RU Cam - An ex-Cepheid Variable Star?

RU Cam

RA = 7h  21m 41.9sec      Size: 2.5'         Distance »  140 pc ?

DEC = +69° 40' 21'',         m = +8.4         

In 1907, M. Ceraski noted the variability of RU Cam from plates taken during the years 1899-1906. He measured the magnitude range from m = +8.0 to +9.1 with a 22.27^d period. RU Cam was later classified as a Cepheid variable.

Cepheids are known for their precise variability which can be measured to a fraction of a second. The cause of the variability lies deep within the structure of the star. In the inner heated layers is a valve-type mechanism. When the valve is open, radiation passes through easily and the star shrinks. The valve then closes, traps radiation which pushes the star apart and makes it expand. The valve mechanism is the ionization of helium and hydrogen in the star.

This pulsation mechanism has been used to determine the absolute brightness of the star. In smaller Cepheids, the radiation wave that moves from the star’s core to the surface moves in a shorter time scale than a larger star, since the larger star has a greater distance for the wave to travel.  The larger stars have longer periods, and the smaller stars have shorter periods. Since larger stars have more surface area and radiate more light, they are brighter. Hence the Period-Luminosity relation was discovered by Miss Henrietta Leavitt in 1912 by examining the Cepheids in the Large Magellanic Cloud.  Harlow Shapley was able to calibrate this relationship and Edwin Hubble used it to determine distances to nearby galaxies.

 In 1928, Roscoe Sanford had discovered a radial velocity and spectrum variability in RU Cam. He found that at maximum light the spectrum was K0, and at minimum light it was R0. He concluded that RU Cam’s rotational axis was pointed toward Earth, or it might be even be a binary star system.

Further investigations of RU Cam by S. Tscherny using the binary star hypothesis in 1942 gave two binary star solutions: One with a  total mass of 43 M_¤ with some giant stars, and a 116 AU diameter orbit, and a second solution with a pair of dwarf stars totaling 0.7 M_¤ with an 18.5 AU  diameter orbit. RU Cam’s original identity as a Cepheid became clear in the later 1940’s as the two types of Cepheid variables were discovered.  

In 1966 a remarkable thing happened. The pulsation of RU Cam began to slow down and eventually ceased over a 4 year  time scale. The Astrophysical Journal article that reported this was titled, “RU Cam - A Cepheid Variable which has Stopped Pulsating.”  No one at the time knew for sure what happened to this star and why it stopped pulsating. Astrophysical theory tells us that even if we stopped the pulsation mechanism, it would take between 1,000-10,000 years for the pulsations to stop !!  But in the case of RU Cam, they declined in less than 4 years !!!  The light curves are shown in Figure 1.

Figure 1.  Light curves of RU Cam 1962-1965. The amplitude went from 1 mag in 1962 to less than 0.1 mag in 1965.

Writing in that famous 1966 paper, astronomers Serge Demers and J. D. Fernie even said they were convinced they had not “misidentified the star.”  They also stated that they had drew up accurate finder charts and compared these charts with older charts of the same region, that their instruments were working perfectly, and their instrumental error did not exceed 0.02 mag.

What caused RU Cam to stop pulsating? Robert Burnham in his celestial handbook series, suggested that the star may have undergone some sort of a “random accident.” Burhnam never gave a clue as to what could have possibly happened. In 1998, John Percy (The Observer’s Handbook Editor) and J. Hale found evidence for an evolutionary period changes in the star. They postulated thet RU Cam had a number of smaller multi-periodicity cycles superimposed on each other. And at the same time, the star had undergone a rapid evolutionary movement on the “loops” of the H-R diagram from the Giant Brach. The superimposing of two periods gave the impression of a single 22 day period, and as the star rapidly evolved out of the giant branch, the two periods merged canceling each other out. This can be visualized by two bathtub waves crashing into one another.

RU Cam’s is a circumpolar star from the Houston, Texas area and is easily visible in small telescopes. A finder chart for it appears as Figure 2.

                                     Figure 2. Finder chart for RU Cam.

REFERENCES

Berdnikov, L., Voziakova, O.,  1995, Commisions 27 and 42 of the IAU, Information Bulletin on Variable Stars, No. 4154, 

Burnham, R., 1978, Burnhams Celestial Handbook, Dover Publications, p. 328

Demers, S., Fernie, J., 1966, RU CAM: A Cepheid Which has Stopped Pulsating?, Astrophysical Journal, 144, p. 440.

Hack, M., 1967, Sky and Telescope, p. 350, June 1967

Percy, J., Hale, J., 1998, Period Changes, Evolution, and Multiperiodicity in the Peculiar Population II Cepheid RU Camelopardalis,  Publications of the Astronomical Society of the Pacific, 110, p. 1428.

Sanford, R., 1928, The Spectrum and  Radial-Velocity Variation of RU Cameloparidalis, Astrophysical Journal, 68, p. 408.

Tscherny, S., 1942, The Nature of RU  Cameloparidalis, Astronomical Journal, 49, p. 147.