Astronomy and Astrophysics – Astronomy
Scientific paper
Jun 1999
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999astl...25..389k&link_type=abstract
Astronomy Letters, Volume 25, Issue 6, June 1999, pp.389-397
Astronomy and Astrophysics
Astronomy
2
Scientific paper
Crimean photographic observations in 1972-1996 are used to study the spectrum of the planetary nebula IC 4997. The flux from the nebula in eleven bright emission lines and in the continuum of the central star was estimated. Most spectral characteristics were found to change with time. The ratio R of the [O III] lambda 4363 and H_gamma fluxes exhibited the greatest change over 1972-1992; this quantity reached a maximum (lg R = +0.28) in 1992. An increase in the degree of excitation and in the degree of ionization of the nebula was detected. The change in the main physical parameters of the object over this period was estimated: the nebula's total electron density increased from N_e = (2-6) x 10^5 to N_e = (1-3) x 10^6 cm^{-3}, and the total electron temperature rose from T_e = 12 000 to T_e = 14 000 K. The set of spectroscopic data suggests that the effective temperature of the central star rose from T* = 40 000 to T* >= 60 000 K. The growth of the nebula's spectroscopic activity, which began in the late 1960s, appears to have stopped in the early 1990s and has given way to a lowering of the activity. Further monitoring of the object is required.
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