Observations of C Swan Bands in Comet Hale-Bopp Near Perihelion
S. M. Hill, D. Brain, M. H. Burger (University of Colorado, LASP)
Observations of the C Swan Bands in comet Hale-Bopp and a simple spectral model were used to study the spatial evolution of C excitation temperatures. The data set provides better spectral resolution than in studies of other comets. The analysis presented here is a first-order look at C temperatures near perihelion in this bright comet.
Comet Hale-Bopp provided an opportunity to obtain observations of the C Swan bands from the 0.6 m telescope at Sommers-Bausch Observatory in Boulder, Colorado. We used a grating spectrograph with an 832 x 832 CCD detector to take data within a month of perihelion at moderate spectral resolution ( 1.2 FWHM). Spatial resolution was limited due to poor seeing at the site (5-10 arcsec). Four nights of usable spectra were obtained overall.
The C Swan bands have been shown to exhibit non-equilibrium vibrational and rotational populations near cometary nuclei due to the long time ( 3000 sec) required to reach fluorescence equilibrium (Rousselot et al., Icarus, 341, 1995; Rousselot et al., A & A, 645, 1994). We used Boltzmann distributions for the upper electronic state (d ) to create model spectra and to estimate relative emission rates. Using fits to the observed spectra and band intensity ratios, we present effective rotational and vibrational excitation temperatures as a function of cometocentric distance. Because the spectrograph slit was aligned at various position angles relative to the Sun-comet axis, we investigate the possibility of excitation temperature variations due to spatial features. With our spectral resolution, individual rotational lines are partially resolved. A number of C lines can be identified, possibly allowing the removal of contaminating lines due to NH fluorescence. Thus, C excitation temperatures may be established to better accuracy than in previous works. Improved understanding of the C emissions allows better determinations of C formation/destruction processes and of carbon isotope ratios.