Rotating Variable Stars

Variable stars with nonuniform surface brightness and/ or ellipsoidal shapes, whose variability is caused by axial rotation with respect to an observer. The nonuniformity of surface brightness distributions may be caused by the presence of spots or by some thermal or chemical inhomogeneity of the atmosphere caused by a magnetic field whose axis is not coincident with the rotation axis.


There are over 900 stars classified as rotating variables within the General Catalog of Variable Stars. The a2 Canum Venaticorum stars are the most numerous, followed by the BY Draconis variables. Only a dozen or so of the FK Coma Berenices type variables are known. As a group, these variables do not receive a great amount of attention from amateur astronomers, probably because they possess small amplitudes and are almost impossible to accurately observe without the aid of instruments. Generally, the range of brightness between maximum and minimum for rotating variable stars is on the order of hundredths to tenths of a magnitude. On the other hand, when observed with the proper instruments these stars will generally reward the patient observer with rapid amplitude changes (periods) on the order of hours in some cases.

Obviously, every star rotates but if the star also has relatively large permanent or semipermanent surface features, similar to sunspots, then the star will appear to vary in brightness, or even color, as the rotation of the star carries the surface features across the observer's line of sight. These change may be so subtle as to not be visually detectable but can be detectable using CCDs or PEP methods. It doesn't take much of a difference in temperature since luminosity varies with temperature raised to the fourth power. This means that a small temperature change, as in the case of a cool "spot" on a star, results in a huge change in luminosity. For this reason, sunspots look black, even though their temperature is many thousands of degrees. The star will not vary of course, if the axis of the rotation points to the observer, or if the surface features are symmetric about the axis of rotation.

As a class, rotating variable stars will provide you with an excellent opportunity to improve your observational abilities using instruments and in the process, provide a valuable service to science. Observing rotating stars, collecting good data, and plotting their light curves requires attention to detail and the application of rigorous observing methods.

a2 Canum Venaticorum stars are main sequence stars (luminosity class V) of spectral types B8p-A7p and they display strong magnetic fields. The "p" in their nomenclature indicates a peculiar chemical composition because their spectra show abnormally strong lines of Si, Sr, Cr, and rare earths that vary with rotation, magnetic field, and brightness changes. These light variations occur over a range from half a day to more than 160 days. The amplitudes of the light changes are usually in the range of O'VOl-O'Vl in V. The best-known group of rotating variables are the peculiar A stars, known as Ap stars. In these stars, the surface features (spots) are caused by a strong magnetic field locking cool chemical structures into relatively stable positions within the stellar gases. The variations in brightness and color are usually small, but in one or two cases they are almost large enough to be seen visually.

BY Draconis stars are usually described as a subset of the classical UV Ceti flare stars. BY Dra stars are commonly described as displaying low amplitudes with periods of a few days, dK or dM spectral type and emission lines of Ca II. You will find within the literature that some astronomers believe that all flare stars are subject to BY Dra-type variability from time to time. To verify this hypothesis will require very careful photometric observations conducted over a period of years. Some BY Draconis stars display amplitudes near 0™5, easily within the observational capabilities of dedicated amateur astronomers using instruments.

^ Rotating variable stars arranged in alphabetical order by designation Designation



Ellipsoidal variable

ptikors SXArietis





B8p-A7p stars showing small variations due to large "starspots" generated by intense magnetic fields. Rapidly oscillating a2 CVn variables. dKe-dMe stars showing quasi-periodic light changes rotating ellipsoidal variables stars. May be any spectral class

G-K III stars that are rapidly rotating, spotted, and in some cases they are binary systems optically variable pulsars B0p-B9p stars with intense He I and Si III lines

Rotating ellipsoidal variable stars are close binary systems that display changes in brightness by periods equal to their orbital motion. Because the two stars are close together, gravity deforms these stars' atmospheres sufficiently to cause variability. Light amplitudes do not exceed 0™1 in V but this is easily detectable using CCD or PEP methods.

FK Comae stars are giant stars (luminosity class III) of spectral type G-K, with rapid rotational velocities. They also display irregular surface features that cause fluctuating brightness. One theory regarding the formation of these stars is that they are coalesced binary systems, perhaps evolved W UMa stars (see Chapter 7, "Close Binary Eclipsing Systems").

Optically variable pulsars are rapidly rotating neutron stars with strong magnetic fields, radiating in the radio, optical, and X-ray regions. Pulsars emit narrow beams of radiation and periods of the light changes coincide with rotational periods (from 05004 to 4s). The amplitudes of the light pulses may reach 0™8.

SX Arietis stars are sometimes called helium variables. Periods of light and magnetic field changes coincide with rotational periods, while amplitudes are ^0™1 in V. These stars are usually described as high-temperature analogs of the ACV variables. The GCVS classifications are listed in Table 6.1.

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