Dust Colour, Phase Behaviour, and Monte Carlo Modelling of Interstellar Comet 3I/ATLAS from 4 au Pre- to 4 au Post-Perihelion

We report multi-band photometric imaging observations and Monte Carlo dust tail modelling of the interstellar comet 3I/ATLAS covering a wide range of heliocentric distances, from about 4 au pre-perihelion to 4 au post-perihelion. The extensive imaging data set allowed us to constrain the dust physical properties, ejection speeds, and production rates as a function of heliocentric distance. The post-perihelion observations, obtained at high cadence in multiple photometric bands (SDSS g, r, i, and luminance filters) and spanning phase angles between approximately 0.7○ and 30○, enabled us to determine the dust color and phase function. The resulting phase curve exhibits a prominent backscattering enhancement, distinct from those derived for Solar System comets, with an opposition surge of 0.1–0.4 mag, a width of 1–3○, and a linear phase coefficient of 0.02–0.04 mag deg−1, consistent with independent pre-perihelion estimates. A possible interpretation of the imaging data, together with independent photometric measurements, indicates a dust size distribution characterized by a power-law index κ ≈ −3.5, with minimum and maximum particle radii of rmin = 10 μm and rmax in the interval 1–10 cm. The reported water production rate correlates well with the dust production rate post-perihelion, but fails to do so pre-perihelion, an effect possibly linked to the high CO2/H2O ratio measured before perihelion. The derived maximum dust-loss rate at perihelion is (0.5–1.8) × 104 kg s−1.