Postperihelion Cometary Activity on the Outer Main-belt Asteroid 2005 XR₁₃₂

Abstract

We report comet-like activity on the outer main-belt asteroid 2005 XR132 discovered by the Lulin One-meter Telescope in early 2021 April. A series of follow-up observations were triggered to characterize the morphology and brightness variation of 2005 XR132. Long-term photometric data of the 2020 perihelion return reveal a 2 mag fading in 120 days, starting 20 days postperihelion, attributed to decreased cometary activity. Even though no variation indicative of the rotational period can be found in our data, we infer an a/b axial ratio of 1.32, given that the lower limit of rotational amplitude is 0.3 mag. A visible spectrum and broadband color support that 2005 XR132 has a reflectance feature similar to a BR-type Centaur object. The syndyne and synchrone simulations reveal a low-speed dust ejecta consisting of millimeter-sized dust grains released shortly after the perihelion passage. We demonstrate that 2005 XR132 has a short dynamical lifetime of 0.12 Myr, with <5% of it in the near-Earth space. Due to the strong gravitational influence from Jupiter and Saturn, the asteroid has followed a random walk orbital migrating process. We also find that since 1550 CE, the perihelion distance of 2005 XR132 has gradually decreased from 2.8 to 2.0 au, likely due to the Kozai–Lidov effect, which potentially reactivated the dormant nucleus. All these dynamical properties support a cometary origin for 2005 XR132 rather than an ice-rich main-belt object kicked out from a stable orbit, although current observational evidence has yet to confirm repeating cometary activities.