超新星和中子星

1. 超新星和中子星 北京师范大学天文系 李宗伟

2. Supernovae are vast explosions in which a whole star is blown up. They are mostly seen in distant galaxies as `new' stars appearing close to the galaxy of which they are members. They are extremely bright, rivalling, for a few days, the combined light output of all the rest of the stars in the galaxy. As most observed supernovae occur in very distant galaxies they are too faint even for the largest telescopes to be able to study them in great detail. Occasionally they occur in nearby galaxies and then a detailed study in many different wavebands is possible.

3. The formation of the neutron star happens extremely rapidly and once neutron-degeneracy pressure is established the core becomes rigid. The collapse of the star is dramatically halted and the infalling material bounces off the core and starts moving up towards the stars surface. A shock wave of tremendous energy is generated moving at supersonic speeds (5-10 000km s-1) blowing off the rest of the star's outer layers. The neutrinos produced by inverse beta decay swiftly travel out of the core, carrying up to 100 times more energy than is emitted as electromagnetic radiation. This gigantic explosion is called a supernova and can produce enough energy to temporarily outshine the whole galaxy! An energy of 1044 J is normally observed in supernova events.

4. The last supernova to be seen in our galaxy, the Milky Way system, was seen in 1604 by the famous astronomer Kepler. The brightest since then was supernova 1987A in the Large Magellanic Cloud, a small satellite galaxy to the Milky Way. The brightest supernova in the northern sky for 20 years is supernova 1993J in the galaxy M81 which was first seen on March 26 1993.

5. Supernovae Type II • This sudden collapse of a massive star's core into a volume over a million times smaller than its original volume is really bad news for the star. The outer layers of the star come raining down onto the core. Somehow this collapse changes into an explosion: a type II supernova. The process by which this happens is still being investigated, but evidently the core collapses to something below its equilibrium radius and then rebounds slightly. That bounce transfers an enormous amount of energy to the layers falling down from above. Just watch that smaller ball take off after they hit the ground!) A strong wave of energy--a shock wave--travels out through the envelope and heats the star so much that the outer layers are blown away. Another important effect is the huge numbers of neutrinos that are produced when the neutron star is formed. Ordinarily, neutrinos don't interact much with matter, but these neutrinos are so numerous and energetic that they help push the outer layers of the star away.

6. The total amount of energy released in a Type II supernova is about 10 53 ergs. About 99% of that energy is emitted as neutrinos, whereas only 1% is converted into the kinetic and heat energy of the ejecta (i.e., outer gas layers). Yet enough light is emitted by a supernova to make it as bright as a billion Suns. The most famous historical Type II SN became visible on July 4, 1054 and was noted by astronomers in Imperial China. It was easily visible in broad daylight for weeks and did not disappear from nighttime skies until 2 years later. At the position where the supernova was observed, we now see a glowing cloud of gas called the Crab nebula which is expanding at thousands of km/s. Near the center of the Crab is a strong source of radio waves and X-rays called the Crab pulsar. We'll discuss pulsars in a minute.

9. The most important supernova that has happened in modern astronomical history is known as SN 1987A, and became visible, as the name suggests, on February 24, 1987. The explosion occurred in a nearby satellite galaxy of the Milky Way called the Large Magellanic Cloud, so named because it was not known to Europeans until Magellan voyaged south of the Equator. Because the LMC, as it's called, is over 100,000 light years away, the explosion actually occurred over 100,000 years ago. (Remember that the further out we look in space, the further back we are looking in time). • By studying the spectrum and the apparent brightness of SN 1987A, astronomers confirmed many of the ideas for how Type II supernovae occur. They even had pictures of the star before it exploded. It was a blue supergiant star with a mass of around 20 and a luminosity of around . They found evidence of radioactive Co in the SN's spectrum. (This isotope of cobalt is radioactive with a short half-life, indicating that it was freshly synthesized in the star.) Experiments on Earth, which look for neutrinos from the Sun, witnessed a sudden burst of neutrinos just before the SN became visible, supporting another theoretical prediction.

12. Recorded explosions visible to naked eye: Year (A.D.) Where observed Brightness 185 Chinese Brighter than Venus 369 Chinese Brighter than Mars or Jupiter 1006 China, Japan, Korea, Europe, Arabia Brighter than Venus 1054 China, SW India, Arabia ->Crab Nebula Brighter than Venus 1572 Tycho Nearly as bright as Venus 1604 Kepler Brighter than Jupiter 1987 Ian Shelton (Chile) . Supernovae • The explosions of stars with the resulting release of tremendous amounts of radiation.

13. Two important effects of Supernovae: • 1) Many elements are ejected into space. 2) Shock wave will trigger new star formation. • Famous Supernovae: • SN 1987A in the Large Magallenic Cloud. • Kepler's Supernova in 1604. • Tycho's Supernova in 1572. • Crab Nebula Supernova in 1054. • What's Left Behind? • 1) Neutron Star = Pulsar • For stars between 8 and 25 solar masses. • 2) Black Hole • For stars greater than 25 solar masses.

16. M81 SN1993J

17. M51 1994/4/8 SN

21. Supernovae of Type II are further subdivided by the way their brightness fades. In many cases the type II will reach maximum brightness, dim slightly, and then stay at almost the same brightness "plateau" for many days before fading at a fairly regular rate and are designated Type IIP (II-Plateau). Other type II supernovae quickly reach maximum brightness and then dim in a linear fashion and are classified as Type IIL (II-Linear).

25. 1885A—2003-10-12 • Supernovae Number ~2667 • ` 1885-1988: 661 • 1997: 1270 • 1998ff: ( 158 ) 1428 • 1999gw ( 204 ) 1632 • 2000ft ( 176 ) 1808 • 2001ke ( 291 ) 2099 • 2002ld ( 316 ) 2415 • 2003ir ( 252 ) 2667

29. The article from Cappellaro et al. presents a synthetic view of the problem. We only give a summary of this paper. The rate of supernovae depends on the supernovae type and the host galaxy type according to the Hubble sequence. The average value for the supernovae rate is 0.68 SNu (1 SNu = 1 SN per century per 1010 L ). Table gives a detailed description of the rates. It should be noticed that SN Ib/c and SN II (which are due to young stars) never occur in elliptical galaxies which contain only old stars.

31. Type II are due to the formation of neutron stars and the subsequent 'core bounce' of infalling matter. In Type I the energy generated is due to thermonuclear processes whereas in TypeII the energy source is indefinite. Astrophysicists are still unsure of the exact processes that give rise to supernovae and what has been described are based on computer models of stellar structure. Supernovae are not frequently seen .An unprecedented opportunity to observe one at close range presented itself in 1987.

34. Supernovae of Type II occur at the end of the evolution of massive stars. The phenomenon begins when the iron core of the star exceeds a Chandrasekhar mass. The collapse of that core under gravity is well understood and takes a fraction of a second. To understand the phenomenon, a detailed knowledge of the equation of state at the relevant densities and temperatures is required. After collapse, the shock wave moves outward, but probably does not succeed in expelling the mass of the star. The most likely mechanism to do so is the absorption of neutrinos from the core by the material at medium distances. Observations and theory connected with SN 1987A are discussed, as are the conditions just before collapse and the emission of neutrinos by the collapsed core. -Bethe H A Rev.Modern Phys 62, 801

37. 较大质量的恒星(M＞8M⊙)演化到晚期，在中心形成一个1.5M⊙左右的铁镍核，在它的外面依次是Si、O、He等元素为主的包层.铁镍核受外部包层影响很小，所以像一个独立的恒星那样演化.在铁镍核中，核反应过程是耗能的，物质比熵很低，主要靠电子简并压平衡引力.相对论电子简并压与密度的关系pe∝ρ4/3，对这样形式的压强，有一个1。4M⊙左右的极限质量，钱氏质量(Chandrasekhar质量)，当质量大于Mch时，天体的力学结构是不稳定的，这个因素造成了铁镍核的初始坍缩.当坍缩进行到中心密度达到109g/cm3时，电子俘获过程开始对坍缩有影响.由于电子俘获过程中的中微子能量损失和电子数浓度的减少会降低电子简并压，铁镍核的流体力学结构更加偏离平衡状态，引起剧烈的爆缩，最大下落速度超过109g/cm3 .当密度达到1011g/cm3以上时，铁镍核的中微子不透明度升高到使一些中微子陷落在其中.这些中微子会点燃电子俘获的逆反应，在3×1012g/cm3左右出现β平衡，这以后电子数浓度将不再降低.密度达到核物质密度(约2×1014g/cm3)以上时，重原子核被压碎，核子非相对论简并压大大加强，

38. 并且由于强相互作用的介入，使得物质突然变得很硬，中心部分的坍缩被突然制止，并在0.7M⊙左右的质量壳层以外造成反弹激波.激波携带着大量的能量，如果能够克服重核离解、中微子能量损失等耗能因素传到表面，且激波具有的能量约1051erg，就会形成能量、质量抛射的Ⅱ型超新星爆发现象，则称为SNⅡ的瞬发爆炸（prompt explosion).如果激波能量不足以克服重核离解和中微子能量损耗，激波缓慢行进、暂驻甚至后退，持续时间较长，由中子星形成过程中发射的大量中微子加热使激波得以复活而变成为延缓爆炸(delayed explosion). • 无论上述哪种爆炸机制，在坍缩反弹后短时期里，中央残留物体就处于准静力学平衡状态.这残留物体是炽热的，富轻子的，经过冷却和消轻子(中子化)过程而形成一个原始中子星.坍缩反弹瞬时爆发期间中微子能量损失约1051erg，绝大部分中微子 • ［(3～4)×1053erg］是在残留物体形成中子星过程中发射的。 • 尽管在坍缩反弹的研究方面已取得了很大的进展，有了共同的认识，但在星核区坍缩反弹后激波传播和爆发过程研究方面仍存在着不少问题.SNⅡ研究的基本问题是星核区坍缩释放的引力势能如何转移到恒星外部而引起爆发的.目前，这个问题(坍缩与爆发之间的耦合)仍然是捉摸不定的.整个SNⅡ过程可分为三个阶段：坍缩阶段、反弹阶段和激波传播阶段.

39. Because little entropy is produced, nucleons remain in nuclei down to nuclear touching densities. Thus, the collapse cannot be stopped above nuclear matter densities. This is illustrated in the following comparison of the adiabatic index against the Newtonian value of 4/3 for gravitational stability,

42. Every 50 years or so, a massive star in our galaxy blows itself apart in a supernova explosion. Supernovas are one of the most violent events in the universe, and the force of the explosion generates a blinding flash of radiation, as well as shock waves analogous to sonic booms.There are two types of supernovas: Type II, where a massive star explodes; and Type Ia, where a white dwarf collapses because it has pulled too much material from a nearby companion star onto itself. Animation of Supernova ExplosionQuicktime Movie

44. Figure 1: Competing processes that determine the destiny of the supernova shock: Gas infall from the collapsing star damps shock expansion. The gas between the neutron star and the shock is cooled and heated by neutrinos. Only when the neutrino heating is strong enough, an explosion can be

47. A 2D simulation of a Type II SNe explosion The matter is heated By neutrinos from The hot proto-neutron Star .Entropies are Given in units of kB/ Nucleon.

48. The explosive processing and nucleosynthesis in the ejecta gives rise to a large fraction of the present day element abundances. Explosive nucleosynthesis calculations require the knowledge of nuclear reaction rates at high temperatures, to a large extent for unstable nuclei, based on theoretical or experimental efforts. The comparison with abundances from specific supernova observations can probe the correctness of the stellar evolution treatment and the 12C( α，γ )16O rate. SN 1987A showed reasonable agreement with C, O, Si, Cl, and Ar abundance observations.

49. An x-ray image of the Crab Pulsar at center of the Crab Nebula. Photograph from CHANDRA website; click picture to go to website. An x-ray image of the Crab Pulsar at center of the Crab Nebula. Photograph from CHANDRA website; click picture to go to website.