Transcript ppt 1.4

Radio Masers in the Galactic Center: Interactions with Molecular Clouds
Collaborators: D. Roberts, M. Wardle, S. Stolovy, M. Burton,
and M. Goss
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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
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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
Background & History: History of OH Molecule
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OH (1720 MHz) masers toward
W28 and W44 (late 60’s)
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Re-discovery
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– OH masers at the edge of W28
– Velocities between 5 and 15 km/s
OH Maser
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Interaction SNR/MC
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A new class of masers due to their
spatial and velocity coincidence
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A total of 20 SNR masers
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A strong correlation of SNR
masers and centrally filled thermal
X-rays
Milne (1968);
Goss (1968)
Frail, Goss & Slysh (1994)
Theory of OH Masers: OH Energy Level Diagram
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Pumping by collisional
excitation of P 3/2
rotational levels at low
temperature in order to
invert the 1720 MHz
line
– T = 50-125 K
– n = 105 cm-3
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Noh ~ 1016-1017cm-2
Td <50 K
Lockett, Gauthier,
Elitzur 1999
Theory of OH Masers
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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
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A total of 16 maser
spots
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The 50 km/s cloud
masers
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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
X-ray
+132 km/s
Radio
FYZ, Roberts, Goss, Frail and Green 1996, 1999
Wardle (1999)
Observations
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Three additional
weak masers
detected by
Karlsson,
Sjouwerman,
Sandqvist and
Whiteoak 2003
CND Masers
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A total of four highly
red-shifted masers
between Vlsr=131.6
and 136.3 km/s
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One maser detected
at –132 km/s
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Mass enclosed is
>7x106 solar mass
assuming the masers
formed in the ring
(r=1.7 pc)
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Clarified the
interaction picture
Observations: Zeeman
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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
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B(OH maser)= -4.48+/-0.46 & 2.80+/-0.37
mG (FYZ, Roberts, Goss, Frail & Green
1999)
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B(HI absorption)=-3+/-0.5 mG (Plante. Lo
and Crutcher (1995)
Optical Depth Profile
OH(1667MHz) Absorption Map
NoH =2.2x1014 Tex t dv ~ 2x1016 cm-2
Observations: HST and AAT
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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
Observations: Scattering
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OH(1720 MHz) masers are
heavily scatter-broadened
– Anisotropic scattering (axial
ratio 0.4)
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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
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