Abstract
ABSTRACT 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.
Highlights
No official definition exists, centaurs such as (95626) 2002 GZ32, are Solar system small bodies that orbit the Sun between the orbits of Jupiter and Neptune
We present the first determination of the size, albedo, projected shape and search for rings around 2002 GZ32 based on the multichord stellar occultation of 2017 May 20
We present an updated rotational light curve for 2002 GZ32 obtained with the 1.23m telescope at Calar Alto Observatory (CAHA) in Spain close to the occultation date
Summary
Mainly the Jupiter family comets, and it is believed that they escaped from the TNO population referred to as the scattered disc objects (SDOs; e.g. Fernandez, Helal & Gallardo 2018). A ring system around the centaur Chariklo (Braga-Ribas et al 2014) and a possible similar structure – still under discussion – around the centaur Chiron (Ortiz et al 2015; Ruprecht et al 2015; Sickafoose et al 2020) have been reported from these multichord occultations This highlights the relevance of observing stellar occultations by the largest centaurs: apart from determining their sizes, shapes, albedos, and other physical properties with high accuracy, detecting new rings might be possible. The large size of 2002 GZ32 (∼240 km), the likely presence of water ice on its surface (see Table 1) and its short rotation period – all these properties present in Chariklo and Chiron – make this centaur a potential candidate to have a ring system.
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