Oort Cloud comets discovered far from the Sun

Abstract

Context. Increasingly, Oort Cloud comets are being discovered at great distances from the Sun and tracked over ever wider ranges of heliocentric distances as observational equipment improves. Aims. Our goal is to investigate in detail how the original semimajor axis for near-parabolic comets depends on the selected data arc and the assumed form of the nongravitational (NG) acceleration. Methods. Among currently known Oort Cloud comets with large perihelion distances (q > 3 au), we selected 32 objects observed over the widest ranges of heliocentric distances in orbital legs before and after perihelion. For each of them, we determined a series of orbits using at least three basic types of datasets selected from available positional data (pre- and post-perihelion data and the entire dataset), and a few forms of NG acceleration representing water ice or CO sublimation. Results. We find that the motion of comets is often measurably affected by NG forces at heliocentric distances beyond 5 au from the Sun. The most spectacular example is C/2010 U3 (Boattini), whose perihelion distance is 8.44 au. NG effects are detectable for 19 of the 32 comets within the positional data. For five comets, we find asymmetric effects of NG forces – in three cases significantly greater before perihelion than afterward (C/2017 M4, C/2000 SV75, and C/2015 O1), and in two others the opposite (C/1997 BA6 and C/2006 S3). We also find that the well-known systematic effect of finding more tightly bound original orbits when including the NG acceleration than in purely gravitational solutions may be related to the specific form of the standard g(r) function describing the sublimation of ices.