2005ApJ...618..543F

A sequential spectroscopic observation of comet C/1995 O1 (Hale-Bopp) was performed from 1996 September to 1997 May. After the first positive detection in early 1997 February, the development of the neutral sodium emission was monitored during about 4 months around the perihelion passage. The ratio of the sodium D-line emission relative to the continuum emission strength varied with the days from perihelion. The sodium D-line emission was weak relative to the continuum near the perihelion because of the Swings effect. We corrected the Swings effect, the phase angle dependence of continuum emission, and the differences in geocentric distance. We found that the corrected sodium-to-continuum ratio varied with the dependence of r^–k; k=5-6, r denotes a heliocentric distance of the comet. Our result supports the hypothesis of thermal desorption of sodium atoms from dust grains.