Transcript type II supernova

超新星和中子星
北京师范大学天文系
李宗伟
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.
• 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.
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.
• 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.
• 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.
• 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.
Supernovae
The explosions of stars with the resulting release of tremendous amounts
of radiation.
Recorded explosions visible to naked eye:
Year (A.D.) Where observed
185
Brightness
1572
Brighter than Venus
Brighter than Mars or
Chinese
Jupiter
China, Japan, Korea, Europe, Arabia Brighter than Venus
China, SW India, Arabia ->Crab
Brighter than Venus
Nebula
Tycho
Nearly as bright as Venus
1604
Kepler
Brighter than Jupiter
1987
Ian Shelton (Chile)
.
369
1006
1054
Chinese
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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.
SN2000cj
NGC6753
35E 8S
14.8mag IIn
M81 SN1993J
M51 1994/4/8 SN
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).
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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
表一:到编号SN2000ek的超新星按照主星系类型的分类统
• 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.
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
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
•
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，
• 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.
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,
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.
Animation of Supernova Explosion
Quicktime Movie
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.
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
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.
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.
An x-ray image of the Crab Pulsar at center of the Crab Nebula.
Photograph from CHANDRA website; click picture to go to website.
Rough estimates:
• Upper limit on mass:
M NS  1.4 M sun (2 /  n ) 2  5.6 M sun
• Radius:
   nuc  1.5 10 kg m
17
3
4 3
M 
R   R  16 km, for M  1.4 M sun
3
• Escape velocity:
V  (2GM / R)
1/ 2
 0.5c
Relativisitic effects must be taken into account
• Many-body QCD problem: equation of state uncertain.
• e.g. free neutrons can exist in neutron stars
• Current best estimate of the mass upper limit is
M NS, upper  3  4 M sun
• From binary pulsar observations
M NS  1.4 M sun
Neutron star interior structure
• Outer crust 56 Fe
• Inner curst: 1S0 neutron
super fluid
• 3P1 neutron super fluid, 1S0
proton superconductivity
Are there stars denser than
neutron stars? Strange stars?
• Strange matter may have lower binding
energy per nucleon
• Strange stars consist of u, d, s quarks and
electrons
• Key signature: similar mass but smaller
radius (difficult to measure)
• Unclear whether strange stars exist
谢谢