ANALYSIS OF THE PLASMA-COMA OF COMET P/HALLEY BY IMAGE PROCESSING TECHNIQUES OF BOCHUM'S PHOTOPLATES

Abstract

ABSTRACT Photographic and photoelectric observations of comet P/Halley's ion gas coma from CO+ at 4250A were part of the Bochum Halley Monitoring Program, conducted from 1986 February 17, to April 17 at the European Southern Observatory on La Silla (Chile). In this spectral range it is possible to watch the continuous formation, motion and expansion of plasma structures. To observe the morphology of these structures 32 CO^+ photos (glass plates) from P/Halley's comet have been analysed. They have a field of view of 28 degrees 6 X 28 degrees 6 and were obtained from 1986 March 29, to April 17 with exposure times between 20 and 120 minutes. All photos were digitized with a PDS 2020 GM (Photometric Data System) microdensitometer at the Astronomisches Institut der Westfalischen Wilhelms-Universitat in Munster (one pixel = 25 microns X 25 microns approximately 46 arcsec 88 X 46 arcsec 88). After digitization the data were reduced to relative intensities, and the part with proper calibrations were also converted to absolute intensities, expressed in terms of column densities using the image data systems MIDAS (Munich Image Data Analysis System; ESO - Image Processing Group, 1988) and IHAP (Image Handling And Processing; Middleburg, 1983). With the help of the Stellingwerf-Theta-Minimum-Method (Stellingwerf, 1978) a period of (2.22 ± 0.09) days results from analysis of structures in the plasma-coma by subtracting subsequent images. The idea behind subtracting subsequent images is that rotation effects are only 10% phenomena on gas distribution. Difference images are than used to supress the static component of the gas cloud. The CO+ column density data (in molecules cm-2) were compared with the data of CN column density from Schulz (1990) in all common days. The results show that the relations between CO+ and CN in average column density values (NCO^+ /NCN) are 11.6 for a circular slit with average diameter (Phi) of 6 arcminute 1 which corresponds to a distance from the nucleus (rho) equal to 6.3 X 104 km; 20.0 for Phi = 7 arcminute 1 and rho = 7.3 X 104 km; 8.1 for Phi = 8 arcminute 5 and rho = 8.7 X 104 km; 35.6 for Phi = 11 arcminute 9 and rho = 1.2 X 105 km; and 31.3 for Phi = 16 arcminute 7 and rho = 1.7 X 105 km. These values are in perfect agreement with the data from Vivekananda et al. (1990) for short distances (rho from 3.9 X 103 to 1.2 X 104 km) and small slit diameters (Phi from 0.4 to 1.2 arcminutes). With the use of slits with large diameters it is possible to get some information about the outer coma of the comet (in this work, from 60000 until 170000 km far away from the nucleus). At these distances (about 105 km) the CO+ column density changes only due to the geometrical dilution, because the CO+ parent molecules are already photoionized or photodissociated (Voelzke, 1989), which means that through different processes the neutral gas that flows to about 105 km from the nucleus is already transformed into ionized gas.