Detection of formaldehyde emission in comet C/2002 T7 (linear) at infrared wavelengths: Line-by-line validation of modeled fluorescent intensities

Formaldehyde (H₂CO) was observed in comet C/2002 T7 (LINEAR) with spectral resolving power λ/Δλ∼ 2.5 × 10 ⁴ using the Cryogenic Echelle Spectrometer (CSHELL) at the NASA Infrared Telescope Facility, on UT 2004 May 5, 7, and 9. The observations, which sampled emission in the v¹ and v₅ rovibrational bands between 3.53 and 3.62 μm, represent the first spectrally resolved detection, at infrared wavelengths, of monomeric H₂CO spanning a range of rotational energies. A comparison of measured line intensities with an existing fluorescence model permitted extraction of rotational temperatures and production rates. Two complementary approaches were used: (1) a correlation analysis that provided a direct global comparison of the observed cometary emissions with the model and (2) an excitation analysis that provided a robust line-by-line comparison. Our results validate the fluorescence model. The overall correlation coefficient was near or above 0.9 in our two principal grating settings. The excitation analysis provided accurate measures of rotational excitation (rotational temperature) on all three dates, with retrieved values of T_rot clustering near 100 K. Through simultaneous measurement of OH prompt emission, which we use as a proxy for H₂O, we obtained native production rates and mixing ratios for H₂CO. The native production of H₂CO varied from day to day, but its abundance relative to H₂O, X_native, remained approximately constant within the errors, which may suggest an overall homogeneous composition of the nucleus. We measured a mean mixing ratio X_native = (0.79 ± 0.09) × 10^-2 for the three dates.