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

TL;DR

This study reports millimeter spectroscopy observations of interstellar comet 3I/ATLAS near perihelion, detecting various gases and analyzing their abundances to understand its composition and outgassing behavior.

Contribution

First detailed millimeter spectroscopic analysis of 3I/ATLAS, revealing gas compositions and outgassing characteristics of an interstellar comet.

Findings

Detected HCN, CH₃OH, CO, and H₂CO lines with high confidence.

Abundances of key gases are at the upper range of Solar System comets.

Observed low gas expansion velocity indicating heavy molecules or sublimating icy grains.

Abstract

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, CH$_3$OH, CO, and H$_2$CO are well detected, and $\sim$4$\sigma$ detections are obtained for CS and CH$_3$CN. The search for H$_2$S was unsuccessful. Abundances of CO, H$_2$CO, CH$_3$OH, and CH$_3$CN 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 CO$_2$, and/or a large fraction of the gaseous production coming from subliming icy grains.