Magnetic Fields in Early-Type Stars

Jason Grunhut (ESO Garching Fellow)

Coralie Neiner (LESIA)

Magnetism in intermediate-mass AB stars

78 Vir (A1p) Babcock (1947)

• >400 magnetic A/B stars (Bychkov+ 2009) – Small fraction of all intermediate-mass A/B stars

(~1.5-8 Mʘ) are magnetic (5-10%)

• Strong chemical peculiarities → spectral signatures → Easily identifiable

Landstreet & Borra (1978)

σ Ori E (B2Vp)

Donati+ 2002

Magnetism in massive OB stars

• Surface fields were not expected in more massive (>8 Mʘ) 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+ 2001

– 𝜃1Ori C (O7V) - Donati+ 2002

• No proxy of magnetism unlike AB stars

The MiMeS Project Magnetism in Massive Stars

• Investigate the magnetic properties and related physics of hot, massive stars

• 3 LPs (2008-2012) 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

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

HD 57682 (O9IV):

Grunhut+ 2012

Field characteristics: Mainly dipolar

Phase 0.0 0.2 0.4 0.6 0.8

• Linear polarization reveals small scale structure and deviations from pure dipole (Kochukhov & Wade 2010)

Field characteristics: Strong fields

Dipole field strength

Nu

mb

er

of

star

s

AB stars: Aurière+ 2007 OB stars: Petit+ 2012,

Wade+ 2014

Bd (G) Log(Bd) (G)

Field characteristics of AB stars: Stable fields

α2 CVn (A0spe) : Silvester+ 2014

ESPaDOnS/Narval (‘06-’10)

MuSiCoS (‘97-’99)

Difference

Magnetically confined winds and magnetospheres

ud Doula+ 2002/08; Petit+ 2013

Townsend+ 2012

Sundqvist+ 2012

Grunhut+ 2012b

Grunhut+ 2012a

Babel & Montmerle (1997)

Field characteristics of OB stars: Stable fields

• Shorter baseline of observations for O stars

• Indirect probe of magnetic field also show field stability

ud Doula+ 2002

Grunhut+ 2012b

HD57682 (O9IV):

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 57682 (Grunhut+ 2009, 2012) – Trumpler 16-22 (Nazé+ 2012) – FORS2, MiMeS related – Plaskett’s Star (Grunhut+ 2013, in prep.)

“The larger our ignorance, the stronger the magnetic field.”

— Lodewijk Woltjer (1966)

Magnetism and (un?)related phenomena

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

Magnetism and N-enrichment in slowly-rotating B stars

• 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

• Theoretical models suggest magnetic breaking -> N-enrichment (Meynet, Eggenberger, Maeder 2011)

Brott+ 2011

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