Re-reduction of CCD observations of Triton in 1996-2006 with Gaia DR2 and comparison with ephemerides

The release of new high-precision and high-density catalogue Gaia DR2 brings new opportunities and challenges to high-precision astrometry. The early CCD images of planet satellites can be re-processed so as to give positions that are much more accurate than those initially obtained. Based on Gaia DR2 catalogue, the early CCD images of Triton taken by Qiao et al. in 1996-2006 were re-reduced. A total of 1007 positions of Triton were derived from the new measurement and reduction, including 66 new observed positions and 941 updated observed positions. The comparison between these re-reduction positions and the theoretical positions, which derived from satellite model of Emelyanov et al. (2015) and planetary ephemeris DE431, shows that the mean residuals (O-C) are 19 mas in right ascension and 25 mas in declination, the standard deviation of residuals (O-C) are 44 mas and 33 mas in right ascension and declination, respectively. Benefiting from Gaia DR2, this astrometric precision is better than that of the original positions. Moreover, we also compared two catalogues of UCAC2 and Gaia DR2, three recent orbit models of Triton as well as seven different planetary ephemerides of Neptune. These comparisons show that the differences among all the three orbit models of Triton are very small within 3 mas. However, there are significant differences between different catalogues (Gaia DR2 and UCC2) and among different planetary ephemerides. These two differences are the main reasons for the presence of some systematic errors of the original observation positions published in Qiao et al. (2007).