Spectroscopic study of the variable planetary nebula IC 4997 |
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Authors: | M A Burlak V F Esipov |
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Institution: | (1) 4 Bedford Place, Scotforth, Lancaster, UK |
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Abstract: | Based on our spectroscopic observations of the variable planetary nebula IC 4997 in 2003–2009, we have obtained the relative
fluxes in optical emission lines. The interstellar extinction c = 0.35 has been found from the Balmer decrement by taking into account the effect of self-absorption in hydrogen lines in
dense nebular regions. The variations in the Balmer decrement point to variability of the self-absorption and circumstellar
extinction. We have investigated the variations in the relative intensities of some spectral lines and their ratios with time.
The drop in the ratios F(λ4363)/F(Hγ) and F(λ363)/F(λ4959) that began back in 1990–1995 has continued, suggesting a decrease in the electron density and temperature in the central
nebular region. The ratio F(λ6731)/F(λ6717) has remained constant. It gives an estimate for the electron density in the outer regions of IC 4997, N
e
∼ 104 cm−3. |
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