Spectral Energy Distribution and Polarimetric Properties of Dust in the Interstellar Comet 3I/ATLAS
The interstellar comet 3I/ATLAS exhibits unusual polarimetric properties compared with Solar System comets, while constraints on its fundamental physical properties, such as dust albedo and grain-size distribution, remain limited. We obtained new optical polarimetry near opposition with the Pirka Telescope, combined it with previously reported polarimetric measurements, and constructed a visible-to-mid-infrared spectral energy distribution (SED) from HST, VLT, SPHEREx, and JWST observations. The SED indicates that the coma is dominated by dark, absorbing dust, with an effective V-band albedo of $p_{\rm eff}\sim0.04$. Modeling the SED with a power-law grain-size distribution yields a size index of $q=3.0$--$3.3$. The mid-infrared spectral structure is better reproduced by adding a minor amorphous-silicate component. The combined polarimetric data further favor a highly porous scattering structure. Despite its unusual polarimetric behavior, the dust of 3I/ATLAS has an albedo and grain-size distribution within the range found for Solar System comets.