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A&A Supplement series, Voll. 123, June I 1997, 353-402

Received July 19; accepted September 9, 1996

The mean magnetic field modulus of Ap starsgiftex2html_wrap_inline4107

G. Mathystex2html_wrap4121 - S. Hubrigtex2html_wrap4123 - J.D. Landstreettex2html_wrap4125gif - T. Lanztex2html_wrap4127gif - J. Manfroidtex2html_wrap4129

Send offprint request: G. Mathys

tex2html_wrap4131  European Southern Observatory, Casilla 19001, Santiago 19, Chile
tex2html_wrap4133  University of Potsdam, Am Neuen Palais 10, D-14469 Potsdam, Germany
tex2html_wrap4135  Department of Astronomy, University of Western Ontario, London, Ontario N6A 3K7, Canada
tex2html_wrap4137  Astronomical Institute, Utrecht University, Princetonplein 5, P.O. Box 80000, NL-3508 TA Utrecht, The Netherlands
tex2html_wrap4139  Institut d'Astrophysique, Université de Liège, avenue de Cointe 5, B-4000 Liège, Belgium


We present new measurements of the mean magnetic field modulus of a sample of Ap stars with spectral lines resolved into magnetically split components. We report the discovery of 16 new stars having this property. This brings the total number of such stars known to 42. We have performed more than 750 measurements of the mean field modulus of 40 of these 42 stars, between May 1988 and August 1995. The best of them have an estimated accuracy of tex2html_wrap_inline4109. The availability of such a large number of measurements allows us to discuss for the first time the distribution of the field modulus intensities. A most intriguing result is the apparent existence of a sharp cutoff at the low end of this distribution, since no star with a field modulus (averaged over the rotation period) smaller than 2.8 kG has been found in this study. For more than one third of the studied stars, enough field determinations well distributed throughout the stellar rotation cycle have been achieved to allow us to characterize at least to some extent the variations of the field modulus. These variations are often significantly anharmonic, and it is not unusual for their extrema not to coincide in phase with the extrema of the longitudinal field (for the few stars for which enough data exist about the latter). This, together with considerations on the distribution of the relative amplitude of variation of the studied stars, supports the recently emerging evidence for markedly non-dipolar geometry and fine structure of the magnetic fields of most Ap stars. New or improved determinations of the rotation periods of 9 Ap stars have been achieved from the analysis of the variations of their mean magnetic field modulus. Tentative values of the period have been derived for 5 additional stars, and lower limits have been established for 10 stars. The shortest definite rotation period of an Ap star with magnetically resolved lines is 34, while those stars that rotate slowest appear to have periods in excess of 70 or 75 years. As a result of this study, the number of known Ap stars with rotation periods longer than 30 days is almost doubled. We briefly rediscuss the slow-rotation tail of the period distribution of Ap stars. This study also yielded the discovery of radial velocity variations in 8 stars. There seems to be a deficiency of binaries with short orbital periods among Ap stars with magnetically resolved lines.

keywords: stars: chemically peculiar -- stars: magnetic fields -- stars: rotation

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