Postperihelion Optical Low-resolution Spectroscopy of Interstellar Comet 3I/ATLAS

Abstract 3I/ATLAS is the third interstellar object discovered on UT 2025 July 1, when it was inbound at a heliocentric distance of r h = 4.51 au. Preperihelion optical spectroscopic observations at r h < 3 au suggested that 3I/ATLAS resembled “carbon-chain-depleted” comets in the solar system. In contrast, postperihelion imaging observations with narrowband filters in the UV and optical wavelength regions indicated that 3I/ATLAS is normal in mixing ratios, at least for CN, C 3 , and C 2 (with respect to OH), but depleted in NH. To verify these results, we conducted optical low-resolution spectroscopic observations of 3I/ATLAS on UT 2025 November 29.8, December 04.8, and 06.8, when 3I/ATLAS was outbound at r h = 1.77–1.95 au. Obtained spectra show prominent CN, C 3 , and C 2 emission bands as well as [O I ] emission lines, while no NH 2 emission bands were detected in our spectra. The mixing ratios of CN, C 3 , and C 2 with respect to H 2 O are consistent with the normal population of the solar system comets, whereas NH 2 is clearly depleted, at least after perihelion. This compositional characteristic probably reflects a depletion of ammonia in the ices incorporated in 3I/ATLAS. The slope of the reflectance spectra of dust continuum in our spectra were 20%–23%/100 nm between 525 and 623 nm, redder than the typical slope for the solar system comets (∼10%/100 nm).