Magnetic field detection in the B2Vn star HR 7355

Th. Rivinius$^1$, Th. Szeifert$^1$, L. Barrera$^2$, R.H.D. Townsend$^3$, S. v{S}tefl$^1$, D. Baade$^4$

1 - ESO - European Organisation for Astronomical Research in the Southern Hemisphere, Casilla 19001, Santiago 19, Chile;
2- Universidad Metropolitana de Ciencias de la Educac'ion (UMCE), Jos'e Pedro Alessandri 774, Santiago, Chile;
3 - Department of Astronomy, University of Wisconsin--Madison, Sterling Hall, 475 N Charter St., Madison 53711, USA;
4 - ESO - European Organisation for Astronomical Research in the Southern Hemisphere, Karl-Schwarzschild-Str.~2, 85748 Garching bei M"unchen, Germany

The B2Vn star HR7355 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 0.52d cycle. A variable magnetic field with strengths between $langle B_z rangle=-2200$ 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 HR7355 unambiguously among the early-type magnetic stars, but confirm its outstanding nature: With $vsin i = 320$,km,s$^{-1}$ the parameter space in which He-strong stars are known to exist has doubled in terms of rotational velocity.

Reference: MNRAS Letter, in press
Status: Manuscript has been accepted


Comments: Also available on astro-ph as arXiv:1003.3601