The magnetic field of the hot spectroscopic binary HD5550
SSF2A 2015
S. Boissier, V. Buat, L. Cambr´esy, F. Martins and P. Petit (eds)
THE MAGNETIC FIELD OF THE HOT SPECTROSCOPIC BINARY HD 5550
C. Neiner , E. Alecian , and the BinaMIcS collaboration Abstract.
HD 5550 is a spectroscopic binary composed of two A stars observed with Narval at TBL inthe frame of the BinaMIcS (Binarity and Magnetic Interactions in various classes of Stars) Large Program.One component of the system is found to be an Ap star with a surprisingly weak dipolar field of ∼
65 G.The companion is an Am star for which no magnetic field is detected, with a detection threshold on thedipolar field of ∼
40 G. The system is tidally locked, the primary component is synchronised with the orbit,but the system is probably not completely circularised yet. This work is only the second detailed study ofmagnetic fields in a hot short-period spectroscopic binary. More systems are currently being observed withboth Narval at TBL and ESPaDOnS at CFHT within the BinaMIcS project, with the goal of understandinghow magnetism can impact binary evolution and vice versa.Keywords: stars: individual: HD 5550, stars: early-type, stars: magnetic field, binaries: spectroscopic,stars: chemically peculiar
HD 5550 is a spectroscopic double-line (SB2) binary system composed of two A-type components (Carrier et al.2002). HD 5550 was previously reported to be an Ap SrCrEu star (Renson et al. 1991). Carrier et al. (2002)also reported that the secondary has chemical peculiarities, but they could not distinguish more precisely thepeculiar type of this component.We observed HD 5550 in the frame of the BinaMIcS (Binarity and Magnetic Interactions in various classesof Stars) project, with the goal to understand the interplay between magnetism and binarity (see Neiner et al.,these proceedings). Twenty-five high-resolution spectropolarimetric observations were obtained with Narval atthe Bernard Lyot Telescope (TBL, Pic du Midi, France) and were used to check for the presence of a magneticfield in both components.We first disentangled the spectra of the two components to be able to analyse them separately. The binaryorbit has a period P orb = 6.82054 d (Carrier et al. 2002) and is almost circularised with an eccentricity e=0.005.We then used Zeeman and Atlas9 LTE models on the disentangled spectra to derive the stellar parameters ofboth components: we confirmed that the primary component is an Ap star and found that the secondary is anAm star, with overabundance of the iron-peak elements, extreme overabundance of Ba, and underabundance ofCa. Finally, we applied the Least-Square Deconvolution (LSD) technique to produce averaged Stokes I and Vspectra of each component and we measured the magnetic field in both stars. We found that the primary Ap star is magnetic with clear Zeeman signatures (see Fig. 1). The longitudinalfield B l values are systematically negative and vary from −
26 to −
12 G, with typical error bars of 4 G.From the variations of the Stokes V profiles, and the corresponding B l values, we found that the field ismodulated by the rotation period P rot ∼ P orb ∼ LESIA, Observatoire de Paris, PSL Research University, CNRS, Sorbonne Universit´es, UPMC Univ. Paris 06, Univ. ParisDiderot, Sorbonne Paris Cit´e, 5 place Jules Janssen, 92195 Meudon, France Univ. Grenoble Alpes, CNRS, IPAG, F-38000 Grenoble, Francec (cid:13)
Soci´et´e Francaise d’Astronomie et d’Astrophysique (SF2A) 2015 a r X i v : . [ a s t r o - ph . S R ] O c t
38 SF2A 2015
Fig. 1.
LSD I/Ic (left) and V/Ic (right) profiles of the primary component of HD 5550, ordered by increasing orbitalphase. Taken from Alecian et al., submitted to A&A.
An oblique dipole model of the Zeeman signatures shows that the polar field strength is only B pol = 65 ±
20 G, with an inclination i ∼ ◦ and an obliquity β ∼ ◦ . This is the weakest magnetic field known in anAp star. Indeed, typical magnetic field strengths in Ap/Bp stars are of the order of 1 kG, with a range between300 G and 30 kG (e.g. Borra & Landstreet 1980; Landstreet 1992; Bagnulo et al. 2006). The dipolar field valueof HD 5550 falls in the dichotomy desert proposed by Auri`ere et al. (2007) between strong and ultra-weak fields. We did not detect a magnetic field in the secondary Am star. The longitudinal field values we measured byintegrating the LSD I and V profiles are all consistent with 0 G, with uncertainties of 3-4 G.To determine the upper limit on the possible undetected magnetic field of the secondary star, we first fittedthe LSD I profiles with Gaussian profiles. We then computed 1000 synthetic Stokes V profiles for various valuesof the polar magnetic field B pol . Each of these models uses a random inclination angle i , obliquity angle β , androtational phase. We added a white Gaussian noise to each modeled profile with a null average and a variancecorresponding to the signal-to-noise of the observed profile. We then computed the detection probability of ahe magnetic field of HD 5550 239magnetic field as a function of B pol for each observation, and combined them to obtain the detection probabilityfunction for the full dataset. Above a 90% detection probability, we consider that we would have detected thefield in our dataset. We therefore established that the upper limit of the magnetic field of the secondary Amcomponent, which could have remained hidden in our observations, is ∼
40 G.Only a few Am stars are known to host a magnetic field so far and all of them have ultra-weak fields, withlongitudinal components of less than 1 G (Petit et al. 2011; Blaz`ere et al. 2015). If such an ultra-weak fieldwere present in the Am component of HD 5550, it would have remained undetected in our observations.
Spectropolarimetric Narval observations of HD 5550 showed that it is a binary system composed of a weaklymagnetic Ap star and an Am star found to be non-magnetic with the achieved precision. With HD 98088(Folsom et al. 2013), this is the second hot magnetic spectroscopic binary studied in details. Studying more hotmagnetic binaries, which is one of the goals of the BinaMIcS project, will allow us to understand the interplaybetween magnetism and binarity in hot systems.
We thank the “Programme National de Physique Stellaire” (PNPS) of CNRS/INSU (France) for their financial support to theBinaMIcS project.