37th DPS Meeting, 4-9 September 2005
Session 11 Comets
Oral, Monday, September 5, 2005, 4:20-6:00pm, Law LG19

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[11.07] The Carbon Monoxide Extended Source in Comet Hale-Bopp Revisited

D. Bockelée-Morvan, J. Boissier, J. Crovisier, F. Henry (Observatoire de Paris), H. A. Weaver (The John Hopkins University)

Infrared long-slit observations of v(1--0) ro-vibrational lines of carbon monoxide near 4.7 micrometers had led DiSanti et al. (2001, Icarus 153, 361) and Brooke et al. (2003, Icarus 166, 167) to suggest that 50 to 90% of the CO was released by a distributed source when comet C/1995 O1 (Hale-Bopp) was within 1.5 AU from the Sun. In contrast to these studies, we suggest that most CO could be of nuclear origin. Indeed, observations of the CO J(1--0) (115 GHz) and J(2--1) (230 GHz) lines were performed on 11 March 1997 at the Plateau de Bure interferometer of the Institut de Radioastronomie Millimétrique (IRAM). The angular resolution was 1.7 and 3 arcsec at 230 and 115 GHz, respectively, i.e., similar to the angular resolution of the infrared observations. The brightness distribution of both the 115 GHz and 230 GHz lines can be fully explained by pure nuclear CO production, providing opacity effects and temperature variations in the coma are taken into account. We re-examined the infrared observations with a radiative transfer model taking into account opacity effects in both the solar pump and outgoing radiation. The relative intensities of the R0 and P2 lines, and of the R1 and P3 lines show direct evidence for strong opacity effects in the emitted radiation near the nucleus. We suggest that the broad extent of the brightness distribution of the IR lines near perihelion, attributed to an extended source, is rather due to opacity effects exacerbated by the CO jet seen both in the radio (Henry et al., 2002, EMP 90, 57) and in the infrared (Brooke et al., 2003, ibid). Cold rotational temperatures estimates near the nucleus may be also affected by opacity effects.

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Bulletin of the American Astronomical Society, 37 #3
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