ICARUS (~, 386-390 (1984) Stellar Occultations by Two Comets: IRAS-Araki-Alcock (1983d) and P/IRAS (1983j) J. LECACHEUX,* W. THUILLOT,t T. ENCRENAZ,* P. LAQUES,$ D. ROUAN,* AND R. DESPIAU$ *Observatoire de Paris-Meudon, 92195 Meudon Principal Cedex, ?Bureau des Longitudes, 77, Au. Denfert-Rochereau, 75014 Paris, and J;Observatoire du Pic du Midi et de Toulouse. 65200 Bagneres de Bigorre, Frant'c Received April 2, 1984: revised June 18, 1984 A stellar appulse of comet 1983d was observed on 1983 May 08.956 using a video photometry technique. Although the miss distance did not exceed 420 -+ 50 km no significant absorption was detected. Another close appulse, of comet 1983j this time, was measured with a photoelectric photometer on 1983 September 14.036, A small feature of the lightcurve, only 1.8% deep, could be attributed to dust surrounding the nucleus within a 350-km radius. The deduced dust ejection rate is 6 x 106 g/sec. © 1984Academic Press, Inc. 1. INTRODUCTION Comets approaching the Earth at dis- tances less than 1 AU are good targets for the occultation sounding method, as their fast apparent motion and large apparent diameter provide frequent stellar occulta- tions. As discussed in detail by Combes et al. (1983), photometric measurement of a stellar occultation by the inner coma is po- tentially a powerful technique: it yields the density of the circumnuclear dust cloud, without any assumption about optical prop- erties of grains, such as albedo. By cou- pling the derived density with global pho- tometry of the coma, an estimate of albedo can be obtained. The apparition of comet 1983d, which ap- proached the Earth on May 11.5 with a min- imum geocentric distance of 0.031 AU, pro- vided an unprecedented opportunity for studying the inner coma. Indeed, an angu- lar distance of 1 arcsec corresponded at perigee time to about 23 kin, allowing a spa- tial resolution 10 to 100 times higher than obtained in typical comets. Although the galactic latitude of comet 1983d exceeded +30 ° on the most favorable nights, many 0019-1035/84 $3.00 ('opyright !i, 1984by AcademicPress, Inc. All rights of reproduction in any form reserved. close appulses of relatively bright stars were expected to occur. Four months later the newly discovered short-periodic comet 1983j approached the Earth with a minimum distance of 0.78 AU, a distance in no way exceptional, but the favorable position in the sky and the rela- tively high velocity (1°.0/24 hr) allowed two or three events to be measurable each night with a telescope of the 2-m class. This paper describes two tentative mea- surements of stellar occultations by these two comets. II. COMET 1983d: OBSERVATION AND DATA REDUCTION On May 8, 1983, at 22h57 m UT, Comet IRAS-Araki-Alcock passed near a 12th magnitude star located 3.6 arcmin south of SAO 17577. The geometry of the event is shown in Fig. 1 and the observation param- eters are given in Table I. At the time of appulse, the distance of the comet was 0.0714 AU and its topocentric velocity (Meudon Observatory) was 0.374 arcsec/ sec. The observation was made with a 60- cm F/12 reflector and a SIT-Vidicon ("Noc- 386