PHYSICAL PROPERTIES OF MAIN-BELT COMET 176P/LINEAR
PHYSICAL PROPERTIES OF MAIN-BELT COMET 176P/LINEAR
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DOI：
10.1088/0004-6256/142/1/29
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发表时间：
2011-05
期刊：
影响因子：
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通讯作者：
H. Hsieh;M. Ishiguro;P. Lacerda;D. Jewitt
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文献类型：
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作者：
H. Hsieh;M. Ishiguro;P. Lacerda;D. Jewitt
我们介绍了 176P/LINEAR 彗星的物理特征，它是新一类主带彗星中第三颗被发现的成员，它表现出彗星活动，但在动力学上与主带小行星无法区分。观察显示，该物体在 2005 年末至少一个月内呈现出扇形尾巴，但在 2006 年初变得不活跃。在此活跃期间，我们测量了宽带颜色 B − V = 0.63 ± 0.02、V − R = 0.35 ± 0.02 和 R − I = 0.31 ± 0.04。使用观察到物体不活动时的数据，我们得出最佳拟合 IAU 相位函数参数 H = 15.10 ± 0.05 mag 和 G = 0.15 ± 0.10，以及最佳拟合线性相位函数参数 m(1, 1, 0) = 15.35 ± 0.05 mag 和 β = 0.038 ± 0.005 mag deg−1。从这个基线相位函数中，我们发现 176P 在其活动期间表现出平均光度过剩 ∼30%，这意味着彗发尘埃总质量大约为 Md ∼ (7.2 ± 3.6) × 104 kg。从 2007 年初获得的非活动数据中，我们发现旋转周期为 Prot = 22.23 ± 0.01 小时，峰谷光度范围为 Δm ∼ 0.7 mag。将我们的光度数据从 176P 的 2005 年活跃期到这个自转期进行定相，我们发现原子核的光度范围比 2007 年要小得多，这不能用慧差阻尼效应来解释，因此，我们将其归因于观察几何效应。对这些几何效应的详细分析表明，176P很可能是一个高度细长的天体，其轴比为1.8<b/a<2.1，轨道倾角为ε∼60°，至点位置为νo=20°±20°的真实异常点。对 176P 尘埃排放的数值模拟发现，其活性只能通过不对称尘埃排放（例如彗星喷流）来重现。我们使用模型假设~10μm的尘埃颗粒在观察到176P活跃期间连续发射，并且射流方向为180°≲αjet≲120°和δjet≈-60°，我们发现与我们的观察结果似乎合理。我们没有找到与我们使用脉冲尘埃发射模型进行的观察的良好契合，即，如果 176P 的活动是由于撞击非挥发性小行星风化层而产生的喷射云，那么我们会预期什么。由于对于旋转体来说，非赤道射流的时间平均方向相当于最近旋转极的方向，因此我们发现等效轨道倾角为50°≲ε≲75°，与光变曲线分析的结果一致。此外，我们的光曲线分析和尘埃模型分析的结果与用于解释 176P 活性调节的季节性加热假设一致。强烈建议进行更多观测，以进一步描述 176P 在 2011 年 7 月 1 日接近近日点时的活跃行为。
We present a physical characterization of comet 176P/LINEAR, the third discovered member of the new class of main-belt comets, which exhibit cometary activity but are dynamically indistinguishable from main-belt asteroids. Observations show the object exhibiting a fan-shaped tail for at least one month in late 2005, but then becoming inactive in early 2006. During this active period, we measure broadband colors of B − V = 0.63 ± 0.02, V − R = 0.35 ± 0.02, and R − I = 0.31 ± 0.04. Using data from when the object was observed to be inactive, we derive best-fit IAU phase function parameters of H = 15.10 ± 0.05 mag and G = 0.15 ± 0.10, and best-fit linear phase function parameters of m(1, 1, 0) = 15.35 ± 0.05 mag and β = 0.038 ± 0.005 mag deg−1. From this baseline phase function, we find that 176P exhibits a mean photometric excess of ∼30% during its active period, implying an approximate total coma dust mass of Md ∼ (7.2 ± 3.6) × 104 kg. From inactive data obtained in early 2007, we find a rotation period of Prot = 22.23 ± 0.01 hr and a peak-to-trough photometric range of Δm ∼ 0.7 mag. Phasing our photometric data from 176P's 2005 active period to this rotation period, we find that the nucleus exhibits a significantly smaller photometric range than in 2007 that cannot be accounted for by coma damping effects, and as such, are attributed by us to viewing geometry effects. A detailed analysis of these geometric effects showed that 176P is likely to be a highly elongated object with an axis ratio of 1.8 < b/a < 2.1, an orbital obliquity of ε ∼ 60°, and a solstice position at a true anomaly of νo = 20° ± 20°. Numerical modeling of 176P's dust emission found that its activity can only be reproduced by asymmetric dust emission, such as a cometary jet. We find plausible fits to our observations using models assuming ∼10 μm dust particles continuously emitted over the period during which 176P was observed to be active, and a jet direction of 180° ≲ αjet ≲ 120° and δjet ≈ −60°. We do not find good fits to our observations using models of impulsive dust emission, i.e., what would be expected if 176P's activity was an ejecta cloud resulting from an impact into non-volatile asteroid regolith. Since for a rotating body, the time-averaged direction of a non-equatorial jet is equivalent to the direction of the nearest rotation pole, we find an equivalent orbital obliquity of 50° ≲ ε ≲ 75°, consistent with the results of our light curve analysis. Furthermore, the results of both our light curve analysis and dust modeling analysis are consistent with the seasonal heating hypothesis used to explain the modulation of 176P's activity. Additional observations are highly encouraged to further characterize 176P's active behavior as the object approaches perihelion on 2011 July 1.