JCMT detection of HCN emission from 3I/ATLAS at 2.1 AU (2510.02817v1)
Abstract: We report the detection of HCN ($J=3-2$) rotational emission from comet 3I/ATLAS at a heliocentric distance of 2.13 AU with the James Clerk Maxwell Telescope (JCMT). Observations were conducted from 07 August 2025 (UT) using the ${\prime}\overline U{\prime}\overline u$ heterodyne receiver and ACSIS spectroscopic backend. The HCN line was detected at $>5\sigma$ on 14 Sep 2025 (UT) and a production rate of $Q({\rm HCN})=(4.0\pm1.7)\times10{25}\ {\rm s}{-1}$ was derived by non-LTE radiative transfer modelling. Preliminary estimates of the HCN/H$_2$O and CN/HCN abundance ratios suggest values similar to Solar System comets.
Summary
- The paper presents the first high-sensitivity detection and analysis of HCN emission in 3I/ATLAS, highlighting its comparable volatile inventory to Solar System comets.
- It employs high-resolution millimeter-wave spectroscopy with JCMT and detailed non-LTE radiative transfer modeling to determine HCN production rates.
- The findings suggest that the HCN abundance and related ratios in 3I/ATLAS imply shared formation pathways with Solar System comets despite different galactic origins.
Detection and Analysis of HCN Emission from Interstellar Comet 3I/ATLAS at 2.1 AU
Introduction
The detection of hydrogen cyanide (HCN) in the coma of the interstellar comet 3I/ATLAS represents a significant addition to the molecular inventory of interstellar objects (ISOs) observed within the Solar System. This paper leverages high-sensitivity millimeter-wave spectroscopy using the James Clerk Maxwell Telescope (JCMT) to characterize the volatile composition of 3I/ATLAS at a heliocentric distance of 2.13 AU. The results provide a direct comparison of the chemical properties of 3I/ATLAS with both Solar System comets and the previously observed interstellar comet 2I/Borisov, with implications for understanding the formation environments and evolutionary histories of ISOs.
Observational Methodology
Observations targeted the HCN rotational transition at 265.8864 GHz, utilizing the JCMT's A-band heterodyne receiver and the ACSIS spectroscopic backend. The system was configured for high spectral resolution (31 kHz, 0.03 km s), essential for resolving narrow cometary emission lines. The observing strategy employed a beam-switching mode with a 90$\arcsec$ throw to mitigate contamination from the extended coma, and Doppler corrections were applied post-observation using JPL/Horizons ephemerides to ensure accurate cometocentric velocity referencing.
Calibration was performed against astronomical standards, and atmospheric opacity corrections were applied using real-time water vapor measurements. Data reduction followed established Starlink routines, with all spectra transformed to the cometocentric velocity frame for aggregation and analysis.
The HCN line was detected at significance on 14 September 2025, with a marginal detection on 7 September 2025. Earlier epochs yielded only upper limits.
Figure 1: HCN() spectra of comet 3I/ATLAS obtained at JCMT on 07 Sep 2025 (top) and 14 Sep 2025 (bottom), shown in the cometocentric velocity frame at 1.0 km s resolution.
Radiative Transfer Modeling and Production Rate Determination
The observed HCN line profile was modeled using the 1D SUBLIME non-LTE radiative transfer code, which incorporates spherically symmetric outgassing, HCN photolysis (using the Huebner & Mukherjee 2015 active Sun rate), collisional excitation with HO (using state-specific rates), electron collisions, and solar radiative pumping. The water production rate was fixed at s, based on contemporaneous OH measurements, and a coma kinetic temperature of K was adopted.
The model fit, optimized via the LMFIT routine, yielded an HCN production rate of s and an outflow velocity of km s. The fit accounted for the hyperfine structure of the HCN transition and instrumental channel binning.
Figure 2: SUBLIME model fit to the HCN() line observed on September 14, with the five strongest hyperfine components labeled.
Comparative Abundance Analysis
The derived HCN/HO abundance ratio for 3I/ATLAS is , which is within the upper range observed for Solar System comets (typically ). This suggests that, at least for HCN, the volatile inventory of 3I/ATLAS is not anomalous relative to Solar System analogs, despite its likely origin in the Galactic thick disk and an extended interstellar sojourn.
Estimates of the CN/HCN production rate ratio, based on published CN measurements and heliocentric scaling, yield . This is consistent with the scenario in which HCN photolysis is the dominant source of CN in the coma, as is typical for Solar System comets.
No contemporaneous CO measurements were available, precluding a direct comparison of CO/HCN ratios with 2I/Borisov, which was notably CO-rich. However, previous infrared observations indicate that CO dominates the coma composition of 3I/ATLAS at larger heliocentric distances.
Implications and Future Directions
The detection of HCN in 3I/ATLAS at a heliocentric distance of 2.1 AU provides a critical data point for comparative planetology of ISOs. The similarity of the HCN/HO and CN/HCN ratios to those of Solar System comets supports the hypothesis that at least some ISOs share a common formation pathway with Solar System comets, despite differences in galactic environment and evolutionary history.
The absence of strong depletion or enhancement in HCN abundance, in contrast to the CO-richness of 2I/Borisov, highlights the compositional diversity among ISOs and underscores the necessity of multi-molecular, multi-epoch studies. The results also reinforce the utility of high-resolution millimeter spectroscopy for constraining volatile inventories in faint, distant comets.
Ongoing and future monitoring of 3I/ATLAS, particularly targeting additional parent volatiles (e.g., CHOH, HCO, NH, CS) and contemporaneous CO measurements, will further elucidate the chemical diversity and evolutionary processing of interstellar comets. The continued development of non-LTE radiative transfer models, incorporating updated collisional and photolytic rates, will enhance the accuracy of production rate determinations for weakly emitting species.
Conclusion
This paper presents the first detection and quantitative analysis of HCN emission from the interstellar comet 3I/ATLAS at 2.1 AU. The derived HCN production rate and abundance ratios are consistent with the upper range of Solar System comets, indicating no significant depletion or enhancement of HCN. These findings contribute to the growing body of evidence that ISOs can possess volatile inventories similar to those of Solar System comets, with implications for models of comet formation and interstellar processing. Continued spectroscopic monitoring and compositional analysis of ISOs remain essential for advancing our understanding of small body chemistry across galactic environments.
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Clear, simple explanation of “JCMT detection of HCN emission from 3I/ATLAS at 2.1 AU”
Overview: What is this paper about?
This paper reports a new discovery about an interstellar comet named 3I/ATLAS. Using a powerful radio telescope in Hawaii (the James Clerk Maxwell Telescope, or JCMT), the scientists detected a gas called HCN (hydrogen cyanide) coming off the comet when it was about 2.1 times farther from the Sun than Earth is (2.1 AU). They measured how much HCN the comet was releasing and compared it to other gases. The big idea: 3I/ATLAS, which came from another star system, seems to have some chemical similarities to comets in our own Solar System.
The big questions the scientists asked
Before observing, the team wanted to find out:
- Does 3I/ATLAS release HCN, a common comet gas?
- How much HCN is coming off the comet per second?
- Is the amount of HCN compared to water (H2O) or to CN (a related fragment molecule) similar to what we see in comets from our Solar System?
- What does this say about how interstellar comets are made and how they change as they travel through space?
How they did it (methods explained simply)
The team used the JCMT to “listen” for a specific radio signal from HCN.
- Molecules like HCN spin and vibrate in special ways. When they change their spin state, they give off radio waves at very precise “notes” (frequencies). For HCN, one of these notes is at 265.886 GHz and is called the J=3–2 line. Think of it like tuning a radio to a specific station to hear just one song.
- The telescope observed the comet on several nights in August and September 2025. The strongest detection happened on September 14.
- They used a technique called beam-switching: the telescope quickly looked at the comet, then a nearby “empty” spot of sky, and subtracted the two. This helps remove background noise, like covering your ears to block a crowd so you can hear one person speaking.
- Because the comet and Earth are moving, they carefully adjusted for the comet’s speed so the signal wouldn’t blur.
- They also corrected for Earth’s atmosphere, especially water vapor, which can block or weaken the signal.
- To turn the signal strength into “how many HCN molecules per second,” they used a computer model (a “non-LTE radiative transfer” model called SUBLIME). In simple terms: the gas near a comet is thin and not in perfect temperature balance, so you need a smart model that includes sunlight, collisions between molecules, and how fast the gas spreads out. This lets them estimate the real production rate.
Key idea: “Production rate” means how many molecules are escaping from the comet each second. For example, 4 × 1025 s-1 means 40,000,000,000,000,000,000,000 molecules every second.
What they found and why it matters
- They clearly detected HCN on September 14, 2025, with a very strong signal (more than “5 sigma,” which means it’s extremely unlikely to be a random noise spike).
- The comet was releasing about Q(HCN) = (4.0 ± 1.7) × 1025 molecules per second.
- The gas was flowing outward at about 0.46 ± 0.14 km/s.
- Comparing to water, they found HCN/H2O ≈ 0.2% (specifically, about 0.2 ± 0.08%). This is within the range seen in many Solar System comets (roughly 0.03%–0.4%).
- They also estimated the ratio CN/HCN to be about 0.1–1. That matters because CN is often created when sunlight breaks HCN apart. Their numbers suggest that simply breaking HCN in sunlight could explain the CN seen around the comet.
Why this is important:
- 3I/ATLAS comes from another star system, yet its HCN level looks a lot like comets from our own Solar System. That hints that comet-making processes in other systems might not be so different from what happened here.
- HCN is a “prebiotic” molecule — it can be involved in chemistry that leads to the building blocks of life. Finding it in an interstellar comet suggests that ingredients important for life’s chemistry might be common in many planetary systems.
What this could mean in the bigger picture
- Interstellar comets may carry familiar mixtures of ices and gases, similar to local comets. That helps scientists understand how planets and comets form across the galaxy.
- If comet chemistry is similar from system to system, then the raw materials for life-like chemistry could be widespread.
- This is just the start for 3I/ATLAS. More observations (for example, of CO, methanol, ammonia, etc.) will help confirm how much it resembles or differs from comets we know, and how its activity changes as it nears or moves away from the Sun.
In short: The team “heard” the radio whisper of HCN from an interstellar comet and found that, at least for this molecule, 3I/ATLAS looks a lot like home. That’s a big clue about how comets — and perhaps the seeds of life’s chemistry — might be common across the galaxy.
Knowledge Gaps
Knowledge gaps, limitations, and open questions
The paper leaves the following gaps and unresolved questions that future studies could address:
- Obtain simultaneous CO and CN measurements near the JCMT HCN detection epoch (R_h ≈ 2.1 AU) to derive Q(CO)/Q(HCN) and Q(CN)/Q(HCN) without relying on extrapolations from earlier, more distant epochs.
- Expand the molecular inventory beyond HCN by attempting detections (and sensitive upper limits) of other parent species detectable in the mm/submm (e.g., CH3OH, H2CO, NH3, CS), enabling a compositional comparison between 3I/ATLAS, 2I/Borisov, and Solar System comets.
- Measure multiple HCN rotational transitions (e.g., J=1–0, 4–3) to directly constrain coma kinetic temperature and optical depth, reducing dependence on the assumed T_kin = 35 ± 15 K and improving the robustness of Q(HCN).
- Map the HCN emission (e.g., on-the-fly or raster mapping) to determine the spatial distribution, identify jets or anisotropic outgassing, and test the assumption of spherically symmetric nucleus release used in the 1D model.
- Incorporate collisions with CO2 (and CO) in the non-LTE excitation modeling, given reports that CO2 dominates 3I/ATLAS’s coma; quantify how these additional collision partners affect the derived Q(HCN).
- Acquire contemporaneous H2O production rates (e.g., via IR water lines) and quantify uncertainties in the OH→H2O conversion and the assumed H2O ortho-to-para ratio (OPR = 3), including their impact on the HCN/H2O abundance ratio.
- Perform a formal sensitivity analysis (and propagate systematics) for Q(HCN) with respect to key model parameters (T_kin, electron density scaling, solar pumping g-factors, photolysis rates, beam efficiency), and report a comprehensive error budget beyond statistical uncertainties.
- Measure electron densities/temperatures in the coma (e.g., via plasma diagnostics or electron-impact sensitive transitions) to replace the adopted comet-generic electron density scaling factor (0.2) with values specific to 3I/ATLAS.
- Monitor HCN across multiple epochs and heliocentric distances to establish a production-rate power law and assess short-term variability; compare trends with those of Solar System comets.
- Obtain simultaneous optical CN observations during the JCMT epochs to test whether HCN photolysis alone explains CN and to quantify contributions from other CN parents (e.g., C2N2, HC3N, CH3CN).
- Verify that the OFF beam is free of coma contamination by testing larger beam throws or alternative position-switching strategies; compute the HCN photodissociation scale length relative to the 90″ throw (≈164,000 km) to confirm adequacy.
- Investigate isotopic composition (e.g., detect H13CN) to constrain formation environment and compare isotopic ratios with 2I/Borisov and Solar System comets.
- Obtain higher signal-to-noise and spectral resolution to resolve HCN hyperfine components more cleanly, refine the outflow velocity (currently 0.46 ± 0.14 km s⁻¹), and search for kinematic substructure indicative of jets or anisotropy.
- Test the sub-solar metallicity/thick-disk origin hypothesis by building a broader volatile inventory (including N-, C-, and O-bearing species) and comparing abundance patterns (e.g., HCN/H2O, CO2/HCN, CO/HCN) with expectations for metal-poor environments.
- Update and validate solar radiation pumping parameters (g-factors) using contemporary solar activity data for 2025, and quantify the sensitivity of the modeled HCN excitation to these updates.
- Assess whether distributed sources (e.g., sublimating icy grains) contribute to HCN by comparing beam-dependent fluxes and radial profiles; explicitly test the nucleus-only release assumption.
- Cross-validate the JCMT main beam efficiency at 265.886 GHz during the observing run and include calibration/efficiency uncertainties in the production-rate error budget.
- Model each epoch individually rather than scaling earlier intensities to Q(HCN) using a single conversion, accounting for changes in excitation, temperature, and geometry as R_h and Δ evolve.
- Coordinate contemporaneous mm/submm and IR observations (e.g., with JWST or ground-based IR facilities) to jointly constrain CO2, CO, H2O, and HCN, probe excitation cross-coupling, and track composition changes with heliocentric distance.
- Measure dust continuum and dust production rates to estimate dust-to-gas ratios, evaluate gas–dust coupling effects on the plasma environment, and improve understanding of coma dynamics relevant to excitation and outflow.
Practical Applications
Immediate Applications
Below are practical, deployable-now applications grounded in the paper’s methods, instrumentation, and findings.
- Optimized observing of fast-moving targets at mm/submm facilities
- Sectors: astronomy/observatories, robotics (telescope control), software
- Application: Adopt short observation segments (<30 minutes) to limit Doppler smearing; use beam-switching (e.g., 90″ throw) to avoid coma contamination; integrate JPL/Horizons ephemerides to achieve <1″ tracking; apply real-time WVM opacity corrections and frequent calibration (every 2–3 hours) against standards.
- Tools/workflows: “Moving-Target SOP” for heterodyne systems; “Ephemeris2Observatory” integration module; WVM-driven extinction correction in the pipeline.
- Assumptions/dependencies: Accurate ephemerides; stable instrument pointing; clear weather (tau225 ≤ 0.15); appropriate OFF positions free of coma emission.
- Rapid, reproducible production-rate estimation for cometary parent molecules
- Sectors: academia (planetary science, astrochemistry), software
- Application: Use non-LTE modeling (SUBLIME 1D) with up-to-date collisional rates, electron scaling, and solar pumping to derive Q(HCN) from single-line detections; apply intensity-to-Q scaling when lines are optically thin and geocentric distance is similar, to estimate upper limits and trends across nights.
- Tools/workflows: “QuickQ” estimator that ingests spectra, calibrations (beam efficiency), OH-derived H2O rates, and returns Q(HCN); templates for hyperfine-resolved fits using
LMFIT. - Assumptions/dependencies: Optically thin emission; availability of H2O/OH production rate (e.g., Swift); kinetic temperature (Tkin ~ 35 ± 15 K) approximate; main-beam efficiency known; spherically symmetric coma approximation.
- Hyperfine-aware spectral fitting and cometocentric velocity alignment
- Sectors: software, astronomy
- Application: Standardize line fitting to include HCN J=3–2 hyperfine components; align and stack spectra in the cometocentric velocity frame to maximize S/N.
- Tools/workflows: “HyperfineFit-LMFIT” library; “CometOCV” (cometocentric velocity) alignment plug-in for Starlink/ACSIS outputs.
- Assumptions/dependencies: Correct channel binning and instrument response; reliable ephemeris velocities; hyperfine component frequencies.
- Cross-facility coordination for coma composition inference
- Sectors: academia, observatories, policy
- Application: Fuse contemporaneous H2O/OH rates (Swift) with mm/submm HCN to obtain robust abundance ratios (HCN/H2O, CN/HCN); prioritize complementary CO and CO2 monitoring (JWST, SPHEREx, ground-based).
- Tools/workflows: “ISO Coma Model Share” dashboard linking JCMT, Swift, JWST/SPHEREx data products and model priors.
- Assumptions/dependencies: Timely data sharing; standardized calibration across facilities; coverage near similar heliocentric distances.
- Comparative chemistry tracking for ISOs vs Solar System comets
- Sectors: academia, policy (program planning)
- Application: Maintain updatable HCN/H2O and CN/HCN ratios; use CN/HCN ~ 0.1–1 as a check for CN production via HCN photolysis; identify targets for deeper molecular surveys (e.g., CH3OH, H2CO, NH3, CS).
- Tools/workflows: “CometChem Tracker” database and visualization.
- Assumptions/dependencies: Consistent scaling laws (e.g., CN ∝ R_h−9.4); reliable photolysis rates (Huebner & Mukherjee).
- Education and outreach using open JCMT comet datasets
- Sectors: education/outreach
- Application: Develop teaching modules showing how mm-wave spectroscopy reveals comet composition; include hands-on exercises in cometocentric velocity transforms and hyperfine fitting.
- Tools/workflows: Classroom kits built from JCMT archive after proprietary period; Jupyter notebooks implementing line-fitting and SUBLIME-based “toy” models.
- Assumptions/dependencies: Public data access post-proprietary period; simplified model parameters for pedagogy.
Long-Term Applications
These applications are feasible but require further research, development, scaling, or cross-institutional investment.
- Continuous Doppler tracking capability for heterodyne receivers
- Sectors: observatory hardware/robotics, software
- Application: Upgrade spectrometer/telescope control to perform continuous Doppler tracking for moving targets, eliminating smearing and enabling longer integrations without sacrificing resolution.
- Tools/workflows: Real-time frequency control modules linked to JPL/Horizons feed; closed-loop tracking software.
- Assumptions/dependencies: Engineering resources and funding; stable frequency references; validation on diverse targets.
- Autonomous ISO rapid-response observation network
- Sectors: observatories/space, policy, software
- Application: Automate triggers from discovery pipelines to schedule multi-band observations (mm/submm, IR, optical) and deliver rapid, standardized composition reports.
- Tools/workflows: “ISO Rapid Response” scheduler; cross-facility APIs; shared priority policies for transient ISOs.
- Assumptions/dependencies: Community agreements on observing priority; interoperable scheduling systems; robust ephemeris propagation.
- Cloud-based non-LTE modeling service (“SUBLIME-as-a-Service”)
- Sectors: software (SaaS), academia, industry (remote sensing analytics)
- Application: Provide turnkey non-LTE modeling with curated collisional databases (e.g., state-specific HCN–H2O rates), solar pumping, electron collisions; extend to other key comet molecules.
- Tools/workflows: Web API for spectra upload; automated parameter sweeps with uncertainty quantification; model provenance tracking.
- Assumptions/dependencies: Sustained maintenance of collisional datasets; compute resources; community validation.
- Expanded collisional-rate databases and hyperfine catalogs
- Sectors: academia (computational chemistry), software
- Application: Compute and disseminate state-specific rates beyond HCN (CH3OH, H2CO, NH3, CS), including electron collision schemes; maintain hyperfine-resolved line lists for cometary molecules.
- Tools/workflows: Open repositories with versioned datasets; citation and DOI frameworks.
- Assumptions/dependencies: HPC access; method benchmarking; periodic updates as new calculations are published.
- Astrobiology mission design informed by ISO chemistry
- Sectors: space missions, policy
- Application: Use ISO abundance patterns (e.g., relatively high HCN/H2O, CO2 dominance) to set payload requirements (spectrometers tuned to key bands), sampling strategies, and target selection for flybys or sample-return missions.
- Tools/workflows: Precursor survey campaigns defining expected volatile inventories; mission concept studies incorporating photochemistry constraints.
- Assumptions/dependencies: Statistics from multiple ISOs; launch opportunities; planetary protection and policy frameworks.
- Cross-application of hyperfine-resolved non-LTE methods to Earth and planetary atmospheres
- Sectors: environmental monitoring, Earth observation, planetary science
- Application: Adapt modeling and fitting methodologies to mm/submm remote sensing of terrestrial trace gases and cold planetary atmospheres, improving retrieval fidelity where non-LTE and hyperfine structure are non-negligible.
- Tools/workflows: Retrieval software that incorporates hyperfine structure and non-LTE; validation with lab spectra.
- Assumptions/dependencies: Spectral line databases for target gases; instrument sensitivity and calibration comparable to astronomy-grade systems.
- Citizen science augmentation for comet photochemistry
- Sectors: education/citizen science
- Application: Organize amateur observations of CN at optical wavelengths and combine with professional HCN mm-wave data to constrain CN/HCN and photolysis models across heliocentric ranges.
- Tools/workflows: Data submission portals; simplified scaling tools for CN vs R_h; training materials.
- Assumptions/dependencies: Sufficient amateur telescope capability; standardized photometric methods; data quality control.
- Community data and metadata standards for comet spectroscopy
- Sectors: software, policy, academia
- Application: Define a metadata schema that encodes cometocentric frames, beam efficiency, Doppler tracking status, channel binning, and hyperfine treatment to ensure reproducible analyses across archives.
- Tools/workflows: Specification documents; validators embedded in archive ingestion pipelines.
- Assumptions/dependencies: Broad community buy-in; alignment among major observatories and archives.
- Rapid characterization to support NEO/ISO risk and science prioritization
- Sectors: policy (planetary defense), observatories
- Application: While ISOs are rarely hazardous, the rapid chemical characterization workflows developed here can be adapted to near-Earth object science triage, informing follow-up priorities and potential resource assessment.
- Tools/workflows: Decision matrices integrating composition indicators (e.g., volatile-rich vs depleted) with orbital risk metrics.
- Assumptions/dependencies: Integration with planetary defense infrastructure; sustained survey coverage; scalable pipelines.
Glossary
- A-band: A frequency range of the telescope’s receiver used for millimeter-wave observations. "in the A-band, namely the transition of HCN at 265.8864~GHz."
- ACSIS: The digital autocorrelation spectrometer used as JCMT’s backend for high-resolution spectral data acquisition. "ACSIS digital autocorrelation spectroscopic backend."
- Abundance ratios: Relative quantities of species in the cometary coma, used to compare compositions. "Preliminary estimates of the HCN/HO and CN/HCN abundance ratios suggest values similar to Solar System comets."
- Apparitions: Observed passages of a comet through the inner solar system. "allowing comparisons to be made with the apparitions of Solar System comets."
- Bandpass: The frequency width over which the spectrometer collects data. "over a bandpass of 250~MHz"
- Beam-switching (BMSW): An observing technique alternating between target and sky to remove background. "We employed {\it beam-switching} (BMSW) between the ON and OFF phases,"
- Beam throw (BMSW throw): The angular separation used in beam-switching between ON and OFF positions. "We used a BMSW throw of 90$\arcsec$"
- Coma: The diffuse gaseous envelope around a comet’s nucleus. "avoid contamination of the spectrum of the sky by emission from the coma."
- Cometocentric velocity frame: A velocity frame centered on the comet to align spectral features. "in the cometocentric velocity frame at 1.0~km~s resolution."
- Daughter molecules: Secondary species produced by photodissociation of parent molecules in the coma. "Of the coma daughter molecules typically observed at optical wavelengths:"
- Doppler shift: The change in observed frequency due to motion along the line of sight. "The overall Doppler shift of the HCN line was allowed to vary,"
- Doppler smearing: Broadening of spectral lines due to changing radial velocity during an integration. "keeping Doppler smearing to less than 0.02~km~s."
- Ephemerides: Predicted positions and velocities of celestial objects used for tracking. "ephemerides 25 {paper_content} 26 from JPL/Horizons,"
- FWHM beam: Full width at half maximum of the telescope beam; a measure of angular resolution. "The FWHM beam at 265.8864 GHz is 18.4\"."
- Galactic thick disk: A kinematic and structural component of the Milky Way with older, metal-poor stars. "With a proposed origin from the Galactic thick disk (Hopkins et al. 2025),"
- Geocentric distance (): The distance from Earth to the comet. "and the heliocentric distance and the geocentric distance, R and (AU), respectively."
- Heliocentric distance (): The distance from the Sun to the comet. "at a heliocentric distance of 2.13 AU"
- Heterodyne receiver: A radio receiver that mixes incoming signals with a local oscillator for high-resolution spectroscopy. "using the \overline{U}\overline{u}$ heterodyne receiver and ACSIS spectroscopic backend."</li> <li><strong>Hyperbolic secondary mirror</strong>: A shaped telescope mirror used to implement rapid beam-switching. "implemented by oscillating the hyperbolic telescope secondary mirror at 1~Hz."</li> <li><strong>Hyperfine components</strong>: Small energy level splittings that produce multiple closely spaced spectral features. "the strongest 5 HCN hyperfine components labeled by tick marks."</li> <li><strong>Interstellar medium (ISM)</strong>: The gas and dust that fill the space between stars. "during the billions of years they spend traversing the interstellar medium"</li> <li><strong>Interstellar objects (ISOs)</strong>: Bodies originating outside the solar system passing through it. "The discovery of the first two interstellar objects (ISOs)"</li> <li><strong>JCMT T$_a^*$ intensity scale</strong>: A calibrated antenna temperature scale used for JCMT spectral line intensities. "effectively on the JCMT T$_{a} intensity scale."
- LMFIT: A non-linear least-squares optimization library used to fit spectral models. "was optimized using the LMFIT routine (Newville et al. 2014),"
- Main beam efficiency: The fraction of the antenna’s response in the main lobe; used to convert measured temperatures to main-beam values. "A JCMT main beam efficiency factor of 0.66 was used"
- Nepers: A logarithmic unit for measuring attenuation (e.g., atmospheric opacity). "less than 0.15~(nepers),"
- Non-LTE: Conditions where particle populations deviate from local thermodynamic equilibrium, requiring specialized modeling. "derived by non-LTE radiative transfer modelling."
- Opacity (tau): A measure of atmospheric attenuation at a given frequency. "the opacity of the atmosphere (tau)"
- Optically thin: A regime where radiation passes through material with negligible absorption or re-emission. "since the emissions are optically thin"
- Ortho-to-para ratio: The population ratio of nuclear spin isomers (e.g., in HO), affecting collisional/excitation processes. "an assumed HO ortho-to-para ratio of 3."
- Outflow velocity: The bulk expansion speed of gas leaving the cometary nucleus. "The retrieved HCN outflow velocity (derived from the spectrally resolved HCN line shape) is ,"
- Perihelion: The point of closest approach of a comet to the Sun in its orbit. "the interval in days prior to perihelion,"
- Photolysis: Chemical dissociation caused by absorption of photons. "The model includes HCN photolysis"
- Production rate : The number of molecules of species X released per second from the comet. "a production rate of ~s"
- Pumping by solar radiation: Excitation of molecular energy levels by absorption of sunlight (radiative pumping). "and pumping by solar radiation (Crovisier {paper_content} Encrenaz 1983)."
- Radiative transfer modelling: Simulation of emission/absorption processes to interpret spectral lines. "the non-LTE radiative transfer modelling described in {\color{blue} Section~\ref{sec:HCNprof},"
- Rotational transition (): A change between rotational energy levels of a molecule, here from to . "HCN() rotational emission"
- Spectroscopic backend: The signal-processing system that records and digitizes spectra from the receiver. "ACSIS spectroscopic backend."
- Spherically-symmetric sublimation: A modeling assumption that gas is emitted uniformly in all directions from the nucleus. "assuming spherically-symmetric sublimation, direct from the nucleus."
- Starlink (software): An astronomical data reduction software suite used for JCMT spectroscopy. "Routine reduction of the spectroscopic data used {\it Starlink} software"
- SUBLIME: A non-LTE radiative transfer code used to model cometary line emission. "SUBLIME model fit to the HCN() line observed on September 14."
- Sub-solar metallicity: A chemical composition with metal abundances below the solar value. "indicative of sub-solar metallicity and an extended sojourn in the Galactic interstellar medium"
- Water Vapor Meter (WVM): An instrument monitoring atmospheric water vapor to measure opacity. "the opacity of the atmosphere (tau) as measured by the JCMT Water Vapor Meter."
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