[Paper Review] Submillimetre observations of comets with Odin: 2001-2005

N. Biver, A. Lecacheux+8 more|SPIRE - Sciences Po Institutional REpository|Oct 26, 2006

Astro and Planetary Science4 citations

TL;DR

This study presents submillimetre observations of 12 comets using the Odin satellite between 2001 and 2005, measuring water production rates via high-resolution 557 GHz H₂O line profiles and detecting H₂¹⁸O and NH₃ lines. Key results include isotropic outgassing constraints, a ¹⁶O/¹⁸O ratio of ~520 (marginally higher than terrestrial), and ammonia abundances of 0.3–0.5% relative to water, consistent with prior measurements.

ABSTRACT

The Odin satellite, launched in Feb. 2001, is equipped with a 1.1-m submillimetre telescope. Odin was used to observe the 557 GHz line of water with high spectral resolution in 12 comets between 2001 and 2005. Line shapes and spatial mapping provide information on the anisotropy of the outgassing and constraints on water excitation, enabling accurate measurements of the water production rate. Five comets were regularly observed over periods of more than one month to monitor the variation of their water outgassing rate with heliocentric distance. Observing campaigns have been generally coordinated with ground-based observations of molecular lines at Nançay, CSO or IRAM 30-m telescopes to obtain molecular abundances relative to water. Thanks to Odin's frequency coverage, it was also possible to detect the H_2^18O 548GHz line, first in comet 153P/Ikeya-Zhang in April 2002 (Lecacheux et al., 2003) and then in comets C/2002 T7 (LINEAR), C/2001 Q4 (NEAT) and C/2004 Q2 (Machholz). The ^16O/^18O isotopic ratio (\approx450) is consistent with the terrestrial value. Ammonia has been searched for in three comets through its J_K = 1_0-0_0 line at 572 GHz and was tentatively detected in C/2001 Q4 and C/2002 T7. The derived abundances of NH_3 relative to water are 0.5 % and 0.3 %, respectively, similar to values obtained in other comets with different techniques.

Motivation & Objective

• To measure water production rates in comets using high-resolution submillimetre observations of the 557 GHz H₂O line.
• To investigate the anisotropy of cometary outgassing through line profile analysis and spatial mapping.
• To determine the ¹⁶O/¹⁸O isotopic ratio in cometary water by comparing optically thick H₂¹⁶O and thin H₂¹⁸O lines.
• To detect and quantify ammonia (NH₃) in cometary comae via its 572.5 GHz fundamental line.
• To coordinate space-based Odin observations with ground-based radio telescopes (Nançay, CSO, IRAM) for relative molecular abundance determination.

Proposed method

• Odin satellite used a 1.1-m submillimetre telescope with dual-band receivers covering 486–504 GHz and 541–580 GHz, enabling observation of H₂O, H₂¹⁸O, and NH₃ lines.
• High-resolution autocorrelators (177 kHz resolution) resolved the 557 GHz water line to study self-absorption and velocity shifts due to optical depth effects.
• Spectral line fitting and radiative transfer modelling were applied to derive water production rates, assuming isotropic outgassing and accounting for optical depth effects.
• Simultaneous observations with ground-based telescopes (Nançay, CSO, IRAM) provided complementary molecular line data for abundance ratios relative to water.
• Pointing accuracy was maintained via star-tracker calibration and monitoring of bright calibrators (e.g., Orion-KL, Jupiter), with typical pointing error <20”.
• Data reduction accounted for receiver frequency drift, especially for the NH₃ line at 572.5 GHz, requiring individual frequency calibration per observation.

Experimental results

Research questions

• RQ1 How does the velocity structure of the 557 GHz H₂O line reveal anisotropy and self-absorption in cometary comae?
• RQ2 What is the water production rate in comets, and how does it vary with heliocentric distance over time?
• RQ3 What is the ¹⁶O/¹⁸O isotopic ratio in cometary water, and how does it compare to terrestrial values?
• RQ4 What is the abundance of ammonia (NH₃) in cometary comae, and how does it compare to values derived from other techniques?
• RQ5 How do the observed line profiles and excitation conditions constrain the radiative transfer and outgassing models?

Key findings

• The 557 GHz H₂O line profile shows redshifted self-absorption, indicating optically thick emission and velocity-dependent optical depth effects in the coma.
• Water production rates were measured with uncertainties up to 50%, primarily due to the assumption of isotropic outgassing in radiative transfer models.
• The ¹⁶O/¹⁸O ratio was derived as ~520, which is marginally higher than the terrestrial value of ~450, though model imperfections may slightly underestimate this ratio.
• H₂¹⁸O at 548 GHz was detected in four comets (153P/Ikeya-Zhang, C/2002 T7, C/2001 Q4, C/2004 Q2), enabling constraints on radiation transfer in water-dominated comae.
• Ammonia was tentatively detected in C/2001 Q4 and C/2002 T7, with NH₃/H₂O abundance ratios of 0.50±0.09% and 0.33±0.08%, respectively, consistent with other observational methods.
• The derived NH₃ abundances are in agreement with upper limits from 24 GHz ground-based observations and previous in situ and optical measurements.