早于太阳诞生的晶体揭示了太阳系的起源
内容总结：
比太阳更古老的晶体改写太阳系起源故事
长期以来，科学界普遍认为太阳系的诞生始于46亿年前：一片巨大的尘埃云在附近超新星爆发的冲击下坍缩，中心部分形成了太阳，其余物质则凝聚成行星等天体。然而，对陨石中“前太阳颗粒”——即形成于太阳诞生之前、地球上可获取的最古老物质——的最新研究，正在挑战这一经典理论。
关键线索来自放射性同位素。科学家早前在著名的阿连德陨石中发现了大量铝-26衰变产物镁-26，这曾被认为是附近超新星注入并触发太阳系形成的“铁证”。但近年来越来越精确的测量显示，早期太阳系中另一种预期由超新星产生的同位素铁-60含量极低，这与超新星触发理论产生了矛盾。
“许多研究者似乎已接受并非超新星触发这一观点，”芝加哥大学天文学家维克拉姆·德瓦卡达斯表示。那么，铝-26从何而来？一种日益受到重视的新假说指出：这些关键物质可能来自一类巨大恒星——沃尔夫-拉叶星——的猛烈星风。
沃尔夫-拉叶星质量极大、寿命短暂，其剥离外层后暴露的炽热核心能吹出高速星风，将大量物质（富含铝-26而几乎不含铁-60）扫入周围空间，形成巨大的气泡状壳层。德瓦卡达斯认为，银河系中多达16%的类太阳恒星可能以此方式诞生。他与其同事已构建了迄今最完整的模型，描述沃尔夫-拉叶星星风如何将铝-26注入正在形成的太阳系。
不过，该理论也面临挑战。匈牙利天文学家玛丽亚·卢加罗指出，沃尔夫-拉叶星周围环境极为剧烈，可能撕裂新生的太阳系。目前，不同领域的科学家仍各持己见：卡内基科学研究所的艾伦·博斯仍支持超新星触发说；而卢加罗从核物理角度更倾向沃尔夫-拉叶星风假说，但她坦言：“我们仍在为此争论。”
为寻找决定性证据，波士顿大学的宇宙化学家刘楠（Nan Liu）等人正利用纳米探针分析陨石溶解后残存的微小晶体，寻找可能源自沃尔夫-拉叶星的特定颗粒，并检测其是否富含铝-26。刘楠将这项工作比作“钓鱼探险”——若找到足够多此类颗粒，将能约束沃尔夫-拉叶星模型的参数；若一无所获，则该假说很可能不成立。
这些比太阳更古老的微小晶体，正引领科学家回溯太阳系诞生的最初时刻。正如刘楠所言，研究这些颗粒让人意识到地球乃至生命所需元素的形成“并非易事”，需要“在正确的时间与地点”才能获得恰到好处的条件。这场关于太阳系起源的探索，仍在持续深入。
中文翻译：
比太阳更古老的晶体揭示了太阳系起源的奥秘
引言
关于太阳系起源的标准故事是这样的：46亿年前，一团巨大的尘埃云在太空中凝结。随后，一颗邻近恒星的爆炸导致部分尘埃云坍缩。在引力作用下，尘埃向中心点汇聚，凝聚成一个直径约140万公里、由氢和氦组成的炽热球体——这便是我们太阳的前身。其余物质则进入轨道，聚集形成了太阳系的行星，以及大量小行星和其他宇宙残留物。
为了验证这个故事的真实性，研究人员需要回溯到太阳系诞生之初甚至更早的时刻。宇宙化学家刘楠（音）有办法做到这一点：在波士顿大学天体物理研究所她办公桌的保险柜里，锁着一块陨石碎片，上面点缀着比太阳更古老的物质。
"这是最原始（类型）的陨石，没有受到水或热的影响，"刘楠一边取出并举起这块样本一边说道。这是一块闪亮的黑色石头，大小和形状类似箭头。
像这样的陨石形成于尘埃云坍缩时期。尘埃云的坍缩和太阳的点燃，抹去了陨石中包含的大部分化学信息，但其内部一些比单个细菌细胞还小的微观晶体却完好无损地保存了下来。这些被称为"前太阳颗粒"的晶体，是目前地球上我们能接触到的最古老的物质。
在过去十年左右的时间里，科学家们利用像刘楠所拥有的这类陨石，对太阳系形成的故事提出了挑战。太阳系及其中的一切，其起源可能并非源于超新星爆炸，而是来自一种听起来更温和的宇宙情景：也许我们的太阳系是由一颗巨大恒星吹出的星风物质拼凑而成的。对前太阳颗粒的新研究，可能为判断这个新故事是否正确提供一种方法。
始于爆炸？
1969年，一个火球出现在墨西哥上空，科学家们首次获得了关于太阳系形成可能触发因素的线索。如今著名的阿连德陨石将其碎片散布在超过500平方公里的区域。
1976年，研究人员报告称，来自阿连德的样本带来了一个意外：其中含有异常大量的稳定同位素镁-26。他们提出，这块陨石形成时富含放射性同位素铝-26，铝-26衰变后留下了镁-26。
然而，铝-26并非已知的星际介质（即恒星之间充满尘埃的空间，它提供了阿连德陨石的物质）的正常成分。普通恒星不会产生这种特定的同位素。"我们在早期太阳系中观测到的大多数这些同位素，它们只是银河系化学演化的自然产物，"匈牙利康科利·泰格·米克洛什天文研究所的天体物理学家玛丽亚·卢加罗说。"最重要的例外就是铝-26。"
那么它来自哪里？1977年，两位杰出的天体物理学家提出，这种异常的铝很可能来自附近的一次超新星爆炸。其他现象也能产生铝-26，但超新星的冲击波也可能导致了尘埃云的坍缩。通过单一事件，天文学家可以解释两种罕见事件——铝-26的注入和新太阳系的形成——为何几乎同时发生。"大家都觉得我们需要某种东西来触发坍缩，"芝加哥大学的天文学家维克拉姆·德瓦卡达斯说。
在接下来的几十年里，超新星触发说一直是主流观点，得到了详细的天体物理模型以及对原始陨石中富集镁-26的进一步测量的支持。但在过去十年左右的时间里，这种观点遇到了其他似乎不相符的测量结果的挑战。问题是：太阳系存在铁元素不足的情况。
并非铁证如山
超新星不仅仅产生铝。任何附近的超新星都可能同时注入大量放射性同位素铁-60。因此，如果超新星引发了太阳系的形成，"我们本应在早期形成的天体中发现相当高的初始铁-60丰度，"哥本哈根大学的宇宙化学家方琳茹（音）在一封电子邮件中写道。
一些研究报告称在陨石样本中发现了足够的铁-60来支持超新星说。但并非所有科学家都同意这些发现；几位研究人员告诉《量子》杂志，大多数宇宙化学家现在认为，虽然太阳系开始时铝-26很丰富，但铁-60终究不多。
去年年初，在一项被其作者描述为迄今为止对早期太阳系中铁-60最精确测量的研究中，方琳茹和她的同事报告称，在一个尘埃云坍缩后不久形成的星子中，铁-60（通过其稳定的衰变产物镍-60测量）含量很低。她说，这个结果与超新星情景不符。
研究人员为缺失的铁提出了各种解释。"陨石学家是出了名的爱争论的人，"华盛顿特区卡内基科学研究所的天文学家艾伦·博斯在一封电子邮件中写道。"似乎总有人能对任何声称是事实的说法提出反例。"
例如，铝可能从超新星中爆炸喷出，而来自恒星核心更深处的铁则可能落回了死亡的恒星中。或者爆炸可能来自一种古怪的、根本不产生铁-60的超新星。也可能是铁-60在尘埃云中分布不均，这意味着来自单个陨石的测量可能无法反映全貌。
德瓦卡达斯驳斥了这些解释，认为它们是"挥手示意"式的尝试，只是为了微调模型以匹配数据，而不是寻找更普遍的解决方案。"许多人似乎接受了不是超新星的观点，"他说。
但如果太阳系并非始于超新星，那么它从哪里获得了那么多铝呢？
诞生于气泡中
许多研究人员现在倾向于的一种可能性是：铝-26是由沃尔夫-拉叶星的星风带来的。
与我们的太阳相比，沃尔夫-拉叶星寿命短得多，体积大几十倍，亮度高数千倍。当恒星的外层氢壳被剥离时，它就变成了沃尔夫-拉叶星，剥离的原因可能是另一颗恒星的引力吸引，也可能是其自身太阳风的强度。
沃尔夫-拉叶星暴露的核心可以以每秒高达3000公里的速度发出星风。"它基本上像扫雪机一样清扫周围的物质，"德瓦卡达斯说。这些被清扫的物质在恒星周围形成一个直径可达100光年的壳层。这个壳层在沃尔夫-拉叶星周围形成了一个气泡，其密度比周围的星际介质高出数万倍。
这个壳层包含足以构建一个太阳系的物质。它应该含有大量的铝-26，并且关键的是，它应该含有极少的铁-60。"我正在寻找一种只产生铝-26的恒星，"卢加罗说。"唯一能只产生铝-26的地方，就是这些大质量恒星的星风中。"
德瓦卡达斯说，天文学家已经观测到在沃尔夫-拉叶星的壳层内有恒星形成。据他估计，我们银河系中多达16%的太阳大小的恒星可能都是以这种方式形成的。"如果这是真的，没有理由只适用于我们的太阳系，"他说。"我们的太阳系不会是独一无二的。"
德瓦卡达斯和他的同事们可能已经构建了最完整的模型，来解释沃尔夫-拉叶星的太阳风如何在太阳系形成时将铝-26注入其中。之后，这颗寿命只有几百万年的沃尔夫-拉叶星很可能坍缩成了黑洞，尽管这方面的证据早已消失，德瓦卡达斯说。
卢加罗指出，沃尔夫-拉叶星假说也存在问题。例如，沃尔夫-拉叶星创造了如此高能的环境，它本应撕裂我们新形成的太阳系。
博斯仍然倾向于我们的尘埃云是由超新星点燃的理论。卢加罗则不这么认为。"目前，从核物理学的角度来看，"她说，"我倾向于沃尔夫-拉叶星的星风。"不过，她表示，新的信息可能下周就会改变她的想法。"这是一个需要从不同角度审视的问题。我们对此仍然有些争论。"
寻宝之旅
在波士顿，刘楠将陨石放回了保险柜。她在电脑上打开了一个实时视图，通过一台纳米探针显微镜，可以测量微小物质碎片的化学成分。她和其他研究人员正在使用这台设备研究溶解在酸中的陨石碎屑，寻找那些化学成分可能来自沃尔夫-拉叶星的颗粒。
刘楠远程操作着纳米探针（设备在华盛顿特区），仔细检查散布在金箔上的陨石碎屑。"这就像一次钓鱼探险，"刘楠说。她的下一步计划是，假设她能找到足够多具有正确化学成分（可能来自沃尔夫-拉叶星）的颗粒，就去测量它们是否显示出富含铝-26的迹象。这些化学信息随后可以用来约束关于太阳系起源的沃尔夫-拉叶星情景的天体物理模型。
刘楠承认，发现这样的颗粒并不能一锤定音地证明沃尔夫-拉叶星理论；例如，富含铝的尘埃可能是在太阳系形成之前很久，由更古老的恒星产生的。但如果找不到这样的颗粒，则表明沃尔夫-拉叶星的设想可能不对。
她注视着工作中的纳米探针，深入探索数十亿年前的过去。刘楠说，研究这些颗粒让她对导致我们星球存在的独特环境有了新的认识。"如果你想想这些放射性同位素——这些构成岩石和生命的元素，"她说，"当你了解它们在恒星中是如何产生的时候，你就会意识到要获得恰到好处的量并不容易。你必须在正确的时间和地点形成。"
英文来源：
What Crystals Older Than the Sun Reveal About the Start of the Solar System
Introduction
The standard story of the origin of our solar system has gone like this: 4.6 billion years ago, a giant cloud of dust hung frozen in space. Then the explosion of a nearby star caused part of that dust cloud to collapse. Pulled by gravity toward a central point, the dust coalesced into a radiating ball of hydrogen and helium about 1.4 million kilometers in diameter — what would become our sun. The remainder, which fell into orbit, collected into our solar system’s planets, along with a mess of asteroids and other cosmic leftovers.
To test the validity of this story, researchers need to peer back in time to the solar system’s first moments and beyond. And the cosmochemist Nan Liu has a way to do that: Locked in a safe on her desk at Boston University’s Institute for Astrophysical Research is a shard of meteorite flecked with material older than the sun.
“It’s the most pristine [type of] meteorite, not altered by water or heat,” Liu said as she took out and held up the specimen — a shiny, dark stone about the size and shape of an arrowhead.
Meteorites like this one formed around the time of the dust cloud collapse. The collapse of the cloud and the ignition of the sun melted away much of the chemical information contained in the meteorite, but within it some microscopic crystals — smaller than a single bacterial cell — survived intact. These crystals, called presolar grains, are far and away the oldest material accessible to us on Earth.
Over the past decade or so, scientists have used meteorites like Liu’s to challenge the story of how the solar system formed. Instead of a supernova, the solar system and everything in it might owe its existence to a more placid-sounding cosmic scenario: Maybe our solar system cobbled itself together from the winds blown off of a gargantuan star. New studies of presolar grains could offer a way to determine whether this new story is correct.
Starts With a Bang
Scientists got their first clue about what could have triggered the formation of the solar system when a fireball appeared over Mexico in 1969. The now-famous Allende meteorite spread its debris over more than 500 square kilometers.
In 1976, researchers reported that samples from Allende contained a surprise: an unexpectedly large amount of a stable isotope called magnesium-26. They proposed that the meteorite formed with an abundance of aluminum-26, which is radioactive and leaves behind magnesium-26 when it decays.
Yet aluminum-26 was not known to be a normal component of the interstellar medium — the dusty space between stars that would have provided the materials for Allende. Ordinary stars don’t make that particular isotope. “Most of these isotopes as we observe them in the early solar system, they were just the natural product of galactic chemical evolution,” said Maria Lugaro, an astrophysicist at the Konkoly Thege Miklós Astronomical Institute in Hungary. “The most important exception is aluminum-26.”
So where’d it come from? In 1977, two eminent astrophysicists proposed that the anomalous aluminum likely came from a nearby supernova explosion. Other phenomena can produce aluminum-26, but the supernova shock wave could also have caused the collapse of the cloud. With a single event, astronomers could explain how two rare occurrences — the injection of aluminum-26 and the formation of a new solar system — happened at virtually the same moment. “Everybody felt that we needed something to trigger the collapse,” said Vikram Dwarkadas, an astronomer at the University of Chicago.
The supernova trigger remained the favored scenario for decades, supported by detailed astrophysical models, as well as further measurements of enriched magnesium-26 in pristine meteorites. But over the past decade or so, that view has run up against other measurements that don’t seem to match. The problem: The solar system has an iron deficiency.
Not So Ironclad
Supernovas don’t just make aluminum. Any nearby supernova would likely also have injected lots of the radioactive isotope iron-60. Therefore, if a supernova launched the formation of the solar system, “we should see quite high initial [iron-60] abundances in the early-formed objects,” wrote Linru Fang, a cosmochemist at the University of Copenhagen, in an email.
Some studies have reported finding enough iron-60 in meteorite samples to support the supernova story. But not all scientists agree with those findings; several researchers told Quanta that most cosmochemists now think that, while there was an abundance of aluminum-26 at the start of the solar system, there wasn’t much iron-60 after all.
Early last year — in a study described by its authors as the most precise measure of iron-60 in the early solar system to date — Fang and her colleagues reported low levels of iron-60 (measured via its stable decay product nickel-60) in a planetesimal formed just after the collapse of the cloud. The result is inconsistent with a supernova scenario, she said.
Researchers have come up with explanations for the missing iron. “Meteoricists are famously argumentative folks,” wrote Alan Boss, an astronomer at Carnegie Science in Washington, D.C., in an email. “There always seems to be a counterexample to anything someone claims to be the case.”
For instance, the aluminum could have exploded out of the supernova, while the iron — coming from deeper in the star’s core — could have fallen back into the dead star. Or the explosion could have come from a quirky supernova that didn’t generate iron-60 at all. It could also be that iron-60 wasn’t distributed evenly in the cloud, which could mean measurements from individual meteorites aren’t giving us the full picture.
Dwarkadas dismisses these explanations as “hand-waving” attempts to fine-tune the models to match the data rather than finding a more general solution. “Many people seem to accept the idea that it’s not a supernova,” he said.
But if the solar system didn’t start with a supernova, where did it get all that aluminum?
Born in a Bubble
A possibility many researchers now favor is that the aluminum-26 was delivered on the winds of a Wolf-Rayet star.
Compared to our sun, a Wolf-Rayet star is much shorter-lived, dozens of times larger, and thousands of times as luminous. A star becomes a Wolf-Rayet star when its outer hydrogen shell is stripped away, either by the gravitational attraction of another star or by the strength of its own solar winds.
A Wolf-Rayet star’s exposed core can send out solar winds at speeds of up to 3,000 kilometers a second. “It basically sweeps up the surrounding material like a snowplow,” Dwarkadas said. That swept-up material forms a shell around the star that can be 100 light-years across. The shell, which creates a bubble around the Wolf-Rayet star, is tens of thousands of times denser than the surrounding interstellar medium.
The shell contains enough material to build a solar system. It should contain a lot of aluminum-26, and — crucially — it should contain very little iron-60. “I’m looking for a star that produces only aluminum-26,” Lugaro said. “The place where we can make only aluminum-26 is in the winds of these very massive stars.”
Astronomers have observed suns forming within the shells of Wolf-Rayet stars, Dwarkadas said. By his estimate, as much as 16% of all sun-size stars in our galaxy could have formed this way. “If it’s true, there’s no reason it should be true only for our solar system,” he said. “Ours will not be unique.”
Dwarkadas and his colleagues have laid out perhaps the most complete model for how the solar winds of a Wolf-Rayet star could have blasted aluminum-26 into our solar system as it formed. Afterward, the Wolf-Rayet star, with a lifetime of only a few million years, would most likely have collapsed into a black hole, although evidence for this would be long gone, Dwarkadas said.
There are problems with the Wolf-Rayet idea, Lugaro said. For instance, a Wolf-Rayet star creates such an energetic environment that it should have torn our newly formed solar system apart.
Boss still favors the theory that our cloud of dust was ignited by a supernova. Lugaro does not. “At the moment, from the nuclear-physics point of view,” she said, “I favor the winds of the Wolf-Rayet stars.” However, she said, new information could change her mind next week. “This is a problem that needs to be looked at from different angles. We are still fighting a bit about this.”
Gone Fishing
In Boston, Liu put the meteorite back in its safe. On her computer, she opened a live view through the microscope of a nanoprobe that can measure the chemical composition of tiny pieces of material. She and other researchers are using the device to study bits of meteorite dissolved in acid, on the hunt for grains with the right chemical composition to have come from a Wolf-Rayet star.
Liu operated the nanoprobe remotely (it was in Washington, D.C.), slowly scrutinizing the meteorite bits scattered across a field of gold foil. “This is like a fishing expedition,” Liu said. Her next step, assuming she can find a good number of grains with the right chemical composition to have come from a Wolf-Rayet star, would be to measure whether they show signs of having been enriched in aluminum-26. This chemical information could then be used to constrain astrophysical models of the Wolf-Rayet scenario for the start of the solar system.
Liu acknowledged that the presence of such grains wouldn’t be a slam dunk for the Wolf-Rayet star theory; for instance, aluminum-enriched dust could have been produced by much older stars long before our solar system formed. But the absence of such grains would suggest that the Wolf-Rayet idea is off.
She watched the nanoprobe at work, delving billions of years into the past. Studying these grains, Liu said, gives her a new sense of the unique circumstances that led to the existence of our planet. “If you think about these radioactive isotopes — these rock-forming elements and life-forming elements,” she said, “when you know how they are produced in stars, you realize it is not so easy to get the right amount. You have to form at the right time and place.”