摘要：We present a study of two newly discovered heliocentric hyperbolic objects—2017 U7 and 2018 C2. Both are possibly thermally evolved comets. Observations of the latter in 2018 March from Xingming Observatory revealed that it has a color similar to those of the long-period comets, Trojans, and D-type asteroids: mB − mV = 0.75 ± 0.03, mV − mR = 0.41 ± 0.02, and mR − mI = 0.37 ± 0.03. A possible extremely faint coma of ∼9″ across was observed. The radial profile of 2018 C2 in comparison with those of the field stars helps confirm its cometary nature. Based on our convolution model, its mass-loss rate is estimated to be 0.7 ± 0.2 kg s−1. Assuming geometric albedo pR = 0.04, its effective radius is 4.4 ± 0.5 km, which means that the fraction of active area is merely ∼10−5–10−4. Our N-body dynamical simulations show that both objects are most likely dynamically old members from the Oort cloud. 2017 U7 has a ∼60% chance to escape the solar system in the 1.5 Myr following its current perihelion passage due to a moderate close encounter with Jupiter in 2020 May. On the condition that no disintegration occurs, 2018 C2 will revisit the inner solar system 0.13–0.14 Myr later, with perihelion distance ∼2 au.
关键词：comets: general;comets: individual (2017 U7, 2018 C2);methods: data analysis
