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 lambda/Delta lambda similar to 2.5; 104 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 nu(1) and nu(5) rovibrational bands between 3.53 and 3.62 mu m, 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 Trot 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 Xnative = ( 0.79 +/- 0.09); 10(-2) for the three dates.