Magnetism and Rotation in Herbig Ae/Be stars

E. Alecian

Laboratoire d’Astrophysique de l’Observatoire de Grenoble

In collaboration with G.A. Wade, C. Catala, C. Folsom, J. Grunhut, et al.

We are starting a session on massive stars and related

objects

Magnetism and rotation in the low-mass stars

Convective dynamo surface magnetic fields …

M < 1.5 M

Magnetism and rotation in the low-mass stars

Convective dynamo surface magnetic fields …

… of complex configuration, …

… interacting with the environment …

M < 1.5 M

Magnetism and rotation in the low-mass stars

Convective dynamo surface magnetic fields …

… of complex configuration, …

… interacting with the environment …

M < 1.5 M

… leading to an evolution of their angular momentum.

Bouvier et al. 1997, A&A 318, 495

The Ap/Bp stars1.5 M < M < 15 M Chemically peculiar stars

~5% of the MS A/B stars No convective envelope, but …

Magnetic fields are observed !!! Very different from the low-mass stars:

-Large-scale magnetic fields: mainly dipolar

-Fields strengths: from 300 G to 30 kG

-Do not correlate with stellar properties

-Stable over many years

Slow rotators compared to the normal A/B star

The origin must be different from the low-mass stars

… … concentrated in the protostar... concentrated in the protostar...

Origin of the Ap/Pb stars magnetic fields

• The fossil field theory

… … producing a strong, slowly producing a strong, slowly decaying stellar magnetic decaying stellar magnetic

field...field...

Galactic magnetic flux Galactic magnetic flux is swept up during star is swept up during star

formation...formation...

……which may survive well which may survive well beyond the MS phase.beyond the MS phase.

PMS ?

Origin of the Ap/Pb stars magnetic fields

• The fossil field theory, implications

• During the pre-main sequence (PMS) phase :

– 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)

Origin of the slow rotation of the Ap/Bp stars

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

– Star-disk magnetic coupling, or– Magnetised winds

• Hypothesis 2 : the magnetic field cannot survive in fast rotators (Aurière et al. 2007)

– Rotational instabilities in young PMS stars would diffuse the magnetic fields

The Herbig Ae/Be stars• Definition (Herbig 1960):

– A and B stars with emission lines– Lies in an obscured region– Association with nebulae

– IR excess• Characteristics associated with

magnetic activity:– Highly ionised species (N V, O

VI), X-ray emission: hot chromospheres or coronae (e.g. Bouret et al. 1997)

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

– magnetospheric accretion (Muzerolle et al. 2004)

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X-rays

Obs.

O VI, NV

AB Aur H variations Catala et al. 1999

The CIR theory Bouret et al. 1997

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)

We were missing: - high-efficiency polarimeter- large-aperture telescope- high spectral resolution- broad spectral range

ESPaDOnS at the 3.6m Canada-France-Hawaii Telescope

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

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

Our HAeBe survey• Our sample:

– Field HAeBes:• Catalogues : Vieira et al . (2003) and

Thé et al. (1994)• Ages: 1Myr - 30 Myr

– Young clusters:• NGC 2264: age ~ 2.6 Myr

Park & Sung (2000)• NGC 2244: age ~ 2.3 Myr

Park & Sung (2002)• NGC 6611: age < 1 Myr

de Winter et al. (1997)

– 128 HAeBe stars

– Mass range: 1.5 – 20 M

• Observations and reductions:– (one or many) Stokes I and V spectra

– Libre Esprit reduction package

– LSD methodAlecian et al., in prep.

NGC 2244 201B1, vsini~25 km/s

Discovery of magnetic fields

A0, vsini~8.6 km/s

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

NGC 2264 83B3, vsini~65 km/s

A2, vsini~5 km/s

128 observed, 7 magnetic ~5% magnetic Herbig Ae/Be stars

NGC 6611 601B1.5, vsini~180 km/s

Catala et al. 2007, Alecian et al. 2008a, Alecian et al. 2008b, Folsom et al. 2008, Alecian et al. 2009

Other detections

• SemelPol +UCLES (AAT) = antecedent of ESPaDOnS• Observations in April 2007 and March 2010• Simple Zeeman signatures consistent with an organised field

HD 104237 HD 101412

A4, vsini = 11.6 km/s Bl = -50 G

A0, vsini = 4.8 km/s Bl = -120 G

• Compute I and V:

– Bl (,) : oblique rotator model

(Stift 1975)

– I(,) : G(instr,v(,) )

– V(,) dI/d Bl (,)

(weak field approximation)

– Integration over the surface : limb-darkening law

• Comparison of the synthetic to observed I and V

• Compute 2 for (P,t0,i,,Bd,ddip)

2 minimisation

Magnetic field characterisation : Method

B

ObsD

ddip

i

Magnetic field characterisation : V380 Ori

P = 4.31276 d. i = 32 ° = 66° Bd = 2.12 kG ddip = 0 R*

On the ZAMS: Bd = 4.5 kG

Alecian et al. 2009

HAeBe Magnetism- Conclusions

• For the 4 stars studied so far (HD 200775, HD 190073, HD 72106, V380 Ori): Magnetic fields mainly dipolar with Bd from 300 G to 2.1 kG Projection on the ZAMS: 1.2 < Bd < 4.5 kG

Catala, Alecian et al. 2007, Alecian et al. 2008a, Folsom et al. 2008, Alecian et al. 2009

• 5% of magnetic HAeBe stars• Expected characteristics in the progenitors of the Ap/Bp stars

Very strong proofs in favor of the fossil field hypothesis

Distribution of vsini

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

• Similar dichotomy on the MS Magnetic stars have already been braked, OR Fast rotators have already dissipated their fields

Magnetic HAeBe stars

Normal HAeBe stars

Alecian et al., in prep.

Abt et al. 1995

Conclusions• Magnetism

– ~ 5% of HAeBe stars are magnetic– with mainly dipolar magnetic fields– with strengths from 300 G to 2 kG, projected on the

ZAMS: from 1.2 to 4.5 kG We find a fossil link between the magnetic fields of PMS and

MS stars

• Rotation– Magnetic stars are more braked than the normal stars– Whatever the mechanism responsible of this dichotomy

is, it must happen during the very early stages of PMS evolution, or even before

Open Issues• Non-homogeneity:

– Why only 5% of the intermediate mass stars are magnetic ?

– Why in a same cluster, only a few percent is magnetic ?

– Why in a close binary system, one is magnetic and the other one is not ?

• What is happening during the intermediate mass T Tauri phase: the partially convective phase ?

• Why do we have already slow rotators in the PMS phase, and what is happening in the core of the proto-stellar phase ?

What is inside the protostar ???

Structure Magnetic fields Internal Dynamic + Impact on magnetic fields Protostar-environment interaction

ReferencesAlecian et al. 2008a, MNRAS 385, 391

Abt et al. 1995, ApJS 99, 135

Alecian et al. 2008b, A&A 481, L99

Alecian et al. 2009a, MNRAS 400 354

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