The magnetic field of the double-lined spectroscopic binary system HD 5550

Abstract

Context. The origin of fossil fields in intermediate-and high-mass stars is poorly understood, as is the interplay between binarity and magnetism during stellar evolution. Thus we have begun a study of the magnetic properties of a sample of intermediate-mass and massive short-period binary systems as a function of binarity properties. Aims. This paper specifically aims to characterise the magnetic field of HD 5550, a double-lined spectroscopic binary system of intermediate mass.Methods. We gathered 25 high-resolution spectropolarimetric observations of HD 5550 using the instrument Narval. We first fitted the intensity spectra using Zeeman/ATLAS9 LTE synthetic spectra to estimate the effective temperatures, microturbulent velocities, and the abundances of some elements of both components, as well as the light ratio of the system. We then applied the multi-line least-square deconvolution (LSD) technique to the intensity and circularly polarised spectra, which provided us with mean LSD I and V line profiles. We fitted the Stokes I line profiles to determine the radial and projected rotational velocities of both stars. We then analysed the shape and evolution of the V profiles using the oblique rotator model to characterise the magnetic fields of both stars. Results. We confirm the Ap nature of the primary, which has previously been reported, and find that the secondary displays spectral characteristics typical of an Am star. While a magnetic field is clearly detected in the lines of the primary, no magnetic field is detected in the secondary in any of our observations. If a dipolar field were present at the surface of the Am star, its polar strength must be below 40 G. The faint variability observed in the Stokes V profiles of the Ap star allowed us to propose a rotation period of 6.84 +0.61 −0.39 d, which is close to the orbital period (∼6.82 d), suggesting that the star is synchronised with its orbit. By fitting the variability of the V profiles, we propose that the Ap component hosts a dipolar field inclined with the rotation axis at an angle β = 156 ± 17• and a polar strength B d = 65 ± 20 G. The field strength is the weakest known for an Ap star.

Fossil magnetic fields in intermediate-mass and massive stars

“…The analysis of the data is still in progress. However, magnetic configurations have already been obtained for 2 systems (HD 98088 and HD 5550) out of the 5 magnetic stars, and they show global properties similar to those of isolated ABO stars (Folsom et al, 2013;Alecian et al, 2016). The statistical analysis of the whole sample is also in progress, but we can already say that the magnetic incidence is much lower than in isolated stars: less than 1.5% (versus 7% in single stars, see Fig.…”

The MiMeS survey of magnetism in massive stars: magnetic properties of the O-type star population

“…This has been recently exemplified by the detection of very weak (at the sub-gauss level) magnetic fields at the surface of some A-type stars (Petit et al 2010(Petit et al , 2011Blazère et al 2016). Clearly, significantly stronger fields would have been noticed -nevertheless, there is only a handful of known main sequence magnetic A-type stars in the 1-100 G field strength bin (Alecian et al 2016;Blazere 2018).…”

Spot distribution and fast surface evolution on Vega

“…Recent studies challenge the lower magnetic threshold proposed by Aurière et al (e.g. Fossati et al 2015;Alecian et al 2016;Blazère et al 2016a), and sometimes highlight a field geometry significantly more complex than a simple dipole (e.g. Kochukhov et al 2004).…”