Dust colour, phase behaviour, and Monte Carlo modelling of interstellar Comet 3I/ATLAS from 4 au Pre- to 4 au post-perihelion

ABSTRACT We report multiband 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 $0.7^\circ$ and $30^\circ$, enabled us to determine the dust colour 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$^\circ$, 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 $\kappa \approx -3.5$, with minimum and maximum particle radii of $r_{\min } = 10~\mu \mathrm{m}$ and $r_{\max }$ 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 CO$_2$/H$_2$O ratio measured before perihelion. The derived maximum dust-loss rate at perihelion is (0.5–$1.8)\times 10^{4}$ kg s$^{-1}$.