Douglas S. Hall

Starspot evolution, differential rotation, and magnetic cycles in the chromospherically active binaries lambda andromedae, sigma Geminorum, II Pegasi, and V711 Tauri

We have analyzed 15-19 yr of photoelectric photometry, obtained manually and with automated telescopes, of the chromospherically active binaries lambda And, sigma Gem, II Peg, and V711 Tau. These observations let us identify individual dark starspots on the stellar surfaces from periodic dimming of the starlight, follow the evolution of these spots, and search for long-term cyclic changes in the properties of these starspots that might reveal magnetic cycles analogous to the Suns 11 yr sunspot cycle. We developed a computer code to fit a simple two-spot model to our observed light curves that allows us to extract the most easily determinable and most reliable spot parameters from the light curves, i.e., spot longitudes and radii. We then used these measured properties to identify individual spots and to chart their life histories by constructing migration and amplitude curves. We identified and followed 11 spots in lambda And, 16 in sigma Gem, 12 in II Peg, and 15 in V711 Tau. Lifetimes of individual spots ranged from a few months to longer than 6 yr. Differential rotation coefficients, estimated from the observed range of spot rotation periods for each star and defined by equation (2), were 0.04 for lambda And, 0.038 for sigma Gem, 0.005 for II Peg, and 0.006 for V711 Tau, versus 0.19 for the Sun. We searched for cyclic changes in mean brightness, B-V color index, and spot rotation period as evidence for long-term cycles. Of these, long-term variability in mean brightness appears to offer the best evidence for such cycles in these four stars. Cycles of 11.1 yr for lambda And, 8.5 yr for sigma Gem, 11 yr for II Peg, and 16 yr V711 Tau are implied by these mean brightness changes. Cyclic changes in spot rotation period were found in lambda And and possibly II Peg. Errors in B-V were too large for any long-term changes to be detectable.

The RS CVn Binaries and Binaries with Similar Properties

This paper will review reasonably thoroughly and comprehensively the many observational properties of the remarkable RS CVn-type binaries. In addition this paper will show that many of these same properties are observed in other types of binary systems.

PHOTOMETRIC VARIABILITY IN A SAMPLE OF 187 G AND K GIANTS

We have used three automatic photoelectric telescopes to obtain photometric observations of 187 G, K, and (a few) M0 —eld giants. We —nd low-amplitude photometric variability on timescales of days to weeks on both sides of the coronal dividing line (CDL) in a total of 81 or 43% of the 187 giants. About one-third of the variables have amplitudes greater than 0.01 mag in V. In our sample the percentage of variable giants is a minimum for late-G spectral classes and increases for earlier and later classes; all K5 and M0 giants are variable. We also obtained high-resolution, red wavelength spectroscopic observations of 147 of the giants, which we used to determine spectral classi—cations, v sin i values, and radial veloci- ties. We acquired additional high-resolution, blue wavelength spectra of 48 of the giants, which we used to determine chromospheric emission —uxes. We analyzed the photometric and spectroscopic obser- vations to identify the cause(s) of photometric variability in our sample of giants. We show that the light variations in the vast majority of G and K giant variables cannot be due to rotation. For giants on the cool side of the CDL, we —nd that the variability mechanism is radial pulsation. Thus, the variability mechanism operating in M giants extends into the K giants up to about spectral class K2. On the hot side of the CDL, the variability mechanism is most likely nonradial, g-mode pulsation. Subject headings: stars: fundamental parametersstars: late-typestars: oscillations ¨ stars: rotationstars: spotsstars: variables: other

PRECISE AUTOMATIC DIFFERENTIAL STELLAR PHOTOMETRY

The factors limiting the precision of differential stellar photometry are reviewed. Errors due to variable atmospheric extinction can be reduced to below 0.001 mag at good sites by utilizing the speed of robotic telescopes. Existing photometric systems produce aliasing errors, which are several millimagnitudes in general but may be reduced to about a millimagnitude in special circumstances. Conventional differential photometry neglects several other important effects, which are discussed in detail. If all of these are properly handled, it appears possible to do differential photometry of variable stars with an overall precision of 0.001 mag with ground based robotic telescopes.

A connection between long-term luminosity variations and orbital period changes in chromospherically active binaries

The eclipsing binary CG Cyg provides observational confirmation of three predictions made by Applegates (1991) improvement on the theory that magnetic cycles cause the quasi-periodic orbital period changes in binaries containing a convective star. The mean brightness outside eclipse and the period vary with the same cycle length of about 50 yr. The light curve and O - C curve are in phase, with maximum light and period increase occurring in early 1980. The chromospherically active star becomes bluer in phase with the brightening. Because a period increase occurs at maximum brightness, the sense of the stars differential rotation is specified: outside rotating faster.

Photometry of 50 suspected variable stars

Fifty stars have been chosen as suspected variable stars and analyzed for variability. A large portion of this sample are stars that are either proved active chromosphere stars or are candidates for such activity. The photometric data base consists of differential V measurements of the Vanderbilt 16 inch (41 cm) automatic photoelectric telescope and 25 observers at 26 observatories worldwide. Published photometric data have also been utilized, with proper adjustments made to ensure that all magnitudes are differential. Searches for photometric period, amplitudes, and times of minimum light showed 68 percent of the sample to be photometrically variable with periods found for 34. Two stars were deemed norvariable for the period of observation. Conclusive statements could not be made concerning the photometric variability of the 14 remaining stars. 81 refs.

Period Changes and Magnetic Cycles

There are periodicities, manifested in a variety of different phenomena in a variety of different types of stars, which fall in the range 10–100 years and might all be consequences of solar-type magnetic cycles: the 11-year solar cycle itself; variable Ca II K-line emission in solar-type dwarfs; variable mean brightness in chromospherically active stars, old novas at quiescence, and dwarf novas between outburts; alternating orbital period variations in Algol binaries, W UMa binaries, cataclysmic binaries, and chromospherically active binaries; occurence of flares in flare stars; outburst interval in recurrent novas; changes in outburst interval in dwarf novas; the high-low states in low-mass x-ray binaries like HZ Her; and alternating changes in pulsation period in RR Lyrae and in Cepheid variables. The similarity in time scales suggests a common mechanism. Magnetic cycles have been suggested previously for many of these but not all. For several of them, there never has been an explanation. Encouragingly, cycle lengths predicted by dynamo theory are very similar for most of these cases. Cepheid and RR Lyr variables present a problem: they have no convective envelopes and would not pulsate if they did. At this meeting Mauder suggested turbulence in the outer layers can concentrate flux tubes, other ingredients for a dynamo are present (rotation, differential rotation, and magnetic fields), and organized motion in the radial direction (normally provided by large- scale convection) can be provided by the pulsation itself.

Observations of the Ca II infrared triplet in chromospherically active single and binary stars

Spectroscopic observations of the Ca II infrared triplet (8498, 8542, 8662 A) have been obtained for 45 stars which are known or suspected to be chromospherically active. The sample includes both single and binary stars of spectral types from F2 to M5 spanning luminosity classes III, IV, and V. Several different types of activity diagnostics were measured, and their relative merits are discussed. Dependence of chromospheric emission upon rotation period, luminosity, temperature, and duplicity are analyzed. Synchronous binaries show a slight trend of increased emission with decreasing period while the asynchronous binaries show abnormally high activity levels for their rotation periods. Several stars exhibit rotationally modulated emission which is anticorrelated with the stellar brightness. Finally, estimates of chromospheric energy losses are presented with the result that the total loss in the infrared triplet is about twice that of the H and K lines.

Line profile asymmetries in chromospherically active stars

A powerful, new probe of chromospheric activity, cross-correlation, has been developed and applied to a variety of stars. In this particular application, an entire CCD spectrum of an active star is correlated with the spectrum of a narrow-line, inactive star of similar spectral type and luminosity class. Using a number of strong lines in this manner enables the detection of absorption profile asymmetries at moderate resolution (λ/Δλ ∼ 40000) and S/N 150:1. This technique has been applied to 14 systems mostly RS CVns, with 10 ≤ v sin i ≤ 50 km/s and P ≥ 7 d. Distortions were detected for the first time in five systems : σ Gem, IM Peg, GX Lib, UV Crb, and ζ And. Detailed modeling, incorporating both spectral line profiles and broad-band photometry, is applied to σ Gem. Profile asymmetries for this star are fitted by two high-latitude spots coverting 5% of the stellar surface

The RS Canum Venaticorum binaries

This review presents a brief up-to-date summary of observational properties of RS CVn binaries, with emphasis on the photometric. A table of the 69 systems known at this time gives the orbital period, mean apparent V magnitude, spectral classification, range of amplitude determinations for the photometric wave, and an indication of known period variations and eclipses. Points considered are (1) the underlying cause of the extreme surface activity, (2) the evolutionary status, (3) the geometric configuration, (4) the temperature, size, shape, location, and lifetimes of the dark spot regions, (5) synchronization of rotational and orbital motion, (6) spectroscopic features in the optical and far ultraviolet, (7) the wave migration, (8) the search for spot cycles, and (9) the orbital period variations produced by the enhanced stellar wind.