A&A Supplement series, Voll. 123, June I 1997, 353-402
Received July 19; accepted September 9, 1996
G. Mathys
- S. Hubrig
-
J.D. Landstreet
- T. Lanz
- J. Manfroid
Send offprint request: G. Mathys
European Southern Observatory, Casilla 19001,
Santiago 19, Chile
University of Potsdam, Am Neuen Palais 10, D-14469 Potsdam, Germany
Department of Astronomy, University of Western Ontario, London,
Ontario N6A 3K7, Canada
Astronomical Institute,
Utrecht University, Princetonplein 5, P.O. Box 80000, NL-3508 TA Utrecht,
The Netherlands
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 
. 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