ABSTRACT The Centaur (60558) Echeclus was discovered on 2000 March 03, orbiting between the orbits of Jupiter and Uranus. After exhibiting frequent outbursts, it also received a comet designation, 174P. If the ejected material can be a source of debris to form additional structures, studying the surroundings of an active body like Echeclus can provide clues about the formation scenarios of rings, jets, or dusty shells around small bodies. Stellar occultation is a handy technique for this kind of investigation, as it can, from Earth-based observations, detect small structures with low opacity around these objects. Stellar occultation by Echeclus was predicted and observed in 2019, 2020, and 2021. We obtain upper detection limits of rings with widths larger than 0.5 km and optical depth of τ = 0.02. These values are smaller than those of Chariklo’s main ring; in other words, a Chariklo-like ring would have been detected. The occultation observed in 2020 provided two positive chords used to derive the triaxial dimensions of Echeclus based on a 3D model and pole orientation available in the literature. We obtained a = 37.0 ± 0.6 km, b = 28.4 ± 0.5 km, and c = 24.9 ± 0.4 km, resulting in an area-equivalent radius of 30.0 ± 0.5 km. Using the projected limb at the occultation epoch and the available absolute magnitude ($\rm {H}_{\rm {v}} = 9.971 \pm 0.031$), we calculate an albedo of pv = 0.050 ± 0.003. Constraints on the object’s density and internal friction are also proposed.

Context. As evidenced by recent survey results, the majority of asteroids are slow rotators (spin periods longer than 12 h), but lack spin and shape models because of selection bias. This bias is skewing our overall understanding of the spins, shapes, and sizes of asteroids, as well as of their other properties. Also, diameter determinations for large (>60 km) and medium-sized asteroids (between 30 and 60 km) often vary by over 30% for multiple reasons.Aims. Our long-term project is focused on a few tens of slow rotators with periods of up to 60 h. We aim to obtain their full light curves and reconstruct their spins and shapes. We also precisely scale the models, typically with an accuracy of a few percent.Methods. We used wide sets of dense light curves for spin and shape reconstructions via light-curve inversion. Precisely scaling them with thermal data was not possible here because of poor infrared datasets: large bodies tend to saturate in WISE mission detectors. Therefore, we recently also launched a special campaign among stellar occultation observers, both in order to scale these models and to verify the shape solutions, often allowing us to break the mirror pole ambiguity.Results. The presented scheme resulted in shape models for 16 slow rotators, most of them for the first time. Fitting them to chords from stellar occultation timings resolved previous inconsistencies in size determinations. For around half of the targets, this fitting also allowed us to identify a clearly preferred pole solution from the pair of two mirror pole solutions, thus removing the ambiguity inherent to light-curve inversion. We also address the influence of the uncertainty of the shape models on the derived diameters.Conclusions. Overall, our project has already provided reliable models for around 50 slow rotators. Such well-determined and scaled asteroid shapes will, for example, constitute a solid basis for precise density determinations when coupled with mass information. Spin and shape models in general continue to fill the gaps caused by various biases.

Context. Trans-Neptunian objects (TNOs) and Centaurs are remnants of our planetary system formation, and their physical properties have invaluable information for evolutionary theories. Stellar occultation is a ground-based method for studying these distant small bodies and has presented exciting results. These observations can provide precise profiles of the involved body, allowing an accurate determination of its size and shape.Aims. The goal is to show that even single-chord detections of TNOs allow us to measure their milliarcsecond astrometric positions in the reference frame of the Gaia second data release (DR2). Accurate ephemerides can then be generated, allowing predictions of stellar occultations with much higher reliability.Methods. We analyzed data from various stellar occultation detections to obtain astrometric positions of the involved bodies. The events published before the Gaia era were updated so that the Gaia DR2 stellar catalog is the reference, thus providing accurate positions. Events with detection from one or two different sites (single or double chord) were analyzed to determine the event duration. Previously determined sizes were used to calculate the position of the object center and its corresponding error with respectto the detected chord and the International Celestial Reference System propagated Gaia DR2 star position.Results. We derive 37 precise astrometric positions for 19 TNOs and four Centaurs. Twenty-one of these events are presented here for the first time. Although about 68% of our results are based on single-chord detection, most have intrinsic precision at the submilliarcsecond level. Lower limits on the diameter of bodies such as Sedna, 2002 KX14, and Echeclus, and also shape constraints on 2002 VE95, 2003 FF128, and 2005 TV189 are presented as valuable byproducts.Conclusions. Using the Gaia DR2 catalog, we show that even a single detection of a stellar occultation allows improving the object ephemeris significantly, which in turn enables predicting a future stellar occultation with high accuracy. Observational campaigns can be efficiently organized with this help, and may provide a full physical characterization of the involved object, or even the study of topographic features such as satellites or rings.

An occultation of TYC 1950-02320-1 by the asteroid (141) Lumen on 2013 December 28 showed this star to be a double star. Both components of the double star were occulted as recorded by one observer, one component of the double star was occulted by three observers, and 9 observers recorded miss observations. The separation of the two components is 0.1529 ± 0.0008 arcseconds at a position angle of 105.8 ± 0.7 degrees. The magnitude of the primary component is estimated to be 11.25 (V). The magnitude of the secondary component is estimated to be 11.47 (V). Observation On 2013 December 28, thirteen observers occupying or operating sites across the United States and Japan observed the asteroid (141) Lumen occult the star TYC 1950-02320-1. See Figure 1 for the path map of the event. One site in Arizona, USA (George) observed two-separate drops in brightness, neither of which had a mag drop as large as predicted, indicating a double star (see Figure 2). Three sites in Japan (Ishida; Owada; Ida) had only a single drop in brightness (see Figures 3, 4, and 5). For these latter sites, the magnitude drop measured was consistent with observation of the secondary star only. Nine sites had a miss. All recorded occultation times and data from the observers can be found in archived IOTA records for the event. The observations were made by the observers located at the sites and with the equipment shown in Table 1. The target star is magnitude 10.60 ± 0.09. This is a V magnitude in the Johnson system derived from the Tycho system magnitudes VT and BT given in the Tycho-2 Catalogue. The asteroid magnitude as predicted by the Minor Planet Center using the magnitude parameter values H = 8.4 and G = 0.15 was 12.54 (V). The combined magnitude of the asteroid and the star was calculated to be 10.43 (V). The expected magnitude drop at occultation was calculated to be 2.11 magnitudes. The star is not listed in the Fourth Interferometric Catalog, nor is it listed in the Washington Double Star Catalog. Analysis The observations were analysed in the standard manner described by IOTA [1]. The finished plot of the double star fit to the data is shown in Figure 6. The double star has a separation of 0.1529 ± 0.0008 arcseconds at a position angle of 105.8 ± 0.7 degrees. Of the data sets that recorded the occul(Continued on page 243) Vol. 10 No. 3 July 1, 2014 Page 241 Journal of Double Star Observations Discovery of Stellar Duplicity of TYC 1950-02320-1 During Asteroidal Occultation by (141) Lumen Figure 1. Occultation Path Figure 2. George light curve showing two distinct events Vol. 10 No. 3 July 1, 2014 Page 242 Journal of Double Star Observations Discovery of Stellar Duplicity of TYC 1950-02320-1 During Asteroidal Occultation by (141) Lumen Figure 3. Ishida light curve Figure 4 – Owada light curve Figure 5 -Ida light curve Vol. 10 No. 3 July 1, 2014 Page 243 Journal of Double Star Observations Discovery of Stellar Duplicity of TYC 1950-02320-1 During Asteroidal Occultation by (141) Lumen tation, George recorded both events with magnitude drops suitable for calculating the stellar component magnitudes. Using the light curve data from all observers, the magnitude drops of the two events were calculated using the brightness measurements derived by ROTE [2], the Magnitude calculator routine in Occult4 [3] (Method 3 – Magnitudes from light curve values), the combined V magnitude from the Tych-2 Catalogue and the predicted V magnitude of the asteroid as explained above. The results are shown in Table 2. Note that the measured magnitude drops are instrumental magnitudes with no filters, and we assume that they are not much different from those in V in calculating the magnitude of each component of the double star. Based on the average magnitude drop estimates for the two components shown in Table 2, the combined magnitude of each component star + asteroid was calculated. The magnitudes of the two component stars were derived by adjusting for the brightness of the asteroid in the light curve. The magnitudes of the two stars are estimated to be 11.25 ± 0.1 (V) primary star and 11.47 ± 0.1 (V) secondary star, and their magnitude difference is estimated to be 0.22 ± 0.06 (V). The event was a BBAA, with the secondary occulted first, then the primary. Based on the data presented in this report, the double star characteristics as shown in the plot in Figure 6 are: Star TYCHO 1950-02320-1 UCAC2 39846549 UCAC4 563-047083 NOMAD 1125-0200778 PPMXL 4181982505129551400 Spectral type F5III [4] Coordinates (UCAC4) RA 09h 05m 39.2553s Dec +22° 34' 52.828" (ICRS(J2000), Epoch 2013 Dec 28) Mag A 11.25 ± 0.1 (Est. Tycho-2 V mag) Mag B 11.47 ± 0.1 (Est. Tycho-2 V mag) Mag Difference 0.22 ± 0.06 Separation 0.1529 ± 0.0008 arcseconds Position Angle 105.8 ± 0.7 degrees (Continued from page 240) Figure 6 Chords Observer Location State Prefecture Country Telescope Type Telescope Dia (cm) Method Result 1 E. Edens Magdalena NM USA/Holland SCT 15 Video + GPS Time Inst Miss 2 A. Yaeza Hitachi Ibaraki Japan SCT 20 Visual + Stop Watch Miss 3 A. Hashimoto Chichibu Saitama Japan SCT 40 Visual + Stop Watch Miss 4 W. Morgan Pleasanton CA USA SCT 20 Video + GPS Time Inst Miss 5 R. Aikawa Sakado Saitama Japan SCT 20 Visual + Stop Watch Miss 6 T. Horaguchi Tsukuba Ibaraki Japan Reflector 50 Video + GPS Time Inst Miss 7 S. Uehara Tsukuba Ibaraki Japan Reflector 20 Visual + Stop Watch Miss 8 S. Uchiyama Kashiwa Chiba Japan SCT 25 Video + GPS Time Inst Miss 9 K. Kitazaki Musashino Tokyo Japan Cass 40 Video + GPS Time Inst Miss 11 M. Ida Odai Mie Japan SCT 20 Video + GPS Time Inst One Event 12 M. Owada Hamamatsu Shizuoka Japan SCT 25 Video + GPS Time Inst One Event 13 M. Ishida Taiki Mie Japan SCT 20 Video + GPS Time Inst One Event 14,15 T. George Scottsdale AZ USA SCT 30 Video + GPS Time Inst Two Events Table 1. Observers, site locations, equipment, methods, and results Observer 1st Event 2nd Event George 0.56 ± 0.03 0.69 ± 0.03