彗星C/2010 X1（Elenin）的消失：带着呜咽声，而不是爆炸声
Disappearance of Comet C/2010 X1 (Elenin): Gone with a Whimper, not a Bang
摘要：
我们研究彗星C/2010 X1（Elenin）在接近近日点时的上升和突然消亡。这颗长周期彗星在4.2天文单位被发现，预计在2011年9月10日接近近日点时会变得非常明亮，在0.48天文单位。从2011年2月开始的观测（日心距离$\sim$ 3.5天文单位）确实显示这颗彗星的亮度增加了约11等，其中大部分增加发生在距离太阳1天文单位的范围内。在UT 2011年8月12.95 $\pm$ 0.50，当距离太阳$\sim$ 0.83 AU时，峰值亮度达到$m_R$ = 6。我们发现，在8月中旬的亮度激增的大部分是由于尘埃粒子的前向散射，而不是从突然增加的活动。一个小得多（$\sim$ 3等）的增亮在UT 2011年8月30日达到最大值，$\pm$ 1（0.56 AU），反映了核心的真正分裂。第二个峰与从中心凝聚到弥散的形态变化相匹配。当在1天文单位时，估计的核横截面为$\sim$ 1 km $^2$，相当于半径为0.6 km的等面积圆。在极限红色星等$r'$ = 24.4，对应半径$\lesssim$ 40 m（假设红色几何星等= 0.04）时，没有发现幸存的碎片。我们的观察结果是一致的，与核分裂成指数$q$ = 3.3 $\pm$ 0.2结合辐射压力的作用的碎片的幂律尺寸分布。我们推测的物理过程，可能会导致原子核分裂的彗星时，仍然0.7 AU的太阳。在这个距离上，潮汐应力和升华引起的原子核的挥发都可以忽略不计。然而，即使在Elenin彗星中测量到的非常低的速率下，由质量损失引起的扭矩也可能大到足以驱动核旋转不稳定。
英文摘要：
We examine the rise and sudden demise of comet C/2010 X1 (Elenin) on its approach to perihelion. Discovered inbound at 4.2 AU, this long-period comet was predicted to become very bright when near perihelion, at 0.48 AU on 2011 September 10. Observations starting 2011 February (heliocentric distance $\sim$3.5 AU) indeed show the comet to brighten by about 11 magnitudes, with most of the increase occurring inside 1 AU from the Sun. The peak brightness reached $m_R$ = 6 on UT 2011 August 12.95$\pm$0.50, when at $\sim$0.83 AU from the Sun. We find that most of the surge in brightness in mid-August resulted from dust particle forward-scattering, not from a sudden increase in the activity. A much smaller ($\sim$3 magnitudes) brightening reached a maximum on UT 2011 August 30$\pm$1 (at 0.56 AU), and reflects the true break-up of the nucleus. This second peak was matched by a change in the morphology from centrally condensed to diffuse. The estimated cross-section of the nucleus when at 1 AU inbound was $\sim$1 km$^2$, corresponding to an equal-area circle of radius 0.6 km. No surviving fragments were found to a limiting red magnitude $r'$ = 24.4, corresponding to radii $\lesssim$40 m (red geometric albedo = 0.04 assumed). Our observations are consistent with disintegration of the nucleus into a power law size distribution of fragments with index $q$ = 3.3$\pm$0.2 combined with the action of radiation pressure. We speculate about physical processes that might cause nucleus disruption in a comet when still 0.7 AU from the Sun. Tidal stresses and devolatilization of the nucleus by sublimation are both negligible at this distance. However, the torque caused by mass loss, even at the very low rates measured in comet Elenin, is potentially large enough to be responsible by driving the nucleus to rotational instability.
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