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 deg and 30 deg, 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 deg, and a linear phase coefficient of 0.02-0.04 mag/deg, 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 of-3.5, with minimum and maximum particle radii of rmin = 10 micrometer 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)E4 kg/s.