The "Binarity and Magnetic Interactions in various classes of Stars" (BinaMIcS) project
SSF2A 2015
S. Boissier, V. Buat, L. Cambr´esy, F. Martins and P. Petit (eds)
THE “BINARITY AND MAGNETIC INTERACTIONS IN VARIOUS CLASSES OFSTARS” (BINAMICS) PROJECT
C. Neiner , J. Morin , E. Alecian , and the BinaMIcS collaboration Abstract.
The “Binarity and Magnetic Interactions in various classes of stars” (BinaMIcS) project is basedon two large programs of spectropolarimetric observations with ESPaDOnS at CFHT and Narval at TBL.Three samples of spectroscopic binaries with two spectra (SB2) are observed: known cool magnetic binaries,the few known hot magnetic binaries, and a survey sample of hot binaries to search for additional hotmagnetic binaries. The goal of BinaMIcS is to understand the complex interplay between stellar magnetismand binarity. To this aim, we will characterise and model the magnetic fields, magnetospheric structure andcoupling of both components of hot and cool close binary systems over a significant range of evolutionarystages, to confront current theories and trigger new ones. First results already provided interesting clues,e.g. about the origin of magnetism in hot stars.Keywords: stars: magnetic field, binaries: spectroscopic, binaries: close, techniques: polarimetric
The goals of the ”Binarity and Magnetic Interactions in various classes of stars” (BinaMIcS) project are tounderstand the impact of magnetic fields on stellar formation and evolution, of tidal effects on fossil and dy-namo magnetic fields, of magnetism on angular momemtum and mass transfers between binary components, aswell as magnetospheric interactions. To address these questions, we are missing observational information onthe magnetic field strengths and topologies of a statistically large sample of magnetic close binary systems, inwhich we expect significant interaction via tidal or magnetospheric interactions. Therefore, BinaMIcS is basedon two large programs of spectropolarimetric observations with ESPaDOnS at CFHT (Hawaii) and Narval atTBL (Pic du Midi, France), in addition to theoretical developments and modelling efforts.Three samples of short-period spectroscopic binaries with two spectra (SB2) are observed: • known selected cool ( < F5) magnetic binaries, to characterise their magnetic properties in details andcompare them to single magnetic cool stars • the few known hot ( > F5) magnetic binaries, to characterise their magnetic properties in details andcompare them to single magnetic hot stars • a survey sample of hot binaries, to discover new hot magnetic binaries and compare the occurence ofmagnetic fields in hot binaries versus single hot stars. 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 LUPM-UMR 5299, CNRS & Universit´e Montpellier, Place Eug`ene Bataillon, 34095, Montpellier Cedex 05, 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
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Fig. 1.
Three examples of LSD Stokes V (top), null (middle), and intensity profiles (bottom) of the cool binary systemBY Dra, taken at various orbital phases. The V and N profiles have been shifted upwards for display purposes. TheStokes V profiles clearly show that both binary components are magnetic.
Six magnetic SB2 systems with at least one O, B, or A component were known to exist and are currently beingcharacterized in the frame of BinaMIcS. These are HD 37017 (V1046 Ori, Bohlender et al. 1987), HD 37061(NU Ori, Petit et al. 2011), HD 136504 ( (cid:15)
Lup, Hubrig et al. 2011; Shultz et al. 2012), HD 47129 (Plaskett’s star,Grunhut et al. 2013), HD 98088 (Folsom et al. 2013), and HD 5550 (Neiner et al., these proceedings, Alecianet al., submitted). Among them, only one is known to host two magnetic stars: (cid:15)
Lup (Shultz et al. 2015).In addition, two newly discovered magnetic SB2 systems with early F stars are being characterized withinBinaMIcS: HD 160922 (Neiner & Alecian 2013) and HD 210027 ( ι Peg, Neiner & L`ebre 2014).No other magnetic OBA SB2 system was discovered among the ∼
200 systems ( ∼
400 stars) observed withinthe survey sample. Among single hot stars, ∼
7% are found to host a magnetic field, with a typical strength of afew hundreds to a few thousands gauss (Grunhut & Neiner 2015). These fields are of simple configuration andstable over decades. Moreover, they are of fossil origin, i.e. remnants from the field present in the molecularcloud at the time of stellar formation, possibly enhanced by a dynamo action during the early phases of thelife of the star (Borra et al. 1982; Neiner et al. 2015). If similar magnetic fields were present with the sameoccurence in hot binaries as in single hot stars, we should have detected 20 to 30 magnetic stars in the surveysample. There is thus a clear and strong deficit of magnetic stars in hot short-period spectroscopic binaries.A possible explanation for this dearth of magnetic fields in hot SB2 systems is provided by stellar formationprocesses. Stellar formation simulations (e.g. Commer¸con et al. 2011) showed that fragmentation of dense stellarcores is inhibited when the medium is magnetic. Therefore, it seems that it is more difficult to form a binarysystem in the presence of a fossil field and, thus, magnetic hot binaries are rarer.
The BinaMIcS sample of magnetic cool SB2 systems includes cool main sequence stars, evolved RS CVn objects,and young T Tauri stars. Several of these systems show signatures of magnetic fields in both components. Thisis the case, for example, of the K4Ve+K7.5Ve system BY Dra (see Fig. 1) or σ CrB (see Neiner & Alecian2013).Single magnetic cool stars show various types of magnetic fields, with various levels of complexity, axisym-metry, and strength. Fig. 2 shows known single magnetic cool stars (filled symbols) in a mass versus rotationperiod diagram. Stars with the strongest fields are the lower mass stars, the slowest rotators, and often havesimple poloidal fields. Weak complex fields are found in two regions of the diagram: slowly rotating very low-mass stars, and rapidly rotating higher mass stars. Open symbols in Fig. 2 indicate the position of the coolmagnetic binary targets of BinaMIcS. Magnetic maps of these binaries will be compared to the maps of singlestars to assess the effect of binarity on stellar dynamo processes.inaMIcS 239
Fig. 2.
Mass versus rotation period diagram for single magnetic stars (filled symbols) and cool binaries of the BinaMIcSsample (open symbols). For single stars, the shape of the symbol indicates the axisymmetry of the magnetic fieldconfiguration, its color indicates how poloidal the field is, and its size indicates the strength of the field.
BinaMIcS aims at studying the effect of magnetism on binary formation and evolution. Individual hot andcool short-period spectroscopic binary (SB2) systems are being studied in great details to infer their magneticproperties. These results will be compared to those obtained for single stars. In addition, the magnetic surveyof hot binaries already provided an important result: hot stars in short-period binaries are less often magneticthan when they are single.
We thank the “Programme National de Physique Stellaire” (PNPS) of CNRS/INSU (France) for their financial support to theBinaMIcS project.
References
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