Near infrared variability has been shown to be present in nearly all the twelve stars studied: however, in the case of HD 10783, HD 12447, HD 116458, HD 147010, HD 166469, HD 170397, and HD 223640 the variations do show a small amplitude and a large dispersion. The star HD 187473, which is one of the largest amplitude variables among CP2 stars, shows in the infrared the largest observed amplitude, although smaller than in the visible.
The typical trend of CP2 stars to present smaller amplitude light variations at increasing wavelength is confirmed: the amplitudes in the near infrared are smaller than in the visible and the variations tend to be in phase with each other in all filters and, at least for the few stars for which we could do contemporaneous uvby photometry, the variations are also in phase at least with the y variation.
The origin of light variations in the ultraviolet and visible part of the spectrum is still unclear, only qualitative considerations have been made based on the assumption that elements are not homogeneously distributed over the surface. Leckrone et al. (1974) and Leckrone (1976), pointing out that CP stars are flux deficient in the ultraviolet if compared to normal stars having the same Balmer jump, have suggested the presence of a greatly enhanced ultraviolet line opacity source, distributed more or less uniformly over the entire Balmer continuum region, and the redistribution of the absorbed UV flux longward of the null-wavelength region, that is the wavelength region with no observed variation.
According to Babcock (1958) and Deutsch (1958), Rare Earths and Fe are mainly concentrated in the positive magnetic pole region, while Cr and Sr are concentrated at the negative one. In a previous paper (CKL) we investigated the effects of high metallicity at the near infrared wavelengths. Because of the numerous metallic absorption lines, the blanketing mechanism steepens the temperature gradient and redistributes the flux from the ultraviolet, where the metallic absorption is strongest, to longer wavelengths, yet conserving the effective temperature. Since the atmospheres of CP stars show locally inhomogeneous metal distributions, the stellar rotation should communicate these optical depth variations as infrared variability. CKL have shown that a Kurucz model atmosphere with a metal content ten times the solar one could explain a three percent variation in the near infrared brightness, which is the typically observed value. The fact that the near infrared variations are in phase with those in the visible, or at least with those in y, may be an indication that the same mechanism is responsible for both.
AcknowledgementsWe would like to thank Dr. P. Bouchet and R. Vega Tello for their help and advice during the observations and reductions. We are also grateful to the referee whose comments have contributed to a better presentation of the paper.
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