Observation of the Magnetic Field

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Observation of the Magnetic Field

• François Ménard
• Equipe FOST
• Laboratoire dAstrophysique
• de Grenoble

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Outline

• I- Introduction
• II- Observation of B
• Methods, limitations
• III- Results at large scales
• From external galaxies to GMCs
• IV- Results at subparsec scales
• Cores, stars, disks and jets
• V- Photospheric Magnetic Fields
• Global photospheric field, Zeeman Doppler
  mapping
• VI- Concluding remarks

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I. Introduction

• The interstellar medium is permeated by a
  magnetic field
• Morphology and strength of B have been actively
  studied for the past 50 years
• Since the discovery of Pis
• Important consequences on the star formation
  process
• Is Magnetic energy density comparable to other
  energy densities???
• Is role of B important or not for support
  against collapse?
• Is role of B important in launching jets 8)

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I. Introduction

• 2 ways to estimate B
• Measure its morphology
• Tangled-gt then turbulence may dominate
• Uniform-gt then might control collapse
• Collapse along field lines
• Linear sheet-like structures
• Helical filaments
• Measure its strength
• Large scales Can B support clouds against
  gravitational collapse?
• Small scales can B help propel jet?

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I. Introduction

• In the rest of the talk
• We will have a brief look at B from large scales
  to small scales starting from the Galaxy, ending
  with accretion disks and stellar photospheres.

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II. How to measure B?

• In the interstellar medium
• can measure either B?? or B?
• But not both at the same time
• Always involves the detection of polarised
  radiation
• Emitted by, or passing through, IS medium
• Emitted by magnetised photosphere, disk
• See, e.g., review by Crutcher, Heiles, Troland
  (2003), LNP, 614 , 155-181.

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II. How to measure B?

• Tracers of B??
• Faraday rotation position angle of linear
  polarisation rotates when passing through a
  medium with magnetic field.
• Need electron density!!!
• Rarely available, except for pulsars

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II. How to measure B?

• Tracers of B?
• Synchrotron radiation an electron with a
  velocity v moving in a medium with a magnetic
  field B will follow a helical path due to Lorentz
  force (v ? B). The acceleration leads to linearly
  polarised radiation.
• GOAL measure the position angle of the linear
  polarisation to derive the position angle of the
  magnetic field in the plane of the sky.

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II. How to measure B?

• Tracers of B?
• Polarisation by dust linear polarisation at
  optical, NIR, and radio wavelengths arises from
  elongated grains with their short axis aligned
  with the magnetic field.
• Davis-Greenstein mechanism charged, spinning IS
  grains are aligned via paramagnetic relaxation
  which damps the component of the grains angular
  momentum perpendicular to the magnetic field,
  gradually aligning the spin axis with the
  magnetic field direction.
• More efficient mechanisms supra-thermal
  rotation and
• radiative torquing.
• See Lazarian for reviews.

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II. How to measure B?

• Tracers of B?
• Polarisation by dust
• In practice The grain properties and the
  alignment mechanisms are not understood well
  enough to infer field strengths reliably.
• However, observations of linear polarisation can
  trace the direction of B projected in the plane
  of the sky.

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II. How to measure B?

• Zeeman Splitting if medium is permeated by B,
  radiation is split by normal zeeman effect into
  three separate frequencies ??? , ?? , ???

??
?-
?
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II. How to measure B?

• Zeeman Splitting
• For B in plane of the sky
• The 3 zeeman components are linearly polarised
• The ?-component polarised parallel to the field
• The ?-components polarised perpendicular to the
  field
• Note sum is null, unless
• ?-component is saturated

?
?
??
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II. How to measure B?

• Zeeman Splitting
• For B along the line-of-sight
• The intensity of ?component is zero, I? 0
• The two ?-components are circularly polarised

?
?
?
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II. How to measure B?

• General considerations
• Zeeman Broadening ???(e / 4?mc2 ) ?2 geff B
• J(J1) S(S1)
  - L(L1)
• Landé factor g 1
• 
  2 J (J1)
• Found empirically (!!!) by Lande in 1923.
• Zeeman splitting increases faster with ? than
  doppler broadening
• In IS medium, the field is too weak to separate
  components. Although possible in theory, cannot
  fully measure B in practice, linear polarisation
  is too weak! Situation is different for stars
  where B 1kG is more typical. With high res.
  spectroscopy, one can measure splitting, and
  sometimes linear polarisation!

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III-Large scales Extra-galactic

• Magnetic field is usually well organised
• Example of NGC 6946
• Magnetic field often follows spiral arms (not
  always)
• Typical values 1-10 ?G
• See Beck (2001) for a review
• Space Science Reviews, 99, 243

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III-Large scales Galactic

• Milky Way is similar
• More difficult to observe globally
• But more angular resolution is available

- Serkowski, Mathewson, Ford - Axon Ellis -
Heiles
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III-Large scales molecular clouds

• Magnetic field permeates star forming regions
• What is its impact the collapse?
• M/?B is critical or not?

See poster by Curran et al. DR 21 (OH) for
another example
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III-Large scales molecular clouds
Collapse drags field lines in
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III-Large scales molecular clouds

• Magnetic fields in star forming regions
• A few Zeeman measurements
• of HI and OH lines available.
• ( a few other (rare) lines)
• Correlation between
• B and density?

Typical Zeeman signature in circular
polarisation. Yields value of B//
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III-Large scales molecular clouds

• Magnetic fields in star forming regions
• To make a long story short
• There is a correlation between
• B and density
• -Heiles law B ? n
• Does not hold for n lt 103
• At low densities B flattens
• to 1 ?G

See also Goodman Myers
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III- Toward smaller scales

• So, the magnetic field seems to play a role in
  Star Formation and guide the collapse at large
  scales
• but certainly not with an iron grip at smaller
  scales
• typical scale is 0.1-1pc

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Goodman, Myers, Bastien, Ménard (1990)
Somewhere in the ? Oph cloud
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III-The Taurus GMC
GALACTIC PLANE
Tamura Sato 1982
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III- Sheets, Cores and Stars in Taurus
Hartmann 2002
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IV- Sub-parsec scales

• Going to smaller scales
• - Do stars remember the original magnetic field?
• - Is there a link between the large scale
  (original?) magnetic field and the one needed to
  drive jets?
• -gtOne possible way to check compare directions
  of stellar flows and disks w/ local magnetic
  field in molecular clouds

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IV- As a reminder

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IV- Sub-pc scales previous claims

• Previous claims
• Young Stellar Objects align with the large scale
  magnetic field"
• Claim is incorrect see Ménard Duchêne 2004
• Results were biased by source selection
• - sources with HH objects only
• e.g, Strom et al. 1986, in L1641
• - embedded sources, direction inferred from
  K-band polarimetry
• e.g., Tamura Sato 1987, 1989 Moneti et al.
  1984,

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IV- Sub-pc scales Jets Disks

• Building a large sample in Taurus

JETS DISKS
DISK NO JET
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IV Orientations of CTTS in Taurus
JET ( disk)
DISK / NO JET
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IV- More Quantitatively

• Orientation of full CTTS sample is random !!!

with outflow
Full sample
Reprints available
No outflow
Ménard Duchêne (2004)
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V- Photospheric Fields TW Hya

• Global photospheric field ? B ? filling factor

Recall ?? ? g?2B
g1.66
g2.08
No B
Magnetically sensitive Titanium lines
g1.58
g2.5
g0.5
See Johns-Krull Valenti
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V- Mapping Photospheric Field

• Zeeman Doppler Imaging (ZDI)
• Intensity and polarisation
• profiles change as star and photospheric features
  rotate

V 410 Tau Joncour et al. (1994)
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V- Mapping Photospheric Field

• Zeeman Doppler Imaging (ZDI)
• RADIAL FIELD
• Polarimetric signature
• Symmetric
• Always same polarity
• maximum _at_ transit
• depends on inclination and position

Animation credit Pascal Petit
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V- Mapping Photospheric Field

• Zeeman Doppler Imaging (ZDI)
• AZIMUTHAL FIELD
• Polarimetric signature
• ANTI - Symmetric
• polarity flip
• maximum _at_ LIMBS
• depends on inclination and position

Animation credit Pascal Petit
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V- Mapping photospheric fields

• A mildly accreting classical T Tauri star

Data from ESPADONS at CFHT
Image unavailable before Publication Watch for
Donati et al. (2006) and for a star named VXXX
Oph!
Image credit J-F DONATI
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V- M-dwarf unexpected topology
Data from ESPADONS at CFHT
Image credit M JARDINE JF DONATI
SEE DONATI ET AL , Science, 2006, V374 Peg
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V- Mapping Fields in Disk FU Ori

• First detection of the field in a disk.
• Topology OK for MHD jets
• Disk is HUGE, is source representative?

Data from ESPADONS at CFHT
B 1kG _at_ 0.05AU
Donati, Bouvier, Paletou, Ferreira (2005)
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VI- Concluding considerations

• Before and during collapse (gt1pc or so)
• B ? n1/2 (Heiles law)
• from 1 ?G to 10mG, from 103 to 107 cm-3
• B may play a role in Star Formation
• role decreasing with smaller scale?
• During PMS phase, at disk scale
• Need organised magnetic field to drive MHD jet
• Is orientation of B same as in neighbour cloud?
• Difference for sources with collimated atomic
  jets and no-jet sources? Good topic for JETSET

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VI- Concluding considerations

• At the photosphere
• Field topology may be more complicated than
  expected
• multipoles, large azimuthal component, etc
• In the disk
• Only measurement available shows presence of
  poloidal component requiered for MHD models
• Complete topology and dynamics complicated.
• Both more data is needed

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