宇宙中的第一代恒星可能是双胞胎
本帖最后由 voyagerbb 于 2009-12-13 20:17 编辑原文这里有一个视频:
http://www.newscientist.com/articlevideo/dn17440/29090525001-universes-first-stars-may-have-been-twins.html
Universe's first stars may have been twins
宇宙中的第一代恒星可能是双胞胎
A good fraction of the universe's first stars may have been born in pairs, a new study suggests.
一项新的研究提出,宇宙中相当大比例的第一代恒星可能是成对诞生的.
Since each star in a pair is likely to be smaller than a single star created from the same natal material, the work may help explain why so far no evidence has been found for exotic physical processes thought to occur in super-heavy stars from the early universe.
既然恒星对中的每一个成员都可能小于从同样的物质中诞生的单个恒星,这项研究可能有助于解释为什么到目前为止还没有发现早期宇宙中的超级重的恒星中发生的非同寻常的物理过程的证据.
It may also mean the star pairs could be detected by the gravitational waves they would emit at the end of their lives.
这还意味着恒星对可能通过引力波来探测,这些引力波是在它们生命结束的时候发出的.
Not much is known about the universe's first stars. It is theoretically possible to see them with telescopes by looking for objects that are extremely far away, since their light takes billions of years to reach Earth. But today's telescopes are not powerful enough to see the dim objects.
人们对宇宙中的第一代恒星还所知不多. 不过既然它们的光线要经过数十亿年才能到地球,理论上,人们可能通过搜寻非常遥远的天体来找到它们.但是现今的望远镜并没有强悍到足以看到这些暗弱的天体.
Computer simulations suggest the ancient beacons, called population III stars, were extraordinarily massive, ranging from 30 to 300 times the mass of the sun. By contrast, the average Milky Way star is just 0.8 times the sun's mass.
数值模拟指出,这些被叫做星族III恒星的远古灯塔质量异乎寻常地大,从30到300太阳质量不等.与此相对,银河系里恒星的平均质量只有0.8个太阳质量.
Natal clouds
诞生云团
But until now, the simulations could not produce a phenomenon that is commonly seen elsewhere – stellar pairs.
但是到目前为止,数值模拟还不能复现一个在其它地方常见的现象--恒星对.
"Multiple star formation had been proposed as a mechanism for forming the first stars in the universe, but this is the first time we've seen it in cosmologically realistic conditions," says Matthew Turk of the Kavli Institute for Particle Astrophysics and Cosmology in Menlo Park, California.
“人们在过去就已经提出多重的恒星形成作为宇宙中第一代恒星形成的一个机制,但这是我们第一次在宇宙的真实情况中看到这一现象”,Matthew Turk说道.他就职于加州门洛帕克市(Menlo Park)的卡弗里粒子天体物理与宇宙学研究所 Kavli Institute for Particle Astrophysics and Cosmology).
Turk and colleagues simulated the conditions 20 million years after the big bang, when the universe was a soup of mostly hydrogen and helium gas. They ran five simulations – each with slightly different initial distributions of gas.
Turk及其同事模拟了宇宙大爆炸之后2千万年内的情况,当时的宇宙就像氢和氦和在一起的汤一样.他们进行了5个模拟--每个模拟中气体的初始分布略有不同.
The simulations modelled what would happen over the first 190 million years after the big bang, enough time for gravity and cooling gas to condense down into cores of dense gas that could eventually evolve to form stars.
这些工作模拟了宇宙大爆炸后前1千900万年内发生的情况,这个时间足够引力和冷气体收缩成致密的气体核,这些气体核最终会演化成恒星.
Two from one
一生二
In one simulation, the team was surprised to see that a cloud had fragmented to form two such cores. Like pizza dough, the spin of the cloud seemed to contribute to tearing it apart. Hydrogen molecules created as the cloud collapsed gravitationally also helped cool the gas, enabling it to clump up even more easily.
在其中的一个模拟中,该团队惊奇地发现一个云团碎裂成了2个这样的核.就想皮萨的生面团一样,自转似乎对云团的撕裂做出了贡献.云团在引力坍缩过程中产生的氢分子也有助于冷却这些气体,使其更容易结团.
Many more simulations will be needed to reveal just how common population III star pairs might be. The fraction of the stars that are born in pairs is "probably less than half but probably more than 5 per cent," Turk says.
人们需要更多的模拟来揭示星族III恒星对到底多么普遍.Turk说,成对产生的恒星的比例”可能小于一半,但可能大于百分之五”.
If a significant fraction of population III stars are born in pairs, it could resolve a strange discrepancy between simulations and observations of the stars' descendents, which can still be found roaming the Milky Way.
如果相当比例的星族III恒星是成对诞生的, 那么数值模拟跟这些恒星后裔的观测之间的一个奇怪的差异就能够得到解决,这些后裔在扫描银河系的时候仍能被发现.
Previous simulations suggested that a large fraction of population III stars may be so massive they are completely torn apart in exotic explosions called pair-instability supernovae.
以前的数值模拟表明,很大一部分的星族III恒星的质量非常大,以至于它们会被称为正负电子对不稳定超新星(pair-instability supernovae)的异常爆发而撕开.
Telltale pattern
典型特征
In stars 140 to 260 times the mass of the sun, the high temperatures and pressure conditions are ideal for converting light into pairs of particles, each comprising an electron and its anti-matter counterpart, a positron.
在质量在140到260太阳质量之间的恒星内,高温和高压非常适合将光转变成粒子对,每个粒子对由一个电子和它对应的反物质粒子,即正电子组成.
This causes a drop in pressure that makes the star unstable and causes it to contract under its own weight. Eventually the process ignites runaway nuclear reactions that rip the star apart.
这引会起压强的突降,使得恒星不稳定并在自己的重力下收缩.最终这个过程引发了雪崩式的核反应并将恒星斯裂开.
These reactions also produce elements with a particular pattern of abundances – elements like nickel with an even number of protons are far more common than those with an odd number of protons.
这些反应还产生了具有独特的丰度特征的元素--像镍这样的具有偶数个质子的元素比那些具有奇数个质子的元素要普遍地多.
Strangely, this pattern has not been seen in the oldest stars in the Milky Way, which descended from population III stars and would have incorporated their remains.
奇怪地是,人们从来没有在银河系的最古老的恒星中发现这一特征,而这些古老的恒星被认为是星族III恒星的后裔并混有它们的遗迹的.
Two smaller stars
两个更小的恒星
Since each of the two stars born from a cloud would likely be less massive than a single star, neither star in the pair might be massive enough to trigger the exotic explosions and the patterns of chemical abundances.
既然从同一个云团中诞生的两个恒星中的任一个都比一个单一的恒星要小,那恒星对中的任何一个成员都没有大到足以引发异常的爆发并给出独特的元素丰度.
Finding that population III stars can be born in pairs "nicely makes this discrepancy go away, which is really exciting", says co-author Brian O'Shea of Michigan State University.
来自密西根州立大学的另一位作者Brian O`Shea说,星族III恒星可以成对诞生这一发现”很好地解决了这个不一致,真是一件兴奋的事情”.
"But there are other ways to get around that problem," says Volker Bromm of the University of Texas in Austin, who was not a part of the team.
“但是还有其它的途径来绕开这个问题”,Volker Bromm说,他来自得克萨斯大学奥斯丁分校,不是这个团队的成员.
He says the reason such chemical patterns have not been found is because existing surveys of Milky Way stars are not looking at the stars that would have them. Existing surveys hunt for stars that are very low in heavy elements, but he says evidence for extremely heavyweight stars would be found in stars that have higher abundances of such elements.
他说人们之所以没有发现这样的元素丰度的特征是因为现存的银河系恒星巡天并没有去寻找有这样特征的恒星.现存的巡天都是在找那些非常低金属的恒星,但是他说,极端超重恒星存在的证据将会在高金属丰度的恒星中找到.
Slingshot away
弹开
If the first stars did form in pairs that orbited each other, over time they would be expected to spiral towards each other as they radiated energy away by emitting gravitational waves, or ripples in space-time. Eventually, they would merge, producing a burst of gravitational waves that could one day be detectable from Earth.
如果第一代恒星确实成对形成并且互相绕转的话,那么他们就会发出引力波,或者叫时空上的涟漪,随着时间的推移他们就会螺旋形地接近对方.最终,他们会并合,产生引力波爆发,这种爆发某天就可能在地球上被探测到.
Bromm adds that stellar pairs might make it difficult for supermassive black holes – which are thought to exist at the heart of large galaxies – to form by mergers between smaller black holes. That's because each star in the pair could become a black hole when it died, and a tightly orbiting pair of black holes could slingshot other black holes away instead of absorbing them, he says.
Bromm补充说道, 恒星对可能给超大质量黑洞带来困难,这样的黑洞被认为存在于大的星系的核心,并且通过小黑洞的并合形成.要是恒星对中的每一个成员在死亡的时候都变成黑洞的话,那么互相紧密绕转的黑洞对会把别的黑洞像弹弓一样弹开而不是吸收进来.
Journal reference: Science (10.1126/science.1173540)
这是Science原文的链接,供参考:
http://www.sciencemag.org/cgi/content/abstract/1173540
英文原文出自:
http://www.newscientist.com/article/dn17440-universes-first-stars-may-have-been-twins.html
先抢沙发，再好好学习~~::003:: 我不太明白的是，如果两个恒星最后合并了，不是立刻就会引发不稳定对爆发吗？
另外，超大质量黑洞是否应该是自己坍缩形成的，像那种10亿太阳质量以上的，靠吸积、合并，时间不够吧，而且貌似现在都观测到100亿太阳质量以上的超级黑洞了。 成对诞生？？ 3# gohomeman1
不是啊,两个小质量的恒星并合在一起并不马上等效为一个大质量的恒星,尤其是中心的核反应状况.剧烈的并合反而可能抛射出去一些物质,最后剩下的东西聚合在一起再反应...那就复杂了.
而且双星通过辐射引力波而损耗能量最后并合在一起的时标恐怕很长,比恒星自己寿命要长的多,除非双星本身处在一个较密的介质里,提供了大量的摩擦力.
超大质量黑洞的增长确实现在有很多不确定性,关于吸积,一个简单的估计就是假设吸积一直顶着爱丁顿极限,也就是:
LED=1.3*10^38(M/M_sun)
LBH=6*10^46*epsilon*dM_BH/dt
可以让LED=LBH来估算一个黑洞的增长过程,是一个指数的形式,取决于初始的质量和epsilon这个参数.
当然增长不可能一直靠近爱丁顿极限的,但是又有人提出"超爱丁顿吸积"的概念,所以...
并合还要麻烦些,因为并合太频繁反而可能把后来者给踢出去.
总之可调的参数很多,一方面说明想要什么样的结果就有什么样的结果,另一方面也说明没有哪个是特别让人信服的. 2# 一分半
欢迎指正. 4# MBR
只能说是一家之言吧,虽然上了Science.
他们要是能够给出具体哪些过程会在多大程度上导致碎裂的发生,并通过控制这些条件来检验其贡献程度,再证明在真实的宇宙中这些过程或者条件是满足的,那就好了,不过感觉现在还没有达到这一步. 5# voyagerbb
貌似现在的大质量黑洞，不是吸积时大量物质都被以高能喷流的方式回到太空了吗，并不是引力范围内都归入黑洞了啊。所以我想那些超级黑洞应该是在物质涨落出现后就直接坍缩的。
我以下这点不了解：最初宇宙是怎么减速的，这点好像是说不清的，对吗？ 8# gohomeman1
是的,不过我上面给出的是一个质量的净增加率.
直接的物质密度涨落是不大可能坍缩成黑洞的,因为最早的时候由于气体有压强,所以暗物质的密度涨落要大于气体的,要是气体都能直接坍缩成黑洞的话,暗物质早就直接坍缩为黑洞了.另外物质密度的涨落是随时间增长的,早期的密度涨落很小,如果现在没有可以直接坍缩为黑洞的区域,早期就更没有了.
下面那个,你是指暴胀是怎么结束的吗? 9# voyagerbb
啊呀，是的说。我以前倒是没想过暗物质在这方面的作用。如果去想黑洞中是否有暗物质，甚至进一步说，我们身边是否就有暗物质呢？这个问题貌似太大了，以后再说吧。
第2点确实在问暴涨是怎么结束的，按理一开始膨胀这么强、这么快（远远超越了光速），后面怎么刹车呢？
最后，能否稍微介绍一个那两个公式，我看到其中的指数值分别为10的38、46次方，是个极大的数，而且质量对比的基数是太阳质量，就是说后面对应的质量都快相当于我们的宇宙了吧。 10# gohomeman1
不过目前人们一般认为是没有的,倒是有所谓的暗物质星的说法,也就是靠暗物质湮灭提供能源的恒星.
这个确实是超出了经典宇宙学的范围吧,我也只是了解一点点.
开始的时候标量场的势演化非常缓慢,即所谓的"慢滚"条件,在这种情况之下,哈勃参数的平方大致是正比于这个势的,可以认为哈勃参数基本不变,由
da/(a*dt)=H
可知,在哈勃参数为常数的情况下,尺度因子a是呈指数增长的,即所谓的"暴涨",等到后来,标量场接近最小值的时候,势就变的陡峭起来,随着时间就快速演化了,此时哈勃参数就不再是常数,也就是退出了暴涨.
那两个公式中,L_ED和L_BH的单位都是尔格/秒,dM_BH/dt的单位是太阳质量/年. 学习一下吧！原来还真不知道这件事！ 11# voyagerbb
哦，我忘了天文学上常用尔格做单位的，这样的能量数值与焦耳比，等于是减去了7个数量级。谢谢啦！ 13# gohomeman1
不必客气,讨论问题对大家都是获益匪浅的. 哪里哪里，我谢你是理所应当的。
关于大团块分裂，是否有图什么的可供直观理解？ 15# gohomeman1
我觉得这个图挺合适的,反应了一个初始的云团在不同的冷却和吸积条件下最终生成的恒星的质量:
本帖最后由 gohomeman1 于 2009-12-15 14:39 编辑
前几步看得懂，大的冷氢云先分裂为团块，然后再吸积为恒星。貌似分子云收缩后形成的团块更大，原子云反而只能形成小团块？
最后一步吸积时，那两个条件不太明白，Macc是吸积速率的导数吗？是不是说吸积核心越迟点燃热核反应，吸积的效果越好，恒星越大？如果吸积核心很早就到达了点燃核反应的条件，反而因为恒星风而把团块中其余物质都给吹散了？ 17# gohomeman1
是指通过氢原子冷却或者通过氢分子冷却.
dMacc/dt 是吸积率,不过不是跟点燃恒星有关,而是在吸积的过程中就会产生辐射(原理跟AGN一样),如果吸积率太强,产生的辐射压就会阻止进一步的吸积和质量增长,最终的恒星质量反而不大.要是吸积率比较低,慢慢来,最终反而可能形成大质量的恒星. 谢谢，我把crit的条件理解错了。是的，吸积时重力势能转化为热能就会变成辐射。
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