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1、第五章 密近双星的演化,5.1 密近双星的分类 5.2 激变变星 5.3 X射线双星,5.1 密近双星的分类,1. 洛希瓣和双星分类 (1) Roche势和等势面 1864年，法国数学家Roche首先提出计算密近双星系统引力势的简化模型。 方法：考虑在两个互相绕转的天体的引力势中一个检验粒子的运动。,基本假设： 1) 每个子星的内部密度分布是球对称的，在动力学上可以认为是质点。 2) 两子星以圆轨道绕公共质心运动（潮汐力）。 3) 两子星的自转和公转一致（潮汐力）。,在以双星公共质心为原点的共转坐标系中，任意质点的Euler方程是：,其中 Coriolis力 Roche势（包含引力和离心力的贡
2、献）,等势面： (r) = 常数的曲面族,等势面的特点： 1) 垂直于本地的有效重力方向，是等压强面和等密度面 2) 等势面的形状取决于子星质量比，大小取决于双星距离 3) 当r a时，等势面是圆形，圆心在公共质心处 4) 当r R时，等势面是圆形，圆心在子星的质心处（物质的运动主要受较近恒星的引力控制） 在 和 处Roche势取极小值,(2) 洛希瓣 (Roche lobe) 临界等势面：同时包络两颗子星并且相接于其间一点（L1）的等势面。 洛希瓣：由临界等势面包围的空间。,在L1（内拉格朗日）点，两颗子星对物质产生的作用力正好相等，Roche势达极大值。 当子星充满洛希瓣后，在内拉格朗日附
3、近的物质处于不稳定状态，受到小扰动就会流向伴星，产生物质交流。,(3) 双星的Kopal分类 根据双星中的一颗或两颗子星是否充满洛希瓣，可以将双星分为 不相接双星 (detached binaries)：两颗子星均未充满洛希瓣， 半相接双星 (semidetached binaries)：一颗子星充满洛希瓣，如天琴， 相接双星 (contact binaries)：两颗子星均充满洛希瓣，如大熊W。,2. 双星中的物质传输 (1) 星风传输（不相接、半相接双星） 大质量恒星在演化过程中通常有强烈的星风。,(2) 洛希瓣渗溢 (Roche-lobe overflow)（半相接、相接双星）,形成吸积
4、盘 (accretion disk),3. 大陵 (Algol) 佯谬 大陵五（英仙） 轨道周期2.867天 主星：B8型主序星 M3.7 M, R3 R 伴星：G5型亚巨星， M0.8 M, R3.4 R，已充满洛希瓣 (2) 大陵佯谬：质量小的恒星反而演化得快？ 根据恒星演化理论，质量越大的恒星主序寿命越短，越早进入巨星阶段。,(3) 解释 亚巨星子星的前身星是一颗双星系统中质量较大的主序星，它先演化充满洛希瓣。 物质传输使得两子星的质量发生逆转。 物质传输在初始阶段进行得较快，当两子星的质量相等后进行得较慢。,5.2 激变变星,1. 激变变星 (cataclysmic variables
5、 ) 白矮星与红矮星构成的半相接双星。 白矮星通过吸积伴星的物质产生辐射。,轨道周期分布在几十分钟至数天，在2至3小时间有一个明显的间隙,分类,B0.1-1 MG 新星 (novae)、再发新星 (recurrent novae)、类新星变星 (nova-like variables) 和矮新星 (dwarf novae) 等 B 1-10 MG Intermediate Polars (DQ Her stars) B 10-100 MG Polars (AM Her stars),Non-Magnetic white dwarf binaries,Magnetic white dwarf b
6、inaries,辐射主要在光学和紫外波段 爆发时的能量释放率 1045-1046 ergs-1 抛射约10-5-10-3 M 的物质，抛射物质速度100-5000 kms-1,2. 新星 (1) 观测特征 在几天到几星期内亮度增加7-16星等，然后缓慢下降，经几个月或几年回复到原先的状态。,例如 Nova Herculis 1934 爆发时星等为3m; (b) 两个月后星等为12m; (c) 40年后向外抛射的物质半径达0.05 ly,Nova Cygni 1975 爆发时星等为2m; (b) 爆发后降为15m,(2) 物理解释,White dwarf siphons off matter f
7、rom companion star, creating accretion disk. Gravity pulls matter in toward surface of white dwarf.,An “ocean“ of hydrogen forms on the white dwarfs surface. Intense pressure and heat build at the bottom of the hydrogen ocean, eventually leading to a massive explosion, which blows off the outer laye
8、rs of hydrogen. Then the siphoning process begins again.,The nova outbursts lasts for tens to hundreds of days. Eventually, as in this HST image of Nova Cygni 1992, the ejected envelope is visible as a limb-brightened shell expanding away from the binary (the central point source in this image) at s
9、peeds of a few hundred to a few thousand km/s.,T Pyxidis 爆发间隔约20年,(3) 再发新星 观测到不止一次类似新星爆发的激变变星。 典型的爆发间隔约10-100年。,The first wave, or shell, of blown-off matter travels at a terrific rate of speed. The shells outer edge moves faster than its inner edge. Gaseous “knots“ observed by Hubble may form from
10、shock waves that occur when matter ejected during the most recent explosion collides with slow-moving material from the previous outburst.,For example, material from the 1966 explosion of T Pyxidis is illustrated in collision with debris from a 1944 eruption.,Cycles of nova shell collisions create c
11、oncentric circles of gaseous knots.,3. Ia型超新星,当吸积白矮星的质量达到Chandrasekha极限，白矮星的爆燃而导致的超新星爆发。,Evolution of CVs,4 M and 1 M Main-sequence stars in binary. Nearing the end of its life, the 4 M star swells, spilling gas onto its companion. Formation of a planetary nebula. as the lower mass star becomes a re
12、d, accretion occurs again. The end point of the system is two white dwarfs of roughly equal mass circling each other.,5.3 X射线双星,1. X射线双星 (X-ray binaries) 由致密星（中子星或黑洞）与正常恒星组成的双星系统。 致密星通过吸积伴星物质产生X射线辐射。,X-ray Astronomy,On the 18th day of June, 1962, the Aerobee sounding rocket began to sweep the sky, l
13、ooking at our Moon. What?! X-rays from the MOON? The Moon shines by reflected light of the Sun, so a certain fraction of the Suns x-rays will be reflected off the lunar surface, back toward the Earth, and the rocket.,X-ray Astronomy,An enormous spike appeared on the chart recorder! The spike, seen o
14、ver and over again, was not from the Moon at all, but actually came from another part of the sky, about 25 degrees away.,X-ray Astronomy,The signal was real, not an equipment malfunction, and not from the Moon. It was from a cosmic source (Sco X-1) of X-rays of truly staggering proportions.,X-ray As
15、tronomy,The Moon in X-ray Light taken by ROSAT,X-ray Observatories,Uhuru,Einstein,ROSAT,ASCA,RXTE,BeppoSAX,Chandra,XMM-Newton,The X-ray Sky,Chandra Sky Map,2. X射线辐射,(1) 能源 吸积物质引力势能动能热能X射线辐射 其中 为吸积率，R为引力半径 (2) 产能率 能量转换效率 （仅与天体的致密程度有关）,对中子星和黑洞，吸积是最为有效的产能形式,3. 分类和观测特征,高质量X射线双星,低质量X射线双星,High-Mass X-ray
16、Binaries,Donor star has early spectral type (O, B), M10M,HMXBs are usually X-ray pulsars,辐射机制： 强磁场吸积流内区截断吸积柱（磁极）束状辐射,Low-Mass X-ray Binaries,Donor star has late spectral type (A and later), I.e. M1.2M,LMXBs are usually X-ray bursters.,4U1705-44,爆发机制： 吸积中子星表面H壳层的失控热核反应. 爆发间隔 几小时. 爆发能 10-5新星. 中子星比白矮星更
17、为致密 表面积小爆发物质量少 表面温度高X射线,Black Hole Candidates,Persistent Black Hole Candidates in HMXBs,Cyg X-1,Transient Black Hole Candidates in LMXBs,Microquasars,BHCs with strong radio jets. Superluminal motion.,The first galactic superluminal source GRS 1915+105,SS 433,吸积产生喷流 轨道周期：13 days 喷流速度：0.26 c 喷流进动周期：164days 致密星： 中子星？黑洞？,Jets from SS 433,Soft X-ray image,