Magnetic fields in maser regions

1
Magnetic fields in maser regions
Jim Cohen
1st December 2004
Collaborators Busaba Hutawarakorn Malcolm
Gray Sandra Etoka
2
Outline of Talk
Masers in Magnetic Fields (Masers in
Circumstellar Envelopes) Masers in Star Forming
Regions
3
Masers are non-linear amplifiers
Microwave Amplification by Stimulated Emission of
Radiation
Population inversion (pump) gives
gain. Exponential amplification, e.g. e25, high
brightness. Line narrowing, beaming, and
polarization develop as maser radiation
propagates. Saturation when stimulated emission
rate pump rate.
4
Why use masers to measure B?

• Many techniques are available to estimate B in
  the ISM. Some measurements give B ?? , some give
  B ? , some give B magnitude, some give the
  direction, some give average B.
• Using masers we get
• Local B field at densities of 105-1010 cm-3
• Projected field direction (neglecting Faraday
  rotation)
• 3-D field vector in favourable cases
• Sub-arcsecond resolution
• High precision astrometry (1-10 milliarcsec)

5
Zeeman Effect in Masers
Polarization and intensity depend on angle of B
to line-of-sight Splitting ? B provided
hyperfine components dont overlap. Otherwise
see Elitzur (1996,8). Saturation causes
competitive gain.

Blended Bcos? Unblended B
6
p
s -
s
W75N
Hutawarakorn Cohen 2002, MNRAS 330, 349
Magnetic Beaming
(Gray Field 1994, AA, 292, 693 ). s-components
grow fastest, along the field lines, and can
suppress p-components . 100 circular
polarization most common. Zeeman shift has same
effect as velocity shift. In a turbulent medium
LHC and RHC trace different molecules in general.
Full Zeeman patterns rare!
Hutawarakorn et al. 2002, MNRAS, 330, 349
7
MERLIN
Multi Element Radio Linked Interferometer Network
10 milliarcsecond astrometry
D 218 km 0.170" 18 cm 0.042 " 4 cm 0.013"
1.4 cm
8
Trinidad et al. 2003, ApJ, 598, 386
AFGL2591
Masers and bipolar outflows
Preibisch et al 2003, AA, 412, 735
near
far
GEMINI Image
E-W bipolar outflow inclined at 35?
9
OH Maser Disc in AFGL2591
1000 au
K-band speckle interferometry Preibisch et al
2003, AA, 412, 735
MERLIN spectropolarimetry Hutawarakorn Cohen,
MNRAS, in press
10
AFGL2591 Disc
Maser disc orthogonal to outflow Radius 750
au Inclined at 55? Vrot 5 km/s
Hutawarakorn Cohen, MNRAS, in press
Magnetic field B 0.5 - 3.8 mG Field
reversal Polarization angles aligned or
orthogonal to outflow B ? E
11
Hutawarakorn Cohen 2003, MNRAS 345, 175
NGC7538 IRS11
2000 au
Polarization angles aligned or orthogonal to
outflow Field reversal Degree of polarization and
alignment may decrease with increasing age.
12
2000 au 0.010 pc
Hutawarakorn Cohen 2002, MNRAS 330, 349
W75N
1665 MHz
Kinematics show a rotating and expanding disc
(torus) orthogonal to the outflow. Strong linear
poln up to 100. Vectors are either parallel to
outflow or perpendicular.

13
1667 MHz and 1720 MHz
W75N continued
Magnetic field reverses on opposite sides of disc
(toroidal component). Field lines twisted in
rotating disc. Uchida Shibata (1985) model is
supported.
14
Model of W75N masers and polarization
Synthetic OH maser spectra generated using
polarization-dependent model of propagation, with
physical conditions scaled from Uchida Shibata
(1985) model. Gray et al. 2003, MNRAS, 343,
1067-1080.
Masers originate at different depths in disc.
15
Excited OH 6.0-GHz
Caswell 2003, MNRAS, 341, 551
Ideal Lande g-factor, only just splitting the
lines, so Zeeman pairs are common. New receivers
for eMERLIN cover OH 6.0-GHz (and also methanol
6.7 GHz) First single baseline results on
W3(OH). First 5-telescope results in Dec 2004.
16
W3(OH) 6-GHz Masers
Etoka, Cohen Gray, in preparation
90 Zeeman pairs. MERLIN astrometry fixes 6-GHz
masers relative to ground state masers. (Wright
et al. 2004, MNRAS 350, 1253 and
1272) Copropagating 6035, 6031, 1665 and 1667
MHz masers, give consistent B values.
RA
17
Some copropagating OH Zeeman pairs
18
Where Next?

• 6-GHz studies can give B and n (from
  copropagation).
• Potential to probe densities to 1010cm-3 (H2O
  etc.).
• 3-d magnetic field studies are sensitivity
  limited for now, but
• Major new IR/submm/mm facilities are coming and
  will overlap with masers at subarcsec resolution.
• Some key questions
• How to treat overlap of hyperfine components?
• Relation to galactic magnetic field?
• Magnetic field evolution?