Rings around a tiny body have changed over the past decade
Rings around a tiny body have changed over the past decade
一个微小天体周围的环在过去十年中发生了变化
For decades, astronomers thought rings were something only giant planets had. That changed in 2013, when a small, dark body orbiting between Saturn and Uranus passed in front of a star and blinked twice on either side of the main event, revealing two narrow rings around an object barely 250 kilometers across. “It was a surprise,” says Pablo Santos-Sanz, an astronomer at the Instituto de Astrofísica de Andalucía in Granada, Spain. 几十年来，天文学家一直认为只有巨行星才拥有行星环。这种情况在2013年发生了改变：当时，一颗在土星和天王星之间轨道运行的小型暗天体从一颗恒星前方经过，在主事件的两侧闪烁了两次，揭示出一个直径仅250公里左右的天体周围竟然存在两个狭窄的环。“这令人惊讶，”西班牙格拉纳达安达卢西亚天体物理研究所的天文学家巴勃罗·桑托斯-桑兹（Pablo Santos-Sanz）说。
Ever since, the question has been what such rings are made of and how long they can last. In a recent study, Santos-Sanz and his colleagues used the James Webb Space Telescope to watch the same body, now known as Chariklo, pass in front of a background star again. They found one of its rings had grown denser and the other had almost vanished. We don’t know exactly why. 从那时起，问题就变成了这些环是由什么组成的，以及它们能持续多久。在最近的一项研究中，桑托斯-桑兹和他的同事们利用詹姆斯·韦伯空间望远镜（JWST）观测了同一个天体（现被称为“女凯龙星”Chariklo）再次从一颗背景恒星前方经过的过程。他们发现其中一个环变得更加致密，而另一个环几乎消失了。我们尚不清楚具体原因。
Shadowing a star
遮蔽恒星
The technique behind the observation is simple. “We predict when a Solar System object passes in front of a star,” Santos-Sanz said. The starlight dims for a moment, and the shape of that dip reveals the size, shape, and surroundings of the object that caused it. “This is particularly challenging for minor bodies, and more challenging for distant minor bodies,” he said. 这种观测背后的技术很简单。“我们预测太阳系天体何时会从恒星前方经过，”桑托斯-桑兹说。星光会瞬间变暗，而这种亮度下降的形状揭示了造成遮挡物体的尺寸、形状和周围环境。“对于小天体来说，这尤其具有挑战性，而对于遥远的小天体来说，挑战性更大，”他说。
The difficulty is that the target’s silhouette on the sky is minuscule, and knowing when it will cross a particular star requires very precise positioning data for both. Still, in 2013, we used ground telescopes to discern Chariklo’s two rings, C1R and C2R. These sat 390 and 405 kilometers from its center and were only a few kilometers wide and about 7 kilometers apart. 困难在于目标在天空中的轮廓极其微小，要确定它何时会穿过特定的恒星，需要两者都具备非常精确的定位数据。尽管如此，在2013年，我们还是利用地面望远镜辨认出了女凯龙星的两个环：C1R和C2R。它们分别位于距离中心390公里和405公里的地方，宽度仅几公里，彼此相距约7公里。
Doing this with a space-based telescope, though, makes lining things up considerably harder. JWST sits at the L2 Lagrange point, and controllers need to nudge the telescope every few weeks to keep its orbit stable. “It’s a kind of tricky task,” Santos-Sanz said. His team identified a possible Chariklo occultation in August 2022 and redid the prediction every week. Between the first prediction and one of the last, the projected line of sight shifted by about 110 kilometers, which was enough to move it off the body entirely. 然而，使用空间望远镜进行这项工作使得对齐变得更加困难。JWST位于拉格朗日L2点，控制人员需要每隔几周对望远镜进行微调以保持其轨道稳定。“这是一项相当棘手的任务，”桑托斯-桑兹说。他的团队在2022年8月确定了一次可能发生的女凯龙星掩星事件，并每周重新进行预测。在第一次预测和最后一次预测之间，预计的视线偏移了约110公里，这足以使其完全偏离该天体。
Unfortunately, JWST requires observations like this to be planned at least 14 days in advance. “We did this maybe a bit blindly, because we didn’t know exactly where the line of sight was,” Santos-Sanz said. “I’m going to move one of the biggest, best telescopes in space, and we don’t know if finally we will catch this or not.” But it all worked out. The occultation came on October 18, 2022. The reconstructed geometry shows JWST’s sightline to the background star skimmed 7.4 kilometers above Chariklo’s surface, missing the body but catching its rings. 不幸的是，JWST要求此类观测至少提前14天进行规划。“我们这样做可能有点盲目，因为我们并不确切知道视线在哪里，”桑托斯-桑兹说。“我要移动太空中最大、最好的望远镜之一，但我们不知道最终是否能捕捉到它。”但一切都很顺利。掩星发生在2022年10月18日。重建的几何结构显示，JWST对背景恒星的视线掠过了女凯龙星表面上方7.4公里处，虽然错过了天体本身，但捕捉到了它的环。
The ring that thickened
变厚的环
JWST recorded the event simultaneously in two near-infrared bands, at 1.5 and 3.2 micrometers, which made it the first time anyone has caught a minor body’s rings in a band beyond three micrometers—a range Earth’s atmosphere puts out of reach for ground telescopes. The inner ring showed up unmistakably, with abrupt, distinct edges, but it was much darker than before. Averaged over roughly 10 previous ground-based occultations, C1R’s normal opacity (the fraction of starlight it blocks) sat at 0.303. JWST measured it at 0.431. JWST在1.5微米和3.2微米两个近红外波段同时记录了这一事件，这是人类首次在超过3微米的波段捕捉到小天体的环——这一范围因地球大气层的阻挡，地面望远镜无法观测。内环清晰可见，边缘突兀且分明，但它比以前暗得多。根据之前约10次地面掩星观测的平均值，C1R的正常不透明度（它阻挡星光的比例）为0.303。而JWST测得的数据为0.431。
“We didn’t believe it at the beginning, so we fought a lot with the data,” Santos-Sanz said. The most straightforward explanation was geometry. Rings are not always uniform, and JWST might simply have cut through a denser clump. To rule this scenario out, the team built a lumpy ring model and ran 10 million simulated occultations. Reproducing an opacity as high as JWST recorded came out at a roughly 1 in a 1,000 chance at 1.5 micrometers, and 4 in 100,000 at 3.2 micrometers for a single measurement. The telescope caught the ring twice, going in and coming out, which made the odds even smaller than that. Santos-Sanz concluded that the inner ring most likely got thicker. “起初我们不相信，所以我们对数据进行了反复核对，”桑托斯-桑兹说。最直接的解释是几何因素。环并不总是均匀的，JWST可能只是穿过了一个更致密的团块。为了排除这种可能性，团队建立了一个块状环模型，并进行了1000万次模拟掩星。要重现JWST记录到的如此高的不透明度，在1.5微米波段的概率约为千分之一，在3.2微米波段单次测量的概率为十万分之四。望远镜在进入和离开时两次捕捉到该环，这使得概率变得更小。桑托斯-桑兹得出结论，内环很可能变厚了。
The ring that faded
褪色的环
At the same time, the outer ring did the opposite. The ring that faded C2R barely registered at 1.5 micrometers and did not appear at all at 3.2, even though the telescope was recording the same stretch of ring in both bands at the same instant. “At the beginning we didn’t even see the outer ring in the light curve,” Santos-Sanz said. “We had to use models. It was really barely visible, so we said, ‘What is happening here?’” 与此同时，外环的情况恰恰相反。C2R环在1.5微米波段几乎没有记录，在3.2微米波段则完全没有出现，尽管望远镜在同一瞬间记录了两个波段中同一段环。“起初我们在光变曲线中甚至没有看到外环，”桑托斯-桑兹说。“我们不得不使用模型。它真的几乎不可见，所以我们说，‘这里发生了什么？’”
The team came up with two possible explanations. The first is that JWST, looking at wavelengths where almost no occultation has ever been recorded, is simply seeing grains that scatter light differently in the infrared. The other is that the rings really have physically changed. Santos-Sanz argues radiative transfer models point toward the latter. The older visible-light observations were consistent with a mixture of ice and silicates, but once the JWST data points are added, no combination of materials and grain sizes could explain what the telescope has seen. “We are witnessing a real evolution of the rings with time,” Santos-Sanz said. “Of course it is not a certainty, but for me it is the preferred explanation.” 团队提出了两种可能的解释。第一种是，JWST观测的波长几乎从未记录过掩星现象，它可能只是看到了在红外线下散射光线方式不同的颗粒。另一种是环确实发生了物理变化。桑托斯-桑兹认为辐射传输模型指向后者。较早的可见光观测结果与冰和硅酸盐的混合物一致，但一旦加入JWST的数据点，没有任何材料和颗粒尺寸的组合能够解释望远镜所看到的情况。“我们正在见证环随时间发生的真实演变，”桑托斯-桑兹说。“当然，这还不是定论，但对我来说，这是最合理的解释。”
What’s more, this evolution most likely was not just the material from the fading outer ring migrating inward to the inner ring. Measured as equivalent width, the inner ring gained about 10 times more than the outer ring lost. “We don’t know where the extra material is coming from, but there are some hypotheses,” Santos-Sanz said. 此外，这种演变很可能不仅仅是褪色的外环物质向内迁移到内环。以等效宽度衡量，内环增加的量大约是外环损失量的10倍。“我们不知道多出来的物质从何而来，但有一些假设，”桑托斯-桑兹说。
The ghost moon
幽灵卫星
The leading explanation, Santos-Sanz explains, is a small shepherd satellite sharing the outer ring’s orbit. Such an object should explain the rings’ stability and their sharp edges and could also shed debris that replenishes C1R. “This satellite has not been detected yet, if it exists,” Santos-Sanz said. A computer model based on JWST data also hints at what the two rings are made of. Santos-Sanz, though, makes it clear that this part of the work is unfinished. 桑托斯-桑兹解释说，主要的解释是一颗与外环共享轨道的小型牧羊卫星。这样的天体可以解释环的稳定性和其锋利的边缘，也可能散落碎片来补充C1R。“如果这颗卫星存在，它目前尚未被探测到，”桑托斯-桑兹说。基于JWST数据的计算机模型也暗示了这两个环的组成成分。不过，桑托斯-桑兹明确表示，这部分工作尚未完成。