天文学家辨认出首次碳爆炸英文原文
著:Robert Irion 译:
Shea


  天文学家非常喜欢观看发生在宇宙空间的爆炸,但是他们却从来也没有观测到过上周所发现的那一次爆炸。一份美国天文学会高能天体物理部的报告称,在一次3小时的热核爆炸中一颗密度极高的恒星的表面的碳发生了爆炸。如果它被证实,那它将是首次被观测到的以碳而不是以氢或氦为燃料的宇宙爆炸。由于它可以证实或者核查现有的碳燃烧模型,所以加州大学的理论家拉斯·比尔德斯登(Lars Bildsten)说:“从核物理的角度看,它的前景是令人兴奋的。”

  这次爆炸来自于一对“低质量X射线双星”。在这样一个系统中,一颗矮星非常靠近地围绕一颗中子星旋转。气体从矮星流入一个绕中子星旋转的炽热的吸积盘中。一些气体会撞击中子星的表面,形成致密的半流质的氢和氦,还有少量的重元素。当逐渐变厚的层中的温度和压力达到足够高时,就会引发热核爆炸,其中的元素会发生瞬间核聚变(flash-fuse)。之后,这一氢层又会被重建,在一定的时间间隔以后,通常为数小时或几天,会再一次出现闪光。尽管由于两颗恒星轨道的动力学因素会使这一时间间隔发生剧烈的改变,但这一过程仍以不确定的方式在继续。

  人造卫星发现大多数源于这些系统的爆发是持续10或20秒的温和的X射线爆。然而,去年天文学家探测到4个X射线爆打破了这一模式。首先,荷兰空间研究组织(the Space Research Organization Netherlands,SRON)的研究人员使用荷兰、意大利联合研制的BeppoSAX卫星发现了已知的3个低质量X射线双星存在长时间爆发的证据。这些爆发持续的时间是短爆发的500倍并且多释放出500至1000倍的能量。“这是挑战现有理论的一种全新的爆发,”SRON的天文学家约翰·汉斯(John Heise)说。然而,他的同事埃瑞克·库科斯(Erik Kuulkers)却不愿逾距宣称这些爆发是碳基的。库科斯相信,那些双星系统中富氢的矮星不会倾倒足够的原料到中子星的表面上。

  但是由NASA的戈达德航天中心的特德·斯特梅耶(Tod Strohmayer)发现的第四个长时间爆发却有些不同。1999年9月,NASA的X射线时变探测器(Rossi X-ray Timing Explorer,RXTE)探测到源自于4U 1820-30——这一对已知最近密的低质量X射线双星——的强大爆发。两颗恒星仅以比木星稍大的间隔互相绕转,周期为11分钟。其中的白矮星用几乎是纯氦来“喂养”那颗中子星,因为它的氢在很久以前就被剥离了。

  斯特梅耶说,那些氦会慢慢地制造出一颗炸弹。这一层积累的氦会在爆炸前达到20至30米厚,而在4U 1820-30上这一过程在一天之内会发生数次。每次爆发都会残留下一些碳,它是氦聚变的主要产物之一。按照斯特梅耶的设想,大约一年后这颗中子星上会覆盖几百米厚的碳。当这一层碳的底部温度达到临界温度阈时,这个碳炸弹就会被点燃,接着它就会爆发数小时。

  “这家伙的威力是氦爆发的1000倍,”斯特梅耶说,“它可能会吹散整个吸积盘。”如果这是真的话,那么天文学家已经得到了当气体盘旋着坠入致密星时吸积盘如何运动变化的最有价值的信息。之后来自快速旋转的白矮星的新物质会迅速地再次充满被吹散的吸积盘,斯特梅耶说。RXTE的数据可能包含有吸积盘再次聚合时所释放出的X射线,它揭示了我们迄今从未见过的物理过程。

  更使理论家跃跃欲试的是,这是他们第一次仔细的观察一次碳爆炸的细节,而不是计算机模拟。而且,它暗示先前的理论计算结果并没有与现实情况相匹配。比尔德斯登和他以前的学生爱德华·布朗(Edward Brown)的最新研究表明中子星的表面温度应该降至不足10亿度,这只能点燃一个相对薄的碳层。一个厚的多得碳层——以及其所导致的底部高压——可能促成了其成为引爆炸弹的诱因。然而,布朗注意到,由4U 1820-30传输物质的速度来看,这样一个碳层需要一个世纪的时间来积累。这使得斯特梅耶的观测发现变得十分的幸运。

  虽然如此,比尔德斯登和布朗同时认为这些爆发最能可能的解释仍是碳爆炸。至于调和理论和观测结果的差别,比尔德斯登则说:“这是值得沉思的有趣问题。”

 

   译自 [Science 17 Nov 2000]

Astronomers Spot Their First Carbon Bomb
Chinese Version
By Robert Irion


  HONOLULU--Astronomers love watching things blow up, but they've never seen a blast quite like the one described here last week. Carbon on the surface of an ultradense star detonated in a 3-hour thermonuclear explosion, according to a report at a meeting of the American Astronomical Society's High Energy Astrophysics Division. If confirmed, the burst would be the first known cosmic explosion fueled solely by carbon rather than hydrogen or helium. That prospect, says theorist Lars Bildsten of the University of California, Santa Barbara, is "very exciting from a nuclear physics standpoint" for its potential to verify or revise models of carbon combustion.

  The blast came from a waltzing pair of stars called a "low-mass x-ray binary." In such a system, a dwarf star orbits closely around a neutron star, a stellar corpse that packs the mass of one or two suns into a dense ball just 20 kilometers wide. Gas from the dwarf flows into a hot spiraling disk around the neutron star. Some gas hits the star's surface, forming a compressed slurry of hydrogen, helium, and a few heavier elements. When pressures and temperatures get high enough within the thickening layer, the elements can flash-fuse in a thermonuclear explosion. Then, the layer rebuilds and the flash repeats after some interval, usually hours or days. This process continues indefinitely, although the timing changes drastically depending on the orbital dynamics of the two stars.

  Satellites see most explosions from such systems as mild x-ray flares that last 10 or 20 seconds. Last year, however, astronomers detected four flares that broke the mold. First, researchers at the Space Research Organization Netherlands (SRON) in Utrecht used the Dutch-Italian BeppoSAX satellite to find evidence for a long burst from each of three known low-mass x-ray binaries. The events lasted 500 times longer and unleashed 500 to 1000 times more energy than the generic short pops. "It's a new class of events that will challenge the theorists," says SRON astronomer John Heise. Heise's colleague Erik Kuulkers, however, stops short of claiming that the explosions are carbon-based. The hydrogen-rich dwarf stars in those binary systems don't dump the necessary raw ingredients onto their neutron star companions, Kuulkers believes.

  That's not the case with a fourth long burst, found by Tod Strohmayer of NASA's Goddard Space Flight Center in Greenbelt, Maryland. On 9 September 1999, NASA's Rossi X-ray Timing Explorer (RXTE) satellite picked up a powerful flare from 4U 1820-30, the tightest known low-mass x-ray binary. The two stars whirl around each other once every 11 minutes within a volume of space just slightly larger than the planet Jupiter. The white dwarf feeds nearly pure helium to the neutron star, as its hydrogen gas was stripped long ago.

  That helium slowly builds the bomb, Strohmayer says. The helium layer grows 20 or 30 meters thick before it explodes, a process that can happen a few times a day at 4U 1820-30. Each blast leaves some carbon, one of the main ashes of helium fusion. Those ashes, in turn, mantle the neutron star with several hundred meters of carbon after about a year, according to Strohmayer's scenario. When the base of the layer reaches some critical temperature threshold, it ignites a carbon bomb that rages for hours.

  "This thing is 1000 times more powerful than the helium bursts," Strohmayer says. "It may blow apart the entire accretion disk." If that's the case, astronomers might gain their best insight yet into how disks of hot gas behave when they spiral into compact stellar remnants. New material from the frantically orbiting white dwarf would quickly replenish the blown-apart disk, Strohmayer says. RXTE's data may contain x-rays flowing from the disk during that reassembly, exposing the physics of the process as never before.

  Even more tantalizing to theorists is their first close look at the details of a real carbon detonation, rather than one based on computer codes. Already, there are hints that prior theoretical calculations don't quite align with the stars. Recent work by Bildsten and former student Edward Brown, now a Fermi Fellow at the University of Chicago, suggests that temperatures within the rind of a neutron star should fall well short of the billion degrees or so needed to ignite a relatively thin layer of carbon. A much thicker layer--and the resulting higher pressures at its base--might suffice to trigger the bomb. However, Brown notes, the rates of matter transfer in 4U 1820-30 imply that such a layer would require a century to accumulate. That would make Strohmayer's observation lucky indeed.

  Even so, Bildsten and Brown concur that the likeliest explanation is a carbon blast. As for reconciling the theoretical and observational differences, Bildsten says, "it's a fun problem for us to ponder." 

 

   From [Science 17 Nov 2000]

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