5. Discussion and conclusion
By analysing the Hipparcos photometry of the magnetic silicon star For and forcing them to be in phase with Renson & Manfroid (1978) data, we have found that the variability period of For is 1.89232 days.
Time-resolved spectra have been obtained in the 400-600 nm range in January and December 1995 to investigate the spectral behaviour of this star. We have found that light curves are in phase with the magnetic field and with the strength of chlorine and iron lines. Out of phase is the variability of helium, magnesium, silicon and chromium lines.
In the hypothesis suggested by Leckrone (1974) that light variations are due to metal rich regions where the ultraviolet flux is blocked and then redistributed toward the visible range, we conclude that in the case of For, iron is the main factor in the flux redistribution.
Assuming that For is a rigid rotator, the angle i between the rotational axis and the line of sight, the stellar radius , the equatorial rotational velocity () and the rotational period are related by the equation:
for the project rotational velocity 505 km s-1 measured by Leone & Manfrè (1996) spectra, the stellar radius (3.20.4) given by North (1998) and the value of the rotational period determined here, we found that the rotational axis and the line of sight forms a angle.
In the hypothesis of dipolar magnetic field, the angle () between the rotational axis and the dipole axis is given from the relation (Preston 1971):
where r is the ratio of the minimum and maximum observed values of the effective magnetic field. The polar strength of the magnetic field () comes from Schwarzschild (1950) relation:
Borra & Landstreet (1980) measurements of the effective magnetic field with are consistent with a inclination of dipole axis with respect to the rotational axis and a polar magnetic field of 1.3 kG.
© European Southern Observatory (ESO) 2000
Online publication: July 7, 2000