1. OH (1720 MHz) Masers: Tracers of Supernova Remnant / Molecular Cloud Interactions Crystal L. Brogan (NRAO) VLBA 10 th Anniversary Meeting June 8-12, 2003

2. The Search for SNR/ Molecular Cloud Interactions SNRs are one of the most energetically important constituents of the Galactic medium - Input of mechanical energy (turbulence) - Responsible for cosmic rays up to 10 14 eV - Possibly trigger new generations of star formation => We need a better tracer! Searching for SNR/molecular cloud interactions is difficult! - There’s lots of ground to cover - Galactic velocity confusion a big pain (HI, CO(1-0)) - Most unambiguous tracers of shocked gas are at high frequencies – difficult to get lots of time

3. The Discovery of OH (1720 MHz) SNR Masers A Brief History: - (1968) Goss & Robinson observe “anomolous” OH (1720 MHz) emission toward SNRs W28, W44, & GC - (1993) Frail, Goss & Slysh identify with maser emission - (1996, 1997) SNR surveys by Frail et al.; Green et al.; Yusef-Zadeh et al. Dame et al. 2001 OH (1720 MHz) masers are found toward 10% of Galactic SNRs (~20) All but one OH maser SNR is inside the Molecular Ring Surveys suggest these masers trace SNR/molecular cloud interactions Green et al. (1997)

4. Properties of SNR OH (1720 MHz) Masers Collisional pump requires strict range of physical conditions (Wardle 1999; Lockett et al. 1999): –Temperature 50 to 125 K –Density 10 4 to 10 5 cm -3 These conditions are easily met when a C- type SNR shock hits a molecular cloud –X-rays from SNR help dissociate H 2 O Shocks are transverse to our line of site to get velocity coherence Can get magnetic field strength from the Zeeman effect (z=0.65 Hz/  G) => Provides only currently known means of *directly* observing B-field in SNRs OH Energy Levels Dipole Selection Rule  F = 0, ±1

5. The Maser/Molecular cloud Connection CTB 37A G349.7+0.2 Greyscale: CO (1-0) emission Reynoso & Mangum (2000)

6. Zeeman Effect in SNR OH Masers SNR OH (1720) masers have line splitting ~ linewidth so that: V ~ c Z B dI/d But this case has not been studied in detail

7. VLA OH (1720 MHz) Observations Toward CTB37 A Brogan et al. (2000) B ~ 0.2 to 1.5 mG and changes sign

8. VLA Zeeman OH (1720) maser Magnetic Fields Implications: Magnetic pressure far exceeds pressure of the ISM or the hot gas interior to the SNR Magnetic pressure ~ ram pressure

9. Next Step… What do we still want to know? => How does observed B change with resolution? => How does the distribution of maser spots compare with shocked gas? => How is the flux distributed in an individual maser spot? - are we only seeing the tip of the iceberg? => What are the properties of the linear polarization? - is there a correspondence between P.A. and shock front? => Resolve Maser Spots & Get Full Stokes However, these masers are weak, narrow, and large: => A few Jy in best cases =>  v ~ 1 km/s => sizes are a few 100 mas with unresolved cores => most SNRs have low dec. => VLBI with Phase Referencing

10. Zeeman magnetic field strengths from the VLA are ~ 0.2 – 0.9 mG Claussen et al. (1997) OH (1720 MHz) Masers in W28 W28 327 MHz Continuum Frail et al. 1993 CO (3-2) Emission [Arikawa et al. 1999] Yusef-Zadeh, Wardle & Roberts (2003) Evidence for extended Non-thermal emission! May originate from face-on shock regions

11. W28 OH (1720 MHz) Masers with MERLIN Moment 0 Maser emission Toward Region F B  = 0.3 ± 0.02 mG B  = 0.7 ± 0.02 mG B  = 1.1 ± 0.03 mG B  = 0.6 ± 0.005 mG B  = 0.8 ± 0.05 mG Hoffman et al. (in prep.) Beam 610 x 250 mas CO (3-2) emission Frail & Mitchell (1998) Linear pol. P.A.

12. VLBA on Strongest OH Masers in W28 Region F B  = 1.5 ± 0.05 mG B  = 1.7 ± 0.08 mG B  = 1.6 ± 0.07 mG B  = 1.3 ± 0.03 mG B  = 0.9 ± 0.02 mG MERLIN Hoffman et al. (in prep.) ~ 100 AU Beam 25 x 10 mas Most of the total flux is recovered B is 2x higher than with MERLIN

13. OH (1720 MHz) Masers in W51C Using the VLA Brogan et al. (2000) B  = 1.9 ± 0.05 mG B  = 1.5 ± 0.05 mG Brogan et al. (in prep.) W51 Complex at 327 MHz

14. MERLIN Beam ~225 x 125 mas W51C Masers with MERLIN B  = 1.9 ± 0.04 mG B  = 1.5 ± 0.03 mG VLA field strengths: 1.9 and 1.5 mG Linear pol. P.A. Brogan et al. (2003, in prep.) + Integrated CO (1-0) emission (Koo 1999)

15. B  = 1.7 ± 0.1 mG B  = 2.2 ± 0.1 mG B  = 1.5 ± 0.2 mG MERLIN Beam ~225 x 125 mas VLBA Toward W51C Maser d SNR ~ 6 kpc Beam ~ 12.5 x 6.3 mas Both regions are missing about half of the total flux L ~ 1.2 x 10 15 cm T b ~ 1.6 x 10 10 K L ~ 3.5 x 10 15 cm T b ~3.1 x 10 9 K B  = 1.9 ± 0.04 mG B  = 1.5 ± 0.03 mG

16. (1)Brogan et al. (2000) (2)Brogan et al. (2003, in prep) (3)Claussen et al. (1997) (4)Claussen et al. (1999) (5)Hoffman et al. (2003, in prep.) (6)Koralesky et al. (1999) (7)Yusef-Zadeh et al. (1999) SNR/OH (1720 MHz) Maser B  Magnetic Fields

17. Conclusions Structures on size scales ranging from tens to a few hundreds of mas => VLBA phase referencing is essential to resolve the cores => MERLIN data are needed to detect extended emission Observed B depends on resolving maser spots spatially and spectrally Excellent correspondence between maser spots and molecular shocks Linear polarization P.A. appears coincident with the shock front => Sparse statistics as yet and assumes no Faraday rotation Largest resolved size ~ 10 16 cm consistent with maser pump theory => but hints that maser emission may be more extended (i.e. W28) Comparison of shape of V with dI/d  suggest that these masers are saturated (not discussed here) => Expect deviation in line wings for unsaturated case => Also, no sign of variability

18. Future Work Near term: Continue to analyze numerous masers in W28 contained in our MERLIN and VLBI data sets Observe the OH maser region in W51C in CO(3-2) transition to determine shock direction and physical parameters New Mexico Array + VLBA Long Term  Accumulate larger sample of masers while retaining most of the total flux with high spectral resolution. Requires going to lower Dec. sources  Test maser polarization theories: linear polarization crucial. Need wider bandwidth  Compare maser locations with molecular shock structures at higher resolution  Do Maser spots move? More epochs SMA, ALMA

21. Simplified Model of SNR/Molecular Cloud Interaction Maser emission zone

22. Saturation Goodness of fit High Brightness temperatures Non-variability Saturated Stokes I=I th =>  v = Doppler width Stokes V th = b dI th /d Unsaturated Stokes I=I th e  =>  v << Doppler width Stokes V= V th e  = b dI th /d  e  = b dI/d

23. OH (1720 MHz) Masers in W44 Using MERLIN Unresolved VLBA image of Region E peak from Claussen et al. (1999) Beam ~ 35 mas Beam 200 mas ~ 1 x 10 16 cm D SNR ~ 3 kpc B ~ - 0.06 to 1.2 mG Brogan, et al. 2002