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Based on long-slit infrared spectroscopic observations, it has been suggestedthat 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 thecoma, whose nature (dust or gas) remains unidentified. We re-assess the originof CO in Hale-Bopp's coma from millimeter interferometric data and are-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 andinterferometric modes. The diversity of angular resolutions (from 1700 to 42000km diameter at the comet) is suitable to study the radial distribution of COand detect the extended source observed in the infrared. We used excitation andradiative transfer models to simulate the observations. Various CO densitydistributions were considered, including 3D time-dependent hydrodynamicalsimulations 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 lengthsof CO parent L_p > 1500 km are rejected.  Based on similar radiation transfer calculations, we show that the CO v = 1-0ro-vibrational lines observed in comet Hale-Bopp at heliocentric distances lessthan 1.5 AU are severely optically thick. The broad extent of the CO brightnessdistribution in the infrared is mainly due to optical depth effects. Additionalfactors can be found in the complex structure of the CO coma, and non-idealslit positioning caused by the anisotropy of dust IR emission.  We conclude that both CO millimeter and infrared lines do not providecompelling evidence for a distributed source of CO in Hale-Bopp's atmosphere.

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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