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  • Anisotropic inflation in Finsler spacetime

    分类: 物理学 >> 核物理学 提交时间: 2016-09-13

    摘要: We suggest the universe is Finslerian in the stage of inflation. The Finslerian background spacetime breaks rotational symmetry and induces parity violation. The primordial power spectrum is given for quantum fluctuation of the inflation field. It depends not only on the magnitude of wavenumber but also on the preferred direction. We derive the gravitational field equations in the perturbed Finslerian background spacetime, and obtain a conserved quantity outside the Hubble horizon. The angular correlation coefficients are presented in our anisotropic inflation model. The parity violation feature of Finslerian background spacetime requires that the anisotropic effect only appears in angular correlation coefficients if l′=l+1. The numerical results of the angular correlation coefficients are given to describe the anisotropic effect.

  • Leap-frog neural network for learning the symplectic evolution from partitioned data

    分类: 天文学 >> 天文学 提交时间: 2023-02-19

    摘要: For the Hamiltonian system, this work considers the learning and prediction of the position (q) and momentum (p) variables generated by a symplectic evolution map. Similar to Chen & Tao (2021), the symplectic map is represented by the generating function. In addition, we develop a new learning scheme by splitting the time series (q_i, p_i) into several partitions, and then train a leap-frog neural network (LFNN) to approximate the generating function between the first (i.e. initial condition) and one of the rest partitions. For predicting the system evolution in a short timescale, the LFNN could effectively avoid the issue of accumulative error. Then the LFNN is applied to learn the behavior of the 2:3 resonant Kuiper belt objects, in a much longer time period, and there are two significant improvements on the neural network constructed in our previous work (Li et al. 2022): (1) conservation of the Jacobi integral ; (2) highly accurate prediction of the orbital evolution. We propose that the LFNN may be useful to make the prediction of the long time evolution of the Hamiltonian system.

  • Search for the correlations between host properties and ${\rm DM_{host}}$ of fast radio bursts: constraints on the baryon mass fraction in IGM

    分类: 天文学 >> 天文学 提交时间: 2023-02-19

    摘要: The application of fast radio bursts (FRBs) as probes to investigate astrophysics and cosmology requires the proper modelling of the dispersion measures of Milky Way (${\rm DM_{MW}}$) and host galaxy (${\rm DM_{host}}$). ${\rm DM_{MW}}$ can be estimated using the Milky Way electron models, such as NE2001 model and YMW16 model. However, ${\rm DM_{host}}$ is hard to model due to limited information on the local environment of FRBs. In this paper, using 17 well-localized FRBs, we search for the possible correlations between ${\rm DM_{host}}$ and the properties of host galaxies, such as the redshift, the stellar mass, the star-formation rate, the age of galaxy, the offset of FRB site from galactic center, and the half-light radius. We find no strong correlation between ${\rm DM_{host}}$ and any of the host property. Assuming that ${\rm DM_{host}}$ is a constant for all host galaxies, we constrain the fraction of baryon mass in the intergalactic medium today to be $f_{\rm IGM,0}=0.78_{-0.19}^{+0.15}$. If we model ${\rm DM_{host}}$ as a log-normal distribution, however, we obtain a larger value, $f_{\rm IGM,0}=0.83_{-0.17}^{+0.12}$. Based on the limited number of FRBs, no strong evidence for the redshift evolution of $f_{\rm IGM}$ is found.

  • Machine learning prediction for mean motion resonance behaviour -- The planar case

    分类: 天文学 >> 天文学 提交时间: 2023-02-19

    摘要: Most recently, machine learning has been used to study the dynamics of integrable Hamiltonian systems and the chaotic 3-body problem. In this work, we consider an intermediate case of regular motion in a non-integrable system: the behaviour of objects in the 2:3 mean motion resonance with Neptune. We show that, given initial data from a short 6250 yr numerical integration, the best-trained artificial neural network (ANN) can predict the trajectories of the 2:3 resonators over the subsequent 18750 yr evolution, covering a full libration cycle over the combined time period. By comparing our ANN's prediction of the resonant angle to the outcome of numerical integrations, the former can predict the resonant angle with an accuracy as small as of a few degrees only, while it has the advantage of considerably saving computational time. More specifically, the trained ANN can effectively measure the resonant amplitudes of the 2:3 resonators, and thus provides a fast approach that can identify the resonant candidates. This may be helpful in classifying a huge population of KBOs to be discovered in future surveys.

  • A Rendezvous Mission to the Second Earth Trojan Asteroid 2020 XL5 with Low-Thrust Multi-Gravity Assist Techniques

    分类: 物理学 >> 地球物理学、天文学和天体物理学 提交时间: 2024-02-01 合作期刊: 《Research in Astronomy and Astrophysics》

    摘要: As the second of Earth's Trojan asteroids, 2020 XL5 is worthy of rendezvous and even sample return missions in many aspects. In this paper, a rendezvous mission to Earth's second Trojan asteroid 2020 XL5 is proposed. However, due to its high inclination and large eccentricity, direct impulsive transfer requires large amounts of fuel consumption. To address this challenge, we explore the benefits of electric propulsion and multi-gravity assist techniques for interplanetary missions. These two techniques are integrated in this mission design. The design of a low-thrust gravity-assist (LTGA) trajectory in multi-body dynamics is thoroughly investigated, which is a complex process. A comprehensive framework including three steps is presented here for optimization of LTGA trajectories in multi-body dynamics. The rendezvous mission to 2020 XL5 is designed with this three-step approach. The most effective transfer sequence among the outcomes involves Earth–Venus–Earth–Venus-2020 XL5. Numerical results indicate that the combination of electric propulsion and multi-gravity assists can greatly reduce the fuel consumption, with fuel consumption of 9.03%, making it a highly favorable choice for this rendezvous mission.

  • The Marginally Stable Circular Orbit of the Fluid Disk around a Black Hole

    分类: 天文学 >> 天文学 提交时间: 2023-08-04

    摘要: The inner boundary of a black hole accretion disk is often set to the marginally stable circular orbit (or the innermost stable circular orbit, ISCO) around the black hole. It is important for the theories of black hole accretion disks and their applications to astrophysical black hole systems. Traditionally, the marginally stable circular orbit is obtained by considering the equatorial motion of a test particle around a black hole. However, in reality the accretion flow around black holes consists of fluid, in which the pressure often plays an important role. Here we consider the influence of fluid pressure on the location of marginally stable circular orbit around black holes. It is found that when the temperature of the fluid is so low that the thermal energy of a particle is much smaller than its rest energy, the location of marginally stable circular orbit is almost the same as that in the test particle case. However, we demonstrate that in some special cases the marginally stable circular orbit can be different when the fluid pressure is large and the thermal energy becomes non-negligible comparing with the rest energy. We present our results for both the cases of non-spinning and spinning black holes. The influences of our results on the black hole spin parameter measurement in X-ray binaries and the energy release efficiency of accretion flows around black holes are discussed.

  • Testing the isotropy of the Universe by using the JLA compilation of type-Ia supernovae

    分类: 物理学 >> 核物理学 提交时间: 2016-09-13

    摘要: We probe the possible anisotropy of the Universe by using the JLA compilation of type-Ia supernovae. We apply the Markov Chain Monte Carlo (MCMC) method to constrain the amplitude and direction of anisotropy in three cosmological models. For the dipole-modulated ΛCDM model, the anisotropic amplitude is consistent with zero at 68% C.L., and has an upper bound AD<1.98×10−3 at 95% C.L. Regardless of much larger uncertainty, we find the dipole direction of JLA is amazingly opposite to that of Union2. Similar results are found for the dipole-modulated wCDM and CPL models. Thus, the Universe is still well consistent with the isotropy according to the JLA compilation.

  • A unified description for dipoles of the fine-structure constant and SnIa Hubble diagram in Finslerian universe

    分类: 物理学 >> 核物理学 提交时间: 2016-09-13

    摘要: We propose a Finsler spacetime scenario of the anisotropic universe. The Finslerian universe requires both the fine-structure constant and accelerating cosmic expansion have dipole structure, and the directions of these two dipoles are the same. Our numerical results show that the dipole direction of SnIa Hubble diagram locates at (l,b)=(314.6∘±20.3∘,−11.5∘±12.1∘) with magnitude B=(−3.60±1.66)×10−2. And the dipole direction of the fine-structure constant locates at (l,b)=(333.2∘±8.8∘,−12.7∘±6.3∘) with magnitude B=(0.97±0.21)×10−5. The angular separation between the two dipole directions is about 18.2∘.

  • Asymmetry in the number of L4 and L5 Jupiter Trojans driven by jumping Jupiter

    分类: 天文学 >> 天文学 提交时间: 2023-02-19

    摘要: Context. More than 10000 Jupiter Trojans have been detected so far. They are moving around the L4 and L5 triangular Lagrangian points of the Sun-Jupiter system and their distributions can provide important clues to the early evolution of the Solar System. Aims. The number asymmetry of the L4 and L5 Jupiter Trojans is a longstanding problem. We aim to test a new mechanism in order to explain this anomalous feature by invoking the jumping-Jupiter scenario. Methods. First, we introduce the orbital evolution of Jupiter caused by the giant planet instability in the early Solar System. In this scenario, Jupiter could undergo an outward migration at a very high speed. We then investigate how such a jump changes the numbers of the L4 (N4) and L5 (N5) Trojans. Results. The outward migration of Jupiter can distort the co-orbital orbits near the Lagrangian points, resulting in L4 Trojans being more stable than the L5 ones. We find that, this mechanism could potentially explain the unbiased number asymmetry of N4/N5~1.6 for the known Jupiter Trojans. The uncertainties of the system parameters, e.g. Jupiter's eccentricity and inclination, the inclination distribution of Jupiter Trojans, are also taken into account and our results about the L4/L5 asymmetry have been further validated. However, the resonant amplitudes of the simulated Trojans are excited to higher values compared to the current population. A possible solution is that collisions among the Trojans may reduce their resonant amplitudes.

  • Asymmetry in the number of L4 and L5 Jupiter Trojans driven by jumping Jupiter

    分类: 天文学 >> 天文学 提交时间: 2023-02-19

    摘要: Context. More than 10000 Jupiter Trojans have been detected so far. They are moving around the L4 and L5 triangular Lagrangian points of the Sun-Jupiter system and their distributions can provide important clues to the early evolution of the Solar System. Aims. The number asymmetry of the L4 and L5 Jupiter Trojans is a longstanding problem. We aim to test a new mechanism in order to explain this anomalous feature by invoking the jumping-Jupiter scenario. Methods. First, we introduce the orbital evolution of Jupiter caused by the giant planet instability in the early Solar System. In this scenario, Jupiter could undergo an outward migration at a very high speed. We then investigate how such a jump changes the numbers of the L4 (N4) and L5 (N5) Trojans. Results. The outward migration of Jupiter can distort the co-orbital orbits near the Lagrangian points, resulting in L4 Trojans being more stable than the L5 ones. We find that, this mechanism could potentially explain the unbiased number asymmetry of N4/N5~1.6 for the known Jupiter Trojans. The uncertainties of the system parameters, e.g. Jupiter's eccentricity and inclination, the inclination distribution of Jupiter Trojans, are also taken into account and our results about the L4/L5 asymmetry have been further validated. However, the resonant amplitudes of the simulated Trojans are excited to higher values compared to the current population. A possible solution is that collisions among the Trojans may reduce their resonant amplitudes.

  • Metasurface-Enabled On-Chip Multiplexed Diffractive Neural Networks in the Visible

    分类: 光学 >> 量子光学 提交时间: 2023-02-19

    摘要: Replacing electrons with photons is a compelling route towards light-speed, highly parallel, and low-power artificial intelligence computing. Recently, all-optical diffractive neural deep neural networks have been demonstrated. However, the existing architectures often comprise bulky components and, most critically, they cannot mimic the human brain for multitasking. Here, we demonstrate a multi-skilled diffractive neural network based on a metasurface device, which can perform on-chip multi-channel sensing and multitasking at the speed of light in the visible. The metasurface is integrated with a complementary metal oxide semiconductor imaging sensor. Polarization multiplexing scheme of the subwavelength nanostructures are applied to construct a multi-channel classifier framework for simultaneous recognition of digital and fashionable items. The areal density of the artificial neurons can reach up to 6.25x106/mm2 multiplied by the number of channels. Our platform provides an integrated solution with all-optical on-chip sensing and computing for applications in machine vision, autonomous driving, and precision medicine.

  • Constraining the Lorentz invariance violation from the continuous spectra of short gamma-ray bursts

    分类: 物理学 >> 核物理学 提交时间: 2016-09-14

    摘要: In quantum gravity, a foamy structure of space-time leads to Lorentz invariance violation (LIV). As the most energetic astrophysical processes in the Universe, gamma-ray bursts (GRBs) provide an effective way to probe quantum gravity effects. We use continuous spectra of 20 short GRBs detected by the Swift satellite to give a conservative lower limit of quantum gravity energy scale MQG. Due to the LIV effect, photons with different energy have different velocities. This will lead to the delayed arrival of high energy photons relative to the low energy ones. Based on the fact that the LIV-induced time delay can't be longer than the duration of a GRB, we present the most conservative estimation of the quantum gravity energy scales from 20 short GRBs. The most strict constraint,MQG>5.05* 1014 GeV, is from GRB 140622A.

  • The Volumetric Extended-Schmidt Law: A Unity Slope

    分类: 天文学 >> 天文学 提交时间: 2023-02-19

    摘要: We investigate the extended-Schmidt (ES) law in volume densities ($\rho_{\rm SFR}$ $\propto$ $(\rho_{\rm gas}\rho_{\rm star}^{0.5})^{\alpha^{\rm VES}}$) for spatially-resolved regions in spiral, dwarf, and ultra-diffuse galaxies (UDGs), and compare to the volumetric Kennicutt-Schmidt (KS) law ($\rho_{\rm SFR}$ $\propto$ $\rho_{\rm gas}^{\alpha^{\rm VKS}}$). We first characterize these star formation laws in individual galaxies using a sample of 11 spirals, finding median slopes $\alpha^{\rm VES}$=0.98 and $\alpha^{\rm VKS}$=1.42, with a galaxy-to-galaxy rms fluctuation that is substantially smaller for the volumetric ES law (0.18 vs 0.41). By combining all regions in spirals with those in additional 13 dwarfs and one UDG into one single dataset, it is found that the rms scatter of the volumetric ES law at given x-axis is 0.25 dex, also smaller than that of the volumetric KS law (0.34 dex). At the extremely low gas density regime as offered by the UDG, the volumetric KS law breaks down but the volumetric ES law still holds. On the other hand, as compared to the surface density ES law, the volumetric ES law instead has a slightly larger rms scatter, consistent with the scenario that the ES law has an intrinsic slope of $\alpha^{\rm VES} \equiv$1 but the additional observational error of the scale height increases the uncertainty of the volume density. The unity slope of the ES law implies that the star formation efficiency (=$\rho_{\rm SFR}$/$\rho_{\rm gas}$) is regulated by the quantity that is related to the $\rho_{\rm star}^{0.5}$.

  • Observations of pores and surrounding regions with CO 4.66 {\mu}m lines by BBSO/CYRA

    分类: 天文学 >> 天文学 提交时间: 2023-02-19

    摘要: Solar observations of carbon monoxide (CO) indicate the existence of lower-temperature gas in the lower solar chromosphere. We present an observation of pores, and quiet-Sun, and network magnetic field regions with CO 4.66 {\mu}m lines by the Cryogenic Infrared Spectrograph (CYRA) at Big Bear Solar Observatory. We used the strong CO lines at around 4.66 {\mu}m to understand the properties of the thermal structures of lower solar atmosphere in different solar features with various magnetic field strengths. AIA 1700 {\AA} images, HMI continuum images and magnetograms are also included in the observation. The data from 3D radiation magnetohydrodynamic (MHD) simulation with the Bifrost code are also employed for the first time to be compared with the observation. We used the RH code to synthesize the CO line profiles in the network regions. The CO 3-2 R14 line center intensity changes to be either enhanced or diminished with increasing magnetic field strength, which should be caused by different heating effects in magnetic flux tubes with different sizes. We find several "cold bubbles" in the CO 3-2 R14 line center intensity images, which can be classified into two types. One type is located in the quiet-Sun regions without magnetic fields. The other type, which has rarely been reported in the past, is near or surrounded by magnetic fields. Notably, some are located at the edge of the magnetic network. The two kinds of cold bubbles and the relationship between cold bubble intensities and network magnetic field strength are both reproduced by the 3D MHD simulation with the Bifrost and RH codes. The simulation also shows that there is a cold plasma blob near the network magnetic fields, causing the observed cold bubbles seen in the CO 3-2 R14 line center image. Our observation and simulation illustrate that the magnetic field plays a vital role in the generation of some CO cold bubbles.

  • The HI Gas Disk Thickness of the Ultra-diffuse Galaxy AGC 242019

    分类: 天文学 >> 天文学 提交时间: 2023-02-19

    摘要: Ultra-diffuse galaxies (UDGs) are as faint as dwarf galaxies but whose sizes are similar to those of spiral galaxies. A variety of formation mechanisms have been proposed, some of which could result in different disk thicknesses. In this study, we measure the radial profile of the HI scale height (h_g) and flaring angle (h_g/R) of AGC 242019 through the joint Poisson-Boltzmann equation based on its well spatially-resolved HI gas maps. The mean HI scale height of AGC 242019 is \approx 537.15 \pm 89.4 pc, and the mean flaring angle is \approx 0.19 \pm 0.03. As a comparison, we also derive the disk thickness for a sample of 14 dwarf irregulars. It is found that the HI disk of AGC 242019 has comparable thickness to dwarfs. This suggests that AGC 242019 is unlikely to experience much stronger stellar feedback than dwarf galaxies, which otherwise leads to a thicker disk for this galaxy.

  • Observations of pores and surrounding regions with CO 4.66 {\mu}m lines by BBSO/CYRA

    分类: 天文学 >> 天文学 提交时间: 2023-02-19

    摘要: Solar observations of carbon monoxide (CO) indicate the existence of lower-temperature gas in the lower solar chromosphere. We present an observation of pores, and quiet-Sun, and network magnetic field regions with CO 4.66 {\mu}m lines by the Cryogenic Infrared Spectrograph (CYRA) at Big Bear Solar Observatory. We used the strong CO lines at around 4.66 {\mu}m to understand the properties of the thermal structures of lower solar atmosphere in different solar features with various magnetic field strengths. AIA 1700 {\AA} images, HMI continuum images and magnetograms are also included in the observation. The data from 3D radiation magnetohydrodynamic (MHD) simulation with the Bifrost code are also employed for the first time to be compared with the observation. We used the RH code to synthesize the CO line profiles in the network regions. The CO 3-2 R14 line center intensity changes to be either enhanced or diminished with increasing magnetic field strength, which should be caused by different heating effects in magnetic flux tubes with different sizes. We find several "cold bubbles" in the CO 3-2 R14 line center intensity images, which can be classified into two types. One type is located in the quiet-Sun regions without magnetic fields. The other type, which has rarely been reported in the past, is near or surrounded by magnetic fields. Notably, some are located at the edge of the magnetic network. The two kinds of cold bubbles and the relationship between cold bubble intensities and network magnetic field strength are both reproduced by the 3D MHD simulation with the Bifrost and RH codes. The simulation also shows that there is a cold plasma blob near the network magnetic fields, causing the observed cold bubbles seen in the CO 3-2 R14 line center image. Our observation and simulation illustrate that the magnetic field plays a vital role in the generation of some CO cold bubbles.

  • The Volumetric Extended-Schmidt Law: A Unity Slope

    分类: 天文学 >> 天文学 提交时间: 2023-02-19

    摘要: We investigate the extended-Schmidt (ES) law in volume densities ($\rho_{\rm SFR}$ $\propto$ $(\rho_{\rm gas}\rho_{\rm star}^{0.5})^{\alpha^{\rm VES}}$) for spatially-resolved regions in spiral, dwarf, and ultra-diffuse galaxies (UDGs), and compare to the volumetric Kennicutt-Schmidt (KS) law ($\rho_{\rm SFR}$ $\propto$ $\rho_{\rm gas}^{\alpha^{\rm VKS}}$). We first characterize these star formation laws in individual galaxies using a sample of 11 spirals, finding median slopes $\alpha^{\rm VES}$=0.98 and $\alpha^{\rm VKS}$=1.42, with a galaxy-to-galaxy rms fluctuation that is substantially smaller for the volumetric ES law (0.18 vs 0.41). By combining all regions in spirals with those in additional 13 dwarfs and one UDG into one single dataset, it is found that the rms scatter of the volumetric ES law at given x-axis is 0.25 dex, also smaller than that of the volumetric KS law (0.34 dex). At the extremely low gas density regime as offered by the UDG, the volumetric KS law breaks down but the volumetric ES law still holds. On the other hand, as compared to the surface density ES law, the volumetric ES law instead has a slightly larger rms scatter, consistent with the scenario that the ES law has an intrinsic slope of $\alpha^{\rm VES} \equiv$1 but the additional observational error of the scale height increases the uncertainty of the volume density. The unity slope of the ES law implies that the star formation efficiency (=$\rho_{\rm SFR}$/$\rho_{\rm gas}$) is regulated by the quantity that is related to the $\rho_{\rm star}^{0.5}$.

  • Reevaluating GPR30: A Paradigm Shift from Estrogen Receptor to Unique Hydrophilic Ligand Activation

    分类: 药物科学 >> 结构生物学 提交时间: 2024-02-24

    摘要: The orphan receptor GPR30, previously classified as a G protein-coupled estrogen receptor (GPER), has been a subject of debate regarding its ligand specificity. Through an integrative approach combining structure elucidation, biochemical binding, and cell signaling assays, we demonstrate that estrogen does not directly bind to or activate GPR30. Cryo-EM structures of GPR30 reveal an unexpected hydrophilic ligand-binding pocket, with striking differences from classical hydrophobic steroid-binding sites, inconsistent with estrogen binding. We further confirmed hydrophilic agonists like Lys05 as true activators of GPR30, providing structural insights into their binding mechanism and receptor activation. Our findings necessitate a paradigm shift in defining GPR30’s role in estrogen signaling, indicating that its activation occurs through mechanisms independent of estrogen binding. This study opens new avenues for developing targeted GPR30 ligands and reinterpreting its role in estrogen-mediated processes.

  • A catalogue of 323 cataclysmic variables from LAMOST DR6

    分类: 天文学 >> 天文学 提交时间: 2023-02-19

    摘要: In this work, we present a catalog of cataclysmic variables (CVs) identified from the Sixth Data Release (DR6) of the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST). To single out the CV spectra, we introduce a novel machine-learning algorithm called UMAP to screen out a total of 169,509 H$\alpha$-emission spectra, and obtain a classification accuracy of the algorithm of over 99.6$\%$ from the cross-validation set. We then apply the template matching program PyHammer v2.0 to the LAMOST spectra to obtain the optimal spectral type with metallicity, which helps us identify the chromospherically active stars and potential binary stars from the 169,509 spectra. After visually inspecting all the spectra, we identify 323 CV candidates from the LAMOST database, among them 52 objects are new. We further discuss the new CV candidates in subtypes based on their spectral features, including five DN subtype during outbursts, five NL subtype and four magnetic CVs (three AM Her type and one IP type). We also find two CVs that have been previously identified by photometry, and confirm their previous classification by the LAMOST spectra.

  • The study of the aging behavior on large area MCP-PMT

    分类: 物理学 >> 核物理学 提交时间: 2016-09-13

    摘要: Aging experiments of a novel type of large area MCP-PMT made by JUNO collaboration were conducted.In these aging experiments, the multi-photoelectron spectrum and single photoelectron spectrum were measured daily,as well as the MCP resistance of the second PMT before and after the experiment. Two PMTs were aged successivelyfor cross check. The first PMT was aged for 52 days, while the other one was aged for 84 days. In order to study the mechanism of the aging process, the high voltage on the second PMT was increased to accelerate its aging process when the cumulative output of charge from its anode was about 4 C. From our study, it can be known that large area MCP-PMT aging had a strong relationship with the related MCPs. In accordance with the PMT aging curve,a PMT aging model was setup and a general aging formula was given.