Magnetic field detection in the B2Vn star HR 7355

TL;DR

This study confirms HR 7355 as a highly magnetic, He-rich star with a strong, variable magnetic field and rapid rotation, expanding the known parameter space for He-strong stars.

Contribution

First spectropolarimetric detection of a variable magnetic field in HR 7355, revealing an oblique dipole topology and unprecedented rotational velocity among He-strong stars.

Findings

Magnetic field varies between -2200G and +3200G.

Star exhibits strong HeI line variability.

Rotational velocity is exceptionally high at 320 km/s.

Abstract

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 disk-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 +3200G was found, with confidence limits of 100 to 130G. The field topology is that of an oblique dipole, while the star itself is seen about equator-on. In the intensity spectra the HeI-lines show the typical equivalent width variability of He-strong stars, usually attributed to surface abundance spots. The amplitudes of the equivalent width variability of the HeI 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 the parameter space in which He-strong stars are known to exist has doubled in terms of rotational velocity.