McDonald Observatory High Spectral Resolution Observations of Comet Hale-Bopp

A. L. Cochran, E. S. Barker, W. D. Cochran, D. L. Lambert (U. Texas)

We observed comet Hale-Bopp with the 2DCoude spectrograph on the 2.7m telescope at McDonald Observatory. This is a cross-dispersed echelle spectrograph located at the coudé focus. It is capable of working in two different resolution regimes: the lower spectral resolving power is R=60,000; the higher spectral resolving power is R=180,000. Comet Hale-Bopp was observed with this spectrograph on 4 nights in fall 1996 with R=60,000. It was observed an additional 17 nights from 21 February through 7 April 1997 using the R=180,000 mode on 8 nights and the R=60,000 mode on 9 nights. The R=180,000 observations were obtained on 28 February through 3 March when the comet was at tex2html_wrap_inline16 1.1 (pre-perihelion) and 1.5AU heliocentric and geocentric distances respectively. Hale-Bopp was also observed at R=180,000 on 4 through 7 April when the comet was at tex2html_wrap_inline16 0.9 (post-perihelion) and 1.4AU heliocentric and geocentric distances. The R=60,000 observations were obtained on 21 through 23 February, 15 and 16 March, and 26 through 29 March, all pre-perihelion.

In the R=60,000 mode, the spectral coverage is complete from tex2html_wrap_inline16 3800-5800Å. From 5800-10,000Å the spectral coverage is broken by increasing interorder gaps. In the R=180,000 mode, spectral coverage is much more limited, with 14-20 spectral orders of approximately 20Å each. Thus, in the higher spectral resolving power regime, specific molecular features were targeted. In this regime, individual molecular lines could be fully resolved and thus we measure the velocity broadening of the various species. On all nights, the slit was placed at different locations in the coma so that differences can be studied between the dustier and less dusty regions of the coma. We will show the effect on the line width of different regions of the coma and of different epochs in the comet's passage. In addition, these spectra will be useful for the study of isotope ratios in the coma, particularly tex2html_wrap_inline22 C/ tex2html_wrap_inline24 C.