Theoretical Modeling of the C₂ Fluorescence Spectrum in Comet Hale-Bopp

Authors: Rousselot P.¹; Hill S.M.²; Burger M.H.²; Brain D.A.²; Laffont C.³; Moreels G.³

Source: Icarus, Volume 146, Number 1, July 2000 , pp. 263-269(7)

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Abstract:

We report on observations and analysis of the C₂ Swan band Delta v=0 sequence in the optical spectrum of Comet Hale-Bopp. One set of observational data was obtained when this comet was at a large heliocentric distance (3.03 AU). This unusual distance provided the opportunity to enhance our knowledge of the intercombination transitions in C₂.

These transitions are forbidden for electric dipole radiation, but they exist due to higher order multipole radiation. Although the transition probabilities are several orders of magnitude smaller than allowed electronic transitions, the intercombination bands play a key role in the fluorescence process of the C₂ radical. This role is to provide a cooling path for rotational and vibrational populations seen in the visible spectrum. Because their exact transition probability is not yet completely clear, better quantitative knowledge of these transitions can help constrain C₂ quantities in comets.

To analyze the data, an equilibrium fluorescence model with 5652 different vibrational levels was created. This model included the triplet and singlet systems of C₂ involved in the fluorescence process. Theoretical spectra corresponding to different values of the electronic transition moments for the a³_u–X¹ Sigma _g⁺ and c³ Sigma _u⁺–X¹ Sigma ⁺_g transitions were computed. These spectra were then compared to the observational data. A good fit is obtained for transition moments of 5×10^-6|D_a-X|²=|D_c-X|²10^-5 atomic units (a.u.), for spectra obtained far from the nucleus where the fluorescent equilibrium is reached. Copyright 2000 Academic Press.

Language: English

Document Type: Research article

Affiliations: 1: Observatoire de Besançon, Besançon Cedex, 25010, France 2: University of Colorado, Boulder, Colorado, 80309-0391 3: Observatoire de Besançon, Besançon Cedex, 25010, France

Publication date: 2000-07-01