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No compelling evidence of distributed production of CO in comet C/1995 O1 (Hale-Bopp) from millimeter interferometric data and a reanalysis of near-IR lines | | D. Bockelée-Morvan
; J. Boissier
; N. Biver
; J. Crovisier
; | | Date: |
12 Aug 2010 | | Abstract: | Based on long-slit infrared spectroscopic observations, it has been suggested
that half of the carbon monoxide present in the atmosphere of comet C/1995 O1
(Hale-Bopp) close to perihelion was released by a distributed source in the
coma, whose nature (dust or gas) remains unidentified. We re-assess the origin
of CO in Hale-Bopp’s coma from millimeter interferometric data and a
re-analysis of the IR lines.
Simultaneous observations of the CO J(1-0) (115 GHz) and J(2-1) (230 GHz)
lines were undertaken with the IRAM interferometer in single-dish and
interferometric modes. The diversity of angular resolutions (from 1700 to 42000
km diameter at the comet) is suitable to study the radial distribution of CO
and detect the extended source observed in the infrared. We used excitation and
radiative transfer models to simulate the observations. Various CO density
distributions were considered, including 3D time-dependent hydrodynamical
simulations which reproduce a CO rotating jet. The CO J(1-0) and J(2-1)
observations can be consistently explained by a nuclear production of CO.
Composite 50:50 nuclear/extended productions with characteristic scale lengths
of CO parent L_p > 1500 km are rejected.
Based on similar radiation transfer calculations, we show that the CO v = 1-0
ro-vibrational lines observed in comet Hale-Bopp at heliocentric distances less
than 1.5 AU are severely optically thick. The broad extent of the CO brightness
distribution in the infrared is mainly due to optical depth effects. Additional
factors can be found in the complex structure of the CO coma, and non-ideal
slit positioning caused by the anisotropy of dust IR emission.
We conclude that both CO millimeter and infrared lines do not provide
compelling evidence for a distributed source of CO in Hale-Bopp’s atmosphere. | | Source: | arXiv, 1008.2101 | | Services: | Forum | Review | PDF | Favorites | |
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