The 2017 May 20th^{\rm th} stellar occultation by the elongated centaur (95626) 2002 GZ32_{32}

We predicted a stellar occultation of the bright star Gaia DR1 4332852996360346368 (UCAC4 385-75921) (m$_{\rm V}$= 14.0 mag) by the centaur 2002 GZ$_{32}$ for 2017 May 20$^{\rm th}$. Our latest shadow path prediction was favourable to a large region in Europe. Observations were arranged in a broad region inside the nominal shadow path. Series of images were obtained with 29 telescopes throughout Europe and from six of them (five in Spain and one in Greece) we detected the occultation. This is the fourth centaur, besides Chariklo, Chiron and Bienor, for which a multi-chord stellar occultation is reported. By means of an elliptical fit to the occultation chords we obtained the limb of 2002 GZ$_{32}$ during the occultation, resulting in an ellipse with axes of 305 $\pm$ 17 km $\times$ 146 $\pm$ 8 km. From this limb, thanks to a rotational light curve obtained shortly after the occultation, we derived the geometric albedo of 2002 GZ$_{32}$ ($p_{\rm V}$ = 0.043 $\pm$ 0.007) and a 3-D ellipsoidal shape with axes 366 km $\times$ 306 km $\times$ 120 km. This shape is not fully consistent with a homogeneous body in hydrostatic equilibrium for the known rotation period of 2002 GZ$_{32}$. The size (albedo) obtained from the occultation is respectively smaller (greater) than that derived from the radiometric technique but compatible within error bars. No rings or debris around 2002 GZ$_{32}$ were detected from the occultation, but narrow and thin rings cannot be discarded.Comment: Accepted for publication in MNRAS (8-Dec.-2020), 15 pages, 9 figure

Scaling slowly rotating asteroids with stellar occultations

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

The multichord stellar occultation on 2019 October 22 by the trans-Neptunian object (84922) 2003 VS

Context. Stellar occultations have become one of the best techniques to gather information about the physical properties of trans-Neptunian objects (TNOs), which are critical objects for understanding the origin and evolution of our Solar System. Aims. The purpose of this work is to determine, with better accuracy, the physical characteristics of the TNO (84922) 2003 VS2 through the analysis of the multichord stellar occultation on 2019 October 22 and photometric data collected afterward. Methods. We predicted, observed, and analyzed the multichord stellar occultation of the Second Gaia Data Release (Gaia DR2) source 3449076721168026624 (mυ = 14.1 mag) by the plutino object 2003 VS2 on 2019 October 22. We performed aperture photometry on the images collected and derived the times when the star disappeared and reappeared from the observing sites that reported a positive detection. We fit the extremities of such positive chords to an ellipse using a Monte Carlo method. We also carried out photometric observations to derive the rotational light curve amplitude and rotational phase of 2003 VS2 during the stellar occultation. Combining the results and assuming a triaxial shape, we derived the 3D shape of 2003 VS2. Results. Out of the 39 observatories involved in the observational campaign, 12 sites, located in Bulgaria (one), Romania (ten), and Serbia (one), reported a positive detection; this makes it one of the best observed stellar occultations by a TNO so far. Considering the rotational phase of 2003 VS2 during the stellar occultation and the rotational light curve amplitude derived (Am = 0.264 ± 0.017 mag), we obtained a mean area-equivalent diameter of $\left( {{D_{{{\rm{A}}_{{\rm{eq}}}}}}} \right)$ = 545 ± 13 km and a geometric albedo of 0.134 ± 0.010. By combining the rotational light curve information with the stellar occultation results, we derived the best triaxial shape for 2003 VS2, which has semiaxes a = 339 ± 5 km, b = 235 ± 6 km, and c = 226 ± 8 km. The derived aspect angle of 2003 VS2 is θ = 59° ± 2° or its supplementary θ = 121° ± 2°, depending on the north-pole position of the TNO. The spherical-volume equivalent diameter is $\left( {{D_{{{\rm{V}}_{{\rm{eq}}}}}}} \right)$ = 524 ± 7 km. If we consider large albedo patches on its surface, the semi-major axis of the ellipsoid could be ~ 10 km smaller. These results are compatible with the previous ones determined from the single-chord 2013 and four-chord 2014 stellar occultations and with the effective diameter and albedo derived from Herschel and Spitzer data. They provide evidence that 2003 VS2’s 3D shape is not compatible with a homogeneous triaxial body in hydrostatic equilibrium, but it might be a differentiated body and/or might be sustaining some stress. No secondary features related to rings or material orbiting around 2003 VS2 were detected

The 2017 May 20 stellar occultation by the elongated centaur (95626) 2002 GZ32

We predicted a stellar occultation of the bright star Gaia DR1 4332852996360346368 (UCAC4 385-75921) (mV = 14.0 mag) by the centaur 2002 GZ32 for 2017 May 20. Our latest shadow path prediction was favourable to a large region in Europe. Observations were arranged in a broad region inside the nominal shadow path. Series of images were obtained with 29 telescopes throughout Europe and from six of them (five in Spain and one in Greece) we detected the occultation. This is the fourth centaur, besides Chariklo, Chiron, and Bienor, for which a multichord stellar occultation is reported. By means of an elliptical fit to the occultation chords, we obtained the limb of 2002 GZ32 during the occultation, resulting in an ellipse with axes of 305 ± 17 km × 146 ± 8 km. From this limb, thanks to a rotational light curve obtained shortly after the occultation, we derived the geometric albedo of 2002 GZ32 (pV = 0.043 ± 0.007) and a 3D ellipsoidal shape with axes 366 km × 306 km × 120 km. This shape is not fully consistent with a homogeneous body in hydrostatic equilibrium for the known rotation period of 2002 GZ32. The size (albedo) obtained from the occultation is respectively smaller (greater) than that derived from the radiometric technique but compatible within error bars. No rings or debris around 2002 GZ32 were detected from the occultation, but narrow and thin rings cannot be discarded. © 2021 2020 The Author(s) Published by Oxford University Press on behalf of Royal Astronomical Society

The 2017 May 20 stellar occultation by the elongated centaur (95626) 2002 GZ32

We predicted a stellar occultation of the bright star Gaia DR1 4332852996360346368 (UCAC4 385-75921) (mV = 14.0 mag) by the centaur 2002 GZ32 for 2017 May 20. Our latest shadow path prediction was favourable to a large region in Europe. Observations were arranged in a broad region inside the nominal shadow path. Series of images were obtained with 29 telescopes throughout Europe and from six of them (five in Spain and one in Greece) we detected the occultation. This is the fourth centaur, besides Chariklo, Chiron, and Bienor, for which a multichord stellar occultation is reported. By means of an elliptical fit to the occultation chords, we obtained the limb of 2002 GZ32 during the occultation, resulting in an ellipse with axes of 305 ± 17 km × 146 ± 8 km. From this limb, thanks to a rotational light curve obtained shortly after the occultation, we derived the geometric albedo of 2002 GZ32 (pV = 0.043 ± 0.007) and a 3D ellipsoidal shape with axes 366 km × 306 km × 120 km. This shape is not fully consistent with a homogeneous body in hydrostatic equilibrium for the known rotation period of 2002 GZ32. The size (albedo) obtained from the occultation is respectively smaller (greater) than that derived from the radiometric technique but compatible within error bars. No rings or debris around 2002 GZ32 were detected from the occultation, but narrow and thin rings cannot be discarded