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

TL;DR

This study characterizes the dust activity and dynamical evolution of two faint active asteroids, revealing impulsive events likely caused by collisions or rotational disruptions, with implications for their origins and activation mechanisms.

Contribution

It provides detailed dust modeling, spectral analysis, and dynamical simulations of P/2019 A4 and P/2021 A5, highlighting their activation processes and possible origins from different regions.

Findings

P/2019 A4 activity linked to impulsive event near perihelion.

P/2021 A5 activity lasted 5-60 days, possibly from sublimation.

Both asteroids have nuclei of a few hundred meters in size.

Abstract

We report on the characterisation of the dust activity and dynamical evolution of two faint active asteroids, P/2019 A4, and P/2021 A5, observed with the 10.4m GTC using both imaging and spectroscopy. Asteroid P/2019 A4 activity is found to be linked to an impulsive event occurring some $\pm$10 days around perihelion, probably due to a collision or a rotational disruption. Its orbit is stable over 100 Myr timescales. Dust tail models reveal a short-term burst producing (2.0$\pm$0.7)$\times$10$^6$ 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 $\sim$50 days after perihelion, lasting $\sim$5 to $\sim$60 days, and ejecting (8$\pm$2)$\times$10$^6$ 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 timescales. 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 ($\sim$0.2 m/s) are consistent with escape speeds from those small-sized objects.