Astronomy and Astrophysics – Astrophysics
Scientific paper
Apr 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994a%26a...284..465v&link_type=abstract
Astronomy and Astrophysics (ISSN 0004-6361), vol. 284, no. 2, p. 465-476
Astronomy and Astrophysics
Astrophysics
13
Astronomical Photometry, Asymptotic Giant Branch Stars, Planetary Nebulae, Stellar Evolution, Stellar Oscillations, Variable Stars, Brightness Distribution, Stellar Color, Stellar Envelopes, Stellar Mass
Scientific paper
We present and discuss VBLUW photometry of the variable star FG Sge, made between 1987 and 1991, which is the central star of a planetary nebula. The pulsational history, the long-term brightness and color variations and the spectral evolution (B5 I-K1 I) of the star during its passage across the HR diagram during the interval 1955-1992 are analyzed. The slope and timescale of this passage are roughly similar to the theoretical track for a low mass star after its final He shell flash. It appears that there exists an inconsistency between the trend of the temperature based on most of the spectral type determinations and other temperature determinations. Adopting the temperature scale based on the spectra and a distance of 2500 pc, the star crossed the population I Cepheid strip between 1972 and 1983, but the period of pulsation of FG Sge was about five times longer than that of Cepheids of the same luminosity. A comparison with recent calculations on the characteristics of post-AGB stars, suggests a possible mass of approximately 0.65 solar mass. The phase of the steep decline by approximately 3.5 m in the visual brightness can be interpreted by a combination of a short evolution to the blue and an obscuration by a circumstellar dust cloud.
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