Detection of magnetic field in the B2 star $\rho$ Oph A with ESO FORS2

TL;DR

This paper reports the detection of a magnetic field in the B2 star $ ho$ Oph A, linking its surface spots and X-ray activity to stellar magnetism, providing evidence of active X-ray regions on young magnetized early-type stars.

Contribution

First detection of a magnetic field in $ ho$ Oph A, establishing a connection between surface spots, X-ray activity, and stellar magnetism in early B-type stars.

Findings

Magnetic field of ~500 G detected in $ ho$ Oph A

X-ray activity linked to surface spots and magnetic field

Stellar parameters and age constrained from spectra

Abstract

Circumstantial evidence suggests that magnetism and enhanced X-ray emission are likely correlated in early B-type stars: similar fractions of them ($\sim$ 10 %) are strong and hard X-ray sources and possess strong magnetic fields. It is also known that some B-type stars have spots on their surface. Yet up to now no X-ray activity associated with spots on early-type stars was detected. In this Letter we report the detection of a magnetic field on the B2V star $\rho$ Oph A. Previously, we assessed that the X-ray activity of this star is associated with a surface spot, herewith we establish its magnetic origin. We analyzed FORS2 ESO VLT spectra of $\rho$ Oph A taken at two epochs and detected a longitudinal component of the magnetic field of order of $\sim500$ G in one of the datasets. The detection of the magnetic field only at one epoch can be explained by stellar rotation which is also invoked to explain observed periodic X-ray activity. From archival HARPS ESO VLT high resolution spectra we derived the fundamental stellar parameters of $\rho$ Oph A and further constrained its age. We conclude that $\rho$ Oph A provides strong evidence for the presence of active X-ray emitting regions on young magnetized early type stars.