Article - GCNEWS, Vol. 8, August 1998
A Newsletter for Galactic Center Research
This Volume was edited by Angela Cotera & Heino Falcke
Volume 8, August 1998
On the Nature of OH (1720 MHz) Masers in the Galactic Center
[A Tex or PS Reprint of the article is available here]
Department of Physics and Astronomy, Northwestern University
The association of 1720 MHz OH masers with supernova remnants, a
possibility which was ignored for more than 25 years (e.g. Goss and
Robinson 1968), was recently
brought to the spotlight by a high-resolution study of the supernova
remnant W28 (Frail, Goss & Slysh
1994). These maser clumps - sometimes called supernova masers -
are observed along
the interface of the supernova remnant and an adjacent molecular cloud
(the extreme evolutionary endpoints of the stars and gas),
these authors to support the idea that
is the pumping source of the
masers as had been suggested by Elitzur (1976) more than 20 years ago.
The OH 1720 MHz masers are
uncommon and appear to be different from those OH emission features seen along
the spiral arms and are in stark contrast to OH 1665 and 1667 MHz
maser spots generally observed toward star-forming regions and in
envelopes of evolved stars.
Theoretical studies of the pumping of
the OH maser lines (e.g.
Elitzur 1976; Pavlakis & Kylafis 1996; Lockett, Gauthier and Elitzur 1998)
that 1665 and 1667 MHz masers are pumped by far-infrared
radiation and are therefore associated with HII regions and
evolved stars. The OH (1720 MHz) maser is collisionally pumped in
molecular gas at temperatures and densities between 15-200 K and
(104-106) cm(-3), respectively. Thus in the absence of the
1665/7 MHz transitions, the OH (1720 MHz) line presumably traces
cooling, shocked gas.
The discovery paper by Frail et al. (1994)
motivated us to search for these masers at the site of
the thermal/nonthermal interaction in the
complex region of the Galactic Center region where a number of
nonthermal features are believed to be interacting with thermal gas (Yusef-Zadeh,
Uchida and Roberts 1995). Figure 1 shows a segment of the
SNR G359.1-0.5 where an OH (1720 MHz) maser is detected at the
location where the Snake crosses the supernova shell.
The detection of these masers in the Galactic Center region
has already provided a wealth of information and has
a potential to be a powerful probe of the interstellar medium of this
region because of the extensive reservoir of dense molecular
observed throughout this region.
Here, highlights of what we have learned from observations of
OH (1720 MHz) masers in this region
are briefly described.
A Probe of the Magnetic Pressure
Direct measurements of the magnetic field is
quite difficult in the Galactic Center region due to the broad
linewidths of molecular clouds and a large
number of unrelated velocity features along the line of sight.
Because, the OH (1720
MHz) maser line traces molecular gas densities on the order
of 104 to 106 cm-3 which are similar to typical molecular
densities of molecular clouds in the Galactic Center region,
their high surface
brightness over a narrow linewidth have made their identification simple
and Zeeman splitting are measured with a high S/N
ratio. The magnetic field measurements of Sgr A East with B~ 2-5
mG with a S/N of 10-30 are roughly an order of
magnitude greater than sources outside the Galactic Center.
Because of the relatively large Zeeman shift in the
line profiles, Elitzur (1998) has recently argued that
these masers show a direct evidence of maser saturation.
We also note extended maser emission clearly associated with
the remnants such as G359.1-0.5 (Yusef-Zadeh, Uchida and
Roberts 1995). Future study of the
extended masing structure should be useful in learning
the structure of the magnetic field behind the shock front.
A Probe of the C-type Shock
The intense UV radiation field exciting the high density, intrinsically
large linewidths of H2 in
a photodissociation region have made the detection of shocked molecular
gas in the Galactic Center region quite difficult.
Shocks driven into the molecular
gas clouds by the ram pressure of SNRs or winds
are considered to be important in two prominent molecular features in
the Galactic Center: the circumnuclear disk (CND) and the Sgr A East
Detection of OH (1720 MHz) masers at the boundary of Sgr A East
and the CND (Yusef-Zadeh et al. 1996) was used to
argue that these masers
are indicators of C-type shocks (Wardle, Yusef-Zadeh and
Geballe 1998). The magnetic pressure behind the shock front
was equated to the pressure of preshock gas, thus constraining the
shock speed to 25-30 km/s.
The X-ray pressure observed within the interior of the Sgr A East
shell and the detection of molecular H2 emission at the boundary of
Sgr A East are all consistent with the C-type shock hypothesis.
Another independent study of the OH (1720 MHz) masers by
Lockett, Gauthier and Elitzur (1998) conclude that the
presence of such masers is a powerful indicator of
A Probe of Low and High Energy Processes
Although shock chemistry predicts that OH is not abundant in
the postshock gas as it is rapidly converted to water within the
shock front, Wardle et al. (1998) argue that the weak
X-ray flux from the interior of the SNR is responsible
for dissociation of H2O to OH molecule.
This situation is unlike the environment of
compact HII regions where OH masers are produced
from water by the photodissociating flux of UV from the star.
The dissociating flux is largely absorbed and reradiated in the FIR by
grains, to pump the 1665/7
MHz transitions which cannot be
the case for the unaccompanied 1720 MHz masers associated with
SNR-molecular cloud interactions. The presence of OH (1720 MHz)
masers near SNRs suggests a strong
coupling between the
low and high-energy activities co-existing with each other.
A Probe of the Scattering Medium
The OH (1720 MHz) masers associated with Sgr A East and the CND
appear to broadened
by the scattering medium toward the Galactic Center. Anisotropic
scattering has been observed toward Sgr A^* and a number of OH/IR
stars in the Galactic Center region. What is potentially
interesting about the scattered size and shape
of OH (1720 MHz) masers is the small
angular separation of maser spots from each other from
which the scattering properties on small
angular scales can be studied. The correlation of
the position angles of scatter-broadened maser spots
are used to measure the length scale of the magnetic fluctuations in the
turbulent medium of the Galactic Center.
Recent study by Yusef-Zadeh et al. (1998) estimates the
size of this fluctuation to be of the order of 0.1-0.2 pc.
A Probe of the Gas Dynamics
The maser sources generally arise at the edge of supernova shells where the
acceleration is tangential to the line of sight and the condition for
velocity coherence is achieved by having a small velocity gradient
along the line of sight. Most sources with OH (1720 MHz) masers
Galactic Center show small
velocity dispersion along the rim of the supernova shells.
This implies that
the radial velocity of the masers
is close to the systemic velocity of the molecular cloud. However,
the masers associated with the shell of Sgr A East show a large
velocity dispersion across the shell. This large velocity
difference is considered to be to due to a strong shear that
Sgr A East is experiencing near the deep gravitational potential of
the Galactic Center. In this scenario, Yusef-Zadeh et al. (1998)
argue that the Sgr A East molecular cloud has to be located within
5 pc behind the Galactic Center if tidal shear
is responsible for the observed velocity difference across
the shell of the SNR.
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