Outline Background & History Theory of OH (1720 MHz) Masers

2. Radio Masers in the Galactic Center: Interactions with Molecular CloudsCollaborators: 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

3. + Frail, Goss & Slysh (1994) Milne (1968); Goss (1968) Background & History: History of OH Molecule • 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

4. Theory of OH Masers: OH Energy Level Diagram • Pumping by collisional excitation of P3/2 rotational levels at low temperature in order to invert the 1720 MHz line • T = 50-125 K • n = 105 cm-3 • Noh ~ 1016-1017cm-2 • Td <50 K 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 > 10-16 s-1 • Noh is sufficient for formation of OH (1720 MHz) masers Wardle (1999)

6. X-ray Radio 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 -132 km/s +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 136.3 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 104-105 cm-3 • Pmag= r vs2 • vs~ 25-30 km/s supports C-type shock models • One of the first B measurements of post-shocked gas • CND Zeeman Measurements • B(OH maser)= -4.48+/-0.46 & 2.80+/-0.37 mG (FYZ, Roberts, Goss, Frail & Green 1999) • B(HI absorption)=-3+/-0.5 mG (Plante. Lo and Crutcher (1995)

10. Optical Depth Profile OH(1667MHz) Absorption Map NoH =2.2x1014 Textdv ~ 2x1016 cm-2

11. Observations: HST and AAT • H21-0 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 > 105cm-3 • The intense UV radiation can also contribute in exciting H2gas • H2filaments link Sgr A East and the CND 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

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