eos vorstellung

Weak magnetic fields
in early-type stars
Jon Braithwaite
Bonn University
Santiago, 7th May 2012
Mittwoch, 9. Mai 12
Contents



Review of observations of early-type stars
− Intermediate-mass stars
− High-mass stars
Hypotheses to explain nature and/or
presence of magnetic fields:
− Fossil fields and failed fossils
− Subsurface convection
− Other possibilities
Summary
Mittwoch, 9. Mai 12
Contents



Review of observations of early-type stars
− Intermediate-mass stars
− High-mass stars
Hypotheses to explain nature and/or
presence of magnetic fields:
− Fossil fields and failed fossils
− Subsurface convection
− Other possibilities
Summary
Mittwoch, 9. Mai 12
Upper-main-sequence stars
spectral type
field detected
Steady, strong,
large-scale fields
Rest of population
Mittwoch, 9. Mai 12
A and late B
O and early B
Upper-main-sequence stars
spectral type
A and late B
field detected
Steady, strong,
large-scale fields
Rest of population
Mittwoch, 9. Mai 12
~10% of stars,
the so-called Ap/Bp stars
200G < B < 100kG
O and early B
A and late B stars: Ap/Bp stars

First magnetic fields found outside solar system (Babcock 1947)

Constant (> decades)

200G-100kG strength

Large-scale
(often dipolar)
53 Cam
(Kochukhov et al 2004)
Mittwoch, 9. Mai 12
α2 CVn
(Kochukhov et al 2002)
Upper-main-sequence stars
spectral type
A and late B
field detected
Steady, strong,
large-scale fields
~10% of stars,
the so-called Ap/Bp stars
200G < B < 100kG
Rest of population Weak fields found in two
bright A stars. Present in
all other stars too?
Mittwoch, 9. Mai 12
O and early B
A and late B stars: the rest


Bimodality:
No stars with fields 10 - 200 G? (Aurière et al 07)
Recent detections:
(Lignières et al 2009, Petit et al 2010, 2011)
name
type
mass
/Msun
v sin i
/km/s
spin
period
/hours
age
/Myr
field
strength
measured
Vega
A0V
~2.5
22
12
~400
0.6 ± 0.3 G
Sirius
A1V
2.1
16
24 - 120? ~200
0.2 ± 0.1 G

Time variability: uncertain

Geometry: uncertain but not very small-scale
Mittwoch, 9. Mai 12
Observations of Vega
Mittwoch, 9. Mai 12
Contents



Review of observations of early-type stars
− Intermediate-mass stars
− High-mass stars
Hypotheses to explain nature and/or
presence of magnetic fields:
− Fossil fields and failed fossils
− Subsurface convection
− Other possibilities
Summary
Mittwoch, 9. Mai 12
Upper-main-sequence stars
spectral type
A and late B
O and early B
field detected
Steady, strong,
large-scale fields
~10% of stars,
Some fraction
the so-called Ap/Bp stars (10, 20%?)
200G < B < 100kG
B ~ 1kG?
Rest of population Weak fields found in two
bright A stars. Present in
all other stars too?
Mittwoch, 9. Mai 12
Observations of main-sequence stars: O and early B

Some fraction have Ap-like fields (all Of?p magnetic?)

Detection limit is much higher than in A stars

Typically kG strength
Field topology of τ Sco, a B0 main-sequence star (MV=2.8)
(Donati et al. 2006, using Zeeman-Doppler imaging)‫‏‬
Mittwoch, 9. Mai 12
Upper-main-sequence stars
spectral type
A and late B
O and early B
field detected
Steady, strong,
large-scale fields
~10% of stars,
Some fraction
the so-called Ap/Bp stars (10, 20%?)
200G < B < 100kG
B ~ 1kG?
Rest of population Weak fields found in two No fields detected, but
bright A stars. Present in indirect suggestions of
all other stars too?
magnetic activity in
whole population
Mittwoch, 9. Mai 12
`Non-magnetic´ massive stars




Most stars have no detected/
detectable field
Various observational phenomena:
discrete absorption components
(DACs), line profile variability (LPV),
wind clumping, solar-like oscillations,
red noise, photometric variability, Xray emission
Some of these phenomena are
ubiquitous -- cannot be explained by
large-scale 'fossil' fields
Could be caused by:
magnetic activity at surface:
reconnection heating etc.

wind shocks caused by linedeshadowing instability, but
seeded by magnetic activity at
surface?

(DACs: Kaper, Henrichs et al. 1999)
Mittwoch, 9. Mai 12
Contents



Review of observations of early-type stars
− Intermediate-mass stars
− High-mass stars
Hypotheses to explain nature and/or
presence of magnetic fields:
− Fossil fields and failed fossils
− Subsurface convection
− Other possibilities
Summary
Mittwoch, 9. Mai 12
Evolution of magnetic field in absence of driving

Put arbitrary
magnetic field
into a star
du
=
dt
U2
1
1
⇤P + g +
(⇤ ⇥ B) ⇥ B
⇥
4 ⇥
cs2
vff2
vA2 (=B2/4πρ)

Lorentz force is balanced by inertia

Therefore evolution takes place on Alfvén timescale τA = L / vA

Reconnection occurs, magnetic energy is destroyed, field strength drops
Magnetic relaxation involves
motion on spherical shells
Mittwoch, 9. Mai 12
Evolution of magnetic field in absence of driving

Put arbitrary
magnetic field
into a star
du
=
dt
U2
1
1
⇤P + g +
(⇤ ⇥ B) ⇥ B
⇥
4 ⇥
cs2
vff2
vA2 (=B2/4πρ)

Lorentz force is balanced by inertia

Therefore evolution takes place on Alfvén timescale τA = L / vA

Reconnection occurs, magnetic energy is destroyed, field strength drops

Eventually, an equilibrium is reached:
Lorentz force balanced by non-spherical deviations of P and ρ
Mittwoch, 9. Mai 12

Put arbitrary
magnetic field
into a star
U2
cs2

From momentnced by inertia

Therefore evolution takes

vff2
Reconnection occurs, mag
Initial conditions
Mittwoch, 9. Mai 12
vA2
Simulations
of magnetic
relaxation to
equilibrium
(Braithwaite
2008)
Evolution of magnetic field in absence of driving

Put arbitrary
magnetic field
into a star
du
=
dt
U2
1
1
⇤P + g +
(⇤ ⇥ B) ⇥ B
⇥
4 ⇥
cs2
vff2
vA2 (=B2/4πρ)

Lorentz force is balanced by inertia

Therefore evolution takes place on Alfvén timescale τA = L / vA

Reconnection occurs, magnetic energy is destroyed, field strength drops


Eventually, an equilibrium is reached:
Lorentz force balanced by non-spherical deviations of P and ρ
Energy (and field strength; E=∫B2/8π dV), fall a lot during relaxation.
Therefore, time taken to reach equilibrium depends on final Alfvén timescale
(confirmed in simulations)

In an Ap star with B ~ 1 kG, τA ~ 10 yr

But during relaxation, age of star t ~ τA
Mittwoch, 9. Mai 12
Importance of rotation

Could fields of Vega and Sirius be dynamically evolving?

Assuming age ~ τA , B ~ 25 and 50 µG in Vega & Sirius

Too weak!
Mittwoch, 9. Mai 12
Importance of rotation

Could fields of Vega and Sirius be dynamically evolving?

Assuming age ~ τA , B ~ 25 and 50 µG in Vega & Sirius

Too weak!

Missing ingredient is rotation
du
=
dt
U2
1
1
⇤P + g +
(⇤ ⇥ B) ⇥ B - 2Ω x u
⇥
4 ⇥
cs2
vff2

In regime Ω << 1/τA , τevol ~ τA

In regime Ω >> 1/τA , τevol ~ τA2 Ω

vA2
(LΩ) U
(τevol = L / U)
Again assuming age ~ τevol , fields of Vega and Sirius should be
~ 10 G. Better!
Mittwoch, 9. Mai 12
Improving the numbers

First estimate gives too strong fields

Possible explanations:



Mittwoch, 9. Mai 12
Field strength higher in interior than at surface
Observations underestimate field strength
(cancellation effects)
Characteristic length scale L is smaller than R
What determines what happens?
Initial conditions?

Strength and form of equilibrium depend on initial conditions,
particularly on magnetic helicity

H ≡ ∫A.B dV, where A is vector potential given by B = ∇xA

Units: H = E L

H is roughly conserved, and at equilibrium Eeq = H / Leq ≳ H / R

Possible scenarios for initial conditions:
−
Large H, large E: high-energy equilibrium forms fast.
At time of observation, τevol << age
−
Small H, large E: evolves quickly at first, then more slowly.
No equilibrium found during lifetime of star. τevol = age.
−
Small H, small E: evolves slowly right from beginning.
No equilibrium found. τevol > age. Problematic for star formation!
Reference: Braithwaite & Cantiello 2012
Mittwoch, 9. Mai 12
Contents



Review of observations of early-type stars
− Intermediate-mass stars
− High-mass stars
Hypotheses to explain nature and/or
presence of magnetic fields:
− Fossil fields and failed fossils
− Subsurface convection
− Other possibilities
Summary
Mittwoch, 9. Mai 12
Subsurface convection



Subsurface convective layer
Possible dynamo
activity
Field could rise to
surface via buoyancy
Cantiello et al. 2008
Mittwoch, 9. Mai 12
Subsurface convection



Subsurface convective layer
Possible dynamo
activity
Field could rise to
surface via buoyancy
Cantiello et al. 2008
Mittwoch, 9. Mai 12
Subsurface convection in massive stars



Dynamo field may fluctuate on timescale ~ τconv ~ day
Field reaches surface
via buoyancy,
since field provides
pressure without mass
Buoyant rise at
Alfvén speed
Mittwoch, 9. Mai 12
Field strengths expected

Assuming equipartition
in convective zone:
B2 / 8π ~ ρ u2 / 2
B (G)
0
500
120 MSun
6.0
2000
2500
150 1000
0 500 0
2000
1500
00
35 MSun
500
5.0
Log L
1500
10
5.5
1000
1000
20 MSun
4.5
4.0
10 MSun
3.5
3.0
4.8
500
0
GAL
7 MSun
4.6
4.4
logTeff
4.2
Cantiello & Braithwaite 2011
Mittwoch, 9. Mai 12
4.0
Field strengths expected

Further assuming
B
∝ρ
B (G)
2/3
on way to surface

Fields can have
significant effects
on wind
10
40
80
320
160
40
16 0
35 MSun
1020 5
40
5
20
20 MSun
4
5
10 MSun
3
4.8
GAL
7 MSun
4.6
4.4
logTeff
4.2
Cantiello & Braithwaite 2011
Mittwoch, 9. Mai 12
> 320
80
120 MSun
20
10
But small-scale
fields are not
directly detectable
with Zeeman
effect
6
5
Log L

0
4.0
Spots on surface?




Magnetic pressure causes gas pressure in magnetic feature to be
lower than in surrounding gas
Consequently photosphere
is lower
So spot is hot
(In the Sun, convection is
inhibited so spot is colder)
For fields of ~ 100 G emerging at the surface this leads to a
temperature increase of ~ 500-1000 K. A hot, bright spot
Mittwoch, 9. Mai 12
Appearance of spots
Spot in convective star
Mittwoch, 9. Mai 12
Spot in radiative star?
Spots in a O8V star?
HD 46149 (Degroote et al. 2010, using CoRoT)
Mittwoch, 9. Mai 12
Spots in a
B0.5IV star?
HD51756 (Pápics et al. 2011, using CoRoT)
Mittwoch, 9. Mai 12
Spots in a
B0.5IV star?
HD51756 (Pápics et al. 2011, using CoRoT)
Mittwoch, 9. Mai 12
Subsurface convection in A and late B stars

Same principle as in massive stars, except

Opacity bump and convective layer due to helium ionisation

Very thin layer, 1% of radius below surface


Can produce fields at photosphere of a few gauss
(subject to assumptions e.g. equipartition field, B~ρ2/3 as field
rises buoyantly through overlying radiative layer, etc.)
Origin of Vega & Sirius fields?
Mittwoch, 9. Mai 12
Comparison of subsurface convection and
failed-fossil fields
Geometry of field: length scale
Failed fossil
Subsurface
convection
L ~ R/5 ?
small-scale?
Geometry: latitude dependence? stronger at pole?
probably
Time variability & (weak) x-rays no
yes
Field strength predicted
in Vega & Sirius
< 10G
~3G ?
Correlation with age
yes
not much
Correlation with spin period
yes
? dynamo theory
Likely in massive stars too?
no
yes
Mittwoch, 9. Mai 12
Comparison of subsurface convection and
failed-fossil fields
Strengths
Failed fossil
Subsurface convection
produces right length scales
we know dynamos work
produces right field strengths right field strengths
Weaknesses
ignores meridional circulation ? dynamo theory ?
etc
still have issue of bimodality
in initial conditions
how to get larger length
scales?
ignorance of initial conditions details of convective
hampers predictive power
layers still uncertain
no confirmation yet of
τevol ~ τA2 Ω from simulations
Mittwoch, 9. Mai 12
Comparison of subsurface convection and
failed-fossil fields
Strengths
Failed fossil
Subsurface convection
produces right length scales
we know dynamos work
produces right field strengths right field strengths
Weaknesses
ignores meridional circulation ? dynamo theory ?
etc
still have issue of bimodality
in initial conditions
how to get larger length
scales?
ignorance of initial conditions details of convective
hampers predictive power
layers still uncertain
no confirmation yet of
τevol ~ τA2 Ω from simulations
Mittwoch, 9. Mai 12
Further thoughts (on [failed] fossils)

Unknown initial radial magnetic energy distribution


Why bimodality in initial helicity?




Assume decaying T-S dynamo and
guess initial rotation profile?
Mergers? But all merger does is create differential rotation.
Why any different from primordial diff. rot.?
Blue stragglers an important clue?
Do simulations to confirm τevol ~ τA2 Ω
Relaxation -- motion on spherical shells:
what forces (components perpendicular to gravity)?

pressure gradient

Lorentz force. How many degrees of freedom?

Coriolis --OR-- inertia
Simulations with barotropic e.o.s.?
Mittwoch, 9. Mai 12
Contents



Review of observations of early-type stars
− Intermediate-mass stars
− High-mass stars
Hypotheses to explain nature and/or
presence of magnetic fields:
− Fossil fields and failed fossils
− Subsurface convection
− Other possibilities
Summary
Mittwoch, 9. Mai 12
Further thoughts regarding Vega & Sirius

Meridional circulation could

drive dynamo directly?

modify a failed fossil

drive differential rotation?

Differential rotation could drive dynamo

BUT: dynamo vanishes towards surface

So how to get field to surface?


bouyancy instability

buoyancy, diffusive
...?
Mittwoch, 9. Mai 12
Contents



Review of observations of early-type stars
− Intermediate-mass stars
− High-mass stars
Hypotheses to explain nature and/or
presence of magnetic fields:
− Fossil fields and failed fossils
− Subsurface convection
− Other possibilities
Summary
Mittwoch, 9. Mai 12
Summary

Weak magnetic fields evolve very slowly in rotating stars

Vega and Sirius fields could be



`failed fossils´, continuously evolving

come from a subsurface convective dynamo
Either way, all A & late B stars without fossil fields should
have fields of at least ~1G unless v.slowly rotating
Subsurface dynamo in O and early B stars could produce
~100 G fields at the surface, and play a role in the various
unexplained observational phenomena in these stars
Mittwoch, 9. Mai 12
Early-type stars:
Summary of observations and theories
spectral type
A and late B
O and early B
field detected
Steady, strong,
large-scale fields
~10% of stars,
Some fraction
the so-called Ap/Bp stars (10, 20%?)
200G < B < 100kG
B ~ 1kG?
Theory: fossil field
Theory: fossil field (?)
Rest of population Weak fields found in two No fields detected, but
bright A stars. Present in indirect suggestions of
all other stars too?
magnetic activity in
whole population
Theories: Failed fossil,
subsurface convection? Theory:
Seems likely all other A Subsurface convection?
stars have these fields.
Mittwoch, 9. Mai 12
Mittwoch, 9. Mai 12
Brainstorming
J: just done
D: develop
F: future
Theoretical considerations
Observational puzzles
Subsurface convection
Drop in flux during Ap star lifetime
F: observational consequences
Fossils and failed fossils
D: formation: what to expect given
different radial energy
distributions and helicity. what are
the degrees of freedom in initial
conditions?
field becomes potential near surface?
meridional circulation?
Origin of Ap-minority, variety in pulsar fields,
etc. Merger hypothesis?
Field strengths in accreting NSs and in
former accretors
D&F: diffusive evolution: different in
different stars. santiago gang
know about hall drift etc etc
F: think about initial conditions? T-S
dynamo dies away leaving what?
Likely radial energy distribution?
Magnetic effects with differential rotation
(v. brief because of henk‘s talk)
mention work of Luis
what else?
Mittwoch, 9. Mai 12
44
Abundance patterns on HR 3831 (Kochukhov et al)
Observations in main-sequence stars:
A and late B



Some fraction display fossil fields, so-called Ap/Bp stars
−
Steady over > decades
−
Strong: 200G to 100kG
−
Large-scale, often roughly dipolar
Rest have fields < 10 gauss
−
No stars with fields 10 to 200 G? (Aurière et al 07)
−
Recent detection of weak fields in Vega and
Sirius: 0.6 and 0.2 G.
Great variety of chemical abundances in all A stars:
lots going on with mixing, radiative levitation, gravitational
settling, selective evaporation, etc. Lots of physics!
Mittwoch, 9. Mai 12
Opacity
46
Mittwoch, 9. Mai 12
Opacity
46
Mittwoch, 9. Mai 12
Opacity
Strong Z-dependency of all
phenomena connected to FeCZ
46
Mittwoch, 9. Mai 12
Surface Turbulence
Matteo Cantiello
Mittwoch, 9. Mai 12
Near-Surface Convection in Massive Stars
KITP – October 24th 2011
3D Hydro Simulations

Pencil Code



Setup: piecewise
polytropic (stableunstable-stable)
Cartesian grid
128 x 128 x 256
Fcon/Frad ~ 0.3
Re ~ 80

Shown is vertical

(Brandenburg & Dobler
2002)
Preliminary, low resolution runs!!!
(Cantiello, Käpylä, Brandenburg et al. In Prep.)
Matteo Cantiello
Mittwoch, 9. Mai 12
Near-Surface Convection in Massive Stars
KITP – October 24th 2011
Microturbulence
Is the additional broadening coming from nonthermal
motions varying on a small scale in the region of line
formation.
Observed line profile
Intrinsic line profile
ΔλD =
Matteo Cantiello
Mittwoch, 9. Mai 12
λ
c
V
2
therm
Near-Surface Convection in Massive Stars
+ξ
2
turb
KITP – October 24th 2011
Microturbulence
Is the additional broadening coming from nonthermal
motions varying on a small scale in the region of line
formation.
Matteo Cantiello
Mittwoch, 9. Mai 12
Near-Surface Convection in Massive Stars
KITP – October 24th 2011
Microturbulence
Is the additional broadening coming from nonthermal
motions varying on a small scale in the region of line
formation.
 To fit stellar spectra of hot stars microturbulence
(~0-25 km/s) is needed
Matteo Cantiello
Mittwoch, 9. Mai 12
Near-Surface Convection in Massive Stars
KITP – October 24th 2011
Microturbulence
Is the additional broadening coming from nonthermal
motions varying on a small scale in the region of line
formation.
 To fit stellar spectra of hot stars microturbulence
(~0-25 km/s) is needed
 Used as a fudge-factor. Unknown physical
origin
Matteo Cantiello
Mittwoch, 9. Mai 12
Near-Surface Convection in Massive Stars
KITP – October 24th 2011
Microturbulence
Is the additional broadening coming from nonthermal
motions varying on a small scale in the region of line
formation.
 To fit stellar spectra of hot stars microturbulence
(~0-25 km/s) is needed
 Used as a fudge-factor. Unknown physical
origin
 But recently a correlation between near-surface
convection and microturbulence has been found!
Matteo Cantiello
Mittwoch, 9. Mai 12
Near-Surface Convection in Massive Stars
KITP – October 24th 2011
Energetic considerations
2
Eg
ρ c ⎛ v c ⎞
≈ M c ⎜ ⎟
Et
ρ s ⎝ ξ ⎠
€
Theoretical models
Cantiello et al. 2009
Matteo Cantiello
Mittwoch, 9. Mai 12
Near-Surface Convection in Massive Stars
KITP – October 24th 2011
Energetic considerations
2
Eg
ρ c ⎛ v c ⎞
≈ M c ⎜ ⎟
Et
ρ s ⎝ ξ ⎠
€
Theoretical models
+ VLT-Flames observations
Cantiello et al. 2009
Matteo Cantiello
Mittwoch, 9. Mai 12
Near-Surface Convection in Massive Stars
KITP – October 24th 2011
Results
ξ = f (L,Teff ,Z)
€
Matteo Cantiello
Mittwoch, 9. Mai 12
Near-Surface Convection in Massive Stars
KITP – October 24th 2011
Results
ξ = f (L,Teff ,Z)
€
Matteo Cantiello
Mittwoch, 9. Mai 12
Near-Surface Convection in Massive Stars
KITP – October 24th 2011
Results
ξ = f (L,Teff ,Z)
€
Matteo Cantiello
Mittwoch, 9. Mai 12
Near-Surface Convection in Massive Stars
KITP – October 24th 2011
Results
ξ = f (L,Teff ,Z)
€
Matteo Cantiello
Mittwoch, 9. Mai 12
Near-Surface Convection in Massive Stars
KITP – October 24th 2011
Solar-like oscillations
Matteo Cantiello
Mittwoch, 9. Mai 12
Near-Surface Convection in Massive Stars
KITP – October 24th 2011
Near-Surface Convection (Solar Z)
Matteo Cantiello
Mittwoch, 9. Mai 12
Near-Surface Convection in Massive Stars
KITP – October 24th 2011
Solar-like oscillations in massive stars
Cantiello et al. 2009
Suggest that near-surface convection in hot, massive stars
could cause stochastically excited pulsations
Belkacem et al. 2009
Corot detection of solar-like oscillations in the massive star
V1449 Aql (B type Star) [However, see Aerts et al. 2011]
Belkacem et al. 2010
Theoretical calculations of stochastically excited modes from
sub-surface convection.
Degroote et al. 2010
Corot detection of solar-like oscillations in an O-type star
Matteo Cantiello
Mittwoch, 9. Mai 12
Near-Surface Convection in Massive Stars
KITP – October 24th 2011
Solar-like oscillations in O star
HD 46149 (Degroote et al. 2010)
Matteo Cantiello
Mittwoch, 9. Mai 12
Near-Surface Convection in Massive Stars
KITP – October 24th 2011
Photometric variability: HRD location
HD46149 (Degroote+ 2011)
Matteo Cantiello
Mittwoch, 9. Mai 12
Near-Surface Convection in Massive Stars
KITP – October 24th 2011
Red Noise
Matteo Cantiello
Mittwoch, 9. Mai 12
Near-Surface Convection in Massive Stars
KITP – October 24th 2011
“Red Noise” in O stars
 Variability in the CoRoT photometry of 3 hot
O-
type stars
(Blomme et al. 2011)
Matteo Cantiello
Mittwoch, 9. Mai 12
Near-Surface Convection in Massive Stars
KITP – October 24th 2011
“Red Noise” in O stars
 Variability in the CoRoT photometry of 3 hot
O-
type stars
 No clear pulsations detected
(Blomme et al. 2011)
Matteo Cantiello
Mittwoch, 9. Mai 12
Near-Surface Convection in Massive Stars
KITP – October 24th 2011
“Red Noise” in O stars
 Variability in the CoRoT photometry of 3 hot
O-
type stars
 No clear pulsations detected
 Variability of stochastic nature
(Blomme et al. 2011)
Matteo Cantiello
Mittwoch, 9. Mai 12
Near-Surface Convection in Massive Stars
KITP – October 24th 2011
“Red Noise” in O stars
 Variability in the CoRoT photometry of 3 hot
type stars
 No clear pulsations detected
 Variability of stochastic nature
 Near-surface convection, granulation or wind
inhomogeneities
O-
(Blomme et al. 2011)
Matteo Cantiello
Mittwoch, 9. Mai 12
Near-Surface Convection in Massive Stars
KITP – October 24th 2011
Photometric variability: HRD location
HD46150
HD46223 (Blomme+ 2011)
HD46966
Matteo Cantiello
Mittwoch, 9. Mai 12
Near-Surface Convection in Massive Stars
KITP – October 24th 2011
Magnetic fields / Spots
Matteo Cantiello
Mittwoch, 9. Mai 12
Near-Surface Convection in Massive Stars
KITP – October 24th 2011
B fields in massive stars (direct evidence)
 About a dozen magnetic OB
stars found (e.g. Donati, Hubrig, Neiner, Petit)
 Detection through Zeeman
spectral signature
 Bias toward strong, large
scale fields
 Origin unclear. Likely Fossil
(Wade et al. 2010)
Tau Sco
Credits: Jardine & Donati
 Important evolutionary
consequences (e.g. ud-doula & Owocki 02,
Meynet et al. 2010)
Matteo Cantiello
Mittwoch, 9. Mai 12
Near-Surface Convection in Massive Stars
KITP – October 24th 2011