Formaldehyde in comets C/1995 O1 (Hale-Bopp), C/2002 T7 (LINEAR), and C/2001 Q4 (NEAT): Investigating the cometary origin of H₂CO

Observations of formaldehyde (H₂CO) have been conducted toward comets C/1995 O1 (Hale-Bopp), C/2001 Q4 (NEAT), and C/2002 T7 (LINEAR) using the Arizona Radio Observatory (ARO) 12 m telescope at 1.2 and 2 mm. Aperture synthesis maps of H₂CO at 3 mm were made using the Berkeley-Illinois-Maryland Association (BIMA) interferometer toward comet Hale-Bopp. These data indicate that the production rate of H₂CO is ∼3.7 × 10²⁸ s^-1 at ∼1 AU in comet Hale-Bopp, using a simple Monte Carlo model, if a nuclear origin for the molecule is assumed. However, maps of H₂CO in Hale-Bopp, in comparison with CO, show an extended distribution (r_s ∼ 15,000 km) with small-scale structure oriented roughly along the comet-Sun direction. This result suggests a source of H₂CO other than the comet nucleus. The extended source of formaldehyde is probably grains composed of a mixture of silicates and organic material. The production rate for H₂CO increases to Q ∼ 1.4 × 10²⁹ s^-1 assuming such an extended grain source. This value implies a Q/Q(H₂O) ∼ 1.4%, which is similar to the production rate ratio of Q/Q(H₂O) ∼ 4% derived from in situ measurements of H₂CO in comet Halley. Production rates for H ₂CO toward comets C/2002 T7 (LINEAR) and C/2001 Q4 (NEAT) are 1.4 × 10²⁷ and 5.6 × 10²⁶ s^-1, respectively, modeled using the extended grain source. The spectra of H ₂CO measured toward comet C/2002 T7 (LINEAR) show evidence for a second velocity component, most likely arising from comet fragmentation.