Physical Properties and Origin of Active Outer Main-belt Asteroid 2005 XR132

We discover a tail-like structure along the anti-solar direction of the outer main-belt asteroid 2005 XR₁₃₂ (hereafter XR₁₃₂) with the Lulin One-meter Telescope (LOT) in early April. XR₁₃₂ is located slightly within the Hilda asteroid group with a semi-major axis equal to 3.76 au and a moderately eccentric orbit (e = 0.432). It is the first report of coma activity since XR₁₃₂ was discovered as an asteroidal object in 2005. We have a brief study to understand the origin and cometary activity of XR₁₃₂ using broad-band photometry and dynamical simulation.

The syndyne and synchrone dust simulation based on our deep exposed image indicate that cometary activity had begun around 120 days before our observation. The Pan-STARRS 1 and ZTF archived image also coincide with this simulating result that XR₁₃₂ seems as a diffused source and the apparent brightness is slightly brighter than the MPC prediction since mid-January of this year. Looking back to the early image before this perihelion passage, the CTIO DECam images taken in 2014 showed a point-like source under co-adding 40s*5 VR-band exposure. So far we do not have enough observing data to support the repetition of the cometary activity of XR₁₃₂. The broad-band multi-color photometry reveals an S-type surface feature on XR₁₃₂. It is an important signature indicating the dust color of an active short-period comet. No significant short-term brightness variation was detected in our 6-hours high cadence observation.

We performed a numerical simulation to investigate the orbital evolution of XR₁₃₂. The 1-Myrs backward orbital integration of XR₁₃₂ and 1000 clone particles based on the covariance matrix of each orbital element showing that XR₁₃₂ is dynamically unstable. It spent most of its lifetime in the Centaur orbit before being traped by the Jupiter resonance. In a shorter timescale, the orbit of XR₁₃₂ is drifting smoothing without any significant close encounters with other planets. The Kozai mechanism steply pushed the perihelion distance of XR₁₃₂ from 3 au to 2 au since 1500 AD. It may be a possible explanation of the reactivation of this ice-rich object since the asteroidal-like object was discovered 15 years ago.