共查询到20条相似文献，搜索用时 15 毫秒
1.
Density functional theory(DFT) is the most versatile electronic structure method used in quantum chemical calculations, and is increasingly applied in astrochemical research. This mini-review provides an overview of the applications of DFT calculations in understanding the chemistry that occurs in star-forming regions. We survey investigations into the formation of biologically relevant compounds such as nucleobases in the interstellar medium, and also cover the formation of both achiral and chi... 相似文献
2.
Qian-Sheng Zhang 《天文和天体物理学研究(英文版)》2015,(4):549-568
We have developed a new stellar evolution and oscillation code YNEV,which calculates the structures and evolutions of stars,taking into account hydrogen and helium burning.A nonlocal turbulent convection theory and an updated overshoot mixing model are optional in this code.The YNEV code can evolve low-and intermediate-mass stars from the pre-main sequence to a thermally pulsing asymptotic branch giant or white dwarf.The YNEV oscillation code calculates the eigenfrequencies and eigenfunctions of the adiabatic oscillations for a given stellar structure.The input physics and numerical scheme adopted in the code are introduced.Examples of solar models,stellar evolutionary tracks of low-and intermediate-mass stars with different convection theories(i.e.mixing-length theory and nonlocal turbulent convection theory),and stellar oscillations are shown. 相似文献
3.
Chun-Guang Zhang Li-Cai Deng Da-Run Xiong Jrgen Christensen-Dalsgaard 《天文和天体物理学研究(英文版)》2013,(9):1127-1140
Local mixing-length theory is incapable of describing nonlocal phenomena in stellar convection, such as overshooting. Therefore standard solar models constructed with local mixing-length theory significantly deviate from the Sun at the boundaries of the convection zone, where convection becomes less efficient and nonlocal effects are important. The differences between observed and computed frequencies mainly come from the region near the surface, while the localized difference in sound speed is just below the convective envelope. We compute a solar envelope model using Xiong’s nonlocal convection theory, and carry out helioseismic analysis. The nonlocal model has a smooth transition at the base of the convection zone, as revealed by helioseismology. It reproduces solar frequencies more accurately, and reduces the localized difference in sound speed between the Sun and standard solar models. 相似文献
4.
Jun Lin 《中国天文和天体物理学报》2007,7(4):457-476
Large-scale magnetic structures are the main carrier of major eruptions in the solar atmosphere. These structures are rooted in the photosphere and are driven by the unceas-ing motion of the photospheric material through a series of equilibrium configurations. The motion brings energy into the coronal magnetic field until the system ceases to be in equilib-rium. The catastrophe theory for solar eruptions indicates that loss of mechanical equilibrium constitutes the main trigger mechanism of major eruptions, usually shown up as solar flares, eruptive prominences, and coronal mass ejections (CMEs). Magnetic reconnection which takes place at the very beginning of the eruption as a result of plasma instabilities/turbulence inside the current sheet, converts magnetic energy into heating and kinetic energy that are responsible for solar flares, and for accelerating both plasma ejecta (flows and CMEs) and energetic particles. Various manifestations are thus related to one another, and the physics behind these relationships is catastrophe and magnetic reconnection. This work reports on re- cent progress in both theoretical research and observations on eruptive phenomena showing the above manifestations. We start by displaying the properties of large-scale structures in the corona and the related magnetic fields prior to an eruption, and show various morphological features of the disrupting magnetic fields. Then, in the framework of the catastrophe theory, we look into the physics behind those features investigated in a succession of previous works, and discuss the approaches they used. 相似文献
5.
《天文和天体物理学研究(英文版)》2017,(8)
The second order perturbation effect of gravitational radiation damping on the periastron advance of binary stars is studied.The second order analytic solution is obtained based on the first order theory in the 2014 article by Li.Theoretical results show that secular variation exists in the periastron advance of binary stars in the second order theory,but secular variation does not exist in the first order perturbation theory.Numerical results for two compact binary stars(PSR J0737–3039 and M33 X-7)are given,demonstrating the theoretical significance even though the effect is very small. 相似文献
6.
HongZhang Cheng－ZhiZhang 《中国天文和天体物理学报》2001,1(3):275-280
The recent Galileo spacecraft explored Jupiter and its satellite system and provided us with new geodetic data. In order to discuss the dynamical parameters and secular tidal effect of Io, the theory of synchronous satellite is described in detail. Using the new geodetic data of Io, two sets of Io‘s internal structure models are constructed based on the asthenosphere assumption. The liberation parameters α，β，γ and dynamical flattening H are calculated for the models of Io. A comparison of Io with the Moon indicates the they are quite different in many characteristics in spite of the fact that they are approximately equal in mass and size and that they both orbit synchronously. 相似文献
7.
Ke-Ke Zhang Xin-Hao LiaoSchool of Mathematical Sciences University of Exeter EX QE UK kzhang@ex.ac.ukShanghai Astronomical Observatory Shanghai xhliao@center.shao.ac.cn 《中国天文和天体物理学报》2003,3(1):12-33
Generation of the Sun‘s magnetic fields by self-inductive processes in the solar electrically conducting interior, the solar dynamo theory, is a fundamentally important subject in astrophysics. The kinematic dynamo theory concerns how the magnetic fields are produced by kinematically possible flows without being constrained by the dynamic equation. We review a number of basic aspects of the kinematic dynamo theory, including the magnetohydrodynamic approximation for the dynamo equation, the impossibility of dynamo action with the solar differential rotation, the Cowling‘s anti-dynamo theorem in the solar context, the turbulent alpha effect and recently constructed three-dimensional interface dynamos controlled by the solar tachocline at the base of the convection zone. 相似文献
8.
A general formalism of the parametric instabilities in the magnetized inhomogencon plasmas is presented. The gyrokinetic and oscillating center methods are adopted For sin plification dipole pump (k0= 0) approximation is considered in the derivation. The WK approximation is used in the inhomogeneous plasma cases. Some properties and applications of the theory arc discussed. 相似文献
9.
1 INTRODUCTIONThe mixing length theory (MLT) for stellar convection originally developed by Vitense(1953, 1958) has been the most popularly used local convection theory in the studies of stellarstructure and evolution. The theory was later modified and revised by many investigators,who suggested some different expressions. In fact, MLT is not a real hydrodynamic theory,rather, it is a simple "ballistic" theory which traces the motion of imaginary convective elements. In reality j stell… 相似文献
10.
As we now know, there are at least two major difficulties with general rel- ativity （GR）. The first one is related to its incompatibility with quantum mechanics, in the absence of a consistent, widely accepted theory that combines the two theo- ries. The second problem is related to the requirement of the dark sectors-inflaton, dark matter and dark energy by the energy-stress tensor, which are needed to explain a variety of astronomical and cosmological observations. Research has indicated that the dark sectors themselves do not have any non-gravitational or laboratory evidence. Moreover, the dark energy poses, in addition, a serious confrontation between funda- mental physics and cosmology. Guided by theoretical and observational evidences, we are led to an idea that the source of gravitation and its manifestation in GR should be modified. The result is in striking agreement with not only the theory, but also the ob- servations, without requiring the dark sectors of the standard approach. Additionally, it provides natural explanations to some unexplained puzzles. 相似文献
11.
周国成 《紫金山天文台台刊》1994,13(3):199-203
In this paper, the new models of the generation mechanism of the observed solar corona type II radio burst and the interplanetary type III radio emission are presented. The synchrotron-maser theory of type II solar is proposed. The electrostatic waves with frequencies close to twice the plasma frequency can be excited,and can naturclly change into electromagnetic waves when they propagate in a plasma in which the plasma density decreases spatially.Emission of electromagnetic waves with frequencies close to the plasma frequency and/or its second harmonic have been frequently observed in solar corona and interplanetary space. In the pass a number of theories have been suggested to explain the generation mechanism of the observed radiation. 相似文献
12.
《紫金山天文台台刊》1998,(2)
On the basis of photogravitational theory, the influence of solar photogravitation on the motion of comet tail is discussed in this paper. It is shown that for plasma tail and some of the dust tail, there are two coplaner libration points L6, L7, and three collinear points L21, L22,L3 at most. The regions in which the dust grain of comet tail can move are also obtained. 相似文献
13.
The interstellar medium, filling the vast space between stars, is a rich reser-voir of molecular material ranging from simple diatomic molecules to more com-plex, astrobiologically important molecules such as vinylcyanide, methylcyanodiac-cetylene, cyanoaUene, etc. Interstellar molecular cyanoallene is one of the most stableisomers of methylcynoacetylene. An attempt has been made to explore the possibilityof forming cyanoallene in interstellar space by radical-radical and radical-moleculeinteraction schemes in the gaseous phase. The formation of cyanoallene starting fromsome simple, neutral interstellar molecules and radicals has been studied using densityfunctional theory. The reaction energies and structures of the reactants and productsshow that the formation of cyanoallene is possible in the gaseous phase. Both of theconsidered reaction paths are totally exothermic and barrierless, thus giving rise to ahigh probability of occurrence. Rate constants for each step in the formation processof cyanoallene in both the reaction paths are estimated. A full vibrational analysishas been attempted for cyanoallene in the harmonic and anharmonic approximations.Anharmonic spectroscopic parameters such as rotational constants, rotation-vibrationcoupling constants and centrifugal distortion constants have been calculated. 相似文献
14.
On the basis of the theory of the grand-canonical ensemble, we have developed a grand-canonical approach in the 'chemical picture" for the equation of state. The approach defines atomic configuration probabilities and gives a conservation equation for the electron number for calculating the occupation numbers of atoms and molecules. The approach also incorporates the perturbations caused by plasma interactions in cool-dense gases into the equation of state formulation and gives the resulting nonidca) terms of thcrmodynamic properties. The resulting formulae have been applied to the study of the properties of cool-dense plasmas and standard solar models. The phase transitions of high temperature plasmas and the influences of pressure ionization effects through solar interior opacities on solar models have been investigated. 相似文献
15.
R. González Felipe H. J. Mosquera Cuesta A. Pérez Martínez H. Pérez Rojas 《中国天文和天体物理学报》2005,5(4):399-411
The equations of state for degenerate electron and neutron gases are studied in the presence of magnetic fields. After including quantum effects in the investigation of the structural properties of these systems, it is found that some hypermagnetized stars can be unstable according to the criterion of stability of pressures. Highly magnetized white dwarfs should collapse producing a supernova type Ia, while superstrong magnetized neutron stars cannot stand their own magnetic field and must implode, too. A comparison of our results with a set of the available observational data of some compact stars is also presented, and the agreement between this theory and observations is verified. 相似文献
16.
Zhan-Kui Huang Xin-Ji Wu Department of Astronomy Peking University Beijing Chinese Academy of Sciences-Peking University Joint Beijing Astrophysics Center Beijing National Astronomical Observatories Urumqi Astronomical Observatory Chinese Academic of Science Urumqi 《中国天文和天体物理学报》2003,3(2):166-174
The initial period of a pulsar is an important factor in our understanding of the formation of neutron stars and of the nature of the equation of state of neutron star matter.Up to now this quantity can only be obtained for a few pulsars for which accurate age and braking index are known.Based on the theory of the offcenter dipole emission,in which pulsars obtain theiry high velocities depending on the initial periods,we calculate the initial period using the proper motion data,Because the orbital velocity of the progenitor and asymmetric kick in the supernova explosion may also contribute to the observed velocity of the pusar,the derived values of initial periods are lower limits.For normal pulsars,the initial periods are in the range of 0.6～2.6ms.For the millisecond pulsars,the initial periods are comparable to their current periods,and the ratio between the initial period and the current period increases with the decrease of the current period.For PSR B1937 21 with the shortest period of 1.56ms,the ratio is 0.77. 相似文献
17.
18.
刘宗礼 《紫金山天文台台刊》1994,13(1):56-56
Chaos is universal phenomenon in the nature. Some scientists think that chaos theory is the third revolution in physics of this century, following relativity theory and quantum mechanics. The δSct type stars are pulsating variable stars with light amplitudes in Ⅴ from 0~m.003to 0~m.9 and periods from 0~d.01 to 0~d.2. The spectral types are A0-F5Ⅲ-Ⅴ. Some stars such as ρ Puppis and AD CMi show light curves which repeat beautifully from cycle to cycle.The light curves of many stars, especially the small-amplitude variables, often appear vari- 相似文献
19.
Ground-based and space-borne observatories used for studying exoplanet transits now and in the future will considerably increase the number of exoplanets known from transit data and the precision of the measured times of transit minima.Variations in the transit times can not only be used to infer the presence of additional planets,but might also provide opportunities to test the general theory of relativity in these systems.To build a framework for these possible tests,we extend previous studies on the observability of the general relativistic precessions of periastron in transiting exoplanets to variations in secular transit timing under parametrized post-Newtonian formalism.We find that if one can measure the difference between observed and predicted variations of general relativistic secular transit timing to 1 s yr-1in a transiting exoplanet system with a Sun-like mass,a period of~1 day and a relatively small eccentricity of~0.1,general relativity will be tested to the level of~6%. 相似文献
20.
Yong Li National Astronomical Observatories Chinese Academy of Sciences Beijing 《中国天文和天体物理学报》2006,6(5)
The Daye Calendar was compiled in AD 597 in the Sui Dynasty. We investigate the records of sunrise and sunset times on the 24 solar-term days in the calendar. By converting the ancient Chinese time units, Chen, Ke and Fen to hour, minute and second, and carrying out a comparison between the ancient records and values computed with modern astronomical theory, we find that the accuracy of solar measurements in the Sui period is remarkably high: for sunrise times, the average absolute deviation is 3.63 min (this value can be further reduced to 3.03 min when erroneous data are excluded), and for sunset times it is 3.48 min. We also find that the observed sunrise and sunset times are strictly symmetrically distributed with respect to both the Winter Solstice and the Summer Solstice, with their deviations showing a similar symmetrical distribution as well. We give a discussion on the date of observation, the feature of the data, and possible reasons of the deviation. 相似文献