Letter to the Editor

Detection of magnetic field in the B2 star ρ Ophiuchi A with ESO FORS2^★

¹ INAF – Osservatorio Astronomico di Palermo G.S. Vaiana, Piazza del Parlamento 1, 90134 Palermo, Italy
e-mail: pilli@astropa.inaf.it
² Harvard-Smithsonian Center for Astrophysics, 60 Garden St., Cambridge, MA, USA
³ Space Research Institute, Austrian Academy of Sciences, Schmiedlstrasse 6, 8042 Graz, Austria
⁴ Department of Astronomy, University of Michigan, 1085 South University Avenue, Ann Arbor, MI 48109, USA
⁵ Institut für Physik und Astronomie, Universität Potsdam Karl-Liebknecht-Strasse 24/25, 14476 Potsdam-Golm, Germany
⁶ Kazan Federal University, Kremlevskaya Str., 18, Kazan, Russia

Received: 11 October 2017
Accepted: 2 December 2017

Abstract

Circumstantial evidence suggests that magnetism and enhanced X-ray emission are likely correlated in early B-type stars: similar fractions of them (~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 ρ 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 analyze spectra of ρ Oph A obtained with the FORS2 spectrograph at ESO Very Large Telescope (VLT) at two epochs, and detect a longitudinal component of the magnetic field of the order of ~500 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 ρ Oph A and further constrained its age. We conclude that ρ Oph A provides strong evidence for the presence of active X-ray emitting regions on young magnetized early type stars.