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