💫王朝陨落谜团💫 Tales the Dynasty’s End
上期我们了解了那些最不起眼的小人物星星，那这次就来看看另一边的大人物吧！
Last time we talked about the smallest of the smallest, so today why not take a look at the other end of spectrum, the powerful ones of the senior group?
如果你错过了上一篇文章，我们这里讨论的是白矮星。它们是曾经充满活力主序恒星的残余，是恒星生命的终结的开始。像我们的太阳一样，所有与它质量相似的恒星总有一天都会变成白矮星。
In case you missed the last article, we are talking about white dwarfs (WD) here. They are the remnants of the once vibrant main sequence star, the end times of stars’ lifetime. Like our Sun, all stars with a similar mass to it will one day become a WD.
虽然宇宙中有几千几万亿颗星星，但每颗恒星都是自己小世界的主宰，陪伴身边的是许多行星臣民们。那么，在王朝终结之际，这些忠诚的追随者们将会迎来什么样的命运呢?
Although there are billions and trillions of them in the universe, the star is the emperor of its own little stellar kingdom with the many of its planet ministers and supporters. So now, as the dynasty’s endapproaches, what would be the fate these loyal followers?
嗯… 说来话长，但在最近的一些研究中发现，他们神秘的故事很有可能会是个悲剧。
Well, the story is complicated, some recent discoveries shows that it might be some mysterious and tragic stories.
近期，天文学家们在一些白矮星的光谱中观察到了出乎意料的金属（比氢和氦更重的元素），而这很有可能是行星的毁灭或者被吸收的迹象。
Astronomers have observed unexpected metals (elements heavier than hydrogen and helium) in the spectra of WDs, indicating signs of the destruction and accretion of planets.
这究竟怎么一回事？我们来一探究竟！
So, what happened? Let's find out!
图为一个IA型超新星，是一颗白矮星与一颗伴星在紧密轨道上的热核爆炸形成的，也是许多白矮星生命最终的结局
A type IA supernova, which is believed to be a thermonuclear explosion of a white dwarf in a tight orbit with a companion star, a common ending for a star
悬案
An Unsolved Case
随着恒星们进入生命最后的一段时光，他们会先脱下沉重的外层，变成最开始恒星的赤裸内核，它们的光谱大多显示含有氢、氦、碳或氧。
As the stars blow off the heavy outer layers at the entry of the last stage of life, they basically become the naked cores of the original stars, their spectra mostly show hydrogen, helium, carbon or oxygen.
图为NASA的哈勃望远镜拍到的PK 329-02.2 星云。是一颗类似太阳的恒星在将要变成白矮星爆炸时的场景，再次之后，这颗恒星将会失去从前作为主序星的外壳
An image of the PK 329-02.2 nebula from NASA's Hubble. It is scene of a Sun like star exploding and blowing away its out layer. After this, the star would have transformed from a main sequence star to a white dwarf
在褪去外壳后，他们会按照内部成分被分组，通常他们会一直保持这样一种状态直到生命的结束。然而，事情并不总是那么简单，他们有些更愿意把疯狂进行到最后一秒。
So, after stripping, the WDs are grouped according to their composition and normally they would stay like that until the end. However, things are not always so simple, some of them prefers to live a crazy life till the last second.
在各种小组中，有一类是氦为主的白矮星，经过科学家们多年对他们光谱的观察，发现其中一些有着神秘的未知来源的氢。
One of the WD groups is the helium dominated WDs, after some years of monitoring, a few of them has started to mysteriously have extra hydrogen with unknown source appearing in their spectra.
针对这个问题，天文学家们提出了三种可能性:
1. 氢一直在那里，它们只是从原来的恒星遗留下来的
2. 氢来自外部物质（如星际介质或行星）
3. 氢是从白矮星深层慢慢渗透挖掘出来的
但到底是哪一个呢?
Therefore, astronomers came up with three possibilities:
1. The hydrogen was always in there, they simply left from the original star
2. The hydrogen came from an external source (e.g. an interstellar medium or a planet)
3. The hydrogen was dredge-up from deeper layers of the core
But which one is it?
在之前的研究中发现，在许多白矮星中，氢在有金属污染的环境出现频率几乎是没有金属污染的恒星的两倍。因此，表明第二个假设是更可能的情况。
Previous research found that, with a number of WDs, hydrogen was almost twice as common in stars with metal pollution than those without. Hence, indicating that option 2 would be the more likely case.
这一想法被另一通过观察白矮星光谱中过量的氧的研究证实了。他们的看法是一致的，因为额外的氧气被解释为来自白矮星吸收水后的结果，所以有了多余的氧也必定会有多余氢的积累。
This idea is confirmed with another study by observing the excesses of oxygen in the WDs’ spectra. They concur each other because the extra oxygen was interpreted as originated from accretion of water, which means there also have been accretion of hydrogen.
但，这些故事的具体情节尚未可知。
However, these are only likely propositions with no clear story line.
因此，为了更好地了解这些行星在白矮星附近消失的离奇过程，以及这些行星的来源身份，一个由Paula Izquierdo领导的，成员来自西班牙、英国、德国和美国的团队成立了。他们决定更详细地研究GD 424，一颗有着金属污染的富含氢的氦白矮星的完美代表。
In order to understand better the history of accreting WDs and origininal identities of the destroyed planets, a team lead by Paula Izquierdo with member across Spain, UK, Germany and USA, decided in to take more detailed look at GD 424. It represents a typical scenario of a metal polluted hydrogen rich helium white dwarf.
谜团重重
The Many Mysteries
想要知晓真相，首先有一系列的谜团需要解开。而第一步要做的就是仔细观察天皇星星本人。
There are a few miseries yet to be solved, however, the first thing would be to take a closer look at the emperor star itself.
使用的是西班牙的威廉·赫歇尔望远镜，研究小组发现GD 424有一个以氦为主的光球层，光谱中还显示含有氢的成分，以及一些属于氧、镁、硅和钙的较窄吸收线。
Using the William Herschel Telescope in Spain, the team found GD 424 to have a helium-dominated photosphere with the presence of hydrogen as well as some narrower absorption lines belonging to oxygen, magnesium, silicon and calcium.
图为GD 424的吸收光谱，图表上不同的凹陷代表了不同物质的存在，其中橙色标记的部分代表了氦的存在，尤为明显
The absorption spectrum of GD 424 showing the existence of different elements with the most obvious orange labelled ones as the helium absorption lines
通过对这些结果的建模，研究小组获得了更多关于光球层属性的细节，例如温度和表面重力。然后通过数据的再次分析，他们最终确定了白矮星中每种物质的丰富程度。
By modeling those result, the team obtained more details on the properties of the photosphere (e.g. the temperature and surface gravity). After which, they were able to finally determine the abundance of each material in the WD.
了解了白矮星本人，下一个挑战是找到那些失踪行星的成分。首先要做的是对行星开始被吸积的时间和吸积的过程做一系列模拟设想来帮助下一阶段的分析。
The next challenge is to find the composition of the potentially destroyed planet. Though before that, the first thing was to make some assumptions on when the accretion of planet started and its progress.
针对这个设想，研究小组提出了一个简单的模型，将吸积过程分为了三个阶段：增加吸积——导致金属丰富度增加；稳态吸积-吸积/扩散达到平衡；减少吸积——金属丰富度由于扩散和下沉而呈指数衰减。
So, the team came up with a simple model, dividing the accretion process into three stages: increasing accretion – leads to increase of metal abundance; steady-state accretion – accretion/diffusion reaches equilibrium; decreasing accretion – metal abundance exponentially decays due to diffusion and sinks into the WD.
所有这三个阶段都表明，金属丰度取决于吸积态（多少进入光球层）和扩散态（多少离开光球）之间的平衡关系。
All three stages indicated that the metal abundance depends on the balanced relationship between accretion state (how much gone into the photosphere) and the diffusion state (how much leaving the photosphere).
因为已经掌握数据没有明确的迹象表明现在GD 424在哪个状态，小组对所有三个阶段都进行了测试。结果表明，增加态和稳定态都是有可能的。
With the data already in hand, there was no clear indication to which state the WD. Therefore all three scenarios were put to test. So, the result suggests that both the increasing state and steady state were possible.
图为被GD 424 吸积的行星的金属元素丰度对比图（以硅为标准），其中深蓝色标记代表在不同吸积阶段会出现的金属丰度，空心标记代表了地球不同区域的指数，以及浅粉和浅蓝标记代表了其他白矮星的指数
The abundances of different metals (relative to silicon) in the accreting planet of GD 424. The dark blue plots refers to the 3 stages in the accretion process, the hollow ones for different parts of the Earth, and the light pink and blue ones for other white dwarfs
不过，GD 424的吸积率是所有观测到的白矮星中最高之一，而且它至少已经吸积了一整个太阳系小行星10 Hygeia（按体积和质量计算，太阳系第四大小行星）的质量。
Though the accretion rate is one the highest of all observed WDs and it has already accreted at least the mass of the Solar System asteroid 10 Hygeia (fourth-largest asteroid in the Solar System by both volume and mass).
结果还表明，被吸积的是一个类似地球，但含有更多的钙的行星。研究小组还试图通过检测非自然出现的过量氧气来计算那颗被吸积的行星上的水含量。有趣的是，结果显示并没有过量的氧气，GD 424似乎正在吸积一颗干燥的岩石行星的碎片，而不是一颗‘水行星‘。
The result also implied an Earth like planet, which with much more calcium, was accreted. Team also tried to calculate the amount of water in that planet originally by checking the excess oxygen that would not naturally appear. Interestingly, there is an absence of excess oxygen, GD 424 appearing to be currently accreting some dry, rocky planetary debris, not a watery one.
图为这次研究中最重要的威廉·赫歇尔望远镜，位于西班牙的加那利群岛（the Canary Islands）拉帕尔马岛（La Palma）上，是牛顿望远镜小组的一部分
The William Herschel Telescope on the La Palma Island in the Canary Islands. It is part of the Isaac Newton Group of Telescopes
所以，那些水到底去哪了?
Where is the water gone?
云开雾散
The Reveal
其实，那不合理数量的氢也不一定是水行星足迹的证据，但一定会有某一个解释。
Although it does not have to be water that existed on the accreted planet for there to be unreasonable amount of hydrogen, there must be another explanation.
幸好我们知道，要有氢出现在光谱中，它一定是最近才被吸积的。原因是白矮星需要降到足够低的温度才能将氢保持在光球层中，否则高温会促使氢与其他物质的化学反应将其全部消耗掉，也就不会出现在我们的观察中了。
Lucky, it is known that for hydrogen to appear in the spectrum, it must have been accreted recently. The reason being the WD has to be sufficiently cool enough to keep the hydrogen in the photosphere, otherwise the high temperature would have encouraged reactions that would use it all up.
所以这表明氢气一定是前一个吸积期遗留下的，白矮星在吞噬了一个富含水的行星以后，氧气在不久后就扩散而消失了，留下了多余的氢。
This indicates that the hydrogen must have been left from the previous accretion period, where the WD consumed a water-rich planet, and the oxygen was gone simply due to diffusion shortly after the accretion process.
图为一颗艺术绘画的白矮星和围绕他的一个行星碎片，以及运动过程中留下的气体
An illustration of a white dwarf and a planetary fragment orbiting it, leaving a trail of gas in its wake
现在我们终于知道了这个谜题的所有细节，谜团可以迎刃而解啦。
Now that we finally know all pieces of the puzzle, the mystery can be unraveled.
离开主序列后，GD 424也决定终结它周围的所有行星。首先是那位水灵灵的御前女官，现在是一位坚硬的石质大臣，那么下一个会是谁呢?
Since leaving the main sequence, GD 424 has decided to also bring an end to its surrounding planets. First, the water-rich lady-in-waiting, now a rocky minister, so who would be next?
一朝天子一朝臣
不止古代的帝王将相
在离开时希望得到陪伴，
星朝始皇们原来也会做出同样的选择！
作为浩瀚宇宙中的一员，我们的命运往往都是相似的。
So it seems,
it's not only us human that have the fear of leaving,
stars too！
As member of this vast universe, our fates all seem similar.
图为一颗艺术绘画的白矮星和正在被吸积的一颗行星
An illustration of a white dwarf and the planet that is being accreted by it
图片来自 NASA, ING, AAAS 官网
文中部分英文信息参考来西班牙、英国、德国和美国学者合作， 由 Paula Izquierdo 带领的团队的 “GD 424 – a helium-atmosphere white dwarf with a large amount of trace hydrogen in the process of digesting a rocky planetesimal" 论文
其余中英文内容为原创
Pictures from official website of NASA, ING, AAAS.
Parts are from “GD 424 – a helium-atmosphere white dwarf with a large amount of trace hydrogen in the process of digesting a rocky planetesimal" by the scientist team lead by Paula Izquierdo from Spain, UK, Germany and USA
the rest of the Chinese and English content is original
