Original Orbits for Long-Period Comets Active Far from the Sun

Nongravitational (NG) accelerations have now been detected in the motion of more than 100 near-parabolic comets (NPCs) - those with original semimajor axes (a_ori) larger than approximately 10,000 au [1,2]. Oort Cloud comets are now being discovered far from the Sun and being tracked for years or even decades, and NG effects have been measured for a few NPCs with perihelion distances (q) beyond 5 au. It has long been known that the values of a_ori inferred using NG orbits for near-parabolic comets are systematically smaller than those obtained with purely gravitational orbits [3,4], but finding the reason why has proven elusive. Resolving this issue is key to determining the size of the Oort Cloud [5]. Here we investigate how a_ori depends on the selected data arc and the assumed form of the NG acceleration [6]. We selected 32 NPCs with q > 3 au observed with long data arcs both before and after perihelion. For each comet, we determined both gravitational and NG orbits using at least three types of positional data (pre- and post-perihelion data and the entire data set) and three forms of NG acceleration representing water ice or CO sublimation. NG effects are detectable for 19 of the 32 comets, including C/2010 U3 (Boattini) [q = 8.44 au]. We also find that difference in a_ori between gravitational and NG orbits is sensitive to the assumed form of the function describing sublimation of ice. LD thanks the New Frontiers Data Analysis Program for support of some of this work. [1] Królikowska, M., Dybczyński, P. A. A&A 640, A97, 2020. [2] Catalogue of Cometary Orbits and their Dynamical Evolution (CODE). https://pad2.astro.amu.edu.pl/comets/index.php [3] Marsden, B. G., Sekanina, Z., Yeomans, D. K., AJ 78, 211, 1973. [4] Marsden, B. G., Sekanina Z., Everhart E., AJ 83, 64–71, 1978. [5] Królikowska, M., Acta Astronomica 56, 385-412. [6] Królikowska, M., Dones, L., submitted to A&A.