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OH (1720 MHz) Masers: Tracers of Supernova Remnant /
Molecular Cloud Interactions
Crystal L. Brogan (NRAO)
VLBA 10th 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
- Input of mechanical energy (turbulence)
- Responsible for cosmic rays up to 1014 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
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)
Properties of SNR OH (1720 MHz) MasersProperties 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/G)
=> Provides only currently known means of *directly* observing B-field in SNRs
OH Energy Levels
Dipole Selection Rule
F = 0, ±1
The Maser/Molecular cloud ConnectionThe Maser/Molecular cloud Connection
CTB 37A
G349.7+0.2
Greyscale: CO (1-0) emission Reynoso & Mangum (2000)
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
VLA OH (1720 MHz) Observations Toward CTB37 AVLA OH (1720 MHz) Observations Toward CTB37 A
Brogan et al. (2000)
B ~ 0.2 to 1.5 mG and changes sign
VLA Zeeman OH (1720) maser Magnetic FieldsVLA 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
Next Step…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=> 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=> VLBI with Phase Referencing
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
W28 OH (1720 MHz) Masers with MERLINMoment 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.
VLBA on Strongest OH Masers in W28 Region FVLBA 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
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
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)
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• dSNR ~ 6 kpc
• Beam ~ 12.5 x 6.3 mas
• Both regions are missing about half of the total flux
L ~ 1.2 x 1015 cm Tb ~ 1.6 x 1010 K
L ~ 3.5 x 1015 cm Tb ~3.1 x 109 K
B = 1.9 ± 0.04 mG
B = 1.5 ± 0.03 mG
(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 BSNR/OH (1720 MHz) Maser B Magnetic Fields Magnetic Fields
ConclusionsConclusions• 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/dsuggest that these masers are saturated (not discussed here)
=> Expect deviation in line wings for unsaturated case
=> Also, no sign of variability
Future WorkFuture WorkNear 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
Simplified Model of SNR/Molecular Cloud Interaction
Maser emission zone
Saturation
• Goodness of fit
• High Brightness temperatures
• Non-variability
Saturated
Stokes I=Ith => v = Doppler width
Stokes Vth= b dIth/d
Unsaturated
Stokes I=Ithe => v << Doppler width
Stokes V= Vthe = b dIth/de = b dI/d
OH (1720 MHz) Masers in W44 Using MERLINOH (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 1016 cm
DSNR ~ 3 kpc
B ~ - 0.06 to 1.2 mG
Brogan, et al. 2002