On the variability of the peculiar A type star HD 49976.

Summary. Photoelectric photometry in 11 colors and spectroscopic observations of the Strontium variable HD 49976 were carried out at La Silla. We calculate a rotational period P =2.9760 days based on (1) Babcock's (1958) verbally described line strengths, (2) the Sr II variation obtained by Pilachowski et aL (1974) and (3) our Sr II line strengths. The corresponding epoch is 2433942.98 J.D. for Sr primary maximum. The measurements in the Str6mgren filters agree well with those of Pilachowski et al. Furthermore we find an asymmetric shape for both maxima of the double wave. This is probably related to the high rotational velocity of this star and to a strong inclination of the magnetic to the rotational axis. Light variaflon can be explained by redistribution of flux which is absorbed in the UV and reemitted in the visible part of the spectrum. The Sr variation shows a double wave like the light curves. However, in contrast to the light curves, its secondary maximum is much weaker than its primary. One will likely have to wait for UV-measurements in order to find the main source of opacity As the flux redistribution raises the visible continuum, the absorption around 4100 and 5200 A is increased. These are the regions for which two pronounced maxima of the line blocking coefficient have been found in Ap stars. In HD 49976 the light variation is strongly reduced at 5200 A and noticeably diminished at 4100 A (v-band). The same situation was found for the short period Ap star HD 203006 by Maitzen et aL (1974). The reauction of the v-amplitude can be entirely explained by a varying line blocking coefficient as measured by Pilachowski et al. Maximum amplitude is observed in the H-Beta-Wide filter (()W060). The main difficulty in explaining the behaviour of HD 49976 based on present theories of Ap stars is the phase difference by which both primary light and line maxima lag the magnetic maximum. The observational evidence now yields values between 0.05 and 0.15 for this phase lag, the uncertainty being due to inadequate phase coverage in the magnetic curve. Contrary to Pilachowski et aL who assume equal polar magnetic field strengths (based on Heff varying between +2 and -2kG and vsini*v) we propose a decentered dipole for HD 49976, as found for a number of other Ap stars (e.g. Preston, 1970). According to the theory of Havnes and Conti (1971) the weaker pole accretes more than the stronger one and produces a spot which in turn is the cause for the primary maxima in the light and line variations. A pronounced difference between both polar surface field strengths together with a nearly rectangular inclination of rotational and magnetic axes and rapid rotation of the Ap star could be the responsible agent for producing the observed phase difference by which the accretion spot at the weaker pole lags the positive magnetic maximum, whereas the opposite spot containing less accreted material seems to be in phase with the (stronger) negative magnetic maximum. Numerical light curves can be obtained from the first author Key words: peculiar A stars light, spectrum, magnetic variations stars, individual