High-Dispersion Spectroscopy of the Sodium (Na) Emission Within the Inner Coma of Hale-Bopp(C/1995 O1)
E.S. Barker, A.L. Cochran, W.D. Cochran (Univ. of Texas at Austin)
We obtained high-dispersion spectra of Comet Hale-Bopp in the February-April 1997 timeframe with the 2DCoude cross-dispersed echelle spectrograph at the f/33 focus of the 2.7-m telescope at McDonald Observatory. On February 27, March 1 and April 5 UT, the Na D-line spectral region was observed at a spectral resolution of 180,000 (1.7 km/s) at spatially resolved locations within 25,000 km of the optocenter of comet Hale-Bopp. Using computer driven offsets and TV guiding, different regions of the inner coma were sampled by a 0.3x8.2 arcsecond slit (0.3x13 on February 27). Doppler shifts of the Na D-lines with respect to the telluric rest frame were consistent with the emission lines being produced by resonant scattering of solar flux by cometary Na atoms. The cometary Na emission lines (intrinsic FWHM = 2-4 km/s) were clearly resolved from their telluric counterparts and were more than 100x stronger than the telluric emissions. On March 1 and April 5 the observed cometary Na D line (5890Å ) was definitely asymmetric with a red shoulder at 10% intensity level. The echelle band pass on April 5 contained the Na D line at 5896Å which was also asymmetric on the red wing. Other cometary lines in our spectra did not show this asymmetry. Na D-line widths and line shapes will be discussed in context of sources of Na atoms
Discovery announcements by the European Hale-Bopp team of an extensive neutral Na tail in IAU Circulars 6631,6334 (April 18/21) were followed by press releases of earlier observations of the Na tail made by Boston University in early March and the POLAR satellite in late March and early April (see this volume). Our Na observations of the inner coma ( km) will be discussed in contrast to the tail observations which concentrated on regions km from the optocenter.