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

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1. Introduction

One of the brightest and well documented bolides detected by the European Network is the Beneov bolide EN070591 (Borovicka & Spurný 1996). This bolide proved very useful for the comparison of observational data with the recently created theoretical model of a large meteoroid entry into the atmosphere (Nemtchinov et al. 1994; Golub' et al. 1996a). In Borovicka et al. (1998, hereafter Paper I), the bolide dynamics, fragmentation and integral radiation has been analyzed. In this paper we will compare the observed and modeled spectra of bolide radiation. The main goal of the study is the assessment of the Beneov bolide characteristics using the above mentioned theory and checking the validity of the theory.

The main result of Paper I was to conclude that several stage fragmentation of the meteoroid occurred along the bolide trajectory. The bolide had already been significantly decelerated at altitudes between 50-40 km, while enormous luminosity was produced below 40 km. This could be explained by an assumption that the meteoroid was already fragmented into 10-30 pieces each with mass 100-300 kg at altitudes 60-50 km. Some of the primary fragments were fragmented again at altitudes 38-31 km, while more compact fragments were disrupted at 24 km. There are direct observational evidences of the fragmentations below 38 km. The products of the disruption at 24 km were photographed down the altitude of 17 km, where they ceased to be visible. It is probable that several fragments with a mass of the order of few kg reached the ground as meteorites. They were, however, not recovered. The initial mass of the Beneov meteoroid was estimated to 3000-4000 kg and the density to 2 g cm-3. The initial velocity was 21 km s-1 (Borovicka et al. 1998).

As described in Paper I, two models of meteoroid fragmentation have been proposed. They differ in whether the fragmented body is still taken as a single body with increasing radius after the break-up or the produced fragments are considered as individual bodies. The former model is called a liquid-like or "pancake" model and assumes that all fragments are embedded in a common shock wave with common deceleration and radiation. By fitting the observed deceleration and the light curve of the Benesov bolide it was found that while some fragments undoubtedly escaped from the common motion, large number of pieces formed at 24 km, and also some formed at 38-31 km, justified the liquid-like description.

The study of the bolide spectra can provide further insight into the process of meteoroid ablation, its interaction with the atmosphere and the production of radiation. The theoretical radiative-hydrodynamic model of Golub' et al. (1996a, 1997) is based on the analogy between one-dimensional non-stationary motion of a cylindrical piston in the air and the two-dimensional quasi-stationary flow around the body. The mass losses are dominated by vaporization due to thermal radiation falling onto the surface of the meteoroid. The model predicts that the spectrum of a large or (and) deeply penetrating body is of a continuum type with superimposed spectral lines.

The observed spectra led Borovicka (1993) to propose another model of meteor ablation and radiation. It was assumed that in the relatively dense layers of the atmosphere the surface of a meteoroid is melting, the temperature of the surface being about 2000 K. The liquid, formed by the well melting siderophile elements, is being separated from the body and carries away also small solid particles of the low melting crust formed by the refractory elements. The material is further heated by the surrounding gas of about 4000 K, formed mainly by the hot air and the evaporated material. The atoms of the refractory elements evaporate incompletely or too late, so they are underabundant in the region of the hot gas relatively to the siderophile elements. This may explain the changes in the composition of the emitting region with the altitude. The high temperature (about 10,000 K) component of the spectrum probably originates in the mixing region of the shock heated air and vapor. This region is relatively thin and this may explain its small role in the flux in the observational passband.

Borovicka (1993) calculated the spectrum of the emerging radiation under simplified assumptions. His model was based on the observations of not very large meteoroids, e.g. EN 151068 cechtice with the size of about 10 cm. Air has not been taken into account, only the vapor lines were considered and a uniform temperature distribution in the emitting volume was assumed. Nevertheless, the observed spectrum could be fitted quite well after subtracting the continuous radiation. The size of the volume was found much larger than the meteoroid size, but no theoretical estimates of the volume shape have been done.

The observed spectrum of the Beneov bolide offers a unique opportunity to compare the spectrum of an actual meter sized meteoroid with the theory. The spectrum has already been described and analyzed at selected points by Borovicka & Spurný (1996). A multitude of atomic spectral lines dominates the spectrum but a relatively strong continuum is also present at lower altitudes. The absolute majority of atomic lines belongs to the meteoric vapor heated to about 5000 K. The origin of the continuous radiation is less clear. Borovicka & Spurný (1996) concluded that the continuum is produced by the same 5000 K vapor. They criticized the air-radiation dominated model of Nemtchinov et al. (1994, 1995), where the shocked air is considered as the primary source of bolide luminosity. However, the addition of the vapor opacity into the model showed that the model predicts the line radiation in qualitative agreement with the observations (Golub' et al. 1996b, 1997). Detailed analysis of the spectra is the goal of this paper.

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© European Southern Observatory (ESO) 1998

Online publication: August 17, 1998