Abstract.
A grating spectrum (45 A/mm) of a fireball of -9th
absolute magnitude and velocity of 18 km/s has been analyzed.
A new simple model for meteor spectra has been developed. Thermal
equilibrium is assumed and self-absorption is taken into account. The
free parameters of the model are the temperature, the column density
of Fe I atoms, the relative abundances of other atoms, and the
visible surface area of meteor radiating volume. The synthetic spectrum
based on this model was computed and compared with the observed spectrum.
The values of the free parameters were calculated by the least squares method.
Then the abundances of neutral atoms were corrected for ionization to obtain
the true ratios of chemical elements. The abundances of Fe, Na, Mg,
Ca, Ti, Cr, Mn, Ni, and Al were determined such way. The computations
were performed independently at 43 points along the fireball trajectory
between heights of 57-35 km.
It was found that thermal equilibrium was relatively well satisfied
at the heights below 50 km. The temperature varied in the range
3500 - 4700 K. But there is also a spectral component with temperature
of about 10,000 K in the spectrum. This component consists of a few
lines of Mg II, Si II and Fe II and originates
probably in the shock wave.
The chemical composition of the radiating gas varied along the fireball
path and does not reflect the chemical composition of the meteoroid itself.
The refractory elements (Al, Ca, Ti) are underabundant in the gas.
The material was ablated by melting in liquid phase and then evaporated
in surrounding hot gas, but the refractory (low melting) elements were
evaporated incompletely or too late. About 95% of the hot gas around
the meteoroid were formed by the air.
Key words: meteors: spectra - ablation - chemical composition
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