magnetism in herbig ae/be stars and the link to the ap/bp stars

Magnetism in Herbig Ae/Be stars and the link to the Ap/Bp stars

E. Alecian, C. Catala, G.A. Wade, C. Folsom, J. Grunhut, J.-F. Donati, P. Petit, S. Bagnulo, S.C. Marsden,

J.D. Landstreet, T. Böhm, J.-C. Bouret, J. Silvester

CNRS Summer schoolLa Rochelle, 24 - 28 September 2007

E. Alecian CNRS Summer School

La Rochelle, 24 - 28 Septembre 2007

Plan

1. Introduction

2. Field Herbig Ae/Be stars study : magnetism

3. Field Herbig Ae/Be stars study : rotation

4. Cluster study

5. Conclusion and Open Issues

1. Introduction



The intermediate mass stars (1)

• Pre-main sequence (PMS): from birthline to ZAMS

Herbig Ae/Be stars (HAEBE)

• Main sequence (MS): around the ZAMS

A/B starsA/B stars

HAEBE



The intermediate mass stars (2)

• HAEBE stars: – radiative inside +

convective envelope, or

– convective core + radiative envelope, or

– totally radiative

• A/B stars– convective core + radiative

envelope

Convective envelope

disappearing

Convective core

apparition



The chemically peculiar stars

• Ap/Bp : ~5% of A/B stars• Abundances anomalies compared to normal A/B stars• Slow rotators

• Ap/Bp: Magnetic stars : 300G to 30kG, large scale organised magnetic field : mostly dipole+quadrupole



Problematic 1

• Origin of the magnetic fields in the Ap/Bp stars– Favoured hypothesis : the fossil field hypothesis

some of the intermediate mass PMS star should be magnetic topology of B(PMS A/B) = topology B(Ap/Bp) intensity B(PMS A/B) compatible with intensity B(Ap/Bp) (assuming

the magnetic flux conservation)

– The core dynamo hypothesis



Problematic 2• Origin of the slow rotation of the Ap/Bp stars

– Hypothesis 1 : magnetic braking during the PMS phase (Stepien 2000)

magnetic PMS A/B stars should existPMS A/B stars should have a diskEvolution of the rotation during the PMS phase

– Hypothesis 2 : the magnetic field cannot survive in fast rotators (Lignières et al. 1996)

No magnetic fast rotators during the PMS phase

We need to observe the PMS intermediate mass stars



The Herbig Ae/Be stars

• A and B stars with emission lines• IR emission• Association with nebulae• Characteristics associated with magnetic activity :

– resonance lines as N V and O VI, X-ray emission : hot chromospheres or coronae (e.g. Bouret et al. 1997)

– magnetospheric accretion (e.g. Mannings & Sargent 1997)

– rotational modulation of resonance lines : wind structured by magnetic field (e.g. Catala et al. 1989, 1999)

} definition (Herbig 1960)

Many indirect signs of magnetic fields



Strategy (1)

• Observation of the field Herbig Ae/Be stars– Detection of magnetic field– Characterisation of their magnetic fields– Compare to the magnetic fields of Ap/Bp stars

Fossil field hypothesis test

– vsini determination– Compare to vsini of Ap/Bp star– vsini as a function of age

Origin of slow rotation hypothesis tests



Strategy (2)

• Observations of the HAEBE stars in young clusters and associations– stars of a single cluster: = age and = initial conditions– ≠ clusters ≠ ages and ≠initial conditions

Disentangle evolutionary effects from initial condition effects

Understand the evolution of the magnetic field during the PMS phase, and its impact on the evolution of the stars



What is our method ?

• The spectropolarimetry: polarisation study inside the spectral lines

• Recall: Zeeman effect in the stars Stokes V parameter ≠ 0

• In the weak field approximation (B<10kG): V dI/d * Bl

We observe the Stokes V spectra



Magnetic fields in Herbig Ae/Be stars ?

• AB Aur : Catala et al. (1993), Catala et al. (1999)no detection

• HD 100546 : Donati et al. (1997)no detection

• HD 104237 : Donati et al. (1997)1st detection (recently confirmed)

• HD 139614 : Hubrig et al. (2004)detection not confirmed with more accurate observations

• HD 101412 : Wade et al. (2007)detection (recently confirmed)

But now we have ESPaDOnS !



ESPaDOnS (CFHT, Hawaii) + LSD: the good formula

• High-resolution spectropolarimeter : R = 65000, broad spectral range (370 - 1080 nm)

• Reduction : Libre-Esprit package (Donati et al. 1997, 2007)

• Least Squares Deconvolution (LSD) method (Donati et al., 1997)

More lines, better S/N ratio, larger magnitude V range of the star

Increase our chances to detect magnetic fields

For more details see the talk of Coralie Neiner

2. Field Herbig Ae/Be stars study : magnetism



Our sample

• Catalogues : Vieira et al . (2003) and Thé et al. (1994)

• 55 Herbig Ae/Be stars

• 1.5 – 20 Msun



Observations and reduction

• For each star: – (one or many) Stokes I and V spectra

– Determination of Teff and log(g)

– LSD method: mask of Teff and log(g) of the star, not including Balmer lines and lines contaminated by emission

– Searching for a Zeeman signature in the LSD V profile



LSD for I

=

*

Spectrum

Mask

Stokes I profile

Donati et al. (1997)



LSD for V

B non détectéB0

Spectra

Stokes V profile

Mask

=

*

Stokes V profile

Zeeman signature



A0, vsini~8.6 km/s

Results

Wonderful Zeeman signatures !!!

55 observed, 4 magnetic ~7% magnetic Herbig Ae/Be stars

B3, vsini~26 km/s B9, vsini~41 km/s

A2, vsini~9.8 km/s



How characterise their magnetic fields ?

1. Model the time variations of Bl

2. Model the time variations of the Stokes V profiles

l

IV B

∂

∝∂

Observations of the stars at different time



The oblique rotator model

• Compute I, V and Bl:

– I(,) : G(instr,v(,) )

– V(,) dI/d Bl (,)

(weak field approximation)

– Bl (,) : oblique rotator model

(Stift 1975)

– Integration over the surface : limb-darkening law

• 5 parameters: (P,0,,Bd,ddip)

B

ObsD

ddip

i



The Oblique rotator model : Example

QuickTime™ and aH.264 decompressor

are needed to see this picture.

i = 50 °

= -60°

Bd = 1000 G



First method: the longitudinal field Bl

[ ]∫∫

−=

dv)v(I1gc

dv)v(vV10x14.2B 11

l

Mean over the lines

Stokes V parameter

Stokes I parameter

(Donati et al. 1997)



Longitudinal field variations of HD 200775

P = 4.328 j

Alecian et al. 2007

2 = 1.25 Estimation of the period



2nd method: Fitting the Stokes V profiles

• Compute a grid of V by varying the 5 parameters: 0 : the reference phase

– P : the rotation period : the magnetic obliquity

– Bd : the dipole intensity

– ddip: the dipole position

2 minimisation



Magnetic field characterisation : HD 200775

P = 4.328 d. i = 13 ° = -102° Bd = 1000 G ddip = 0.10 R*

Alecian et al. (2007)



Magnetic field characterisation : V380 Ori

P = 7.6 d.i = 34° = -95°

Bd = 1.4 kG

ddip = 0 R*

P = 9.8 d.i = 47° = -95°

Bd = 1.4 kG

ddip = 0 R*

2 dipole solutions



Magnetic field characterisation : HD72106

P = 0.63995 d. i = 23° = 60° Bd = 1300 G ddip = 0 R*

Folsom et al. (2007)



Catala et al. (2007)

Magnetic field characterisation : HD 190073

• 3 different hypothesis :– Pole-on star = 0

– Long Period

• In all cases:– Simple dipolar Zeeman signature

– Signature stable over more than 2 years

strong probability for an organised magnetic field

• Bd = 100 - 1000 G



Other detections

• SemelPol +UCLES (AAT) = antecedent of ESPaDOnS• Simple Zeeman signature consistent with an organised field

HD 104237 HD 101412

A4, vsini = 11.6 km/sBl = -50 G

A0, vsini = 4.8 km/sBl = -120 G

Thanks to S. Bagnulo and S.C. Marsden



Fundamental parameters of the stars

• Position in the HR diagram compared to evolutionary tracks

M, R, age, PMS timeProportion of PMS

time performed: gives the evolutionary status (independent of the mass)

R on the ZAMS



First conclusions on the magnetic field

• 7% magnetic HAEBE stars• Projection of magnetic Ap/Bp stars on the PMS

phase prediction of 5-10% magnetic HAEBE stars• Large scale organised magnetic field in HAEBE stars• Magnetic intensity of the HAEBE projected on the

ZAMS : same order of the intensity of B(Ap/Bp): (assuming the magnetic flux conservation)

HD 200775: on the ZAMS Bd = 3.6 kGV380 Ori: on the ZAMS Bd = 2.4 kGHD 72106: already on the ZAMS Bd = 1.3 kGHD 190073: on the ZAMS Bd = 400 - 4000 G

Strong arguments in favour of the fossil field theory

3. Field Herbig Ae/Be stars study : rotation



Distribution of vsini

• All field magnetic HAEBE are slow rotators• No magnetic HAEBE are fast rotators

• Magnetic HAEBE stars seem to have been braked more than the non-magnetic HAEBE stars

Magnetic HAEBE stars

Non magnetic HAEBE stars



Period in function of time

• No clear evolution of the period• Majority of HAEBE: between 40 and 80% of their PMS track• To study period evolution we need younger stars than our sample



Evolution of vsini to the ZAMS

• vsini HAEBE on the ZAMS close to normal A/B stars• No clear indications of braking from HAEBE age to MS

Norm A/B starsNon magnetic HAEBENon magnetic HAEBE

on the ZAMSRoyer et al. (2002)

4. Cluster study



NGC 6611 sample

• Age = ~1 MyrYounger than the field

HAEBE

• 3 - 20 MsunFill the whole in the

HRD



NGC 2244 Sample

• Age ~ 8 Myr

• 2 - 20 Msun



NGC 2264 sample

• Age = 9Myr

• 1.5 - 9 Msun



Cluster resultsNGC6611 W601 NGC 2264 NGC2244 201

12 observed stars

1 magnetic

12 observed stars

1 magnetic

18 observed stars

0 magnetic

Does the initial conditions play a role ?

?

B1.5, vsini~180 km/s B1, vsini~25 km/s



vsini of the cluster magnetic stars

• NGC6611 W601 180 km/s ~ 1 Myr B1.5

• NGC2244 201 25 km/s ~ 8 Myr B1

• Can we see a sign of the evolution of the rotation in the magnetic HAEBE stars?

vsini age Sp.T.



Conclusions (1) : Field HAEBE study

• Magnetism:– 7% magnetic HAEBE

– HAEBE magnetism in favour of the fossil field hypothesis

• Rotation:– vsini(magnetic HAEBE) < vsini(non magnetic HAEBE)

– Magnetic HAEBE: slow rotators and very youngA braking mechanism acts very early during the PMS phase

– Dvsini(HAEBE on ZAMS) = Dvsini(A/B Norm)Constant angular momentum evolution from the age of

HAEBE to the MS



Conclusions (1): preliminary cluster study

• Magnetism– Detections in 2 clusters, none in one cluster

The initial conditions may play a role on the presence (or on the intensity) of magnetic fields

• Rotation– At 1Myr, one magnetic star with vsini~180 km/s

Promising for the study of the angular momentum evolution, as well as the impact of magnetic field on the rotation evolution of HAEBE stars



Conclusion (2): Fossil Field against Convective Core hypothesis• 5 magnetic stars are in the totally

radiative phase• These stars have the same type of

magnetic field of the stars with a convective core

Core convection does not appear to be responsible for the presence of magnetic fields in HAEBE stars

The magnetic fields of the intermediate mass stars are very likely FOSSIL



Open Issues

• Unanswered questions :– Only a fraction of stars is magnetic : why all the

stars are not magnetic ?– Clusters– Binaries : one magnetic + one non-magnetic– Protostellar phase : is the field able to survive

during that phase ?– Decentered dipole (or dipole + quadrupole) :

how the molecular cloud contraction can form that field topology ?



Thank you for your attention

magnetism in herbig ae/be stars and the link to the ap/bp stars

Documents

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