转动潮汐变形双星理论模型研究

转动和潮汐效应是影响双星系统结构和演化不可忽略的重要物理因素. 根据大质量双星系统V478 Cyg的观测结果, 检验了提出的转动潮汐变形的理论模型. 将转动潮汐变形的模型与传统的双星演化模型对比发现: 转动和潮汐效应使恒星外层(低于平均密度区)发生的形变远大于内层; 恒星两极点重力加速度变大, 赤道面上重力加速度变小; 转动潮汐变形模型具有较大的半径、赤道速度和中心集中度, 较低的氢核能产生率, 恒星向低温和低光度端演化. 此外, 大质量双星系统V478 Cyg由潮汐形变造成的拱线运动速率大于转动形变造成的拱线运动速率, 广义相对论效应造成的拱线运动最小. 由于主星具有较高的中心集中度, 次星潮汐、转动形变造成的拱线运动速率均大于主星相应的拱线运动速率.     

辐射压对非同步转动双星系统洛希势函数的影响

辐射压是影响大质量恒星结构和演化不可忽视的重要物理因素. 根据辐射压对非同步转动的洛希势函数的影响, 数值计算了洛希瓣的大小和3个拉格朗日点的位置和相应的势函数, 并与同步转动的洛希模型计算的结果做了对比. 结果发现: 辐射压可以整体地减小大质量恒星表面的重力加速度, 而转动离心力能最大减少赤道附近的重力加速度. 辐射压和非同步转动均可以明显地改变洛希瓣的大小和3个拉格朗日点的位置和势函数, 影响双星系统物质交换的时间. 因此, 研究辐射压, 非同步转动等物理因素对大质量双星系统洛希势函数的影响, 对密近双星的演化具有重要意义. 关键词： 恒星结构与演化 转动 辐射压     

转动效应对恒星热力学结构与演化的影响研究

转动效应对恒星结构与演化的两大方面的影响体现在动力学效应和元素混合上.本文利用国际MESA程序,研究了转动效应对20 M_⊙恒星在主序阶段热力学结构的影响.发现转动的动力学效应能使恒星中心温度变低,减少了恒星表面的不透明度,中心平均分子量和表面熵.由于动力学效应对恒星热结构的调整,降低了恒星的氢燃烧率,恒星向低温和低光度端演化.转动的元素混合效应减小了恒星的中心温度、密度、压强、紧密度、表面不透明度和平均分子量,使恒星表面氦元素、氮元素明显超丰,中心燃烧核区域变大,延长恒星在主序阶段的寿命.     

临界转动恒星Achernar的斜压结构与引力昏暗的精细研究

转动和潮汐效应是影响恒星结构和演化的非常重要的物理因素. 根据对Achernar的观测数据, 用扰动理论推导了临界转动恒星Achernar分别作为单星和双星的斜压结构的特征, 给出Achernar等压面上的密度等物理量的分布. 利用考虑转动和潮汐及形变效应的单、双星模型研究了Achernar的引力昏暗现象. 结果表明正剪切增强离心力、减小赤道的重力加速度和温度, 反剪切结果则与之相反. 反剪切和刚性转动情况并不符合对Achernar的引力昏暗观测结果. 发现转动双星模型比单星模型虽更符合Achernar赤道和极半径之比的观测值, 但理论计算的角速度比观测值小. 对比理论计算和观测结果发现, 当Achernar的自转角速度为4.65× 10^-5 s^-1, 正剪切率Ω/Ω_s为0.7851时, Achernar的极点温度为16041 K, 赤道温度为12073 K. 所有理论计算与观测值的相对误差不超过7%.     

转动恒星中的元素扩散效应研究

计算了影响元素扩散过程快慢的3个系数Deff,Dshear和Dh,根据转动恒星中的角动量传输和元素扩散方程,给出了转动恒星中的元素扩散效应.结果表明:子午环流是大质量恒星角动量转移的主要物理机制,而剪切湍流是传输化学元素的主要机制.增加水平湍流将增加子午环流和剪切湍流传输化学元素的效率.元素扩散效应造成恒星表面有4He和14N元素的超丰和较大的对流核心,使恒星具有较高的光度和较低的中心温度,对恒星结构与演化有极其重要的影响.     

光深对大质量恒星形成团块化学性质研究的影响（英文）

本文使用文献中的N_2H+(1-0)、H~(13)CO+(1-0)、HCN(1-0)和HN~(13)C(1-0)谱线数据研究大质量恒星形成团块的化学性质和演化,发现H~(13)CO~+和HN~(13)C的丰度受H_2柱密度的影响.由于从A阶段到B阶段这两个丰度的中值增加了近10倍,H~(13)CO~+和HN~(13)C适合追踪大质量恒星形成团块的演化.从A到B阶段四种分子丰度增长速度从高到低依次为H~(13)CO~+、HCN、HN~(13)C、N_2H~+.结果表明进行光学薄分子谱线的高分辨率观测对于研究大质量恒星形成团块的化学演化是必要的。     

双星运行轨道的研究

文章首先把双星运动看做二体问题,研究以一个星体为参照系,另一个星体的轨迹为圆锥曲线,以系统的质心为参照系,每一个星体的轨迹也是圆锥曲线,然后提出了行星与恒星之间也是双星现象,地球与月球之间也是双星现象,解释了困扰物理学与天文学多年的行星运行轨道的共面性、公转的同向性难题,指出了研究行星运行轨道时惯性质量应当用折合质量计算。     

转动双星模型及演化

提出了转动双星的结构和演化模型,给出了主星的重力势,并模拟其加速度在等势面上的分布.结果表明在忽略自引力势高阶项的情况下,极地处重力加速度最大,赤道上长轴顶点处重力加速度最小.自转、公转和伴星潮汐作用力的联合效应降低了子星的有效温度,延长了子星在氢主序上的演化时间.     

近太阳17颗年轻恒星锂丰度: 丽江1.8 m望远镜高色散光谱观测

应用中国科学院云南天文台丽江天文观测站1.8 m望远镜及其折轴光谱仪,对18颗近太阳样本恒星进行了高色散光谱观测,获得这些恒星高质量的高色散光谱。表明原中国科学院国家天文台兴隆观测站2.16 m折轴光谱仪搬迁、改造获得成功,达到并超过了预期的效果。以丽江天文观测站1.8 m望远镜及其折轴光谱仪观测获得的高质量的高色散光谱为基础,应用高斯拟合方法测量了样本中17颗年轻类太阳恒星锂吸收线的等值宽度,并计算了锂元素丰度。应用Hall给出的经验公式,计算了这些年轻恒星的Ca ⅡΗ和Κ(λ=395.0 nm)辐射强度。讨论了锂元素丰度和表明恒星色球活动的Ca H和K线辐射强度之间的关系。发现锂元素丰度值高的年轻恒星,其Ca H和K线辐射更强。考虑到恒星自转速度随恒星年龄的变化,小质量主序前恒星随着恒星年龄的增加,自转明显的加快;主序恒星随着恒星年龄的增加,自转逐渐变慢。我们的结果支持恒星自转越快,其活动性也就越强,以及色球活动较强的恒星具有高的锂元素丰度值。     

银河系中的老寿“星”——兼论对第一代恒星的探索

 天文学家们认为我们银河系内隐藏着年龄大于100亿岁的老年星。按照恒星演化理论,恒星越老,所含金属越少(在天文学中,“金属”一词泛指重于氦的元素),故目前识别老年恒星的惟一途径是审视它们的光谱;光谱中某种金属的特征谱线越暗淡说明该恒星所含此种金属的量越稀少,无此谱线说明该恒星不含此金属。但知易行难,欲从银河系2000亿颗恒星中挑选出老寿“星”不啻是大海捞针。迄今为止,较易做到的是观察数百万颗孤立单星的光谱,并从中筛选出金属特征谱线既少且暗淡的星。     

On the non-synchronous rotation of binary systems

During the evolution of the binary system,many physical processes occur,which can influence the orbital angular velocity and the spin angular velocities of the two components,and influence the non-synchronous or synchronous rotation of the system.These processes include the transfer of masses and angular momentums between the component stars,the loss of mass and angular momentum via stellar winds,and the deformation of the structure of component stars.A study of these processes indicates that they are closely related to the combined effects of tide and rotation.This means,to study the synchronous or non-synchronous rotation of binary systems,one has to consider the contributions of different physical processes simultaneously,instead of the tidal effect alone.A way to know whether the rotation of a binary system is synchronous or non-synchronous is to calculate the orbital angular velocity and the spin angular velocities of the component stars.If all of these angular velocities are equal,the rotation of the system is synchronous.If not,the rotation of the system is non-synchronous.For this aim,a series of equations are developed to calculate the orbital and spin angular velocities.The evolutionary calculation of a binary system with masses of 10M⊙+6M⊙shows that the transfer of masses and angular momentums between the two components,and the deformation of the components structure in the semidetached or in the contact phase can change the rotation of the system from synchronous into non-synchronous rotation.     

An alternative evolution scenario for OAO 1657-415 and IGR J18483-0311

The relation between the spin period and the orbital period in high-mass X-ray binaries is investigated. Be/X-ray binaries and SG/X-ray binaries occupy different areas in Corbet’s diagram. The two SG/X-ray binaries, IGR J18483-0311 and OAO 1657-415, however, appear to occupy a transition region between Be/X-ray binaries and wind-fed SG/X-ray binaries. This can be understood by noting that the neutron stars in the two SG/X-ray binaries have not reached their equilibrium spin periods when the donors evolved off the main sequence. Here, we provide an alternative scenario to explain their location in Corbet’s diagram: the SG/X-ray binaries are the descendants of Be/X-ray binaries, i.e. the supergiants in these systems were formed from O-type emission line stars instead of normal main-sequence stars. Furthermore, we suggest that there are two types of SG/X-ray binaries: one is directly evolved from a normal OB-type main sequence and neutron star system without an X-ray history, and the other evolved from a Be/X-ray binary with an X-ray history.

     

Effect of Tidal Torques on Rotational Mixing in Close Binaries

The effect of tidal torques on rotational mixing in close binaries is investigated. It is found that spin angular momentum can attain a high value due to a strong tidal torque. Nitrogen and helium enrichment occurs early in the binary system that is triggered by tides. The stellar radius can reach a high value in the single star model with high initial velocities at the early stage of the evolution, but efficient rotational mixing can inhibit stellar expanding at the subsequent evolution. Central compactness is increased by the centrifugal force at the early stage of evolution but is reduced by rotational mixing induced by strong tides. The binary models with weak tides have high values of central temperature and stellar radius. Rotational mixing in single stars can slow down the shrinkage of convective cores, while convective cores can be expanded by strong tides in the binary system.Efficient rotational mixing induced by tides can cause the star to evolve towards high temperature and luminosity.     

On the Tidally Induced Gravitational Collapse of a Particle Cluster

An important issue in the dynamics of neutron star binaries is whether tidal interaction can cause the individual stars to collapse into black holes during inspiral. To understand this issue better, we study the dynamics of a cluster of collisionless particles orbiting a non-rotating black hole, which is part of a widely separated circular binary. The companion body's electric- and magnetic-type tidal fields distort the black hole and perturb the cluster, eventually causing the cluster to collapse into the hole as the companion spirals in under the influence of gravitational radiation reaction. We find that magnetic-type tidal forces do not significantly influence the evolution of the cluster as a whole. However, individual orbits can be strongly affected by these forces. For example, some orbits are destabilized due to the addition of magnetic-type tidal forces. We find that the most stable orbits are close to the companion's orbital plane and retrograde with respect to the companion's orbit.     

Millisecond pulsars with r-modes as steady gravitational radiators

Millisecond pulsars (MSPs) probably achieve their fast rotation by mass transfer from their companion stars in low-mass x-ray binaries (LMXBs). The lack of MSPs and LMXBs rotating near breakup has been attributed to the accretion torque being balanced, at fast rotation, by gravitational radiation, perhaps caused by an unstable oscillation mode. It has been argued that internal dissipation involving hyperons may cause LMXBs to evolve into a quasisteady state, with nearly constant rotation rate, temperature, and mode amplitude. We show that MSPs descending from these LMXBs spend a long time in a similar state, as extremely steady sources of gravitational waves and thermal x rays, while they spin down due to gravitational radiation and the standard magnetic torque. Observed MSP braking torques already place meaningful constraints on this scenario.     

The influence of magnetic field on short-wavelength instability of Riemann ellipsoids

We address the question of stability of the so-called S-type Riemann ellipsoids, i.e. a family of Euler flows in gravitational equilibrium with the vorticity and background rotation aligned along the principal axis perpendicular to the flow. The Riemann ellipsoids are the simplest models of self-gravitating, tidally deformed stars in binary systems, with the ellipticity of the flow modelling the tidal deformation. By the use of the WKB theory we show that mathematically the problem of stability of Riemann ellipsoids with respect to short-wavelength perturbations can be reduced to the problem of magneto-elliptic instability in rotating systems, studied previously by Mizerski and Bajer [K.A. Mizerski, K. Bajer, The magneto-elliptic instability of rotating systems, J. Fluid Mech. 632 (2009) 401-430]. In other words the equations describing the evolution of short-wavelength perturbations of the Riemann ellipsoids considered in Lagrangian variables are the same as those for the evolution of the magneto-elliptic-rotational (MER) waves in unbounded domain. This allowed us to use the most unstable MER eigenmodes found in Mizerski et al. [K.A. Mizerski, K. Bajer, H.K. Moffatt, The α-effect associated with elliptical instability, J. Fluid Mech., 2010 (in preparation)] to provide an estimate of the characteristic tidal synchronization time in binary star systems. We use the idea of Tassoul [J.-L. Tassoul, On synchronization in early-type binaries, Astrophys. J. 322 (1987) 856-861] and that the interactions between perturbations significantly increase the effective viscosity and hence the energy dissipation in an Ekman-type boundary layer at the surface of the star. The results obtained suggest that if the magnetic field generated by (say) the secondary component of a binary system is strong enough to affect the flow dynamics in the primary, non-magnetized component, the characteristic tidal synchronization time can be significantly reduced.     

Dynamical determination of the innermost stable circular orbit of binary neutron stars

We determine the innermost stable circular orbit (ISCO) of binary neutron stars (BNSs) by performing dynamical simulations in full general relativity. Evolving quasiequilibrium (QE) binaries that begin at different separations, we bracket the location of the ISCO by distinguishing stable circular orbits from unstable plunges. We study Gamma=2 polytropes of varying compactions in both corotational and irrotational equal-mass binaries. For corotational binaries, we find an ISCO orbital angular frequency somewhat smaller than that determined by applying turning-point methods to QE initial data. For the irrotational binaries, the initial data sequences terminate before reaching a turning point, but we find that the ISCO frequency is reached prior to the termination point. Our findings suggest that the ISCO frequency varies with compaction but does not depend strongly on the stellar spin.     

Maximum gravitational recoil

Recent calculations of gravitational radiation recoil generated during black-hole binary mergers have reopened the possibility that a merged binary can be ejected even from the nucleus of a massive host galaxy. Here we report the first systematic study of gravitational recoil of equal-mass binaries with equal, but counteraligned, spins parallel to the orbital plane. Such an orientation of the spins is expected to maximize the recoil. We find that recoil velocity (which is perpendicular to the orbital plane) varies sinusoidally with the angle that the initial spin directions make with the initial linear momenta of each hole and scales up to a maximum of approximately 4000 km s-1 for maximally rotating holes. Our results show that the amplitude of the recoil velocity can depend sensitively on spin orientations of the black holes prior to merger.