LARGE-SCALE MOTIONS OF IONIZED CALCIUM IN THE STELLAR ATMOSPHERE *)

The authors of the paper under review have recently succeeded in spectroscopically detecting large-scale motions of ionized calcium in the atmosphere of one of the stars classified as a cool giant. The object of observation was the star 31 Cygni (of the fourth magnitude). This star is a spectroscopic binary and consists of the aforementioned giant (belonging to class $K$), whose diameter is approximately 150 times greater than the diameter of the Sun, and a comparatively small (with a diameter approximately 3 times greater than the solar one) hot star of class $B$ (the period of revolution is 3,800 days). Observations were carried out during the eclipse of the smaller component, which took place in 1951. First of all it was found that the giant is surrounded by an atmosphere extending from its surface to a distance of about 150 solar diameters—the effect of the atmospheric eclipse was noticed on June 1, 1951, and ceased to be noticed on January 8, 1952 (from August 12 to October 12, 1951, a total eclipse took place).

) A. McKeller, C. J. Odgers, L. H. Aller and D. B. McLanglin, Nature 169*, 990 (1951).

Without giving a complete analysis of the spectrograms obtained, the authors dwell on only one of the effects. When the small (hot) component, with an almost continuous spectrum, moves behind the atmosphere of the giant, it plays the role of a point source for analyzing the atmosphere by the ordinary methods of absorption spectroscopy. In the present case the object of observation was the absorption Ca II \(K\)-line (\(\lambda = 3973\) Å), clearly expressed in the giant’s spectrum but absent from the spectrum of the small star (the authors note that they obtained analogous data also for the Ca II \(H\)-line). It was found that, during emergence from eclipse, the gradual decrease in the intensity of the \(K\)-line (caused by the decreasing translucent thickness of the giant’s atmosphere) was accompanied by sharply expressed concomitant changes in its structure. (In the period preceding total eclipse, these phenomena were expressed more weakly.)

In the period between the second and fourth weeks after total eclipse, when the \(K\)-line was still very broad, these changes were manifested in the appearance of a weak absorption satellite whose position and intensity changed rapidly. Thus, on October 26 the satellite was observed on the short-wavelength side, and its position corresponded to a relative velocity of 85 km/sec. On October 27 the satellite formed a weak wing of the main line at its long-wavelength edge. On October 28 its shift into the long-wavelength region corresponded to a velocity of 110 km/sec; on October 30, to a velocity of 130 km/sec (in the same direction); and on November 1 it was again observed in the form of a weak long-wavelength wing of the main line.

The authors note that the phenomenon described must be interpreted as evidence for the presence of large-scale motions of Ca ions, occurring at high velocities and in different directions. During this period the small star was behind the outer region of the giant’s lower chromosphere. Since the small star was displaced relative to the giant’s atmosphere by about 2.5 solar diameters per day, observations (even those immediately following one another night after night) cannot be referred to one and the same region of the giant’s atmosphere; thus the question is probably not of temporal, but of spatial variability in the character of the motion.

In the period between November 1 and 14 the character of the picture changed substantially. The \(K\)-line narrowed sharply because the small star had moved into the region of the more rarefied atmosphere of the giant. On November 14 the line split noticeably. On November 16 the red component disappeared, leaving only a weakly expressed wing. On November 23 the \(K\)-line again became double, with both components of equal intensity, while on December 7 and 14 the long-wavelength component proved noticeably more intense than the short-wavelength one and was separated from the latter by a distance corresponding to a velocity of 35 km/sec. (The authors note that the short-wavelength component is the unshifted one.)

All these data testify not only to powerful variable motions in the outer atmosphere of the giant, but also to the complexity of the structure of the atmosphere itself.

Thus the authors have obtained direct evidence of the activity of the stellar atmosphere for the indicated type of giants; moreover, the streams apparently embrace the overwhelming part of the star’s atmosphere (in contrast to the Sun). The possibility of such powerful streams is evidently due to the comparatively weak force of gravity (in comparison with the Sun).

R. G.