The rapid magnetic rotator HR7355

TL;DR

This paper reports the discovery and detailed analysis of HR7355, a rapidly rotating magnetic B2 star with a complex magnetosphere and surface inhomogeneities, challenging previous assumptions about magnetic star rotation speeds.

Contribution

It provides the most detailed spectral variability analysis of a rapidly rotating magnetic star, revealing a complex magnetosphere and surface abundance inhomogeneities.

Findings

Detection of a strong magnetic field in a fast rotator

Identification of a magnetosphere causing hydrogen emission

Observation of surface abundance inhomogeneities

Abstract

For early type magnetic stars slow, at most moderate rotational velocities have been considered an observational fact. The detection of a multi-kilogauss magnetic field in the B2Vpn star with P approx. 0.52d and v sin approx. 300km/s has brought down this narrative. We have obtained more than 100 high-resolution, high-S/N echelle spectra in 2009. These spectra provide the most detailed description of the variability of any He-strong star to date. The circumstellar environment is dominated by a rotationally locked magnetosphere out to several stellar radii, causing hydrogen emission. The photosphere is characterized by surface chemical abundance inhomogeneities, with much stronger amplitudes, at least for helium, than slower rotating stars like \sigma Ori E. The highly complex rotational line profile modulations of metal lines are probably a consequence the equatorial gravity darkening of HR 7355, and thus may offer an independent measurement of the von Zeipel parameter \beta .