A&A 645, A48 (2021) Astronomy https://doi.org/10.1051/0004-6361/202038776 & c ESO 2021 Astrophysics New precise positions in 2013–2019 and a catalog of ground-based astrometric observations of 11 Neptunian satellites (1847–2019) based on Gaia-DR2 Ye Yuan (袁è)1,2, Fan Li (Ná)1,3, Yanning Fu (傅燕宁)1, and Shulin Ren (û树林)1 1 Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing 210033, PR China e-mail: [email protected] 2 School of Astronomy and Space Science, University of Science and Technology of China, Hefei, Anhui 230026, PR China 3 University of Chinese Academy of Sciences, Beijing 100049, PR China Received 29 June 2020 / Accepted 1 October 2020 ABSTRACT Context. Developing high-precision ephemerides for Neptunian satellites requires not only the continuation of observing campaigns but also the collection and improvement of existing observations. So far, no complete catalogs of observations of Neptunian satellites are available. Aims. We aim to provide new, precise positions, and to compile a catalog including all available ground-based astrometric observations of Neptunian satellites. The observations are tabulated in a single and consistent format and given in the same timescale, the Terrestrial Time (TT), and reference system, the International Celestial Reference System (ICRS), including necessary changes and corrections. Methods. New CCD observations of Triton and Nereid were made at Lijiang 2.4-m and Yaoan 0.8-m telescopes in 2013–2019, and then reduced based on Gaia-DR2. Furthermore, a catalog called OCNS2019 (Observational Catalog of Neptunian Satellites (2019 version)) was compiled, after recognizing and correcting errors and omissions. Furthermore, in addition to what was considered for the COSS08 catalog for eight main Saturnian satellites, all observed absolute and relative coordinates were converted to the ICRS with corrections for star catalog biases with respect to Gaia-DR2. New debiasing tables for both the modern and old star catalogs, which were previously not provided based on Gaia-DR2, are developed and applied. Treatment of missing positions of comparison bodies in conversions of observed relative coordinates are proposed. Results. OCNS2019 and the new debiasing tables are publicly available online. OCNS2019 includes 24996 observed coordinates of 11 Neptunian satellites obtained over 3741 nights from 1847 to 2019. All observations are given in TT and ICRS. The star catalog biases are removed, which are significant for Nereid and outer satellites. We obtained 880 (5% of total now available) new coordinates for Triton over 41 nights (1% of total observation nights so far), and 790 (14%) for Nereid over 47 nights (10%). The dispersions of these 00 00 new positions are about 0: 03 for Triton and 0: 06 for Nereid. Conclusions. OCNS2019 should be useful in improving ephemerides for the above-mentioned objects. Key words. catalogs – astrometry – ephemerides – planets and satellites: individual: Neptune – methods: observational – methods: data analysis 1. Introduction in formats which are specific to their respective publications, and are consequently in various formats of data type, timescale, coor- To support current and future scientific investigations of the dinates, and so on. Neptune system and relevant space exploration missions (e.g., New CCD observations of Triton and Nereid were made at NASA’s Trident, Mitchell et al. 2019; Prockter et al. 2013), Lijiang 2.4-m and Yaoan 0.8-m telescopes in 2013–2019, and high-precision ephemerides for Neptunian satellites are needed then reduced based on Gaia-DR2 (Gaia Collaboration 2018). (Jacobson 2009; Brozovic´ et al. 2011, 2020; Emelyanov & Arlot Among all the available observed coordinates of Triton, we 2011; Jacobson et al. 2012; Emelyanov & Samorodov 2015). The obtained 880 (5% of total now available) new coordinates for precision of ephemerides and determined dynamical parame- Triton over 41 nights (1% of total observation nights so far), ters depends both on the precision of observations and on the and 790 (14%) for Nereid over 47 nights (10%). Our analysis length of the time interval covered by observations (Desmars et al. shows that the standard deviations of the residuals with respect to 2009a; Emelyanov 2010; Jacobson et al. 2012; Arlot et al. 2012; the ephemeris positions given with DE430/NEP081xl1 are about Emelyanov & Samorodov 2015; Lainey 2016). Therefore, not 000:03 for Triton and 000:06 for Nereid. only the continuation of observing campaigns but also the collec- For the compilation of the observational catalog, we extract tion and improvement of existing observations are required. In the most data from three databases: the Natural Satellite Data present work, we provide new precise positions, and compile an observational catalog including all available ground-based astro- metric observations of Neptunian satellites. So far, no complete 1 The planetary ephemerides DE438 (Folkner & Park 2018) and satel- catalog tabulating all observations of Neptunian satellites in a con- lite ephemerides NEP081xl (Jacobson 2009). They can be retrieved sistent format and in the same timescale and reference system is from the JPL Solar System Dynamics website https://ssd.jpl. available. In fact, most of the available observations are presented nasa.gov/. Article published by EDP Sciences A48, page 1 of 18 A&A 645, A48 (2021) Center (NSDC2; Arlot & Emelyanov 2009), the Minor Planet Table 1. Instrumental details for the used reflectors and CCD detectors. Center (MPC3), and the JPL Solar System Dynamics Website 4 (JPL-SSD ). Although these databases are frequently updated, Years 2013 2013, 2017 2018–2019 a few errors and omissions can be recognized by using the SAO/NASA Astrophysics Data System (ADS). Site O44 O44 O49 Since the discovery of the first Neptunian satellite, Triton Diameter of primary mirror 2.4 m 2.4 m 0.8 m (more than 170 years ago), many observations of Neptunian CCD detector PI VersArray YFOSC PIXIS CCD field of view (effective) 50 × 50 90 × 90 110 × 110 satellites have been made and published in different formats. Size of CCD array (effective) 1340 × 1340 1900 × 1900 2048 × 2048 However, for convenience of use, it is preferable that all observa- Size of pixel 20 µm 13.5 µm 13.5 µm tions are tabulated in a single and consistent format, as done by Angular extent per pixel 000:215 000:285 000:346 Desmars et al.(2009b) for their COSS08, an observational cat- alog of eight main Saturnian satellites. Furthermore, we convert Notes. The “Site” row provides the IAU observatory code. all the observed coordinates to the International Celestial Refer- ence System (ICRS). Table 2. Details of the astrometric reduction. Each star catalog has its unique systematic relations with the ICRS (Schwan 2001). For absolute coordinates actually ref- Satellite Triton Nereid erenced to a star catalog, the biases induced by star position site O44 O49 O44 O49 errors should be corrected. Considering the Gaia-DR2 catalog as an extremely accurate realization of the ICRS, Eggl et al. Nm∗ 19–87 7–118 40–87 30–124 (2020) determined the biases with respect to the Gaia-DR2 for Mean(Nm∗) 33 52 68 84 the 26 modern star catalogs commonly used as a reference for S/N 20–700 50–1800 14–123 10–55 the ICRS. Here, we apply their provided debiasing tables 5 to Median(S/N) 370 380 70 27 the Neptunian satellites for the first time. In addition, we also Notes. N develop and apply debiasing tables to the currently used mod- The “Site” row gives the IAU observatory codes. m∗ is the num- ber of matched Gaia-DR2 reference stars. S/N is the measured signal- ern star catalog, Tycho-1 (ESA 1997), and the old ones, the Yale to-noise ratio of a satellite. Zone Catalogs (Yale University 1997), which were not studied by Eggl et al.(2020). Conversions of observed relative coordinates, except for the distances. However, some frames can be used to provide both separations independent of coordinate systems, require com- observed positions of Triton and Nereid. According to the stan- plementary positions of comparison bodies. However, these dard procedure of astrometric observations, the flat-field, bias, positions are usually unavailable in observational data. Here, and dark frames were also taken. a treatment is proposed for this based on the ephemeris posi- tions given with DE438/NEP081xl. Analytical estimations and 2.2. Astrometric reduction based on Gaia-DR2 test calculations show that the introduced error of < ∼ 5 µas is negligible. A robust and stable astrometric reduction procedure was car- The new precise positions are described in Sect.2. The com- ried out according to Ofek(2019). The observed positions of pilation procedure is described in Sect.3. The final observational Triton and Nereid were obtained with respect to the reference catalog is presented and analyzed in Sect.4. We call this catalog stars in the same fields of view. Here, following the advice of OCNS2019, which stands for the Observational Catalog of Nep- Arlot et al.(2012) and Ofek(2019), Gaia-DR2 is used as a ref- tunian Satellites (2019 version). Conclusions are given in Sect.5. erence catalog. After matching the reference stars with the posi- tions in Gaia-DR2, a plate model was applied between the pixel and standard coordinates for each reference star as suggested by 2. New precise positions of Triton and Nereid Ofek(2019). Specifically, the plate model is selected based on (2013–2019) the number of matched stars (Nm∗) by applying the following 2.1. CCD observations rules: 5 ≤ N < 10: an affine transformation; All CCD observations are made with the 0.8-m reflector at the m∗ 10 ≤ N < 75: an affine transformation + tip/tilt terms; Yaoan station (IAU observatory code, O49, assigned by MPC6) m∗ 75 ≤ N < 120: an affine transformation + tip/tilt terms + 3rd of Purple Mountain Observatory and the 2.4-m reflector at the m∗ order Chebyshev polynomials of the second kind; Lijiang station (O44) of Yunnan Astronomical Observatory.