The Remarkable Object FG Sagittae
FG Sagittae
RA: 20h 11m 56.1 sec m = +10.0, central star, m = +16.3 planetary
DEC: +20° 20' 4" Type = variable star/planetary nebulae
Distance = 2,500 pc
The supergiant star FG Sge, is the nucleus of the faint planetary nebula PK 60-7°1. On first glance, FG Sge looked rather ordinary. It was recognized as a variable star only recently in 1943, even though it was first photographed in 1894. In 1894 the star was at photographic magnitude +13.6, in 1950 it reached +11, and by 1960 it has surpassed +10. FG Sge continued to brighten and peaked in 1968 around m = +9.5. Between August 1992 and September 1992 a dramatic decline from m= +9.3 to +12.87 was observed. Within these large increases in brightness were small irregular variations. As it turns out, FG Sge has been in constant evolution since the beginning of this century.
The light output of FG Sge was strange enough, and in 1960 Lick Observatory astronomer George Herbig (of Herbig-Haro objects) found that the photographic image of FG Sge was fuzzy. More powerful telescopes revealed a planetary nebula 36 arc seconds in diameter. (Astronomer Karl Heinze, who later finished his career at NASA’s Johnson Space Center in Houston, independently discovered the planetary in 1955). Later measures showed the nebula to be expanding at 34 km/sec, fairly typical for planetaries, thus the distance of FG Sge was computed as 2500 pc, and a diameter of 1.4 light years for the planetary, implying an ejection age of 6,000 years. Since its discovery in 1955, at the current 34 km/sec expansion rate, the nebula has expanded by 325 AU's, or 0.056", too small to be measured directly by ground based telescopes. A Digitized Sky Survey image of FG Sge appears as Figure 1.
More puzzles plagued astronomers as they began to measure the spectrum of FG Sge. In 1955 spectrograms showed FG Sge to be spectral type B4. By 1960 FG Sge had advanced to B9, and in 1967 it had reached A5 !! In 1972 it was spectral type F6, by 1980 it had reached G9 and by 1992 it had reached K2. These rapid changing spectral types indicated that FG Sge's temperature was plunging - falling by nearly 3 OO'K per year !! The star appeared to be expanding enormously - from 10 times the radius of the Sun in 1958 to over 55 solar radii in 1973. FG Sge changed from a hot blue star to a moderate temperature yellow star in just 20 years. Continued observationsin the 1970’s showed FG Sge’s luminosity to be in the same phase with its radius variations. Many astronomers feel that this is when FG Sge crossed over into becoming a Cepheid pulsating variable star.
A history of FG Sagittae's changing properties is given below:
Year Spectral Type Temp. Mag.
1894 13.4
1955 B4 10,000 11.0
1960 B9 9,700 10.0
1965 A3 8,700 10.0
1966 A3 to A4 8,500
1967 A5 8,300
1968 A3 8,700
1969 F0 7,500
1970 F2 7,200
1970 F3 to F6 6,500 9.4
1971 F4 6,300
1972 F6 6,300
1974 G0 5,800
1975 G2 5,500
1977 G3 to G4 5,300
1979 G8 to G9 5,000
1980 G8 to K0 5,000
1992 K2 4,250 12.8
Figure 1. Digitized Sky Survey image of FG Sagittae and the surrounding planetary nebula. Photographed with the 48" Schmidt telescope on September 21, 1951.
Actually, FG Sge's total luminosity at all wavelengths has remained about the same over the time interval, but the wavelength where the radiation is most intense has increased.
In the 1974, s-process elements in large abundances were discovered in Fg Sge's atmosphere. These s-process (slow process) elements are believed to be formed when free neutrons are available for capture by an atomic nucleus. The process is slow as calculations show that it could be 100 years before two successive neutrons are captured by an atomic nucleus to form a new heavier element inside red giant stars. On the other hand, the reprocess (rapid) occurs when the neutron flux is very high and many neutrons are eventually captured by atomic nuclei and element formation is rapid, since millions of neutrons can bombard atomic nuclei every second !! Astronomers believe this reprocess is what happens in supernova explosions.
In order to figure out what's happening with FG Sge, it was modeled by the famed astrophysicist Icko lben, Jr. and Mariao Livio in 1992. They concluded that:
1. FG Sge underwent a helium flash a few centuries in the past, ejecting a shell.
2. It became a hot star after it ceased hydrogen burning, now burning helium at the core.
3. Next, hydrogen is ignited in the outer layers of the star.
4. As the star rapidly evolves to the red end of the spectrum, hydrogen burning mixed with helium burning causes the s-process elements to appear near the surface.
5. Ejection of a shell caused deeper layers of the star to be observed making it to appear fainter-in the visual and appear bluer. This makes an increase in the ultraviolet (UV) flux.
Then predicted as the increased UV flux moves outward, it should "lite up" the shell, perhaps on the order of 3 - 10 years. Helium burning would continue for about 10,000 years and FG Sge should then evolve into a white dwarf. An alternate explanation for the rapid decline in FG Sge would be the formation of dust in an ejected shell.
In 1992, observations of Fg Sge just before its rapid 3 mag brightness decrease indicated 2 expanding circumstellar shells. The shells are 2 and 11 R¤ from the star and are expanding at 42 km/sec and 21 km/sec respectively. The cause of the shell outbursts can be attributed to regular pulsations in Fg Sge's radius which began in the early 1970's. Estimates of Fg Sge's mass from these observations came to 0.4 M¤ to 0.8 M¤, in accord with the notion that FG Sge is the central star of a planetary nebula and has already undergone substantial mass loss. Hinkle, Joyce and Smith in their 1992 Astrophysical Journal paper suggest that this renewed mass loss appears to be part of the transformation of FG Sge into a R CrB type star, i.e., a star that begins carbon burning in the atmosphere resulting in a "carbon-soot" ejection. (More about this interesting kind of star in a future RUT).
The observations of FG Sge may also be explained by multiple helium shellflashes. In this scenario proposed 20 years ago, a first helium shell flash ejected the planetary nebula we see today 6,000 years ago. In 1894, the star underwent another helium shell flash, one that started the cooling trend in the star's atmosphere. This flash triggered the s-process (heavier) elements now seen in the spectral lines. With a 6,000 year interflash period, this would imply a core mass of about 70 to 80% M¤ . Based upon the estimated mass of the star's core, it should take about 100 years for the surface temperature of FG Sge to return to that of a "normal" planetary central star.
In July 1994 there was another surprise: a rapid brightening observed was attributed to a new outburst of material. Analysis of absorption line profiles in its spectrum showed mass loss rates of about 3 x 107 M¤ /yr. It could be that multiple ejections take place before a rapidly evolving star such as FG Sge calms down.
Finder Chart from MEGASTAR for FG Sagittae. It is very near the Dumbell Nebula M27 and the globular cluster M71.
REFERENCES
Croswell, K., 1983, FG Sagittae: One Piece of the Puzzle, Astronomy, October 1983 p 74.
Hinkle, K., Joyce, R., 1995, Circumstellar CO in FG Sagittae, Astronomical Journal, 109, p. 808-811.
Iben, I., Livio, M., 1993, A Model for the Dramatic Decline of FG Sagittae, Astrophysical Journal, 406, L15-L16.
Kipper, T., Kipper, M., Klochkova, V., 1995, The Spectrum of FG Sge in 1994, Astronomy and Astrophysics, 297, L33-L36.
Mayor, M., Acker, A., 1980, Radial Velocity Curve, and Radius of the Pulsating Star FG Sge, Astronomy and Astrophysics, 92, p. 1-5.
Van Genderen, A., Gautschy, A., 1995, Deductions from the Reconstructed Evolutionary and Pulsational History of FG Sagittae, Astronomy snd Astrophysics, 294, p. 453-468.
Van Genderen, A., 1994, On the Pre-Decline and Decline Phases of FG Sagittae, the Post-AGB Star and Central Star of the Planetary Nebula PK 60 -7°.1, Astronomy and Astrophysics, 284, p. 465-476.