半人马(太阳系小天体)Centaur (small Solar System body)

【机器翻译自wiki,也手动翻译了很多】

Centaurs are small Solar System bodies with either a perihelion or a semi-major axis between those of the outer planets. They generally have unstable orbits because they cross or have crossed the orbits of one or more of the giant planets; almost all their orbits have dynamic lifetimes of only a few million years,[1] but there is one Centaur, 514107 Kaʻepaokaʻawela, which may be in a stable (though retrograde) orbit.[2][how is this a centaur if the Trojans aren’t?] Centaurs typically behave with characteristics of both asteroids and comets. They are named after the mythological centaurs that were a mixture of horse and human. Observational bias toward large objects makes determination of the total Centaur population difficult. Estimates for the number of Centaurs in the Solar System more than 1 km in diameter range from as low 44,000[1] to more than 10,000,000[3][4]

半人马座是太阳系中的小天体,其近日点和半长轴在外行星之间。 它们通常具有不稳定的轨道,因为它们已经穿越或已经穿越了一个或多个巨型行星的轨道。 它们几乎所有的轨道都只有几百万年的动态寿命,[1]但是有半人马天体,即  514107 Kaʻepaokaʻawela ,它可能处于稳定的(尽管逆行)轨道。[2]半人马天体通常表现出小行星和彗星的特征。 它们以马和人混合的神话人物命名。 对大型物体的观测偏差使确定半人马天体总数困难。 估计直径超过1 km的太阳系中半人马天体的数量约是从44,000 [1]到大于10,000,000 [3] [4]。

The first Centaur to be discovered, under the definition of the Jet Propulsion Laboratory and the one used here, was 944 Hidalgo in 1920. However, they were not recognized as a distinct population until the discovery of 2060 Chiron in 1977. The largest confirmed Centaur is 10199 Chariklo, which at 260 kilometers in diameter is as big as a mid-sized main-belt asteroid, and is known to have a system of rings. It was discovered in 1997. However, the lost Centaur 1995 SN55 may be somewhat larger. The transneptunian object 2018 VG18, which is a Centaur under the broader definition, may be quite a bit larger.

根据JPL的定义,发现的第一个半人马座是1920年944 的Hidalgo。但是,直到1977年发现2060 Chiron之后,它们才被确认为独特的天体群。最大的已确认的半人马天体 是10199 Chariklo,它的直径260公里,与一个中型主带小行星一样大,并且已知具有环系。 它于1997年被发现。但是失踪的Centaur 1995 SN55可能更大。 跨海王星天体2018 VG18(在更广泛的定义下为半人马座)可能要大得多。

No Centaur has been photographed up close, although there is evidence that Saturn‘s moon Phoebe, imaged by the Cassini probe in 2004, may be a captured Centaur that originated in the Kuiper belt.[5] In addition, the Hubble Space Telescope has gleaned some information about the surface features of 8405 Asbolus.

还没有近距离拍摄到半人马天体过,尽管有证据表明2004年由卡西尼号探测器拍摄的土星的卫星Phoebe,可能是起源于柯伊伯带,后被木星捕获的半人马天体。 此外,哈勃太空望远镜还收集了有关8405 Asbolus表面特征的一些信息。

1 Ceres may have originated in the region of the outer planets,[6] and if so might be considered an ex-Centaur, but the Centaurs seen today all originated elsewhere.

1谷神星可能起源于外行星区域,[6]如果被认为是前半人马天体,但是今天看到的半人马都起源于其他地方。

Of the objects known to occupy Centaur-like orbits, approximately 30 have been found to display comet-like dust comas, with three, 2060 Chiron60558 Echeclus, and 29P/Schwassmann-Wachmann 1, having detectable levels of volatile production in orbits entirely beyond Jupiter[7]. Chiron and Echeclus are therefore classified as both asteroids and comets, while Schwassmann-Wachmann 1 has always held a comet designation. Other Centaurs, such as 52872 Okyrhoe, are suspected of having shown comas. Any Centaur that is perturbed close enough to the Sun is expected to become a comet.

在已知占据半人马轨道的天体中,已发现约有30个显示彗星状尘埃,其中三个,即  2060 Chiron60558 Echeclus, 和 29P/Schwassmann-Wachmann 1 ,在整个轨道上都具有可检测水平的挥发物产生,超过了木星[7]。 因此,Chiron和Echeclus被归类为小行星和彗星,而Schwassmann-Wachmann 1一直被认为是彗星。 其他半人马天体,例如52872 Okyrhoe,也被怀疑表现出 彗发 【注:Coma, 彗发是彗核的蒸发物 】。 任何离到太阳足够近的半人马天体都将成为彗星。

定义Classification

The generic definition of a Centaur is a small body that orbits the Sun between Jupiter and Neptune and crosses the orbits of one or more of the giant planets. Due to the inherent long-term instability of orbits in this region, even Centaurs such as 2000 GM137 and 2001 XZ255, which do not currently cross the orbit of any planet, are in gradually changing orbits that will be perturbed until they start to cross the orbit of one or more of the giant planets.[1] Some astronomers count only bodies with semimajor axes in the region of the outer planets to be Centaurs; others accept any body with a perihelion in the region, as their orbits are similarly unstable.

半人马天体的一般定义是,在木星和海王星绕太阳公转,并穿过一个或多个巨型行星的轨道的小天体。 由于该地区固有的长期轨道不稳定性,即使是目前尚未穿越任何行星轨道的半人马天体(例如2000 GM137和2001 XZ255)也处于逐渐变化的轨道,直到它们开始穿越一个或多个巨型行星的轨道时才会受到干扰。 。[1] 一些天文学家仅将外行星区域中具有半长轴的物体算作半人马座。 其他人则认为该区域带有近日点的任何天体都是半人马天体,因为它们的轨道同样不稳定。

差异标准Discrepant criteria

However, different institutions have different criteria for classifying borderline objects, based on particular values of their orbital elements:

但是,不同的机构根据其轨道元素的特定值对边界对象进行分类有不同的标准:

  • The Minor Planet Center (MPC) defines Centaurs as having a perihelion beyond the orbit of Jupiter (5.2 AU < q) and a semi-major axis less than that of Neptune (a < 30.1 AU).[8] Though nowadays the MPC often lists Centaurs and scattered disc objects together as a single group.
  • 小行星中心(MPC)将半人马定义为具有木星轨道以外的近日点(5.2 AU <q),半长轴小于海王星的半长轴(a <30.1 AU)。[8] 尽管如今,MPC经常将半人马和分散的圆盘对象作为一个组列出。
  • The Jet Propulsion Laboratory (JPL) similarly defines Centaurs as having a semi-major axis, a, between those of Jupiter and Neptune (5.5 AU ≤ a ≤ 30.1 AU).[9]
  • 喷气推进实验室(JPL)类似地将半人马天体定义为在木星和海王星之间具有半长轴a的天体(5.5 AU≤a≤30.1 AU)。[9]
  • In contrast, the Deep Ecliptic Survey (DES) defines Centaurs using a dynamical classification scheme. These classifications are based on the simulated change in behavior of the present orbit when extended over 10 million years. The DES defines Centaurs as non-resonant objects whose instantaneous (osculating) perihelia are less than the osculating semi-major axis of Neptune at any time during the simulation. This definition is intended to be synonymous with planet-crossing orbits and to suggest comparatively short lifetimes in the current orbit.[10]
  • 相反,深黄道调查(DES)使用动态分类方案定义了半人马座。 这些分类基于当下轨道超过1000万年时的模拟行为变化。 DES将Centaurs定义为非共振物体,其瞬时(振荡)近日点数小于模拟期间任何时候海王星的振荡半长轴。 该定义旨在与跨行星轨道同义,并建议当前轨道的寿命相对较短。[10]
  • The collection The Solar System Beyond Neptune (2008) defines objects with a semi-major axis between those of Jupiter and Neptune and a Jupiter-relative Tisserand’s parameter above 3.05 as Centaurs, classifying the objects with a Jupiter-relative Tisserand’s parameter below this and, to exclude Kuiper belt objects, an arbitrary perihelion cut-off half-way to Saturn (q ≤ 7.35 AU) as Jupiter-family comets, and classifying those objects on unstable orbits with a semi-major axis larger than Neptune’s as members of the scattered disc.[11]
  • 《海王星之后的太阳系》(2008)将半长轴与木星和海王星之间的半长轴以及与木星相对的Tisserand参数大于3.05的对象定义为半人马天体,将与木星相对的Tisserand参数在此以下的对象分类,并且, 排除柯伊伯带天体,将距土星途中的任意近日点截止点(q≤7.35 AU)归类为木星家族彗星,并将那些半长轴大于海王星的不稳定轨道上的那些天体归类为分散的成员 光盘。[11]
  • Other astronomers prefer to define Centaurs as objects that are non-resonant with a perihelion inside the orbit of Neptune that can be shown to likely cross the Hill sphere of a gas giant within the next 10 million years,[12] so that Centaurs can be thought of as objects scattered inwards and that interact more strongly and scatter more quickly than typical scattered-disc objects.
  • 其他天文学家更喜欢将半人马定义为与海王星轨道内的近日点不共振的天体,这可以证明在未来一千万年内可能穿越天然气巨人的希尔球,[12] 被认为是向内散布的对象,与典型的散布盘对象相比,它们的相互作用和散布更快。
  • The JPL Small-Body Database lists 452 Centaurs.[13] There are an additional 116 trans-Neptunian objects (objects with a semi-major axis further than Neptune’s, i.e. 30.1 AU ≤ a) with a perihelion closer than the orbit of Uranus (q ≤ 19.2 AU).[14]
  • JPL小天体数据库列出了452个半人马天体。[13] 还有另外116个跨海王星天体(半长轴比海王星的半长轴,即30.1 AU≤a)和近日点比天王星的轨道更近(q≤19.2 AU)。[14]

两线之间的物体Objects caught between the lines

The Gladman & Marsden (2008)[11] criteria would make some objects Jupiter-family comets: Both Echeclus (q = 5.8 AU, TJ = 3.03) and Okyrhoe (q = 5.8 AU; TJ = 2.95) have traditionally been classified as Centaurs. Traditionally considered an asteroid, but classified as a Centaur by JPL, Hidalgo (q = 1.95 AU; TJ = 2.07) would also change category to a Jupiter-family comet. Schwassmann-Wachmann 1 (q = 5.72 AU; TJ = 2.99) has been categorized as both a Centaur and a Jupiter-family comet depending on the definition used.

Gladman&Marsden(2008)[11]的标准会使一些木星家庭彗星:传统意义上被认为是小行星的Echeclus(q = 5.8 AU,TJ = 3.03)和Okyrhoe(q = 5.8 AU; TJ = 2.95)都归类为半人马座。 。但被JPL归类为半人马座的Hidalgo(q = 1.95 AU; TJ = 2.07)也会将类别更改为木星族彗星。 Schwassmann-Wachmann 1(q = 5.72 AU; TJ = 2.99)既是半人马天体又是木星家族彗星。

Other objects caught between these differences in classification methods include 944 Hidalgo which was discovered in 1920 and is listed as a Centaur in the JPL Small-Body Database(44594) 1999 OX3, which has a semi-major axis of 32 AU but crosses the orbits of both Uranus and Neptune is listed as an outer Centaur by the Deep Ecliptic Survey (DES). Among the inner Centaurs, (434620) 2005 VD, with a perihelion distance very near Jupiter, is listed as a Centaur by both JPL and DES.

在分类方法的这些差异之间发现的其他对象包括944 Hidalgo,它在1920年被发现,在JPL小天体数据库中被列为半人马座。 (44594)1999 OX3,它的半长轴为32 AU,但与天王星和海王星的轨道相交,被深黄道测量(DES)列为半人马天体。 在内部半人马中,(434620)2005 VD的近日点距非常接近木星,被JPL和DES列为半人马天体。

A recent orbital simulation[3] of the evolution of Kuiper Belt Objects through the Centaur region has identified a short-lived “orbital gateway” between 5.4 and 7.8 AU through which 21% of all Centaurs pass, including 72% of the Centaurs that become Jupiter-family comets. Four objects are known to occupy this region, including 29P/Schwassmann-WachmannP/2010 TO20 LINEAR-GrauerP/2008 CL94 Lemmon, and 2016 LN8, but the simulations indicate that there may of order 1000 more objects >1 km in radius that have yet to be detected. Objects in this gateway region can display significant activity[15][16] and are in an important evolutionary transition state that further blurs the distinction between the Centaur and Jupiter-family comet populations.

最近的柯伊伯带天体穿过半人马区域的轨道模拟[3]确定了一个介于5.4和7.8 AU之间的短暂“轨道通道”,所有半人马天体中有21%通过会通过此处,其中72%成为木星家族的彗星。 已知有四个物体占据了该区域,包括29P / Schwassmann-Wachmann,P / 2010 TO20 LINEAR-Grauer,P / 2008 CL94 Lemmon和2016 LN8,但模拟表明,可能有1000个以上> 1 km 的天体尚未检测到。 这个通道区域内的物体会显示出重要的活动[15] [16],并且处于重要的演化过渡状态,这进一步模糊了半人马和木星家族彗星种群之间的区别。

还没有官方No official word, yet

The Committee on Small Body Nomenclature of the International Astronomical Union has not formally weighed in on any side of the debate. Instead, it has adopted the following naming convention for such objects: Befitting their Centaur-like transitional orbits between TNOs and comets, “objects on unstable, non-resonant, giant-planet-crossing orbits with semimajor axes greater than Neptune’s” are to be named for other hybrid and shape-shifting mythical creatures. Thus far, only the binary objects Ceto and Phorcys and Typhon and Echidna have been named according to the new policy.[17]

国际天文学联合会小机构命名委员会尚未正式参与辩论的任何方面。 取而代之的是,它对这类天体采用了以下命名约定:使其与TNO和彗星之间的半人马天体过渡轨道相适应,“半长轴大于海王星的不稳定,非共振,巨行星相交轨道上的天体”应为 因其他混合和变形的神话生物而得名。【??】 到目前为止,根据新方法,仅命名了二元对象Ceto和Phorcys以及Typhon和Echidna。[17]

可能的矮行星有更多的问题Possible dwarf planet raise further issues

Centaurs with measured diameters listed as possible dwarf planets according to Mike Brown‘s website include 10199 Chariklo(523727) 2014 NW652060 Chiron, and 54598 Bienor.[18]

根据麦克·布朗(Mike Brown)的网站,有些已经测出直径的半人马天体可能是矮行星,包括10199 Chariklo,(523727)2014 NW65、2060 Chiron和54598 Bienor。[18]

轨道Orbits

分布Distribution

Orbits of known Centaurs[note 1]

已知半人马天体的轨道

The diagram illustrates the orbits of known Centaurs in relation to the orbits of the planets. For selected objects, the eccentricity of the orbits is represented by red segments (extending from perihelion to aphelion).

该图显示了已知半人马天体相对于行星轨道的轨道。 对于特定的天体,轨道的离心率由红色部分表示(从近日点延伸到近日点)。

The orbits of Centaurs show a wide range of eccentricity, from highly eccentric (PholusAsbolusAmycusNessus) to more circular (Chariklo and the Saturn-crossers Thereus and Okyrhoe).

半人马座的轨道表现出广泛的偏心率,从高度偏心(Pholus,Asbolus,Amycus,Nessus)到更圆形(Chariklo和土星穿越者Thereus和Okyrhoe)。

To illustrate the range of the orbits’ parameters, the diagram shows a few objects with very unusual orbits, plotted in yellow :

为了说明轨道参数的范围,该图显示了一些轨道非常不寻常的物体,以黄色绘制:

  • 1999 XS35 (Apollo asteroid) follows an extremely eccentric orbit (e = 0.947), leading it from inside Earth‘s orbit (0.94 AU) to well beyond Neptune (> 34 AU)
  • 2007 TB434 follows a quasi-circular orbit (e < 0.026)
  • 2001 XZ255 has the lowest inclination (i < 3°).
  • 2004 YH32 is one of a small proportion of Centaurs with an extreme prograde inclination (i > 60°). It follows such a highly inclined orbit (79°) that, while it crosses from the distance of the asteroid belt from the Sun to past the distance of Saturn, if its orbit is projected onto the plane of Jupiter’s orbit, it does not even go out as far as Jupiter.
  • 1999 XS35(阿波罗小行星)遵循一个极其偏心的轨道(e = 0.947),使其从地球轨道内(0.94 AU)到达远超海王星(> 34 AU)的轨道
  • 2007 TB434遵循准圆形轨道(e <0.026)
  • 2001 XZ255具有最小的倾斜度(i <3°)。
  • 2004 YH32是一小部分半人马座之一,具有极端的前倾角(i> 60°)。 它遵循这样一个高度倾斜的轨道(79°),当它从小行星带到太阳的距离越过土星的距离时,如果它的轨道投影到木星的轨道平面上,它甚至不会走 直到木星。

A dozen known Centaurs follow retrograde orbits. Their inclinations range from modest (e.g., 160° for Dioretsa) to extreme (i < 120°; e.g. 105° for (342842) 2008 YB3[19]).

一些已知的的半人马天体遵循逆行轨道。 它们的倾斜范围从适度(例如Dioretsa为160°)到极端(i <120°;例如(342842)2008 YB3 [105]为105°)。

变化的轨道Changing orbits

The semi-major axis of Asbolus during the next 5500 years, using two slightly different estimates of present-day orbital elements. After the Jupiter encounter of year 4713 the two calculations diverge.[20]

图:在未来5500年中,使用两个当今略有不同的推测方法计算得到Asbolus的半长轴。 在4713年与木星相遇之后,这两种计算方法出现了分歧。[20]

Because the Centaurs are not protected by orbital resonances, their orbits are unstable within a timescale of 106–107 years.[21] For example, 55576 Amycus is in an unstable orbit near the 3:4 resonance of Uranus.[1] Dynamical studies of their orbits indicate that being a Centaur is probably an intermediate orbital state of objects transitioning from the Kuiper belt to the Jupiter family of short-period comets.

由于半人马天体不受轨道共振的保护,因此它们的轨道在106-107年的时间范围内是不稳定的。[21] 例如, 55576 Amycus 在天王星3:4共振附近的不稳定轨道上[1]。 对它们轨道的动力学研究表明,半人马天体可能是天体从柯伊伯带过渡到木星家族的短周期彗星的中间轨道状态。

Objects may be perturbed from the Kuiper belt, whereupon they become Neptune-crossing and interact gravitationally with that planet (see theories of origin). They then become classed as Centaurs, but their orbits are chaotic, evolving relatively rapidly as the Centaur makes repeated close approaches to one or more of the outer planets. Some Centaurs will evolve into Jupiter-crossing orbits whereupon their perihelia may become reduced into the inner Solar System and they may be reclassified as active comets in the Jupiter family if they display cometary activity. Centaurs will thus ultimately collide with the Sun or a planet or else they may be ejected into interstellar space after a close approach to one of the planets, particularly Jupiter.

天体可能在从柯伊伯带受到干扰,因此它们跨越海王星并与该之发生引力相互作用(请参阅起源理论)。 然后它们被归类为半人马天体,但它们的轨道是混乱的,随着半人马天体与一个或多个外行星的反复接近,其变化相对较快。 一些半人马天体将穿越木星的轨道,届时它们的近日点可能会低到太阳系内部,如果它们显示出类似彗星活动,它们可能会被重新归类为木星家族中的活动彗星。 因此,半人马天体最终将与太阳或行星发生碰撞,或者在接近其中一个行星,特别是木星后,它们可能被抛入星际空间。

物理特征Physical characteristics

The relatively small size of Centaurs precludes remote observation of surfaces, but colour indices and spectra can provide clues about surface composition and insight into the origin of the bodies.[21]

半人马座相对较小,因此无法远程观察表面,但是颜色指数和光谱可以提供有关表面组成和对物体起源的见解的线索。[21]

颜色Colours

Colour distribution of Centaurs

半人马天体的颜色分布

The colours of Centaurs are very diverse, which challenges any simple model of surface composition.[22] In the side-diagram, the colour indices are measures of apparent magnitude of an object through blue (B), visible (V) (i.e. green-yellow) and red (R) filters. The diagram illustrates these differences (in exaggerated colours) for all Centaurs with known colour indices. For reference, two moons: Triton and Phoebe, and planet Mars are plotted (yellow labels, size not to scale).

半人马天体的颜色非常多样,这对任何简单的表面组成模型都构成了挑战。[22] 在上面图中,颜色索引是通过蓝色(B),可见(V)(即绿黄色)和红色(R)滤镜对物体的视在大小的度量。 该图说明了具有已知颜色索引的所有半人马的这些差异(以夸张的颜色表示)。 作为参考,绘制了两个卫星:海卫一Triton和土卫Phoebe,以及火星(黄色标签,大小未按比例绘制)。

Centaurs appear to be grouped into two classes:

半人马天体被分为两类

  • very red – for example 5145 Pholus
  • blue (or blue-grey, according to some authors) – for example 2060 Chiron
  • 非常红——例如 5145 Pholus
  • 蓝(按作者来说,还可以叫蓝灰)——例如 2060 Chiron

There are numerous theories to explain this colour difference, but they can be divided broadly into two categories:

也有很多理论可以解释这些颜色差异,也可以被大致分为两类

  • The colour difference results from a difference in the origin and/or composition of the Centaur (see origin below)
  • 颜色差异是因为不同半人马天体的起源和组成不同
  • The colour difference reflects a different level of space-weathering from radiation and/or cometary activity.
  • 颜色差异是由于受到辐射和彗星活动的不同

As examples of the second category, the reddish colour of Pholus has been explained as a possible mantle of irradiated red organics, whereas Chiron has instead had its ice exposed due to its periodic cometary activity, giving it a blue/grey index. The correlation with activity and color is not certain, however, as the active Centaurs span the range of colors from blue (Chiron) to red (166P/NEAT).[23] Alternatively, Pholus may have been only recently expelled from the Kuiper belt, so that surface transformation processes have not yet taken place.

作为第二类的例子,Pholus的红色被解释为可能的受辐照红色有机物的地幔,而Chiron则由于其周期性的彗星活动而暴露了冰,因此它具有蓝色/灰色指数。 然而,与活动性和颜色的相关性尚不确定,因为活跃的半人马涵盖了从蓝色(凯龙星)到红色(166P / NEAT)的颜色范围。[23] 或者,Pholus可能只是最近才从柯伊伯带被驱逐出,因此尚未进行表面转化过程。

Delsanti et al. suggest multiple competing processes: reddening by the radiation, and blushing by collisions.[24][25]

Delsanti等。 认为存在多种竞争过程:通过辐射变红,通过碰撞变红。[24] [25]

光谱Spectra

The interpretation of spectra is often ambiguous, related to particle sizes and other factors, but the spectra offer an insight into surface composition. As with the colours, the observed spectra can fit a number of models of the surface.

光谱的解释通常含糊不清,与粒径和其他因素有关,但是光谱提供了对表面组成的深入了解。 与颜色一样,观察到的光谱可以适合许多表面模型。

Water ice signatures have been confirmed on a number of Centaurs[21] (including 2060 Chiron10199 Chariklo and 5145 Pholus). In addition to the water ice signature, a number of other models have been put forward:

许多半人马天体已经确认有了水冰 谱线 [21](包括2060 Chiron,10199 Chariklo和5145 Pholus)除了水冰 谱线 以外,还提出了许多其他模型:

  • Chariklo’s surface has been suggested to be a mixture of tholins (like those detected on Titan and Triton) with amorphous carbon.
  • Pholus has been suggested to be covered by a mixture of Titan-like tholinscarbon blackolivine[26] and methanol ice.
  • The surface of 52872 Okyrhoe has been suggested to be a mixture of kerogens, olivines and small percentage of water ice.
  • 8405 Asbolus has been suggested to be a mixture of 15% Triton-like tholins, 8% Titan-like tholin, 37% amorphous carbon and 40% ice tholin.
  • 有人认为,Chariklo的表面是tholins(类似于在Titan和Triton上检测到的)和无定形碳的混合物。
  • 有人认为类似泰坦的Tholin,炭黑,橄榄石[26]和甲醇冰的混合物覆盖着Pholus。
  • 有人认为52872 Okyrhoe的表面是 kerogens ,橄榄石和少量水冰的混合物。
  • 已经提出8405 Asbolus是由15%的Triton样的Tholin,8%的Titan的样Tholin,37%的无定形碳和40%的冰Tholin组成的混合物。

Chiron appears to be the most complex. The spectra observed vary depending on the period of the observation. Water ice signature was detected during a period of low activity and disappeared during high activity.[27][28][29]

Chiron似乎是最复杂的。 观察到的光谱根据观察周期而变化。 在活动量少的时期检测到水冰谱线,在活动量高的时候消失了。[27] [28] [29]

与彗星的相似性Similarities to comets

Comet 38P exhibits Centaur-like behavior by making close approaches to Jupiter, Saturn, and Uranus between 1982 and 2067.[30]

彗星38P在1982年至2067年之间通过接近木星,土星和天王星,表现出类似半人马天体的行为。[30]

Observations of Chiron in 1988 and 1989 near its perihelion found it to display a coma (a cloud of gas and dust evaporating from its surface). It is thus now officially classified as both a comet and an asteroid, although it is far larger than a typical comet and there is some lingering controversy. Other Centaurs are being monitored for comet-like activity: so far two, 60558 Echeclus, and 166P/NEAT have shown such behavior. 166P/NEAT was discovered while it exhibited a coma, and so is classified as a comet, though its orbit is that of a Centaur. 60558 Echeclus was discovered without a coma but recently became active,[31] and so it too is now classified as both a comet and an asteroid. Overall, there are ~30 Centaurs for which activity has been detected, with the active population biased toward objects with smaller perihelion distances. [32]

1988年和1989年Chiron在近日点附近的观测发现它显示出彗发(从其表面蒸发的气体和尘埃云)。 因此,尽管它比典型的彗星大得多,但现在仍被正式归类为彗星和小行星。 其他半人马天体也监视到类似彗星的活动:到目前为止,60558 Echeclus和166P / NEAT表现出这种行为。 166P / NEAT表现彗发时被发现,因此被归类为彗星,尽管它的轨道是半人马太难题。 60558 Echeclus被发现没有彗发,但最近活跃了[31],因此它现在也被归类为彗星和小行星。 总体来说,大约有30个半人马天体被发现有活动,活跃天体群偏向近日点距离较小的物体。 [32]

Carbon monoxide has been detected in 60558 Echeclus[7] and Chiron [33] in very small amounts, and the derived CO production rate was calculated to be sufficient to account for the observed coma. The calculated CO production rate from both 60558 Echeclus and Chiron is substantially lower than what is typically observed for 29P/Schwassmann–Wachmann[15], another distantly active comet often classified as a Centaur.

在60558 Echeclus [7]和Chiron [33]中检测到的一氧化碳数量非常少,计算得出的CO生成率足以解决所观察到的彗发。 从60558 Echeclus和Chiron算出的CO产生率大大低于通常被定义为半人马天体的另一颗遥远活跃的彗星29P / Schwassmann–Wachmann的观测值[15]。

There is no clear orbital distinction between Centaurs and comets. Both 29P/Schwassmann-Wachmann and 39P/Oterma have been referred to as Centaurs since they have typical Centaur orbits. The comet 39P/Oterma is currently inactive and was seen to be active only before it was perturbed into a Centaur orbit by Jupiter in 1963.[34] The faint comet 38P/Stephan–Oterma would probably not show a coma if it had a perihelion distance beyond Jupiter’s orbit at 5 AU. By the year 2200, comet 78P/Gehrels will probably migrate outwards into a Centaur-like orbit.

半人天体与彗星之间没有明确的轨道区别。 29P / Schwassmann-Wachmann和39P / Oterma都被称为半人马天体,因为它们具有典型的半人马天体轨道。 39P / Oterma彗星目前处于非活动状态,只有在1963年木星将其扰动到半人马天体轨道之前才被视为活跃。[34] 如果微弱的38P / Stephan–Oterma彗星的近日点距离超过5 AU的木星轨道,它可能不会显示彗发。 到2200年,78P / Gehrels彗星可能会向外迁移到半人马天体式轨道。

轮换期Rotational periods

A periodogram analysis of the light-curves of these Chiron and Chariklo gives respectively the following rotational periods: 5.5±0.4~h and 7.0± 0.6~h.[35][35]

对这些Chiron和Chariklo光曲线的周期图分析分别给出以下旋转周期:5.5±0.4〜h和7.0±0.6〜h。[35] [35]

尺寸,密度,反射性Size, density, reflectivity

A catalogue on the physical characteristics of Centaurs can be found at http://www.johnstonsarchive.net/astro/tnodiam.html. Centaurs can reach diameters up to hundreds of kilometers. The largest Centaurs have diameters in excess of 100 km, and primarily reside beyond about 13.11 AU[36]

有关半人天体的特征的目录可在http://www.johnstonsarchive.net/astro/tnodiam.html上找到。 半人马天体的直径可达数百公里。 最大的半人马天体直径超过100公里,主要在约13.11 AU以上。 [36]

起源推测Hypotheses of origin

The study of Centaurs’ origins is rich in recent developments, but any conclusions are still hampered by limited physical data. Different models have been put forward for possible origin of Centaurs.

对半人马天体起源的研究在最近的研究中非常多,但是任何结论仍然受到有限的物理数据的阻碍。 针对半人马天体的起源提出了不同的模型。

Simulations indicate that the orbit of some Kuiper belt objects can be perturbed, resulting in the object’s expulsion so that it becomes a Centaur. Scattered disc objects would be dynamically the best candidates (For instance, the Centaurs could be part of an “inner” scattered disc of objects perturbed inwards from the Kuiper belt.[37]) for such expulsions, but their colours do not fit the bicoloured nature of the Centaurs. Plutinos are a class of Kuiper belt object that display a similar bicoloured nature, and there are suggestions that not all plutinos’ orbits are as stable as initially thought, due to perturbation by Pluto.[38] Further developments are expected with more physical data on Kuiper belt objects.

模拟表明,一些柯伊伯带天体的轨道可能会受到扰动,导致天体被逐出,使其成为半人马天体。 对于这种驱逐,散射盘对象将是动态最佳的候选对象(例如,半人马可能是从柯伊伯带向内扰动的“内部”分散对象的一部分。[37]),但它们的颜色不适合半人马天体双色的性质。 Plutinos是一类显示出相似双色性质的柯伊伯带天体,而且有人提出,并非所有Plutinos的轨道在冥王星的扰动下都还能像最初认为的那样稳定。[38]更多有关柯伊伯带天体的物理数据将使该理论得到进一步发展。

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