Detection of magnetic field in the B2 star ρ Oph A with ESO FORS2

I. Pillitteri (1,2), L. Fossati (3), N. Castro Rodriguez (4), L. Oskinova (5), S. J. Wolk (2)

1- INAF - Osservatorio Astronomico di Palermo G.S. Vaiana, Palermo, Italy,
2- Harvard-Smithsonian Center for Astrophysics, Cambridge (MA), USA,
3- Space Research Institute, Austrian Academy of Sciences, Graz, Austria,
4- Department of Astronomy, University of Michigan, Ann Arbor, MI, USA,
5- Institut für Physik und Astronomie, Universität Potsdam, Potsdam-Golm, Germany

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 analyzed FORS2 ESO VLT spectra of ρ Oph A taken at two epochs and detected a longitudinal component of the magnetic field of 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.

Reference: A&A Letter in press, arxiv:1712.00728
Status: Manuscript has been accepted