Post-fragmentation Morphological and Broadband Color Evolution of C/2025 K1 (ATLAS)

Abstract

Comet C/2025 K1 (ATLAS) underwent major fragmentation after its 2025 October 8 perihelion passage at q ~ 0.33 au. We present four epochs of Johnson-Cousins BVR imaging obtained with the Mount Laguna Observatory 40-inch reflector on 2025 November 26, November 29, December 5, and December 8, sampling the subsequent evolution of the integrated fragment-coma complex. The images were homogenized to common circular two-dimensional effective point-spread functions before morphology was measured along the projected fragment axis. We evaluated 324 profile-extraction configurations and removed solutions whose localized peak landed on the search-window boundary. The fragment-axis surface-brightness enhancement relocates inward between November 26 and 29 and outward between December 5 and 8; the sign of both changes is recovered in every valid configuration and at all tested common target PSFs from 5.3 to 6.0 arcsec. Calibrated BVR photometry using Gaia DR3 synthetic Johnson-Kron-Cousins standards independently shows a repeatable November 26-29 change in broadband spectral shape: B-V decreases while V-R increases. The curvature index S'_(V-R) - S'_(B-V) changes by +3.47 (+1.50/-1.65) percent per 1000 Angstrom, with 99.1 percent of Monte Carlo realizations positive, while the full B-R reflectivity gradient remains approximately constant. Repeating the color comparison at a fixed projected radius of 2205 km produces nearly identical changes. The combined results are consistent with continued restructuring of the unresolved post-fragmentation inner coma, while broadband imaging alone cannot uniquely separate dust-color evolution, molecular emission, and changing contributions from unresolved fragments.