3. Stellar parameters
Spectral, photometric and magnetic variations of MCP stars are characterized by a single period. Stibbs (1950) proposed that these stars present a dipolar magnetic field, whose symmetry axis is tilted with respect to the rotational axis, and a non homogeneous distribution of chemical elements on the stellar surface. Thus the variability period is the rotational one.
The rotational period of CU Vir was firstly determined by Deutsch (1952) equal to 0.52067 days. Several authors have later re-determined this period with values ranging from 0.52067 to 0.5207 days, see Catalano & Renson (1998) for a complete list of references. Recently, Pyper et al. (1998) analyzed all the photometric data covering 40 years. They suggested that the rotational period of CU Vir increased abruptly between 1983 and 1987 of a factor . This unexpected jump of the rotational period was discussed by Stepie (1998). He suggested that the spin down occurs in the outher stellar envelope alone, due to a change of the moment of inertia of the envelope itself because of the internal magnetic field strength. The new ephemeris given by Pyper et al. (1998), referred to light minimum, are:
valid for a rigid rotator, we measure an inclination of the rotational axis with respect to the line of sight .
Borra & Landstreet (1980) found that the effective magnetic field (), measured from 1976 to 1978, is variable with the period established by Winzer (1974) from photometry. is the average of the component of the local magnetic field along the line of sight over the whole stellar disk. From the value of the inclination i and Borra & Landstreet (1980) measurements of , we obtain an obliquity of the dipole and a polar magnetic field equal to gauss. Pyper et al. (1998) give new values of ; however, we prefer to infer the magnetic geometry from Borra & Landstreet measurements which are based on hydrogen lines. In fact, Pyper and co-workers measurements are based on the 634.7 and 637.1 nm lines of silicon and this element is not homogeneously distributed on the surface of CU Vir (Kusching et al. 1999). Even if we cannot exclude a more complex topology of the magnetic field, like a dipole+quadrupole or a decentered dipole, as proposed by Hatzes (1997), the assumption of a simple dipole at an height above the photosphere where the radio emission is generated seems reasonable, as the strength of the quadrupole field decreases one order of magnitude faster than the dipole field.
A summary of the adopted and derived parameters for CU Vir is reported in Table 1.
Table 1. Stellar parameters
© European Southern Observatory (ESO) 2000
Online publication: October 30, 19100