Radio Masers in the Galactic Center: Interactions with Molecular Clouds Collaborators: D. Roberts, M. Wardle, S. Stolovy, M. Burton, and M. Goss Outline.

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

Frail, Goss & Slysh (1994)

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

Milne (1968); Goss (1968)

OH Maser

Theory of OH Masers: OH Energy Level Diagram

• Pumping by collisional excitation of 3/2 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

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)

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

X-ray

Radio

-132 km/s

+132 km/s

Wardle (1999) FYZ, Roberts, Goss, Frail and Green 1996, 1999

Observations

• Three additional weak masers detected by

Karlsson, Sjouwerman, Sandqvist and Whiteoak 2003

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

Observations: Zeeman

• These are the highest S/N measurements of B at densities of 104-105 cm-3

• Pmag= 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)

Optical Depth Profile OH(1667MHz) Absorption Map

NoH =2.2x1014 Tex v ~ 2x1016 cm-2

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

FYZ, Stolovy, Burton, Wardle, Ashly 2002

• 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

Observations: Scattering

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