We have presented near infrared photometric observations of the CP2 SrCrEu stars HD 3980, HD 24712, HD 49976, HD 72968, HD 83368, HD 96616, HD 98088, HD 101065, HD 111133, HD 118022, HD 125248, HD 126515, HD 137949, HD 148898, HD 153882, HD 164258, HD 203006, HD 206088, HD 220825, and HD 221760.
Infrared variability has been observed in all program stars but HD 137949, which has been found to be constant within 0.01 mag. Variability of the stars HD 101065 and HD 206088 has been detected for the first time, although their periodicity has to be confirmed.
From the analysis of the visible light variations we know that the amplitudes shown by CP2 of the SrCrEu subgroup are in the average smaller than those of hotter CP stars, but they show almost the same very complicated morphology of different behavior in different filters, in the sense that the shape of the variations and the phases at which their extrema occur do change with the wavelength. This result has to be taken into consideration when looking for the interpretation of the observed variations.
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. In most cases we find variations which appear to be in phase with each other in all filters.
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. However the complex behavior of the visible light curves of some stars, as for example HD 125248, is a direct evidence that this mechanism cannot fully explain the observed light variations.
Another possible origin of the light variations is the local line blocking. However Pilachowski & Bonsack (1975) have examined the influence of local line blocking on the light variations of HD 125248, and concluded that line blocking is certainly important but not sufficient to explain the observed amplitudes.
According to Babcock (1958a,b) 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 possibility that the magnetic pressure could have non-negligible influence on the hydrostatic equilibrium of the stellar atmosphere has been suggested in the past to explain the light variations of CP stars, although it does not always match the observations. The characteristics of the observed infrared light variations could be considered supporting the magnetic pressure influence on the hydrostatic equilibrium pressure since the infrared radiation comes from the outermost layers where the gas pressure becomes lower. However, because of the overabundances in the magnetic pole regions, the expected modification of the atmospheric temperature gradient produced by the overabundant elements could be the origin of the infrared variations.
We would like to thank Dr. P. Bouchet and R. Vega Tello for their help and advice during the observations and reductions.