Preliminary estimates of the Zwicky Transient Facility 'Ayló'chaxnim asteroid population completeness

Abstract

Near-Earth asteroids (NEAs) are organized into five main classes: Amor, Apollo, Aten, Atira and 'Ayló'chaxnim. Asteroids belonging to the 'Ayló'chaxnim class are located entirely within the orbit of Venus making them difficult to detect by ground-based observatories. The first-known asteroid of this class, (594913) 'Ayló'chaxnim, was discovered by the Zwicky Transient Facility (ZTF) in 2020 January during a twilight search for asteroids at small solar elongations that ran between September 2019 and January 2020. Due to its large diameter of ∼2 km, the discovery of (594913) 'Ayló'chaxnim is surprising because contemporary NEA population models predict a scarcity of asteroids of this size located inside the orbit of Venus. To compare the discovery of (594913) 'Ayló'chaxnim by ZTF with the predictions of NEA population models, we estimated the ZTF survey completeness at detecting 'Ayló'chaxnim asteroids and the number of 'Ayló'chaxnim asteroids expected to have been discovered by simulating observations of synthetic 'Ayló'chaxnim asteroids. We find that the 'Ayló'chaxnim population completeness of the survey is ∼18% and there is only a 5% probability that a single 'Ayló'chaxnim asteroid would have been discovered. Given the small chance for (594913) 'Ayló'chaxnim to have been discovered, its presence is either a statistical fluke or it implies that asteroid population models may need to be revised.