1. Thanks for inviting me again and giving me the opportunity to talk about my work.
The core of what I am going to talk about is shown
In this schematic diagrm sshowing the exansion of a snr into an adjacent MC.
The interplay between supernovae and interstellar gas is of fundamental part of the evolution of the Galaxy.
Expansion of supernova remnants running into parent molecular clouds generate a
A Shock front moving into the cloud. Masers are produced behind this shock signaling the
Interaction of the two.
The expansion is slowed and the remnant is distorted.
Heating my fill the interior and a reflected shocj may run into the interior of the remnant.

2. Radio Masers in the Galactic Center: Interactions with Molecular Clouds Collaborators: D. Roberts, M. Wardle, S. Stolovy, M. Burton, and M. Goss
Outline
Background & History
Theory of OH (1720 MHz) Masers
Collisional pumping within a shock wave
Physical conditions for the inversion
X-ray emission responsible for enhanced OH abundance
Observations of SNR masers
CND masers (5components)
The 50 km/s molecular cloud masers (11 components)
SNR Sgr A East is interacting with the CND and the 50 km/s cloud
H2 emission and OH (1665 MHz) absorption consistent with theory
Scattering of OH(1720 MHz) masers
Conclusions
Review of some of the work that has been done in the last few yearly. Most of the work on Sgr A has been done with
D. Roberts, M. Wardle,
All the topics that were discussed were relevant to Sgr A. However, OH masers apply to SNRs in general
The idea of OH masers as signposts of interation molecular clouds with SNRs is particularily rewarding because
Of the confusion and complex phenomenology at the galactic center.
Evidence for interacting systems is a difficult in the direction of the galactic center because of the confusion.

3. Background & History: History of OH Molecule
Milne (1968); Goss (1968) +
OH (1720 MHz) masers toward W28 and W44 (late 60’s)
Re-discovery
OH masers at the edge of W28
Velocities between 5 and 15 km/s
Interaction SNR/MC
A new class of masers due to their spatial and velocity coincidence
A total of 20 SNR masers
A strong correlation of SNR masers and centrally filled thermal X-rays
OH Maser
Frail, Goss & Slysh (1994)
Until recently the class of relatively rare, weak masers in the 1720 MHz OH line associated with SNR masers was
Neglected as a curiosity
The difference reflects pumping mechanism
It was only interferometric measurements of the VLA showing many discrete spots around the
Remnants where it was believed to be interacting with a molecular cloud

4. Theory of OH Masers: OH Energy Level Diagram
Pumping by collisional excitation of P 3/2 rotational levels at low temperature in order to invert the 1720 MHz line
T = K
n = 105 cm-3
Noh ~ cm-2
Td <50 K
1720MHz line, it is necessary to selectively excite PI3/2 more than PI1/2 than other state.
A natural way to convert this is at low temperature
Iversion of the 1720MHz requires a preferential excitation of Pi3/2
Lockett, Gauthier, Elitzur 1999

5. Theory of OH Masers Model X-rays from SNR interior penetrate cloud
Eject electrons
Excite electronic transition of H2
Secondary FUV photons produced from radiative decay
Advantages:
Weak UV field
Small FIR radiation field
Ionization rate > s-1
Noh is sufficient for formation of OH (1720 MHz) masers
A model for production of OH in molecular clouds associated with SNRs. A c-type shock is driven into a molecular cloud
Adjacent to a SNR. The X-ray emission from thje SNR interior from the left
permeates the cloud inducing a weak FUV flux that is
Produced locally throughout the cloud.
The shock wave incorporates atomic and molecular oxygen into wate. Once the shocked gas cools then water is
Dissociated into OH and then OI by the secondary UV flux.
Prediction: a strong correlation between thermal X-ray emitting remnants and OH masers
Weak uv field gets soacked up by dust grains and then reradiate in FIR.
Wardle (1999)

6. FYZ, Roberts, Goss, Frail and Green 1996, 1999
Observations
A total of 16 maser spots
The 50 km/s cloud masers
Shell-like SNR (blue)
Center-filled in X-ray
Clarified interaction of Sgr A East with the 50 km/s molecular cloud and the circumnuclear molecular ring
A total of xx OH masers have been detected.
-132 km/s
X-ray
Radio
+132 km/s
FYZ, Roberts, Goss, Frail and Green 1996, 1999
Wardle (1999)

7. Observations Three additional weak masers detected by
Karlsson, Sjouwerman, Sandqvist and Whiteoak 2003

8. CND Masers
A total of four highly red-shifted masers between Vlsr=131.6 and km/s
One maser detected at –132 km/s
Mass enclosed is >7x106 solar mass assuming the masers formed in the ring (r=1.7 pc)
Clarified the interaction picture

9. Observations: Zeeman
These are the highest S/N measurements of B at densities of cm-3
Pmag= r vs2
vs~ km/s supports
C-type shock models
One of the first B measurements of post-shocked gas
CND Zeeman Measurements
B(OH maser)= /-0.46 & /-0.37 mG (FYZ, Roberts, Goss, Frail & Green 1999)
B(HI absorption)=-3+/-0.5 mG (Plante. Lo and Crutcher (1995)
Considering that OH and HI probe different density regime and different spatial structures,
It is remarkable that that the two measurements agree with each other.
Equating the magnteic pressur and the ram presssure of the gas driven by the overpressue from the
Interior of the remnant

10. Optical Depth Profile OH(1667MHz) Absorption Map
The large column density of OH needed to produce the OH(1720 MHz) masers is sufficent for thermal
Absorption of OH if there is a strong backgrund
A profile from 0.5 asecond toward the maser position.
NoH =2.2x1014 Tex t dv ~ 2x1016 cm-2

11. Observations: HST and AAT
H2 1-0 S(1) should be shock excited as well.
H2 filament is shock excited by the expansion of SNR Sgr East
Intensity of H2 emission is consistent with C-type shock with vs=20 km/s, nH2 > 105 cm-3
The intense UV radiation can also contribute in exciting H2 gas
H2 filaments link Sgr A East and the CND
If the OH masers are shock excited then H2 should be shock excited as well.
Winds from RS 16 is too weak to account for H2 emission
FYZ, Stolovy, Burton, Wardle, Ashly 2002

12. Observations: Scattering
OH(1720 MHz) masers are heavily scatter-broadened
Anisotropic scattering (axial ratio 0.4)
A correlation exists between the position angle distribution of scattered sources
The scale length of the magnetic fluctuations is ~0.1 to 0.8 pc
Supports ``interface’’ model (lazio and Cordes 1998)
Similar scale length estimated from the variation of RM measurements
The orientation and shape of the scattering differs between different lines of sight separated by 1pc differs
The conclusions that can be drawn from a small number of samples is that the the shape and orientation
Of the broadening differs by less tan a pc.
The scattering medium is patchy
If the intervening scattering medium were thicker than a few parsec, the integration along
The line of sight would wash them out.

13. Conclusions
OH (1720 MHz) masers are an unambiguous sign of SNR/MC interaction
Observations and theory consistent with C-type shocks
The role of X-rays in ionization and chemistry of MCs
The origin of mixed-morphology SNRs and SNR masers
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