1. Magnetic Fields in the Envelopes of Late-Type Stars: Circular Polarization of H2O Masers
Wouter Vlemmings, Cornell University
Phil Diamond, Jodrell Bank
Huib Jan van Langevelde, JIVE
4/7/2019

2. Role of Magnetic Fields
Mass loss
Alfvén waves can drive stellar winds and produce clumpy mass loss
Outflows
Shaped by magnetic fields
Magnetic pressure dominates
the thermal/kinetic pressure
for high magnetic fields
Planetary nebulae
4/7/2019

3. Previous Observations
SiO Masers:
Highly ordered Magnetic Fields
Field Strengths (Zeeman):
Supergiants: up to 100 G
Miras: Gauss
But: non-Zeeman interpretation:
Fields factor 1000 less
OH Masers:
Some indication of alignment with CSE structure.
Field Strengths:
Both Supergiants and Miras show a few mG fields
Discuss non Zeeman
4/7/2019
Kemball and Diamond, 1997, ApJ 481 L111

4. H2O Masers H2O maser 616 – 523 rotational transition. 22.235 GHz
6 Hyperfine transitions
Non-paramagnetic:
Factor 103 weaker than for radicals like OH.
Expected splitting 10-3 times typical maser line width (20 kHz).
Hyperfines are close, magnetic substates are close, all because of the closed shell
4/7/2019

5. Observation Calibration
VLBA observations of 4 late type stars.
(S Per, U Her, VY CMa and NML Cyg)
Correlated twice:
All 4 polarizations, 0.1 km/s resolution.
RR and LL only, km/s resolution.
Calibration:
First calibration on low spectral resolution
Apply solutions on high resolution data
VLBA observations from dec 98,
Polarization quality of VLBA, limited spectral resolution on crosses.
4/7/2019

6. Polarization Analysis
LTE method:
Calculate Zeeman splitting
For 3 dominant hyperfine lines
Create Synthetic Circular Polarization Spectrum
Proportional to derivative of total power, I’
Determine AF-F’ in:
PV  ( Vmax – Vmin ) / Imax =
AF-F’ · B[Gauss] / v [km/s]
4/7/2019

7. Magnetic Fields Results
Clear detections
Only few %
Rule out systematics:
Varying values and directions
B|| = 207 ±30 mG
But:
V spectrum narrower than thermal Zeeman
No linear polarization
Value derived from 7-6 dominated value WHY?
No linear component, seems to point to pure Zeeman effect.
4/7/2019
Vlemmings, Diamond, van Langevelde, 2001, A&A 375 L1

8. Polarization Analysis
Non-LTE method:
Calculate Equations of State
Linear maser geometry
Including interaction between:
3 dominant Hyperfine lines
Their magnetic substates
Total of 99 non-linearly related equations
Solve for various thermal line widths of the maser medium
Directly fit the observations to the models
Partly explains narrowing
(2D or 3D could provide solution)
4/7/2019

9. Results S Per: VY CMa: NML Cyg: U Her: H2O: 150 mG / 200 mG
OH: 1 mG (Masheder et al. 1999)
VY CMa:
H2O: 175 mG / 200 mG
SiO: 65 G (Barvainis et al.1987)
OH: 2 mG (Cohen et al. 1987)
NML Cyg:
H2O: 500 mG / 500 mG
OH: 2 mG (Cohen et al. 1987)
U Her:
H2O: 1.5 G / 2.5 G
OH: 1 mG (Palen & Fix 2000)
4/7/2019

10. Magnetic Fields in CSEs
Observations trace
Inner edge of the maser region
High density clumps
Favors Solar Type (r-2) magnetic fields
Surface field of 100 G (Miras) to 1 kG (Supergiants)
Magnetic pressure can drive outflows and help shape nebulae
4/7/2019
Vlemmings, Diamond, van Langevelde, 2002, A&A 394, 589

11. Planetary Nebulae 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.
Asymmetric nebulae possibly due to:
magnetic shaping of the outflow (García-Segura, 1999)
wind interaction with a warped circumstellar disk (Icke, 2003)
warped disk may be caused by high magnetic fields (Lai, 1999)
4/7/2019

12. Conclusions Zeeman interpretation is favored
No linear polarization
LTE models appear too simple
Coupled transfer models (non-LTE) promising
Constraints on saturation & beaming
Inferred magnetic fields fit nicely
Compared to OH & SiO values and solar type magnetic field
Indicate surface fields of  1 kG
Comparable to dynamo-produced fields (Blackman et al. 2001)
The one Mira star in sample appears to have a stronger field
Indicates H2O maser in thick shell, closer to the star
New VLBA observation will expand sample
observed: VX Sgr, R Cas, U Ori
Masers in P-PNe can provide clues on evolution of the magnetic fields
proposed observations on: IRAS and K3-35
4/7/2019