Snapshot of a new interstellar comet: 3I/ATLAS has a red and featureless spectrum
2025/07/07 by Cyrielle Opitom, Colin Snodgrass, Opitom, Cyrielle +41 · 8 voices · 18 citations
#astro-ph.EP #astro-ph.GA
paper · pdf · doi:10.48550/arxiv.2507.05226
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 (18±4)%/1000~Å, 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 C2, NH2, CN, and [OI], which is consistent with volatile non-detections for Solar System comets at this heliocentric distance. At present, the coma appears entirely dusty. Future observations of 3I as it comes closer to the Sun will provide an invaluable opportunity to witness the evolution of its activity, study its composition, test predictions of interstellar object population models, and compare 3I to Solar System comets.
Cited by
Discussions
- Interstellar comet 3I/ATLAS papers! 1. Spectroscopy: Opitom ... @astrokiwi.bsky.social et al. arxiv.org/abs/2507.05226 2. Photometry: Bolin et al. arxiv.org/abs/2507.05252 3. Revised version of Seligm [bsky, 26 points, 2 comments]
- And here's the preprint of the other VLT observations on the same night, with MUSE! arxiv.org/abs/2507.05226 [bsky, 19 points, 1 comments]
- Confirmation of a coma from Opitom et al.: arxiv.org/abs/2507.05226 [bsky, 18 points, 1 comments]
- Today on the #arXiv: Opitom et al. 2025, "Initial VLT/MUSE spectroscopy of the interstellar object 3I/ATLAS" - arxiv.org/abs/2507.05226 #3I/ATLAS [bsky, 12 points, 0 comments]
- Yep! Right now the tail is being viewed at a head-on perspective from Earth (& Hubble), so the tail's behind the nucleus and is foreshortened (said in arxiv.org/pdf/2507.05226). Sorta like looking dir [bsky, 5 points, 0 comments]
- remarkable speed for release of results for optical #spectroscopy of just-discovered interstellar comet 3I/ATLAS on #arXiv, arxiv.org/abs/2507.05226 (preprint's corr auth at @edinburgh-uni.bsky.social [bsky, 2 points, 0 comments]
- On en a quelques idées grâce à des premières observations avec MUSE ! A priori, pas de C2, NH2, CN ou [OI] arxiv.org/abs/2507.052... [bsky, 1 points, 1 comments]
- Today on the # arXiv : Opitom et al. 2025, "Initial VLT/MUSE spectroscopy of the interstellar object 3I/ATLAS" - https:// arxiv.org/abs/2507.05226 # 3I [mastodon, 0 points, 0 comments]
Related