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

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

Previous Observations 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 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

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

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, 0.027 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

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

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

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

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

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

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

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 19296+2227 and K3-35 4/7/2019