哈勃空间望远镜对主带彗星133 P/Elst-Pizarro的观测
Hubble Space Telescope Investigation of Main-Belt Comet 133P/Elst-Pizarro
摘要：
我们报告的原型主带彗星（活跃的小行星）133 P/埃尔斯特-皮萨罗采取高角分辨率使用哈勃太空望远镜的新观测。该天体有三个主要组成部分：a）一个点状的核心，b）一个长而窄的反日尘埃尾，以及c）一个短而向阳的反日尾。昏迷未消退。该核的平均绝对星等H_V = 15.70 ± 0.10，光变范围0.42等，后者对应于投影尺寸3.6x5.4公里（轴比1.5：1），先前测量的几何半径为0.05+/-0.02。我们探索了一系列的连续和脉冲发射模型，同时适合测量的表面亮度轮廓，宽度和位置角的反日尾巴。较好的拟合表明，在150天或更短的时间内，尘埃颗粒的排放遵循指数为3.25 < q < 3.5的微分幂律粒度分布，并且具有广泛的粒度范围。向太阳方向投射的超低表面亮度尘埃是以前轨道上发生的发射活动的残余物，由最大（>厘米大小）的粒子组成。一微米大小的粒子的喷射速度与原子核的约1.8 m/s的引力逃逸速度相当，而较大的粒子以低于引力逃逸速度的速度释放。观测结果与混合假设一致，但不能证明这一假设，即质量损失是由近地表水冰升华产生的气体阻力驱动的，但逃逸是由细长核旋转产生的向心加速度辅助的。未确定合理的备择假设。
英文摘要：
We report new observations of the prototype main-belt comet (active asteroid) 133P/Elst-Pizarro taken at high angular resolution using the Hubble Space Telescope. The object has three main components; a) a point-like nucleus, b) a long, narrow antisolar dust tail and c) a short, sunward anti-tail. There is no resolved coma. The nucleus has a mean absolute magnitude H_V = 15.70+/-0.10 and a lightcurve range 0.42 mag., the latter corresponding to projected dimensions 3.6 x 5.4 km (axis ratio 1.5:1), at the previously measured geometric albedo of 0.05+/-0.02. We explored a range of continuous and impulsive emission models to simultaneously fit the measured surface brightness profile, width and position angle of the antisolar tail. Preferred fits invoke protracted emission, over a period of 150 days or less, of dust grains following a differential power-law size distribution with index 3.25 < q < 3.5 and having a wide range of sizes. Ultra-low surface brightness dust projected in the sunward direction is a remnant from emission activity occurring in previous orbits, and consists of the largest (>cm-sized) particles. Ejection velocities of one micron-sized particles are comparable to the ~1.8 m/s gravitational escape speed of the nucleus, while larger particles are released at speeds less than the gravitational escape velocity. The observations are consistent with, but do not prove, a hybrid hypothesis in which mass loss is driven by gas drag from the sublimation of near-surface water ice, but escape is aided by centripetal acceleration from the rotation of the elongated nucleus. No plausible alternative hypothesis has been identified.
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