Basic parameters and properties of the rapidly rotating magnetic helium strong B star HR7355

TL;DR

This study characterizes the magnetic, spectral, and rotational properties of the helium-strong B star HR7355, revealing complex surface abundance patterns, a dense magnetosphere, and significant deviations from typical spectral type expectations.

Contribution

It provides detailed modeling of HR7355's photospheric parameters, magnetic field, and magnetosphere, highlighting its unique properties among rapidly rotating magnetic B stars.

Findings

Star rotates at 90% of critical velocity.

Magnetic field strength is about 11-12 kG.

Magnetosphere is dense and similar to  Ori E.

Abstract

The spectral and magnetic properties and variability of the B2Vnp emission-line magnetosphere star HR7355 were analyzed. The object rotates at almost 90% of the critical value, meaning it is a magnetic star for which oblateness and gravity darkening effects cannot be ignored any longer. A detailed modeling of the photospheric parameters indicate that the star is significantly cooler than suggested by the B2 spectral type, with T_eff=17500K atypically cool for a star with a helium enriched surface. The spectroscopic variability of helium and metal lines due to the photospheric abundance pattern is far more complex than a largely dipolar, oblique magnetic field of about 11 to 12kG may suggest. Doppler imaging shows that globally the most He enriched areas coincide with the magnetic poles and metal enriched areas with the magnetic equator. While most of the stellar surface is helium enriched with respect to the solar value, some isolated patches are depleted. The stellar wind in the circumstellar environment is governed by the magnetic field, i.e. the stellar magnetosphere is rigidly corotating with the star. The magnetosphere of HR7355 is similar to the well known \sigma Ori E: the gas trapped in the magnetospheric clouds is fairly dense, and at the limit to being optically thick in the hydrogen emission. Apart from a different magnetic obliquity, HR7355 and the more recently identified HR5907 have virtually identical stellar and magnetic parameters.