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JCMT Constraints on the Early-Time HCN and CO Emission and HCN Temporal Evolution of 3I/ATLAS

Jason T. Hinkle, Bin Yang, Karen J. Meech, Andrew Hoffman, Benjamin J. Shappee, W. B. Hoogendam, James J. Wray

TL;DR

The paper addresses how early-time volatile emission from the interstellar object 3I/ATLAS evolves with heliocentric distance. Using JCMT observations of HCN J=3-2 and CO J=2-1, it derives 3σ upper limits on production rates and combines these with later HCN detections to model the evolution as $Q(\mathrm{HCN}) \propto r_h^{-n}$ with $n = 12.7^{+6.9}_{-2.5}$. The steep HCN ramp-up contrasts with typical Solar System comets and aligns with other species in 3I/ATLAS, with CN likely tracing HCN photolysis. These results provide the earliest sub-millimeter constraints on interstellar cometary activity and motivate continued monitoring as more ISOs are discovered and characterized.

Abstract

Interstellar objects (ISOs), particularly those with cometary activity, provide unique insight into the primordial physical and chemical conditions present during the formation of the planetary system in which they originated. Observations in the sub-mm regime allow for direct measurements of several parent molecules released from the comet nucleus into the coma. Here we present observations of the third ISO, 3I/ATLAS, with the `Ū`ū heterodyne receiver on the James Clerk Maxwell Telescope (JCMT), which targeted emission from HCN($J = 3 - 2$) and CO($J = 2 - 1$). Our observations, taken between 16 July 2025 and 21 July 2025 (UT), when 3I/ATLAS was at a heliocentric distance between 4.01 and 3.84 au, provide the earliest sub-mm constraints on its activity. We do not detect HCN or CO in these epochs, with 3$σ$ upper-limits on the production rates of $Q(HCN) < 1.7 \times 10^{24}$ s$^{-1}$ at $r_h = 4.01 - 3.97$ au and $Q(CO) < 1.1 \times 10^{27}$ s$^{-1}$ at $r_h = 3.94 - 3.84$ au, respectively. We combine this HCN limit with later JCMT observations of HCN to constrain its temporal evolution. Fitting the HCN detections with a $Q(HCN) \propto r_h^{-n}$ model and accounting for the upper-limits yields $n = 12.7^{+6.9}_{-2.5}$. This slope is steeper than those of typical Solar System comets, but consistent with the production rate slopes measured for other species in the coma of 3I/ATLAS.

JCMT Constraints on the Early-Time HCN and CO Emission and HCN Temporal Evolution of 3I/ATLAS

TL;DR

The paper addresses how early-time volatile emission from the interstellar object 3I/ATLAS evolves with heliocentric distance. Using JCMT observations of HCN J=3-2 and CO J=2-1, it derives 3σ upper limits on production rates and combines these with later HCN detections to model the evolution as with . The steep HCN ramp-up contrasts with typical Solar System comets and aligns with other species in 3I/ATLAS, with CN likely tracing HCN photolysis. These results provide the earliest sub-millimeter constraints on interstellar cometary activity and motivate continued monitoring as more ISOs are discovered and characterized.

Abstract

Interstellar objects (ISOs), particularly those with cometary activity, provide unique insight into the primordial physical and chemical conditions present during the formation of the planetary system in which they originated. Observations in the sub-mm regime allow for direct measurements of several parent molecules released from the comet nucleus into the coma. Here we present observations of the third ISO, 3I/ATLAS, with the `Ū`ū heterodyne receiver on the James Clerk Maxwell Telescope (JCMT), which targeted emission from HCN() and CO(). Our observations, taken between 16 July 2025 and 21 July 2025 (UT), when 3I/ATLAS was at a heliocentric distance between 4.01 and 3.84 au, provide the earliest sub-mm constraints on its activity. We do not detect HCN or CO in these epochs, with 3 upper-limits on the production rates of s at au and s at au, respectively. We combine this HCN limit with later JCMT observations of HCN to constrain its temporal evolution. Fitting the HCN detections with a model and accounting for the upper-limits yields . This slope is steeper than those of typical Solar System comets, but consistent with the production rate slopes measured for other species in the coma of 3I/ATLAS.

Paper Structure

This paper contains 9 sections, 3 figures.

Table of Contents

  1. Introduction
  2. Observations and Data
  3. Overview of JCMT Observations
  4. HCN($J = 3 - 2$) Measurements
  5. CO($J = 2 - 1$) Measurements
  6. Temporal Evolution of HCN in 3I/ATLAS
  7. Comparison with CN
  8. Production Rate Dependence on $r_h$
  9. Summary

Figures (3)

  • Figure 1: Stacked HCN($J = 3 - 2$) spectrum of 3I/ATLAS obtained with JCMT 'Ū'ū. Left: Broadband spectrum shown in the cometocentric velocity frame at a binning of $1$ km s$^{-1}$ (black) and the scatter within each bin (gray). Right: Spectrum centered on the expected line velocity at a binning of $0.5$ km s$^{-1}$ (black) and the scatter within each bin (gray). The red line indicates a Gaussian with a velocity FWHM of $0.5$ km s$^{-1}$, and an integrated intensity matching the 3$\sigma$ upper-limit on HCN emission. The vertical dashed lines mark zero velocity, and the horizontal lines mark the continuum level of zero K.
  • Figure 2: Stacked CO($J = 2 - 1$) spectrum of 3I/ATLAS obtained with JCMT 'Ū'ū. Left: Broadband spectrum shown in the cometocentric velocity frame at a binning of $1$ km s$^{-1}$ (black) and the scatter within each bin (gray). Right: Spectrum centered on the expected line velocity at a binning of $0.5$ km s$^{-1}$ (black) and the scatter within each bin (gray). The red line indicates a Gaussian with a velocity FWHM of $0.5$ km s$^{-1}$, and an integrated intensity matching the 3$\sigma$ upper-limit on CO emission. The vertical dashed lines mark zero velocity, and the horizontal lines mark the continuum level of zero K.
  • Figure 3: Left: Temporal evolution of HCN($J = 3 - 2$) emission from 3I/ATLAS. Our constraint is shown in red, and data from Coulson2025 are shown in gray. Downward-facing triangles indicate 3$\sigma$ upper limits, and circles are detections with corresponding 1$\sigma$ uncertainties. The solid gray line is the best-fit power-law model, and the dashed gray lines represent the 1$\sigma$ confidence band around this fit. Right: Histogram of power-law slopes resulting from 20,000 Monte Carlo iterations. Dashed vertical lines show typical slopes such as the $r_h^{-0.5}$ temperature scaling (red), the $r_h^{-2}$ scaling from Solar insolation (gold), and measured dependences of CN (teal) and Ni (purple) in 3I/ATLAS from Rahatgaonkar2025 and SalazarManzano2025.