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RE: Sgr A*
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Title: Limits on the Position Wander of Sgr A*
Authors: Mark J. Reid, Avery E. Broderick, Abraham Loeb, Mareki Honma, Andreas Brunthaler

We present measurements with the VLBA of the variability in the centroid position of Sgr A* relative to a background quasar at 7-mm wavelength. We find an average centroid wander of 71 45 micro-arcsec for time scales between 50 and 100 min and 113 50 micro-arcsec for timescales between 100 and 200 min, with no secular trend. These are sufficient to begin constraining the viability of the hot-spot model for the radio variability of Sgr A*. It is possible to rule out hot spots with orbital radii above 15GM_SgrA*/c , which contribute more than 30% of the total 7-mm flux. However, closer or less luminous hot spots remain unconstrained. Since the fractional variability of Sgr A* during our observations was ~20% on time scales of hours, the hot-spot model for Sgr A*'s radio variability remains consistent with these limits. Improved monitoring of Sgr A*'s centroid position has the potential to place significant constraints upon the existence and morphology of inhomogeneities in a supermassive black hole accretion flow.

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Title: Massive Star Formation Near Sgr A* and Bimodal Star Formation in the Nuclear Disk
Authors: F. Yusef-Zadeh, M. Wardle

The history of star formation in the strong gravitational potential of the Galactic centre has been of much interest, recently. We propose that the sub-parsec-scale disk of massive stars orbiting the massive black hole at the Galactic centre can be interpreted in terms of partial accretion of extended Galactic centre clouds, such as the 50 \kms molecular cloud, as these clouds envelop Sgr A* on their passage through the inner Galactic centre. The loss of angular momentum of the captured cloud material by self-interaction subsequent to gravitationally focusing by Sgr A* naturally creates a compact gaseous disk of material close to Sgr A* in which star formation takes place. On a larger scale the formation of massive clusters such as the Arches and Quintuplet clusters or on-going massive star formation such as Sgr B2 could also be triggered by cloud-cloud collisions due to gravitational focusing in the deep potential of the central bulge.
Unlike the violent and high-pressure environment of clustered star formation triggered by cloud-cloud collision, there are also isolated pockets of star formation and quiescent dense clouds. These sites suggest an inefficient, slow mode of star formation. We propose enhanced cosmic rays in the nuclear disk may be responsible for inhibiting the process of star formation in this region. In particular, we argue that the enhanced ionisation rate due to the impact of cosmic-ray particles is responsible for lowering the efficiency of on-going star formation in the nuclear disk of our Galaxy. The higher ionisation fraction and higher thermal energy due to the impact of these electrons may also reduce MHD wave damping which contributes to the persistence of the high velocity dispersion of the molecular gas in the nuclear disk.

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Monitoring a region around the centre of our Galaxy, astronomers have indeed found evidence for a surprisingly large number of variable x-ray sources - likely black holes or neutron stars in binary star systems - swarming around the Milky Way's own central supermassive black hole.



Chandra Observatory combined x-ray image data from their monitoring program is shown above, with four variable sources circled and labelled A-D. While four sources may not make a swarm, these all lie within only three light-years of the central supermassive black hole known as Sgr A* (the bright source just above C). Their detection implies that a much larger concentration of black hole systems is present. Repeated gravitational interactions with other stars are thought to cause the black hole systems to spiral inward toward the Galactic Centre region.

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