jrasc june 1998 final - The Royal Astronomical Society of Canada

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jrasc june 1998 final - The Royal Astronomical Society of Canada

ionization of Fe in the main spectrum, we estimate the mass ratioof the two components to be about 30. That corresponds to a smallerfractional share for the second component than has been found inbright Perseid meteors (Borovicka & Betlem 1997).The above parameters are valid for the last flare. Nevertheless,the spectrum developed along the trajectory. This is demonstratedin figures 3 and 4. In figure 3 the spectra at six selected points alongthe trajectory, marked A–F, are given. Point A falls at the beginningof the meteor trajectory, where all emissions are very faint. At pointB the lines of Mg i, Na i and N 2 are clearly visible. Point C falls at apoint on the trajectory after the sudden increase of meteor brightnesswhere the lines of Ca ii are also present, at comparable brightnessto Mg i. The spectrum is not very different just prior to the firstflare, at point D. At the instant of the flare (point E), the lines of Caii and other lines of the second spectral component (Mg ii, Si ii andFe ii) brightened dramatically. The lines then quickly decreased instrength again following the flare (point F).Monochromatic light curves in three representative lines aregiven in figure 4. The line of Na i is typical of the main spectralcomponent, Mg ii of the second component and Ca II is the brightestmultiplet in the spectrum. Only Na i has significant intensity at thebeginning. Ca ii increases slowly and bursts in the flares. Mg ii isbright only in the flares, while Na i decreases less between the flares.Ca ii is intermediate in that respect.The increase in brightness for lines of ionized atoms towardthe end of the trajectory and especially in the flares was notedpreviously by Cook et al. (1971). However, an approximate physicalanalysis of the spectra at different points showed that the massratio of both component gases does not change as dramatically inthe flares as one could expect solely on the basis of the monochromaticlight curves. The share of the second component gas just beforeand between the flares is smaller than in the flares only to withina factor of two. The reason for the much more dramatic changesin brightness of lines like Mg ii in comparison with Na i, for example,is that Mg ii and the other lines of the second component, exceptCa ii, are optically thin while the lines of the main component areoptically thick. The changes in column density in the flares produce,of course, more pronounced changes of brightness in optically thinlines. The lines of Ca ii are more optically thick than other lines ofthe second component, but more optically thin than the brightlines of the main component.The physical interpretation of the observational features listedabove is not quite clear. Borovicka (1994) proposed that the hightemperature component is produced in the meteor shock wave.The problem with such an interpretation is that the Ca ii lines arepresent at high altitudes. By comparison with other Perseid meteorswe can say that the offset for Ca ii must have occurred above analtitude of 90 km. At such altitudes the shock wave is not supposedto be developed (Bronshten 1993). It could be argued that the Caii lines are weakly present also in the low temperature component(Borovicka 1993, 1994). Nevertheless, the N 2 bands and the infraredN i and O i lines (Millman & Halliday 1961) begin even earlier inthe trajectory. More observational and theoretical work is neededto fully understand the hydrodynamics and radiation of meteorsand the formation of two distinct temperature components.The authors would like to thank Dr. David G. Turner, editor of theJRASC for his special efforts and time taken in getting this paperpublished. Thanks are also due to the anonymous referees for theirsuggestions which helped to improve the quality of the paper.Jirí BorovickaAstronomical Institute251 65 Ondrejov ObservatoryCzech RepublicEdward P. Majden1491 Burgess RoadCourtenay, British ColumbiaV9N 5R8ReferencesBorovicka, J. 1993, Astron. Astrophys., 279, 627Borovicka, J. 1994, Planet. Space Sci., 42, 145Borovicka, J. & Betlem, H. 1997, Planet. Space Sci., 45, 563Bronshten V. A. 1993, Physics of Meteoric Phenomena (D. Reidel Pub.:Dordrecht)Cook, A. F., Halliday, I. & Millman, P. M. 1971, Canadian J. Phys., 49, 1738Evans, S. J. & Ridley, H. B. 1993, J. Br. Astron. Assoc., 103, 27Halliday, I. 1960, Astrophys. J., 131, 25Halliday, I. 1961, Publ. Dominion Obs. Ottawa, 25, No. 1Halliday, I. 1968, from Physics and Dynamics of Meteors, IAU Symp. 33,ed. L. Kresak & P. Millman, (Reidel: Dordrecht), p. 91Halliday, I. 1969, Publ. Dominion Obs. Ottawa, 25, No. 12Millman, P. M. & Halliday I. 1961, Planet. Space Sci., 5, 137JIRÍ BOROVICKA is an astronomer at the Ondrejov Observatory of the Astronomical Institute of the Academy of Sciences of the Czech Republic. He obtainedhis bachelor’s degree and Ph.D. in astronomy and astrophysics from Charles University in Prague in 1987 and 1993, respectively. His research involves thephysics of meteors, meteor spectroscopy, and small bodies in the solar system, and he is also interested in the study of variable stars. His hobbies are travel andcycling.EDWARD P. MAJDEN is a retired electronics armament systems technician for RCAF/CAF, now called com/radar/systems. He was first introduced to meteorstudies and spectroscopy while a student member of the Regina Astronomical Society in the 1950s. He set up his own program of meteor spectroscopy in 1972.He is a life member of the Victoria Centre, an AMS affiliate and a Meteoritical Society member. He was recently elected an associate member of the Meteoritesand Impacts Advisory Commitee and is also part of Jeremy Tatum’s fireball interviewing network on Canada’s west coast.156JRASC June/juin 1998

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