Periodic Light Variations in the Near-Nucleus Zone of Comets Giacobini-Zinner and P/halley

Photoelectric photometric measurements in the near nucleus zone of comets P/Halley and P/Giacobini-Zinner have been performed at the Wise Observatory during the recent apparitions of these two comets. The results reveal periodic light variations in the two objects, with periods of 52 h and 9.5 h respectively. The variations observed in P/Halley are detected mainly in the radiation in emission lines. They are interpreted as resulting from a periodic modulation of the ejection of the line emitting molecules from the nucleus into the near zone, due to the rotation of the nucleus. A production duration of 30 h is derived for the daughter molecule C₂ at the time of the authors' observations. In the P/Giacobini-Zinner case, the variations are found mostly in the continuum radiation and they are interpreted as the reflection of sunlight from the rotating solid nucleus of the comet.