Magnetic fields of (young) massive stars

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1 Magnetic fields of (young) massive stars Gregg Wade!! Royal Military College of Canada! Queen s University! Véronique Petit!! Florida Institute of Technology! From Stars to Massive Stars, April 2016

2 Acknowledgments! - RMC/Queen s graduate students: Alexandre David-Uraz *, Zsolt Keszthelyi, Matt Shultz, James Sikora!! - The MiMeS, BinaMIcS and LIFE collaborations!

3 Outline! 1. Stars as astrophysical magnets! 2. Measuring stellar magnetic fields! 3. Magnetic characteristics of hot stars! 4. The fossil field hypothesis!

4 Introduction!

6 Magnetism and stellar structure! Surface m / M Rad. Core + Conv. Env Core Fully Convective Higher mass stars Conv. Core + Rad. Env.

7 Measurement of stellar magnetic fields!

8 Measuring stellar magnetic fields! High resolu:on op:cal spectropolarimetry CFHT

9 Measuring stellar magnetic fields! Courtesy O. Kochukhov

10 Measuring stellar magnetic fields! Hα Strength Line-of-sight magnetic field (G) Rotational phase Magne:c field geometry and rota:onal period inferred from periodic varia:ons Magne:c wind confinement yielding a corota:ng magnetosphere Optical brightness Rotational phase Wade et al. (2011)

11 Measuring stellar magnetic fields! Kochukhov & Wade (2016)

12 What are the characteristics of magnetic fields of hot, massive stars?!

13 Recent surveys! q MiMeS: Magnetism in Massive Stars ( )! - Wade et al. (2015)! - Magnitude-limited survey of >550 OB stars! q BoB: B-fields of OB stars ( )! - e.g. Fossati et al. (2016)! - Rotation-selected, 100 «normal» stars! q BinaMIcS: Binarity & Magnetic Interactions ( )! - Alecian et al. (2015)! - Magnitude-limited survey of 220 close binaries!

14 Characteristics of surface B fields! - Strong, stable, organised fields detected! in ~10% of MS ABO stars! - Oblique to rotation axis! - Periodic variability: rotational modulation! - Uncorrelated with basic stellar properties! - Characteristics independent of mass, rotation! - Shortage of magnetic stars in short-period binaries! - Identical properties on the pre-main sequence! - Growing evidence for much weaker (<1 G) fields in (many?) others! - Working paradigm: Fossil fields!

15 Characteristics of surface B fields! Strength Increasing rotation phase kg strength Orientation Global dipolar structure Silvester et al. (2014)

$5 M Number / fraction (%) 10 1 Constant$ stars d<100 pc ~10% from ~3-40 M 0.1 2 2.

16 Incidence of surface B fields! 1000 Fields first appear 100at 1.5 M Number / fraction (%) 10 1 Constant All stars d<100 incidence pc of Magnetic stars d<100 pc ~10% from ~3-40 M Stellar mass Power et al. (2007) Sikora et al. (in prep)

17 Characteristics Masses up to of surface B fields! 50 M Detected fields in OB supergiants are rare Log L/L 11 known Galac:c magne:c O stars ~70 known magne:c OB stars Shultz et al. (in prep) Log T eff

18 Characteristics of surface B fields! Mean B of 3 kg, 70% of magne:c stars host fields from kg A stars B stars Number of stars A, B and O stars show compa:ble distribu:ons Dipole surface intensity in kg Shultz et al. (in prep)

19 Characteristics of surface B fields! - Strong, stable, organised fields detected! in ~10% of MS ABO stars! - Oblique to rotation axis! - Periodic variability: rotational modulation! - Uncorrelated with basic stellar properties! - Characteristics independent of mass, rotation! - Shortage of magnetic stars in short-period binaries! - Identical properties on the pre-main sequence! - Growing evidence for much weaker (<1 G) fields in (many?) others! - Working paradigm: Fossil fields!

20 Cool stars: P rot -activity relation! Strong increase in magne:c ac:vity with increasing rota:on Magnetic activity Increasing rotation Wright et al. (2012)

21 Hot stars: P rot -activity relation! Slow and rapid rotators show similar distribu:ons of fields Magnetic activity Increasing rotation Shultz et al. (in prep)

Characteristics of surface B fields! - Strong, stable, organised fields detected! in ~10% of MS ABO stars! - Oblique to rotation axis! - Periodic variability: rotational modulation!

22 Characteristics of surface B fields! - Strong, stable, organised fields detected! in ~10% of MS ABO stars! - Oblique to rotation axis! - Periodic variability: rotational modulation! - Uncorrelated with basic stellar properties! - Characteristics independent of mass, rotation! - Shortage of magnetic stars in short-period binaries! - Identical properties on the pre-main sequence! - Growing evidence for much weaker (<1 G) fields in (many?) others! Veronique Pe:t s - Working paradigm: Fossil fields! talk, coming up next!

23 Characteristics of surface B fields! - Strong, stable, organised fields detected! in ~10% of MS ABO stars! - Oblique to rotation axis! - Periodic variability: rotational modulation! - Uncorrelated with basic stellar properties! - Characteristics independent of mass, rotation! - Shortage of magnetic stars in short-period binaries! - Identical properties on the pre-main sequence! - Growing evidence for much weaker (<1 G) fields in (many?) others! - Working paradigm: Fossil fields!

24 Characteristics of surface B fields! Weak, complex signatures detected in Vega and several other bright A stars Longitudinal fields below 1 G! Animations by C. Folsom Petit et al. (2014)

25 Magnetic desert! Organised magne4c fields Field strength «Magne4c desert» Vega-like fields Lignières et al. (2014)

26 What does it all mean?!

Could it be a dynamo?! - Hot stars exhibit strong core convection! - Radiative zone shear dynamos are proposed! q Strong, stable, organised, oblique fields!

27 Could it be a dynamo?! - Hot stars exhibit strong core convection! - Radiative zone shear dynamos are proposed! q Strong, stable, organised, oblique fields! q Detected in ~10% of MS stars. Identical on! the pre-ms, disappearing on post-ms! q Uncorrelated with mass, rotation! q? Shortage in short-period binaries! q Existence of a «magnetic desert»!

28 The fossil field hypothesis! - Relaxed remnant fields! in radiative zones! Toroidal Poloidal Braithwaite & Nordlund (2006) Duez & Mathis et al. (2010)

29 Could it be a fossil field?! - Fossil fields evolve due to Ohmic diffusion and mechnical stresses! q Strong, stable, organised, oblique fields! q Detected in ~10% of MS stars. Identical on the pre-ms, disappearing on post-ms! q Uncorrelated with mass, rotation! q? Shortage in short-period binaries! q Existence of a «magnetic desert»!

30 The fossil field hypothesis! - Magnetic fields of ABO stars are considered to be fossil Where fields! do the fossil - Fields of ABO stars behave magne:c similarly fields of to A, B those of WDs and NSs! and O stars come from? - Cores of late-type stars have structure similar to envelopes of early-type stars! - Fossil magnetism is likely an important phenomenon in stars across the span of stellar evolution!

Rotation, Emission, & Evolution of the Magnetic Early B-type Stars

Contrib. Astron. Obs. Skalnaté Pleso 48, 175 179, (2018) Rotation, Emission, & Evolution of the Magnetic Early B-type Stars M.Shultz 1, G.A.Wade 2, Th.Rivinius 3, C.Neiner 4, O.Kochukhov 1 and E.Alecian