IFU Observations of Comet 3I/ATLAS = C/2025 N1 (ATLAS) using the Hobby-Eberly Telescope with the VIRUS Instrument
Abstract
We observed interstellar object 3I/ATLAS = C/2025 N1 (ATLAS) on 9 December 2025 UT with the VIRUS spectrograph on the Hobby-Eberly Telescope. VIRUS is a low spectral resolving power, multi-IFU, optical spectrograph. We used a single IFU for the observations and were able to obtain spectra spaced every one arcsec (1327 km) from the optocenter out to about 85,000 km in the sunward direction. The spectra include molecular features normally seen in the optical for spectra of Solar System comets, with the addition of many very strong lines of FeI and NiI. We modeled these observations using the Haser (1957) model framework and find that the data can only be fit if we assume that the outflow velocity of the gas is half that of comets in our Solar System observed at the same heliocentric distance. We derived Haser model scale lengths for three NiI and four FeI lines. We discuss the implication of the similarities and differences between this object and normal Solar System comets. The excellent spatial resolution of VIRUS, coupled with a FOV of 50X50 arcsecs and a geocentric distance of 1.83 au, allowed us to probe the coma of 3I in great detail.
Related
- Very Large Telescope observations of interstellar comet 3I/ATLAS III: High-resolution monitoring of CN and forbidden oxygen emission across the perihelion passage with ESPRESSO
- JWST/MIRI Observations of Amorphous Silicates in Interstellar Comet 3I/ATLAS
- A Search for Persistent Rotational Modulation in the Dedicated TESS Observations of Interstellar Comet 3I/ATLAS
- Characterization of the anisotropic dust activity of interstellar comet 3I/ATLAS
- High-resolution optical spectroscopy reveals the distinctive volatile composition of the interstellar comet 3I/ATLAS
BibTeX
@misc{cochran2026observations,
title = {{IFU Observations of Comet 3I/ATLAS = C/2025 N1 (ATLAS) using the Hobby-Eberly Telescope with the VIRUS Instrument}},
author = {Anita Cochran and Gregory Zeimann and Emmanuel Jehin and Hideyo Kawakita and Adam McKay},
howpublished = {arXiv},
year = {2026},
month = sep,
doi = {10.48550/arxiv.2609.01785},
eprint = {2609.01785},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2609.01785},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - IFU Observations of Comet 3I/ATLAS = C/2025 N1 (ATLAS) using the Hobby-Eberly Telescope with the VIRUS Instrument
AU - Anita Cochran
AU - Gregory Zeimann
AU - Emmanuel Jehin
AU - Hideyo Kawakita
AU - Adam McKay
PY - 2026
DA - 2026/09/01
PB - arXiv
DO - 10.48550/arxiv.2609.01785
UR - https://arxiv.org/abs/2609.01785
AB - We observed interstellar object 3I/ATLAS = C/2025 N1 (ATLAS) on 9 December 2025 UT with the VIRUS spectrograph on the Hobby-Eberly Telescope. VIRUS is a low spectral resolving power, multi-IFU, optical spectrograph. We used a single IFU for the observations and were able to obtain spectra spaced every one arcsec (1327 km) from the optocenter out to about 85,000 km in the sunward direction. The spectra include molecular features normally seen in the optical for spectra of Solar System comets, with the addition of many very strong lines of FeI and NiI. We modeled these observations using the Haser (1957) model framework and find that the data can only be fit if we assume that the outflow velocity of the gas is half that of comets in our Solar System observed at the same heliocentric distance. We derived Haser model scale lengths for three NiI and four FeI lines. We discuss the implication of the similarities and differences between this object and normal Solar System comets. The excellent spatial resolution of VIRUS, coupled with a FOV of 50X50 arcsecs and a geocentric distance of 1.83 au, allowed us to probe the coma of 3I in great detail.
ER -