Letter to the Editor

X-SHOOTER spectrum of comet 3I/ATLAS: Insights into a distant interstellar visitor

Instituto de Astrofísica de Andalucía – Consejo Superior de Investigaciones Científicas (IAA-CSIC), Glorieta de la Astronomía S/N, E-18008 Granada, Spain

^⋆ Corresponding author.

Received: 9 July 2025
Accepted: 25 July 2025

Abstract

Context. Comets are primitive remnants of the early Solar System whose composition offers fundamental clues to their formation and evolution. High-resolution broad-wavelength spectroscopy is crucial for identifying volatile species and constraining the physical conditions within the coma.

Aims. We characterize the gas composition and physical environment of the newly discovered comet 3I/ATLAS through optical and near-infrared spectroscopy.

Methods. We used a medium-resolution spectrum of comet 3I/ATLAS with X-shooter at the ESO Very Large Telescope that covers the 300–2500 nm wavelength range. Standard data reduction and flux calibration were applied.

Results. Although the object is clearly active, only upper limits to the production rates of OH and CN can be estimated: 8.2 × 10²⁶ s⁻¹ and 5.6 × 10²³ s⁻¹, respectively. We obtained red spectral slopes consistent with those of typical D-type asteroids and objects in the outer Solar System.