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Astron. Astrophys. 337, 591-602 (1998)

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3. The observed spectrum

The observed spectrum of the Beneov bolide was described by Borovicka & Spurný (1996). Here we repeat the basic facts with the emphasis on the instrumental effects which are important for a comparison with the theoretical spectra.

The spectrum was photographed on two plates and covers the trajectory of the bolide from an altitude of 90 km to 19 km. The wavelength range is given by the sensitivity of panchromatic material, i.e. 3600-6750 Å. The dispersion is 50-70 Å mm-1 in the first order spectra. Unfortunately, the blue part of the spectrum was not well focused. At some altitudes the blurred section extends up to 4500 Å. The use of the second order spectra can sometimes make the situation better.

Even in the well focused parts, the spectral resolution is limited. Individual emission lines exhibit an instrumental profile with the width much larger than the natural width of the lines. Apart from the limited resolution of the spectrograph, the reason may be the detection of the true geometrical extent of the radiating volume. The instrumental profiles were well described by a Gaussian function with a width of 3-5 Å.

The measured spectra were corrected to the spectral sensitivity of the spectrograph. However, absolute calibration was not possible from the spectral records alone. We used a direct image of the bolide taken at another station to determine the bolide absolute magnitude as a function of altitude. The spectrum was then scaled to obtain the luminosity in ergs s-1 ster-1 Å-1 consistent with the derived magnitude.

The spectrum consists of atomic emission lines, molecular bands and continuum radiation. The following species have been identified: Fe I , Na I , Mg I , Al I , Ca I , Ca II , Ti I , Cr I , Mn I , Ni I , Li I , N II , Si I , Si II , Co I , MgO, AlO, CaO, and FeO. The line spectrum was analyzed at selected altitudes in Borovicka & Spurný (1996) by the method of Borovicka (1993). It was concluded that the majority of the lines and the continuum can be fitted by a model containing the radiation of meteoritic vapors heated to 4000-5000 K. Some additional effects were also found. They are briefly listed here:

  • The abundance of calcium was found to be very low in the upper part of the trajectory. This was ascribed to the effect of incomplete evaporation.

  • Very strong radiation from the bolide wake is present at altitudes of 70-55 km. The wake spectrum cannot be described in terms of thermal equilibrium. Low excitation intercombination lines are very strong.

  • Relatively weak lines of ionized silicon are present at altitudes of 75-40 km. These lines exhibit short flares and can be ascribed to the high temperature (10,000 K) spectral component. Surprisingly, at altitudes of 65-57 km, the Si II flares are accompanied by flares of ionized nitrogen lines which cannot be explained even by the high temperature component.

  • A persistent radiating cloud was detected near the position of the bolide brightest flare at 24 km. The spectrum of the cloud consists almost exclusively of a continuum and molecular bands.

  • Thermal continuum from the meteoroid surface was registered at the beginning of the event.

In the next section we will compare the observed spectra at altitudes 63.5 km, 40 km and 24.5 km with the theoretical spectra.

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© European Southern Observatory (ESO) 1998

Online publication: August 17, 1998