主带彗星259 P/加拉德（P/2008 R1）的再活化
The Reactivation of Main-Belt Comet 259P/Garradd (P/2008 R1)
摘要：
我们使用双子座南北望远镜对主带彗星259 P/Garradd 2017年近日点通过前四个月至近日点后五个月进行了观测。该物体被证实在此期间处于活动状态，使其成为七个被证实具有经常性活动的MBC之一。我们发现2017年259 P的平均净近日点前尘埃产生率为dM/dt = 4.6+/-0.2 kg/s（假设颗粒密度为rho = 2500 kg/m^3，平均有效粒径为a_d = 2 mm），可检测放射性的最佳起始日期为2017年4月22日± 1，当该天体的日心距为rh = 1.96 + 0.03au，真距平为nu = 313.9+ 0.4 °时，观测到了该天体的日面角。我们估计259 P在2017年活跃期开始时，其表面积的有效活性分数为f_act ~ 7 x10 ^-3至f_act ~ 6x 10 ^-2（相当于有效活性面积A_act ~ 8x10^3 m^2至A_act ~ 7x10^4 m^2）。比较2008年和2017年测量的259 P估计总尘埃质量，没有证据表明这两个活动幽灵之间的活动强度发生了变化。259 P活动起始点的日心距离比其他的MBC小得多，这表明它的冰库可能比离太阳更远的MBC位于更深的地方，增加了太阳辐射驱动的热波到达地下冰所需的时间。我们认为，259 P上更深的冰可能是由于与其他MBC相比，该物体更接近太阳而导致的更快的冰消耗。
英文摘要：
We present observations of main-belt comet 259P/Garradd from four months prior to its 2017 perihelion passage to five months after perihelion using the Gemini North and South telescopes. The object was confirmed to be active during this period, placing it among seven MBCs confirmed to have recurrent activity. We find an average net pre-perihelion dust production rate for 259P in 2017 of dM/dt = 4.6+/-0.2 kg/s (assuming grain densities of rho = 2500 kg/m^3 and a mean effective particle size of a_d = 2 mm) and a best-fit start date of detectable activity of 2017 April 22+/-1, when the object was at a heliocentric distance of r_h = 1.96-/+0.03 au and a true anomaly of nu = 313.9+/-0.4 deg. We estimate the effective active fraction of 259P's surface area to be from f_act ~ 7x10^-3 to f_act ~ 6x10^-2 (corresponding to effective active areas of A_act ~ 8x10^3 m^2 to A_act ~ 7x10^4 m^2) at the start of its 2017 active period. A comparison of estimated total dust masses measured for 259P in 2008 and 2017 shows no evidence of changes in activity strength between the two active apparitions. The heliocentric distance of 259P's activity onset point is much smaller than those of other MBCs, suggesting that its ice reservoirs may be located at greater depths than on MBCs farther from the Sun, increasing the time needed for a solar irradiation-driven thermal wave to reach subsurface ice. We suggest that deeper ice on 259P could be a result of more rapid ice depletion caused by the object's closer proximity to the Sun compared to other MBCs.
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