Magnetic Fields in the Starforming Region Cepheus A

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Magnetic Fields in the Star-forming Region
Cepheus A

• from H2O maser polarization
• Wouter Vlemmings, JBO
• Phil Diamond, MERLIN/JBO
• Huib Jan van Langevelde, JIVE/Leiden
• José-Maria Torrelles, CSIC-IEEC

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Outline

• Background
• Magnetic fields in star-formation
• Background II
• Water Masers in (high-mass) star-formation
• Previous maser polarization observations
• Observations
• Including some maser theory
• Cepheus A overview
• Results on Cepheus A HW2
• Conclusions
• Extra
• A magnetically collimated jet around an evolved
  star

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Background

• Question (How) do magnetic fields influence the
  dynamics in star-forming regions ?
• Current theory
• low-mass star-formation
• Magnetic fields support against and regulate
  molecular cloud collapse
• Involved in outflow creation and disk support
• high-mass star-formation
• Still many unknowns
• Merging low-mass stars ?
• Recent observations show accretion disks around
  high-mass proto-stars ? points to similar
  formation mechanism as in low-mass star-formation
• Role of magnetic fields unknown
• - turbulence instead of magnetic fields regulate
  infall ?

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Background (II)

• Water masers in star-forming regions
• With VLBI observations we probe small scales (lt1
  mas)
• Circular/linear polarization provides magnetic
  field strength and direction
• Likely occur in shock in disks or outflows
• Typical densities nH2 108 - 1010 cm-3
• Shocks increase pre-shock magnetic field
• Previous observations
• Water maser magnetic fields in star-forming
  regions 5-80 mG (e.g. Fiebig
  Güsten 1987 Sarma et al. 2001,2002)
• Most observations at low resolution underestimate
  field strengths due to blending of maser features
• OH maser observations in lower density regions of
  Cepheus A 5-15 mG (MERLIN, Bartkiewicz et al.
  2005)
• Question (How) do magnetic fields influence the
  dynamics in star-forming regions ?

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Observations

• Polarimetric VLBA observations of the water
  masers around Cepheus A HW2
• Performed at Oct 3 2004
• Determine Linear and Circular polarization
• Linear polarization
• Typically non-existent or weak (lt1)
• Strongly dependant on angle between magnetic
  field and line-of-sight (?) as well as maser
  saturation level
• Either parallel or perpendicular to magnetic
  field direction on the sky, dependant on ?
• Circular polarization
• Caused by the Zeeman effect however the
  splitting in water is small giving rise to
  circular polarization often ltlt1
• Magnetic field strength along the line-of-sight
  BB cos(?) determination depends on accuracy of
  theoretical water maser models
• Dependence on ? changes from cos(?) for higher
  saturation levels
• Velocity and magnetic field gradients along the
  maser influence measured polarization
• Magnetic field can be underestimated by as much
  as a factor 2
• But typically approximately 25

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Masers in the high mass star formation region Cep
A
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Cepheus A HW2 Field I

• Expanding shockwave through rotating
  proto-stellar disk (Gallimore et al. 2003).
• Strong magnetic fields (650mG) with small scale
  reversals.
• Magnetic field direction follows disk
  (inclination 51º).
• Toroidal field ?
• Shock compression of field lines ?
• Field enhancement through nearby Dynamo.

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Cepheus A HW2 Field II

• Newly detected filamentary maser structure.
• Typical magnetic field strengths (80 mG).
• Structure indicative of shocked nature.
• Velocity, Linear polarization and magnetic field
  strength gradient along the filament.
• Shocked interaction between outflow from HW2 and
  surrounding molecular cloud.

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Cepheus A HW2 Field III

• Expanding shell around embedded proto-star.
• (Torrelles et al. 2001).
• Magnetic fields typical for those in the water
  maser regions of SFRs
• (50-100 mG).
• Circular polarization determined field
  overestimates for saturating masers at certain
  field angles.
• Magnetic field direction toward central star.
• Strong linear polarization indicates magnetic
  field perpendicular to polarization vectors.

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Conclusions

• H2O maser magnetic fields consistent with density
  scaling from previous OH maser measurements.
• Field decreases away from HW2.
• Additional source of magnetism in circumstellar
  disk
• Toroidal field (1/r) in disk would imply 2.5
  Gauss near embedded protostar.
• In all regions of different density (both near OH
  and H2O masers) magnetic pressure is
  approximately equal to the gas pressure
• ? magnetism plays important role in the
  dynamics of high-mass star-formation.