Distant activity of Chiron-type comet C/2023 RS61 (PanSTARRS)

Abstract

We report on new multi-band photometric and long-slit spectroscopic observations of the active Chiron-type comet C/2023 RS61 (PanSTARRS), performed on November 25, 2025 at a heliocentric distance of 8.3 au. Spectral analysis reveals a dust continuum, without gas emission features down to the 3σ detection limit. The measured colour of the comet (g − r = 0.78 ± 0.01 mag near the nucleus, decreasing to 0.69 ± 0.01 mag at cometocentric distances of ~20 000 km) places C/2023 RS61 (PanSTARRS) between the red and gray colour groups of Centaurs. Image enhancement confirms an asymmetric coma morphology with a prominent jet-like structure at a position angle of 205○. Geometric modelling of this feature constrains the sub-solar latitude of the active source to the southern hemisphere (φ = −84○ ± 3○). Rotational modulation analysis restricts the nuclear rotation period to 32–640 h. Monte Carlo dust-tail modelling suggests that the observed coma morphology and brightness evolution were driven by a major outburst at T ≈ −1208 d before perihelion, releasing ~1010 kg of dust, followed by a sustained activity level of ~60–125 kg s−1.