Interferometry of Circumstellar maser Envelopes

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Interferometry of Circumstellar maser Envelopes

• Wouter Vlemmings, Cornell University

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Circumstellar Masers

• Onion model
• Dust at few AU
• Molecules until
• dissociation by UV
• Excitation varies
• SiO at few AU
• Water at lt500 AU
• OH at 1.000 10.000 AU
• As Vexp increases
• radial amplification starts to dominate

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Radio Interferometers
D 8611 km 0.005 18 cm 0.0003 1.3 cm 0.00015
0.7 cm
D 3000 - 10000 km 0.0050.015 18
cm 0.00030.001 1.4 cm 0.00015 0.0005
0.7 cm

• D 218 km
• 0.170 18 cm
• 0.013 1.3 cm

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SiO Masers

• 43 86 GHz
• 0.7 0.3 cm
• tangential beaming
• gas density ?109 cm-3
• temperature ?1500 K
• shell monitoring
• comparison with IR diameters
• at ?2-3 R
• within dust formation zone

Diamond Kemball 2004
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SiO polarization

• SiO Masers
• Highly ordered Magnetic Fields
• Field Strengths (Zeeman)
• Supergiants up to 100 G
• Miras 10-30 Gauss
• But non-Zeeman interpretation
• Fields factor 1000 less

TX Cam
Kemball and Diamond, 1997, ApJ 481 L111
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H2O masers

• 22 GHz
• also various sub-mm maser transitions
• at few hundred AU
• gas density?108-9 cm-3
• temperature ?200-2000 K
• H2O masers in acceleration envelope
• no preference for radial or tangetial beaming
• stellar position often assumed

Bains et al. 2003
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H2O Maser Astrometry

• Astrometry 3 mas
• Estimated from consistency check
• Other references too weak
• Compare with OH and optical positions
• Positions transformed to J1991.25 (Hip.)
• Maser matches stellar
• Error dominated by proper motion uncertainty
• Water maser amplified stellar image?

Vlemmings et al, AAL 2002
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H2O polarization

• no linear polarization
• high degree of circular polarization
• magnetic field

• U Ori
• H2O 2-3 G
• OH 10 mG (Reid et al. 1979)

• VX Sgr
• H2O 0.3 3 G
• OH 1-2 mG
• (Szymczak et al. 2001)
• SiO 80 G (Barvainis et al. 1987)

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VX Sgr

• Fit dipole magnetic field
• using VX Sgr stellar outflow
• model (Chapman Cohen, 1986)
• Vexp 20 km/s

• Fit results
• Polar angles
• - ? 35 deg
• - ? 230 deg
• Surface field
• - B 2 kG
• indication of
• elongated outflow
• along the equator
• But need better
• outflow model

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Proto-Planetary Nebulae

• Collimated and precessing jet from W43a
• Formed approximately 28 years ago
• H2O masers at 500 AU
• (Imai et al. 2002)

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OH masers

• Radial amplification
• 500 2000 AU shells
• gas density ?104-5 cm-3
• temperature ?100 K
• Very persistent
• Tie to stellar position ?
• Amplification of star
• Radio-sphere behind maser, special spot
• If this is common
• lt 1AU accuracy in astrometry
• Usable to 1 kpc
• Astrometry fairly standard

Chapman et al. in prep, 2002
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OH 127.8
2 arcsec
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U Her

• Position of bluest spot
• Observed 14 epochs over 8 years
• 2 epochs unusable due to solar maximum
• No self-cal in these images!

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Results
U Her
W Hya
R Cas
S CrB
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Comparison with Hipparcos stellar position
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OH Maser Results

• Confirmed OH maser amplified stellar image theory
  in U Her and W Hya
• no blue-shifted spot in S Crb and R Cas
• Able to do OH maser astrometry with ?3 mas
  accuracy
• also on red-shifted spots
• improved parallaxes/distances
• get distances to heavily enshrouded stars

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Compare results
from Whitelock 2003
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OH Maser polarization

• Polarimetric map of 1612 and 1665 MHz OH masers
  shows clear alignment with the CSE (Etoka
  Diamond 2004)
• 2-3 mG field strength

Note the map shows only the redshifted 1612 MHz
emission
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Magnetic Fields in CSEs

• H2O Observations trace
• Inner edge of the maser region
• High density clumps
• Solar Type (r -2) and dipole magnetic fields fit
  data
• Surface field of 100 G (Miras) to several kG
  (Supergiants)
• Magnetic pressure in the H2O maser region
• ? ? 8 ? nH k T / B²
• (ratio of thermal and magnetic pressure)
• ? ? 0.05 the magnetic pressure dominates by a
  factor of 20 for B ? 250 mG.
• Magnetic pressure can drive outflows and help
  shape nebulae

Vlemmings, Diamond, van Langevelde, 2002, AA
394, 589
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Future Instrumentation

• ALMA sub-mm masers
• SKA sensitivity (up to 25 GHz) will present
  unique capabilities for maser studies in Local
  Group Galaxies
• could resolve maser shells out to 1 Mpc