Astrochemistry

Publication Abstract

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

Stefanie N. Milam, Anthony J. Remijan, Maria Womack, Leif Abrell, L. M. Ziurys, Susan Wyckoff, A. J. Apponi, D. N. Friedel, L. E. Snyder, J. M. Veal, Patrick Palmer, L. M. Woodney, Michael F. A'Hearn, J. R. Forster, M. C. H. Wright, I. de Pater, S. Choi, and M. Gesmundo

Observations of formaldehyde (H2CO) have been conducted towards comets C/1995 O1 (Hale-Bopp), C/2001 Q4 (NEAT), and C/2002 T7 (LINEAR) using the Arizona Radio Observatory (ARO) 12m 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 towards Comet Hale-Bopp. These data indicate the production rate of H2CO is ~ 3.7 x 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 (rs ~ 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 x 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 Halley. Production rates for H2CO towards Comets C/2002 T7 (LINEAR) and C/2001 Q4 (NEAT) are 1.4 x 1027 s-1 and 5.6 x 1026 s-1, respectively, modeled using the extended grain source. The spectra of H2CO measured towards Comet C/2002 T7 (LINEAR) shows evidence for a second velocity component, most likely arising from comet fragmentation.

Published in the Astrophisical Journal, vol. 649, pp. 1169-1177



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© 2015, D. N. Friedel