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OH (1720 MHz) Masers: Tracers of Supernova Remnant / Molecular Cloud Interactions

OH (1720 MHz) Masers: Tracers of Supernova Remnant / Molecular Cloud Interactions. Crystal L. Brogan (NRAO). VLBA 10 th Anniversary Meeting June 8-12, 2003. The Search for SNR/ Molecular Cloud Interactions. SNRs are one of the most energetically important constituents of the Galactic medium

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OH (1720 MHz) Masers: Tracers of Supernova Remnant / Molecular Cloud Interactions

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  1. OH (1720 MHz) Masers:Tracers of Supernova Remnant / Molecular Cloud Interactions Crystal L. Brogan (NRAO) VLBA 10th 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 1014 eV • - Possibly trigger new generations of star formation • 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 => We need a better tracer!

  3. 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) 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.

  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 104 to 105 cm-3 • These conditions are easily met when a C-type SNR shock hits a molecular cloud • X-rays from SNR help dissociate H2O • 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/mG) => Provides only currently known means of *directly* observing B-field in SNRs OH Energy Levels Dipole Selection Rule DF = 0, ±1

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

  6. Zeeman Effect in SNR OH Masers SNR OH (1720) masers have line splitting ~ linewidth so that: V ~ c Z B dI/dn 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 mGand 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. 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? Next Step… => Resolve Maser Spots & Get Full Stokes However, these masers are weak, narrow, and large: => A few Jy in best cases=> Dv ~ 1 km/s=> sizes are a few 100 mas with unresolved cores => most SNRs have low dec. => VLBI with Phase Referencing

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

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

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

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

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

  15. L ~ 3.5 x 1015 cm Tb ~3.1 x 109 K Bq = 1.7 ± 0.1 mG Bq = 1.5 ± 0.2 mG L ~ 1.2 x 1015 cm Tb ~ 1.6 x 1010 K MERLIN Bq = 2.2 ± 0.1 mG Bq = 1.5 ± 0.03 mG Bq = 1.9 ± 0.04 mG Beam ~225 x 125 mas • dSNR ~ 6 kpc • Beam ~ 12.5 x 6.3 mas • Both regions are missing about half of the total flux VLBA Toward W51C Maser

  16. SNR/OH (1720 MHz) Maser Bq Magnetic Fields • Brogan et al. (2000) • Brogan et al. (2003, in prep) • Claussen et al. (1997) • Claussen et al. (1999) • Hoffman et al. (2003, in prep.) • Koralesky et al. (1999) • Yusef-Zadeh et al. (1999)

  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 ~ 1016 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/dn suggest that these masers are saturated (not discussed here) • => Expect deviation in line wings for unsaturated case • => Also, no sign of variability

  18. 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 New Mexico Array + VLBA SMA, ALMA 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

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

  20. Saturation Saturated Stokes I=Ith=> Dv = Doppler width Stokes Vth= b dIth/dn Unsaturated Stokes I=Ithet(n)=> Dv << Doppler width Stokes V= Vthet(n) = b dIth/dn et(n) = b dI/dn • Goodness of fit • High Brightness temperatures • Non-variability

  21. Unresolved VLBA image of Region E peak from Claussen et al. (1999) Beam ~ 35 mas OH (1720 MHz) Masers in W44 Using MERLIN Beam 200 mas ~ 1 x 1016 cm DSNR ~ 3 kpc B ~ - 0.06 to 1.2 mG Brogan, et al. 2002

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