Discovery of a magnetic field in the B pulsating system HD 1976

Abstract

The presence of a magnetic field can have a strong impact on the evolution of a binary star. However, only a dozen of magnetic OB binaries are known as of today and available to study this effect, including very few magnetic pulsating spectroscopic OB binaries. We aim at checking for the presence of a magnetic field in the B5IV hierarchical triple system HD 1976 with spectropolarimetric data obtained with Narval at the Bernard Lyot Telescope (TBL). We use orbital parameters of HD 1976 available in the literature to disentangle the Narval intensity spectra. We compute Stokes V profiles with the Least Square Deconvolution (LSD) technique to search for magnetic signatures. We then derive an estimate of the longitudinal magnetic field strength for each observation and for various line lists. Our disentangling of the intensity spectra shows that HD 1976 is a double-lined spectroscopic (SB2) binary, with the lines of the secondary component about twice broader than the ones of the primary component. We do not identify the third component. Moreover, we find that clear magnetic signatures are present in the spectropolarimetric measurements of HD 1976 and seem to be associated with the primary component. We conclude that HD 1976 is a magnetic slowly-pulsating double-lined spectroscopic binary star, with an undetected third component. It is the second such example known (with HD 25558).Comment: Accepted in A&A Letter to the Editor, 4 pages, 2 figures, 2 table