Perihelion observations of interstellar comet 3I/ATLAS with the IRAM 30-m telescope

Comet 3I/ATLAS is the third interstellar comet identified as passing through the Solar System. Its high outgassing activity and favourable perihelion passage on October 29, 2025 UT provided an excellent opportunity to investigate the composition of its coma gases through millimeter spectroscopy. We present observations undertaken with the IRAM 30-m telescope on November 1–3, 2025 at an heliocentric distance of 1.36–1.37 au. Lines of HCN, CH3OH, CO, and H2CO are well detected, and ∼4σ detections are obtained for CS and CH3CN. The search for H2S was unsuccessful. Abundances of CO, H2CO, CH3OH, and CH3CN relative to HCN are in the upper ranges of values measured in Solar System comets. The sulfur-to-carbon abundance ratio in 3I/ATLAS’s coma is at most the minimum value observed in comets. The unusually low expansion velocity of coma gases suggests a near-nucleus gas flow driven by heavy molecules such as CO2, and/or a large fraction of the gaseous production coming from subliming icy grains.