Snapshot of a new interstellar comet: 3I/ATLAS has a red and featureless spectrum
Cyrielle Opitom, Colin Snodgrass, Emmanuel Jehin, Michele T. Bannister, Erica Bufanda, Sophie E. Deam, Rosemary Dorsey, Marin Ferrais, Said Hmiddouch, Matthew M. Knight, Rosita Kokotanekova, Brayden Leicester, Micha\"el Marsset, Brian Murphy, Vincent Okoth, Ryan Ridden-Harper

TL;DR
This paper presents initial optical spectroscopic observations of the interstellar comet 3I/ATLAS, revealing a red, dust-dominated coma with no detectable gas emissions, providing a baseline for future studies of its activity and composition.
Contribution
First spectroscopic analysis of 3I/ATLAS shortly after discovery, establishing its red, dusty coma and setting a baseline for future interstellar comet studies.
Findings
Comet 3I/ATLAS has a red, featureless spectrum.
No gas emissions detected from C2, NH2, CN, or [OI].
Coma appears entirely dusty at current distance.
Abstract
The interstellar comet 3I/ATLAS is only the third interstellar object to be discovered. Pre-perihelion measurements provide a unique opportunity to study its activity and composition, which may alter as it is heated in the coming months. We provide an initial baseline from optical spectroscopic observations obtained only two days after discovery, using the MUSE instrument on the VLT on 2025 July 3, while 3I was at 4.47 au from the Sun and 3.46 au from the Earth. These observations confirm the cometary nature of 3I, and reveal a red coma with a spectral slope of ~\AA, redder than most Solar System comets but similar to the surface colour of some Solar System Trans-Neptunian Objects or Centaurs. We searched for but did not detect gas emission from C, NH, CN, and [OI], which is consistent with volatile non-detections for Solar System comets at this heliocentric…
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Taxonomy
TopicsAstro and Planetary Science · Astrophysics and Star Formation Studies · Astronomy and Astrophysical Research
