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

N. Biver, D. Bockelée-Morvan, R. Moreno, J. Crovisier, G. Paubert, V. Zakharov, J. Boissier, M. A. Cordiner, N. X. Roth

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$4$σ$ 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.

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

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, CHOH, CO, and HCO are well detected, and 4 detections are obtained for CS and CHCN. The search for HS was unsuccessful. Abundances of CO, HCO, CHOH, and CHCN 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, and/or a large fraction of the gaseous production coming from subliming icy grains.
Paper Structure (11 sections, 3 equations, 8 figures, 5 tables)

This paper contains 11 sections, 3 equations, 8 figures, 5 tables.

Table of Contents

  1. Introduction
  2. Observations
  3. Results
  4. Discussion
  5. Log of the observations
  6. Detected lines and their characteristics
  7. Synthetic line profiles
  8. CH$_3$OH rotation diagram
  9. Histograms of abundances relative to water
  10. Histogram of the Sulfur/Carbon ratio
  11. Expansion velocity

Figures (8)

  • Figure 1: Methanol lines near 242 GHz and 252 GHz observed in comet 3I/ATLAS on November 1--3, 2025.
  • Figure 2: Sample of spectra obtained in comet 3I/ATLAS on November 1--3, 2025. Line characteristics are given in Table \ref{['tabarea']}. The plot with the HCN $J$(3--2) spectrum shows the position and relative intensities of the hyperfine components.
  • Figure 3: Histograms of the abundances relative to HCN measured from observations at radio wavelengths. Dynamically new (DN) and long-period (LP) comets from the Oort cloud (OCC), Halley-family (HFC) and Jupiter-family (JFC) comets, are indicated in green, cyan, blue, and red colors, respectively. The orange color is for 3I/ATLAS. See Biv24 and references therein.
  • Figure 4: Synthetic line profiles (blue) superimposed on observed line profiles (black) of HCN (left panel) and CH$_3$OH (middle and right panels). The inserts in the upper right of the plots show the assumed geometry of the outgassing with respect to the Sun direction, with the production rates given in units of 10$^{25}$ molec. s$^{-1}$ for HCN, and in units of 10$^{26}$ molec. s$^{-1}$ for CH$_3$OH. The assumed expansion velocities (km s$^{-1}$) in the sunward and antisunward hemispheres are also indicated. Thermal broadening assuming a kinetic temperature of 60 K (see main text) is considered.
  • Figure 5: Same as Fig. \ref{['figabunHCN']} for abundances relative to H$_2$O
  • ...and 3 more figures