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
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