[Paper Review] Water in the Near IR spectrum of Comet 8P/Tuttle

R. J. Barber, S. Miller+4 more|Sep 22, 2008

Astro and Planetary Science1 references4 citations

TL;DR

This study presents the first detection and identification of weak, high-energy water emission lines—termed 'SH' lines—in the near-infrared spectrum of Comet 8P/Tuttle, observed at high spectral resolution using UKIRT. The lines originate from vibrationally excited states and suggest non-thermal excitation mechanisms, with a derived water production rate of (1.4 ± 0.3) × 10²⁸ molecules s⁻¹ and a rotational temperature of 62 ± 5 K, challenging existing cometary coma models.

ABSTRACT

High resolution spectra of Comet 8P/Tuttle were obtained in the frequency range 3440.6-3462.6 cm-1 on 3 January 2008 UT using CGS4 with echelle grating on UKIRT. In addition to recording strong solar pumped fluorescent (SPF) lines of H2O, the long integration time (152 miutes on target) enabled eight weaker H2O features to be assigned, most of which had not previously been identified in cometary spectra. These transitions, which are from higher energy upper states, are similar in character to the so-called 'SH' lines recorded in the post Deep Impact spectrum of comet Tempel 1 (Barber et al., 2007). We have identified certain characteristics that these lines have in common, and which in addition to helping to define this new class of cometary line, give some clues to the physical processes involved in their production. Finally, we derive an H2O rotational temperature of 62+/- K and a water production rate of (1.4+/-0.3)E28 molecules/s.

Motivation & Objective

• To identify and characterize previously undetected, weak H₂O emission lines in the near-infrared spectrum of Comet 8P/Tuttle.
• To investigate the physical mechanisms responsible for the formation of these high-energy 'SH' lines, which are not explained by standard solar-pumped fluorescent (SPF) models.
• To derive key coma parameters, including water production rate and rotational temperature, from high-resolution spectral data.
• To explore the role of non-thermal processes such as electron impact, dissociative recombination, and photodissociation in populating vibrationally excited states of H₂O.
• To establish a foundation for understanding the differences in excitation mechanisms between short-period comets of different dynamical classes, such as 8P/Tuttle (a near-isotropic comet) and typical ecliptic comets.

Proposed method

• High-resolution near-infrared spectroscopy was conducted using the CGS4 echelle grating on the United Kingdom Infrared Telescope (UKIRT) at a spectral range of 3449.0–3462.2 cm⁻¹.
• A long on-source integration time of 152 minutes enabled the detection of eight weak H₂O transitions from high-energy upper states, previously unassigned in cometary spectra.
• Spectral line identification was performed by matching observed transitions to laboratory and theoretical data for H₂O ro-vibrational states, particularly those in the 3ν polyad.
• Rotational temperature was derived from the relative intensities of the observed transitions, assuming local thermodynamic equilibrium in the rotational levels.
• Water production rate was calculated using the observed line strengths and radiative transfer modeling, accounting for the comet's heliocentric distance and nuclear distance.
• Theoretical analysis explored potential excitation pathways, including electron impact, dissociative recombination of H₃O⁺, and O(¹D) quenching by H₂O, to explain the observed population inversion in upper states.

Experimental results

Research questions

• RQ1 What causes the population of high-energy upper states in H₂O molecules in the coma of Comet 8P/Tuttle, given that standard SPF models do not account for these transitions?
• RQ2 How do the physical conditions in the inner coma—such as electron density, temperature, and local density—affect the excitation of these SH lines?
• RQ3 What is the water production rate and rotational temperature of Comet 8P/Tuttle based on high-resolution near-IR spectroscopy?
• RQ4 Can the observed SH lines be explained by non-thermal processes such as electron impact, dissociative recombination, or photodissociation-induced cascades?
• RQ5 How do the excitation mechanisms in 8P/Tuttle compare to those in other comets, particularly in light of its classification as a near-isotropic comet (NIC) rather than a typical ecliptic comet (EC)?

Key findings

• Eight previously unassigned H₂O emission lines were identified in the near-infrared spectrum of Comet 8P/Tuttle, all originating from high-energy upper states, marking the first detection of such features in this comet.
• The observed lines exhibit characteristics of a 'water maser backbone series', indicating population inversion in the upper states, with transitions preferentially populating Kₐ ≥ J−1 states, suggesting cascade decay from higher vibrational levels.
• The rotational temperature of the coma was determined to be 62 ± 5 K, consistent with cold, rarefied conditions in the outer coma where collisional de-excitation is inefficient.
• The water production rate was measured as (1.4 ± 0.3) × 10²⁸ molecules s⁻¹, providing a key constraint on the comet's activity level during the observation.
• The presence of SH lines—originating from higher vibrational states than SPF lines—cannot be explained by standard SPF models, indicating the need for additional excitation mechanisms.
• Three potential excitation mechanisms were identified: electron impact, dissociative recombination of H₃O⁺, and O(¹D) quenching by H₂O, with the latter two being particularly promising for populating vibrationally excited states in the inner coma.