Dynamical characterisation of small asteroid family members by an occultation astrometry survey
Authors: Liberato Luana , Tanga Paolo , Leiva Rodrigo , Spoto Federica 
Published: July 2021

Up to now, we know that there are more than 100 asteroid families in the main asteroid belt with over 130000 objects as members[1]. These families are created from impact(s)on a (parent) asteroid, which generates countless fragments with approximately the same initial orbit. The study of the asteroid families can provide information on the events that originated the family, the dynamical evolution of their members, and the evolution of other bodies in the main asteroid belt. The classification and age estimation of these families is a massive work that requires the knowledge of many physical properties of their members, such as the albedo, diameter, mass, and the determining parameter, the Yarkovsky drift[2]. The Yarkovsky effect is an important parameter that can be used as a clock to the family age estimation, and also to derive asteroid density. The lack of data, mainly the Yarkovsky drift, makes it hard to obtain unbiased estimates of the ages of the families, especially for those with a small number of members[3]. As the ground-based direct imaging astrometry accuracy deteriorates considerably with the asteroid&#8217;s distance. Hence, the Yarkovsky drift used to calibrate the ages of asteroid families in the main belt comes from measurements on Near-Earth Asteroids[4]. These are better obtained since the NEAs usually have accurate orbits, which is crucial for Yarkovsky drift measurements. Therefore, to obtain the drift rate we need very precise astrometric measurements from the asteroids, currently provided in the Gaia DR2 catalogue for about 14000 asteroids [5]. But to increase the sample, we take advantage of the stellar occultation technique, which consists of observing the exact time of the event where an object passes in front of a star and block its light for a brief moment. That is a powerful observation technique that can provide high accuracy astrometric measurements, reaching uncertainties on the order of a few milliarcseconds, without the need for significantly large telescopes and special equipment. That level of precision is only possible thanks to the Gaia mission because having a precise position of the stars decreases the uncertainty in the astrometry for other bodies. GaiaDR3, planned for next spring (See Paolo Tanga&#8217;s presentation), will considerably improve the amount of epoch astrometry of asteroids already available in Gaia DR2, by a factor of10. Using Gaia data and stellar occultation technique, we can obtain direct high accuracy astrometry and measure the size, position, shape and even the presence of a companion or a ring around the occulting body. If the uncertainty in the astrometry is small enough, we can measure the Yarkovsky drift on the asteroid. Therefore, in the present work, we aim to observe stellar occultations for as many asteroid family members as possible to obtain the physical properties needed to estimate the families ages or improve the current estimates. We select all families members with a diameter of at least 1 km, resulting in a list with approximately 100000 bodies. Using the package for stellar occultations reduction and analysis SORA, star positions from GaiaDR2 and ephemerides from JPL Horizons[6], we obtain predictions for all events in a chosen 1-year timespan for the selected bodies. On average, all around the Earth&#8217;s surface occurs approximately 27 events of stellar occultation per body per month, for stars with magnitude up to 16. However, that number considers all the events, even those not much observable as in the cases of a too-small magnitude drop, event duration, or in locations hardly accessible (e.g. middle of the ocean or remote places). Also, when we look for events where the shadow path of the occultation passes on an exact location, the number of events is much smaller. But even after the constraints, we have a substantial number of stellar occultations per night in each chosen site due to a large number of bodies in our survey. Therefore, we classify as high-priority the events in which the body has less than 10 km diameter, is from a family with age already determined, is a binary body or is close to the V-shape lines of its family. Additionally, we include in the priority list another 225 bodies with one or two companions, with and without family membership, for their significant scientific interest. For better coverage, we need observations from some different locations around the Earth. But we focus on making use of the recently built 50 cm UniversCityTelescope at the Plateau de Calern Observatory. The automatization of this telescope is under development and soon will become a fully robotic instrument. It will be capable of observing many events in one night, every night, reading the targets from a list of coordinates and configurations, and giving as results the images obtained, the astrometry and the photometry of the observed events (See Rodrigo Leiva&#8217;s presentation). This telescope will make possible our goal of observing so many stellar occultations in a short period. As both the events predictions and priority selection are in the final phase, we intend to start obtaining data in the forthcoming weeks, then begin to measure and determine the asteroids families features that we look for.
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[1] AstDyS-2 https://newton.spacedys.com/astdys
[2] Milani, A., Spoto, F., Kne&#382;evi&#263;, Z., Novakovi&#263;, B., \&amp; Tsirvoulis, G. (2015). Proceedings of the International Astronomical Union, 10(S318), 28-45.
[3] Spoto, F., Milani, A., \&amp; Kne&#382;evi&#263;, Z. (2015). Icarus, 257, 275-289.
[4] Vokrouhlick&#253;, D., Milani, A., \&amp; Chesley, S. R. (2000). Icarus, 148(1), 118-138.
[5] Brown, A. G. A., Vallenari, A., Prusti, T., De Bruijne, J. H. J., Babusiaux, C., ... &amp; Bertone, S. (2018). Astronomy &amp; astrophysics, 616, A1.
[6] JPL HORIZONS.https://ssd.jpl.nasa.gov/?horizons
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This study was financed in part by the Coordena&#231;&#227;o de Aperfei&#231;oamento de Pessoal de N&#237;vel Superior &#8211; Brasil (CAPES) &#8211; Finance Code 001, also by CAPES-PRINT Process 88887.570251/2020-00, and by Cote d&#8217;Azur Observatory.
This research made use of SORA, a python package for stellar occultations reduction and analysis, developed with the support of ERC Lucky Star and LIneA/Brazil.