration mission, targeting Mars, and includes orbiting, landing, and rover phases. Similar to previous Mars missions, the Tianwen-1 orbiter was designed for polar orbits during the scientific mission period but has an exceptional eccentricity of approximately 0.59. We provide the first independent eight-degree Martian gravity field model in this paper, which was developed exclusively by a team working in China with our independent software as well, based on about two months of radiometric Doppler and range data from only the Tianwen-1 mission. This model is independent from the models created by the groups at NASA Jet Propulsion Laboratory and Goddard Space Flight Center in the United States, as well as the Centre National d'Etudes Spatiales in France. Furthermore, in order to optimize the engineering and scientific benefits, we proposed a number of potential orbits for the extended Tianwen-1 mission. In order to solve a higher-degree independent Mars gravity field model, the viability of modifying the perigee height was investigated, with the priority considerations of fuel savings and implementation hazards being controlled.

Most asteroids and comets are formed in the early stages of the solar system and therefore contain a wealth of information about their birth. The asteroid exploration mission planned in the coming years by China will likely target the celestial body named 133P/Elst-Pizarro (estimated diameter of about 4 km). The orbit of this asteroid stays within the asteroid belt, but nevertheless, it displays a comet-like dust tail. In this study, we used differential tracking data between two simulated probes and the data from an Earth station to estimate 133P gravity field model. This observation mode is similar to how the gravity field was estimated for large celestial objects in the GRAIL and GRACE missions, but here the object is the very small 133P asteroid. We compared the estimated gravity fields obtained for 133P from the satellite-to-satellite combined with the Earth-based two-way range-rate observation mode, with only the Earth-based two-way range rate mode. The results show that the accuracy of the low-degree (4 degree and order) estimate of the gravity field is improved by one order of magnitude by using the satellite-to-satellite combined with the Earth-based two-way range-rate observation mode with respect to the Earth-only tracking. Furthermore, another order of magnitude improvement in the gravity field solution is gained by decreasing the orbit altitude from 12 to 8 km.

The influence of a third-body's orbital elements on the second-body's motion in a hierarchical triple system is a crucial problem in astrophysics. Most prolonged evaluation studies have focused on a distant zero-inclined third-body. This study presents a new perspective on second-body motion equations that addresses a perturbing-body in an elliptic orbit derived with consideration of the axial-tilt (obliquity) of the primary. The proposed model is compared by the dual-averaged method and the N-body problem algorithm. After validation, a generalized three-body model is derived to investigate the effects of the third-body's orbital elements on secondary-body motion behavior. The proposed model considers short-time oscillations that affect secular evaluation and applies to exoplanets with all the primary and third body eccentricities, inclinations, and mass ratios. It is shown that the obliquity of the primary (or third-body's inclination) must be considered for precise long-term assessment, even in highly-hierarchical systems.

Applying functional differentiation to the density field with Newtonian gravity, we obtain the static, nonlinear equation of the three-point correlation function ζ of galaxies to the third order density perturbations. We make the equation closed and perform renormalization of the mass and the Jeans wavenumber. Using the boundary condition inferred from observations, we obtain the third order solution ζ(r, u, θ) at fixed u = 2, which is positive, exhibits a U-shape along the angle θ, and decreases monotonously along the radial r up to the range r ≤ 30 h⁻¹ Mpc in our computation. The corresponding reduced Q(r, u, θ) deviates from 1 of the Gaussian case, has a deeper U-shape along θ, and varies non-monotonously along r. The third order solution agrees with the SDSS data of galaxies, quite close to the previous second order solution, especially at large scales. This indicates that the equations of correlation functions with increasing orders of density perturbation provide a stable description of the nonlinear galaxy system.

In the vicinity of a massive black hole, stars move on precessing Keplerian orbits. The mutual stochastic gravitational torques between the stellar orbits drive a rapid reorientation of their orbital planes, through a process called vector resonant relaxation. We derive, from first principles, the correlation of the potential fluctuations in such a system, and the statistical properties of random walks undergone by the stellar orbital orientations. We compare this new analytical approach with numerical simulations. We also provide a simple scheme to generate the random walk of a test star's orbital orientation using a stochastic equation of motion. We finally present quantitative estimations of this process for a nuclear stellar cluster such as that of the Milky Way.

The stellar initial mass function plays a critical role in the history of our universe. We propose a theory that is based solely on local processes, namely the dust opacity limit, the tidal forces, and the properties of the collapsing gas envelope. The idea is that the final mass of the central object is determined by the location of the nearest fragments, which accrete the gas located farther away, preventing it from falling onto the central object. To estimate the relevant statistics in the neighborhood of an accreting protostar, we perform high-resolution numerical simulations. We also use these simulations to further test the idea that fragmentation in the vicinity of an existing protostar is a determinant in setting the peak of the stellar spectrum. We develop an analytical model, which is based on a statistical counting of the turbulent density fluctuations, generated during the collapse, that have a mass at least equal to the mass of the first hydrostatic core, and sufficiently important to supersede tidal and pressure forces to be self-gravitating. The analytical mass function presents a peak located at roughly 10 times the mass of the first hydrostatic core, in good agreement with the numerical simulations. Since the physical processes involved are all local, occurring at scales of a few 100 au or below, and do not depend on the gas distribution at large scale and global properties such as the mean Jeans mass, the mass spectrum is expected to be relatively universal.

Yen et al. advanced a direct approach for the calculation of self-gravitational force to second-order accuracy based on uniform grid discretization. This method improves the accuracy of N-body calculation using exact integration of kernel functions and employing the Fast Fourier Transform to reduce the complexity of computation to be nearly linear. This direct approach is free of artificial boundary conditions; however, the applicability is limited by the uniform discretization of grids. We report here an advancement in the direct method with the implementation of adaptive mesh refinement and maintaining second-order accuracy, which breaks the barrier set by uniform grid discretization. The adoption of graphic process units can significantly speed up the computation and make application of this method possible for the astrophysical systems of gaseous disk galaxies and protoplanetary disks.

SNe Ia could arise from mergers of carbon–oxygen white dwarfs (WDs) triggered by Lidov–Kozai (LK) oscillations in hierarchical triple-star systems. However, predicted merger rates are several orders of magnitude lower than the observed SNe Ia rate. The low predicted rates can be attributed in part to the fact that many potential WD-merger progenitor systems, with high mutual orbital inclination, merge or interact before the WD stage. Recently, evidence was found for the existence of natal kicks imparted on WDs with a typical magnitude of 0.75 km s⁻¹. In triples, kicks change the mutual inclination and in general increase the outer orbit eccentricity, bringing the triple into an active LK regime at late stages and avoiding the issue of pre-WD merger or interaction. Stars passing by the triple can result in similar effects. However, both processes can also disrupt the triple. In this paper, we quantitatively investigate the impact of WD kicks and flybys on the rate of WD mergers using detailed simulations. We find that WD kicks and flybys combine to increase the predicted WD merger rates by a factor of ∼2.5, resulting in a time-integrated rate of ≈1.1 × 10⁻⁴M_⊙⁻¹. Despite the significant boost, the predicted rates are still more than one order of magnitude below the observed rate of ∼10⁻³M_⊙⁻¹. However, many systematic uncertainties still remain in our calculations, in particular the potential contributions from tighter triples, dynamically unstable systems, unbound systems due to WD kicks, and quadruple systems.