ACTA ASTRONOMICA Vol. 43 (1993) pp. 353±357
FG Sagittae as a New R Coronae Borealis Variable
by Johanna J u r c s i k
Konkoly Observatory, Hungarian Academy of Sciences P.O. Box 67, H-1525 Budapest XII, Hungary
ABSTRACT
The 1992±93 dimming of FG Sagittae can be interpreted as a blue type R CrB decline. This favors the ®nal ¯ash scenario of the R CrB behavior and clears up the evolutionary connection between the planetary nebula nuclei and the R CrB phenomenon.
The observation of direct evidence of stellar evolution has severe dif®culties as its timescale is many orders of magnitude larger than modern astronomy exists. Therefore to observe evolution connected changes of one single star twice during one century does not seem to be probable but this is the case for FG Sagittae. The central star of the planetary nebula He1±5 evolved across the HR diagram from the end of the last century. Its unique behavior was interpreted as a consequence of a ®nal helium shell ¯ash of the pre-white-dwarf nucleus (as a review see Montesinos et al. 1990). Besides the secular changes of the spectral type (from B to K) drastic abundance anomaly of the surface chemical composition has also occurred in the early seventies (Langer, Kraft, and Anderson 1974). The s-process elements enhanced 25 times the solar value, while the abundances of the Fe peak elements remained normal ± the rapid expansion of the star to supergiant dimension was followed by a mixing episode of the freshly processed material. During the last decade signi®cant changes of the physical parameters of FG Sagittaewere notreported, thestarseemedto reachthe endof its redward evolution, there were even hints that evolution towards the opposite direction started. How- ever, the interpretation of the observations in the case of a very diluted atmosphere of a supergiant with severe and anomalous line blanketing effects, has dif®culties and leads to contradicting results. From the 1960's FG Sge is known as a pulsating variable too. The period of the small amplitude variation has increased, in accordance with the continuous 354 A. A. expansionofthestar. Fig1showsthelightcurvecollectedfrom all the photoelectric
observationsof FG Sge. The mean brightness still decreased in the80's. The period
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Fig. 1. The light curve of FG Sagittae between 1968 and 1992 collected from all the published photometric observations (the references are listed in Jurcsik and Szabados 1989, Arkhipova 1988, and Arkhipova, Zaitseva and Noskova 1988, 1991). The period increased from about 60 days (in the 1970's) to about 160 days (1989±90), but onwards both the amplitude and the period dropped. Note the continuous decrease of the mean brightness. The drop in 1992 is the beginning of the 4 mag decline. in1985was110dayswhereasin1989±90it wasaslongas160days. This indicates that the dilution of the atmosphere has not stopped yet and the opposite behavior observed spectroscopically is an effect caused by the different optical depths seen in different wavelengths. From August 1992 FG Sge undergoes unexpected rapid changes again as the 5 color photometry obtained with the 1 m telescope of the Piszkestet oÈ Mountain
Station of Konkoly Observatory shows (Fig. 2). The star dimmed more than 4
B magnitudes in 40 days while the colors became bluer (the U decreased more than 1 magnitude). This was neither the result of the contamination of the 12.4 magnitude companion star (as suggested by Iben and Livio 1993) which is a highly reddened G star and was excluded from the diaphragm at each occasion, nor of the
nebular emission (suggested by Stone, Kraft and Prosser 1993) as no differences 00 larger than 0.2 mag were obtained using different (10 00 , 15 ) size apertures even at the faintest state of the star. The ®rst impression that the backward evolution of FG Sge to its pre-¯ash state shortened so dramatically (Jurcsik 1992, Iben and Livio 1993), could not be held any longer. IUE observations showed the ultraviolet ¯ux to decrease as
Vol. 43 355
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Fig. 2. The light and color curves of FG Sge in 1992±93 (obtained with the 1 meter telescope of Konkoly Observatory) represent a blue type R CrB decline (see e.g. the 1988 decline of R CrB, Cottrell, Lawson and Buchhorn 1990). On the basis of observations obtained with different size
apertures the correction for the contribution of the nebular emission does not exceed 0.3 mag in
B U which is much smaller than the observed 1 mag blueing. Open circles represent the CCD photometry of Woodward et al. (1993). 356 A. A. well, contradicting the blueward evolution of the star (Gonzalez-Riestra, Jùrgensen
and Grunnet 1992). The infrared colors changed signi®cantly between 1989 and
H K 1992. Prior to 1989 the observed J magnitudes (Arkhipova and Taranova 1990, Montesinos et al. 1990, Woodward et al. 1993) corresponded to those of the stellar continuum with slight anomalies (the infrared excess reported by Arkhipova and Taranova does not seem to be real) but according to observations obtained between1992Septemberand 1993January, an infrared excess of 1000K blackbody radiation appeared (Woodward et al. 1993). Optical spectroscopyof FG Sge during the decline does not show any evidence of changes in the stellar spectra, only the emission lines of the surrounding nebula became more prominent as the stellar ¯ux dimmed (Stone, Kraft and Prosser 1993). The only plausible explanation remained that we are witnessing the obscuration of the star by a dusty cloud, i.e. an R CrB type decline. It seems plausible to connect the appearance of the IR excess with the change of the pulsational properties between 1990 and 1991 reported by Jurcsik (1992). The smaller amplitude and shorter period of the light variation occurred ± which matches the corresponding properties of other R CrB variables (see e.g. Weiss 1987) ± dating the appearance of the R CrB dust ejection episodes about two years before. The mass ejection is known to happen to R CrB stars in different directions with limited solid angle in every pulsational cycle and some puffs are enough to explain the observed IR excess (Feast 1986). Therefore, it is suggested that prior to 1991 some changes in the envelope structure and/or a new mixing episode should have taken place. Resulting changes in the pulsation of the star have favored directed mass ejections. Between 1980 and 1990 the continuous evolution of the
colors slowed down but after 1990 the color changes were considerable again
U B ) = (B V ) = ( ( 0 25 mag, 0 15 mag) supporting this idea, too. High resolution spectroscopy obtained only 2 weeks before the decline (Kipper and Kipper 1993) de®nitely shows the appearance of the C2 Swan bands whose existence was argued before. The chemical composition of the R CrB variables shows wide variety, even their extreme H de®ciency cannot be hold after the ®nding of considerable amount of H in the case of V854 Cen. Signi®cant s- process enhancement in the case of U Aqr was also reported, therefore the surface abundance of FG Sge is not in contradiction with the description its present state as an R CrB variable. Two different hypotheses exist about the evolutionary state of R CrB stars: the ®nal helium ¯ash of a single, post-AGB star or the merger of two white dwarfs (see e.g. Renzini 1990). Observations which can be interpreted in the frame of the connection of planetary nebula nuclei and R CrB type variables (e.g. the knots of A30, A78, the nebula around V348 Sgr, and the peculiar outburst of V605 Aqr supposed to be the central star of A58) seemed to favor the ®rst one. The 1992-93 R CrB decline of FG Sge, a central star of a PN which just undergoes a He shell ¯ash, strongly supports the ®nal ¯ash scenario of the R CrB variables. Vol. 43 357
Acknowledgements. This work has been supported by the Hungarian OTKA grant No. T7438.
REFERENCES
Arkhipova, V.P. 1988, Perem. Zvezdy, 22, 631. Arkhipova, V.P., and Taranova, O.G. 1990, Pis'ma Astron. Zh., 16, No.9, 808. Arkhipova, V.P., Zajtseva, G.V., and Noskova, R.I. 1988, Astr. Tsirk., No.1526. Arkhipova, V.P., Zajtseva, G.V., and Noskova, R.I. 1991, Astr. Tsirk., No.1547. Cottrell, P.L., Lawson, W.A., and Buchhorn, M. 1990, MNRAS, 244, 149. Feast, M.W. 1986, Hydrogen De®cient Stars and Related Objects, eds. K. Hunger et al. (ASSL Vol. 128), p. 151. GonzÂalez-Riestra, R., Jùrgensen, H.E., and Grunnet, C. 1992, IAU Circ., No.5610. Iben, I.Jr., and Livio M. 1993, Astrophys. J. Letters, 406, L15. Jurcsik, J. 1992, IBVS, No.3775. Jurcsik, J., and Szabados, L. 1979, IBVS, No.1722. Jurcsik, J., and Szabados, L. 1989, Astrophys. Space Sci., 153, 45. Kipper, T.A., and Kipper, M.A. 1993, Astron. Astrophys., in press. Langer, G.E., Kraft, R.P., and Anderson, K.S. 1974, Astrophys. J., 189, 509. Montesinos, B., Cassatella, A., GonzÂalez-Riestra, R., FernÂandez-Castro, T., Eiroa, C., and JimÂenez- Fuensalida, J. 1990, Astrophys. J., 363, 245. Renzini, A. 1990, Confrontation between Stellar Pulsation and Evolution, eds. C. Cacciari, G. Clementini (ASPCS Vol. 11), p. 549. Stone, R.P.S., Kraft, R.P., and Prosser, C.F. 1993, P.A.S.P., 105, 755. Taranova, O.G. 1986, Astro®zika, 25, 453. Weiss, A. 1987, Astron. Astrophys., 185, 178. Woodward, C.E., Lawrence, G.F., Gehrz, R.D., Jones, T.J., Kobulnicky, H.A., Cole, J., Hodge, T., and Thorson, H.A.Jr. 1993, Astrophys. J. Letters, 408, L37.