Magnetic Fields in Early-Type Stars

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1 Magnetic Fields in Early-Type Stars Jason Grunhut (ESO Garching Fellow) Coralie Neiner (LESIA)

2 Magnetism in intermediate-mass AB stars >400 magnetic A/B stars (Bychkov+ 2009) Small fraction of all intermediate-mass A/B stars (~1.5-8 M ʘ ) are magnetic (5-10%) Strong chemical peculiarities spectral signatures Easily identifiable 78 Vir (A1p) Babcock (1947) σ Ori E (B2Vp) Landstreet & Borra (1978)

3 Magnetism in massive OB stars Surface fields were not expected in more massive (>8 M ʘ ) OB stars due to stronger turbulence & outflows Destroy large scale structure & stability Motivation for B-fields in massive stars X-ray emission; DACs/CIRs; LPVs Detected fields β Cep (B1 III) Henrichs+ 2000/01, Donati θ 1 Ori C (O7V) - Donati No proxy of magnetism unlike AB stars Donati+ 2002

4 The MiMeS Project Magnetism in Massive Stars Investigate the magnetic properties and related physics of hot, massive stars 3 LPs ( ) with new generation of spectropolarimeters (ESPaDOnS@CFHT, Narval@TBL, HarpsPol@ESO) PI: Gregg Wade (CFHT), Coralie Neiner (TBL), Evelyne Alecian (ESO 3.6m) 50+ collaborators, 13 countries Survey of 545 OB stars to determine magnetic incidence fraction and establish connection between magnetism and observed phenomena

5 Magnetism in massive OB stars Magnetic detection 7±1% of MiMeS survey No correlation with spectral type/mass Fully consistent with A and B stars

6 Field characteristics: Mainly dipolar HD (O9IV): Grunhut Phase Linear polarization reveals small scale structure and deviations from pure dipole (Kochukhov & Wade 2010)

7 Number of stars Field characteristics: Strong fields AB stars: Aurière OB stars: Petit+ 2012, Wade B d (G) Log(B d ) (G) Dipole field strength

8 Field characteristics of AB stars: Stable fields MuSiCoS ( 97-99) ESPaDOnS /Narval ( 06-10) Difference α 2 CVn (A0spe) : Silvester+ 2014

9 Magnetically confined winds and magnetospheres Sundqvist Grunhut+ 2012b Babel & Montmerle (1997) ud Doula+ 2002/08; Petit Townsend Grunhut+ 2012a

10 Field characteristics of OB stars: ud Doula Stable fields Shorter baseline of observations for O stars Indirect probe of magnetic field also show field stability Grunhut+ 2012b HD57682 (O9IV):

11 Magnetism in O-type stars: from MiMeS Tripled # of confirmed O-type stars (Grunhut+ in prep.) No photospheric chemical peculiarities associated with magnetism (Martins+ 2014) Established first class of magnetic O-stars with distinct spectral characteristics that likely reflect wind-field interaction - Of?p stars (Walborn 1972) Detected in all 5 Galactic examples 3 extra-galactic examples (Walborn+ 2010) Not the only magnetic O-stars HD (Grunhut+ 2009, 2012) Trumpler (Nazé+ 2012) FORS2, MiMeS related Plaskett s Star (Grunhut+ 2013, in prep.)

12 Magnetism and (un?)related phenomena The larger our ignorance, the stronger the magnetic field. Lodewijk Woltjer (1966)

13 Magnetism in classical Be stars Observational support for low-resolution studies using FORS (e.g. Hubrig+ 2007, 2009, Yudin+ 2009, 2011) Later shown as spurious (Bagnulo+ 2012) Magnetically torqued disk model (Cassinelli+ 2002) MiMeS no detections from observations of 85 Be stars (Neiner, Grunhut+ in prep.) Median σ ~ 100 G 30% of sample <30 G 10% of sample <10 G Incompatible with MTD model Consistent with other MHD studies

14 Magnetism and N-enrichment Brott in slowly-rotating B stars Theoretical models suggest magnetic breaking -> N- enrichment (Meynet, Eggenberger, Maeder 2011) MiMeS Observed 15 stars from Nieva & Przybilla (2012) 9 stars identified as N-rich 5 stars very N-rich B-field detected in 3 stars Extended study to 19 N-rich stars by Wade+ (2014) All slow magnetic stars N-rich, but not all N-rich magnetic

15 Summary The basic physics of magnetism in stellar radiative zones remains unchanged across 1.5 decades of mass Simple large scale fields -> mainly dipolar Strong fields -> Surface polar field strength ~ 1 kg Stable fields over the lifetime of observations Magnetic field confines winds Magnetospheres often observable Magnetism not involved in formation of Keplerian discs of classical Be stars