Stellar  Rings

In 1968 a new form of stellar aggregation was discovered by the West German astronomer Theodore Schmidt-Kaler at Ruhr University, Bochum. While studying the dark clouds of the Milky Way galaxy, Dr. Schmidt-Kaler became aware of large numbers of stellar rings, which appear on photographs as regular elliptical groups of stars that are sharply bounded on the outside. He found that typical rings contain between 25 and 200 stars with an average of 70 stars) and that the stellar density averages four times higher than the surrounding field.

 A complete inspection of the Palomar Observatory Sky Survey (POSS) yielded over 1,000 stellar rings, and three more were discovered on the plates of the Lick Observatory Sky Atlas. Statistical tests show that 80 to 90% of the rings are real, and only 10 - 20% of the rings are accidental groupings of stars. 

Accurate distances within individual rings could only be determined in  few cases since they contained stars of known absolute and apparent magnitudes. The diameters of the four rings turned out to be very similar, roughly 7.2 pc (23 light years) across, a value supported by determinations in nine other cases. This lead to the hypothesis that  all rings have nearly equal minor diameters.

                                             Figure 1. Stellar Ring No. 373.

 Figure 1 shows the Stellar ring No. 373. from Schmidt-Kaler and J. Isserstedt's   1970 paper, in which they performed photoelectric photometry on the stars of this ring and of two adjacent fields and concluded that the distance modulus was m -  M = 12.2, giving a distance of 2.78 kpc (9,000 light years).  Using this distance the minor diameter of the ring was 7.0 pc which was in good agreement with earlier work. They also found an Ultra-Violet (UV) deficit among the brighter ring members of unexplained origin.

 Schmidt-Kaler has pointed out in an earlier paper that star formation continues to occur in certain gaseous nebulae. In NGC 6888 (Figure 2) he says we are apparently observing the evolution from a gaseous shell to a stellar ring.  In 1968 he found such stellar rings in a later stage of their evolution without correlated H II regions. Many of the rings show an elongated shape similar to the current shape of NGC 6888.

 NGC 6888 is listed as a wind blown expanding nebula in K. Lang’s  Astrophysical Data: Stars and Planets.  Its mass is estimated at 5 M_> , with an expansion velocity of 75 km/sec. This data gives a presumed age of about 240,000 years and a minor diameter of 6.3 pc.  The nebula is surrounding the Wolf-Rayet star HD 192163 which may have been the driving force for the expansion. Wolf-Rayet stars are thought to be massive stars in the advanced stages of hydrogen burning. These stars are also believed to have hydrogen rich envelopes through mass transfer. In the case of NGC 6888, its expanding shell could have been the result of an ejected shell from HD 192163 as part of its later evolutionary stage. Normally one would think that NGC 6888 is a planetary nebula, it could be a very late stage of a special type planetary. Schmidt-Kaler and his colleague I. Isserstedt postulate that the stellar rings formed out of the expanding shells of such stars.  This they suggested after they found that the chemical composition of the stellar rings was very similar to the nebula NGC 6888.

                      Figure 2. NGC 6888 in Cygnus.  Size: 18’ x 12’, distance = 1.2 kpc.

If  the expanding NGC 6888 begins to forms stars, possibly the ring might stabilize at a diameter of around 7.1 pc from its current 6.3 pc diameter. Interesting idea, right?

Archinal 2003 has stated that there appears to be no direct relation of Isserstedt's minimum diameter of rings vs the actual size of several clusters in which the rings reside.

No papers on Stellar Rings appeared after 1970 as the theory of their existence was debunked by several astronomers. The rings at best were asterisms in the plane of the sky that appeared "ringlike" using enough imagination. 

REFERENCES

Archinal, B., Hynes, S., 2003, Star Clusters, Willmann-Bell, Inc., Richmond, VA, ISBN 0-943396-80-8.

Isserstedt, J. Schmidt-Kaler, T., 1970, Photometrische Untersuchung des Sternrings 373, Astronomy & Astrophysics, 7, p. 481-487.

Isserstedt, J., 1969, Lichtelektrische Untersuchchungen eines nahen Sternrings in Aquila, Astronomy & Astrophysics, 3, p. 210-213.

Isserstedt, J., 1970, P. Cygni und der Sternting 274, Astronomy & Astrophysics, 8, p. 168-170. 

Isserstedt, J., 1970, 53. Stellar Rings and Galactic Structure, The Spiral Structure of Our Galaxy, International Astronomical Union Series, p. 287-289.