Dust environment of active asteroids P/2019 A4 (PANSTARRS) and P/2021 A5 (PANSTARRS)

Fernando Moreno,Javier Licandro,Antonio Cabrera-Lavers,Daniel Guirado,David Morate

doi:10.1093/mnras/stab1841

Abstract

ABSTRACT We report on the characterization of the dust activity and dynamical evolution of two faint active asteroids, P/2019 A4, and P/2021 A5, observed with the 10.4 m GTC using both imaging and spectroscopy. Asteroid P/2019 A4 activity is found to be linked to an impulsive event occurring some ±10 d around perihelion, probably due to a collision or a rotational disruption. Its orbit is stable over 100 Myr time-scales. Dust tail models reveal a short-term burst producing (2.0 ± 0.7) × 106 kg of dust for maximum particle radius rmax = 1 cm. The spectrum of P/2019 A4 is featureless, and slightly redder than the Sun. P/2021 A5 was active ∼50 d after perihelion, lasting ∼5 to ∼60 d, and ejecting (8 ± 2) × 106 kg of dust for rmax = 1 cm. The orbital simulations show that a few percent of dynamical clones of P/2021 A5 are unstable on 20–50 Myr time-scales. Thus, P/2021 A5 might be an implanted object from the JFC region or beyond. These facts point to water-ice sublimation as the activation mechanism. This object also displays a featureless spectrum, but slightly bluer than the Sun. Nuclei sizes are estimated in the few hundred meters range for both asteroids. Particle ejection speeds (≈0.2 m s−1) are consistent with escape speeds from those small-sized objects.

Highlights

Active asteroids constitute a new class of objects in the Solar system
We present images and spectra of P/2019 A4 and P/2021 A5 obtained with the Gran Telescopio Canarias (10.4-m aperture) on the island of La Palma
The full width half maximum (FWHM) is always constrained to FWHM < 20 d, and the total dust mass-loss is given by Mt = (2.0 ± 0.7) × 106 kg

Summary

INTRODUCTION

Active asteroids constitute a new class of objects in the Solar system. They are characterized by being located in the main asteroid belt, but, contrary to most objects in the belt, display cometary appearance, i.e. dust comae and tails. Some 40 objects of this kind have been discovered so far The reasons of their activity are rather diverse, including impact-induced, rotational break-up, thermal fracture, or ice sublimation (Jewitt, Hsieh & Agarwal 2015). While most of those objects seem native to the main belt, some of them have been shown to become unstable on time-scales of a few tens of Myr Its orbital elements (a = 3.047 au, e = 0.14, and i = 18.19◦) give TJ = 3.147, and place the object close to the outer main belt where ice sublimation presumably starts to become dominant over other mechanisms.

ORBITAL DYNAMICS SIMULATIONS

O B S E RVAT I O N S

DUST MODELLING

RESULTS AND DISCUSSION

CONCLUSIONS