Magnetic field detection in the B2 Vn star HR 7355☆

The B2Vn star HR 7355 is found to be a He-rich magnetic star. Spectropolarimetric data were obtained with FORS1 at UT2 on Paranal Observatory to measure the disc-averaged longitudinal magnetic field at various phases of the presumed 0.52 d cycle. A variable magnetic field with strengths between << B(z)>> = -2200 and +3200 G was found, with confidence limits of 100 to 130 G. The field topology is that of an oblique dipole, while the star itself is seen about equator-on. In the intensity spectra, the He i lines show the typical equivalent width (EW) variability of He-strong stars, usually attributed to surface abundance spots. The amplitudes of the EW variability of the He i lines are extraordinarily strong compared to other cases. These results not only put HR 7355 unambiguously among the early-type magnetic stars, but confirm its outstanding nature: with v sin i = 320 km s-1, the parameter space in which He-strong stars are known to exist has doubled in terms of rotational velocity.