The Wardle Instability in Interstellar Shocks: I. Nonlinear Dynamical Evolution

The nonlinear evolution of unstable C-type shocks in weakly ionized plasmas is studied by means of time-dependent magnetohydrodynamical simulations. This study is limited to shocks in magnetically dominated plasmas (in which the Alfven speed in the neutrals greatly exceeds the sound speed), and microphysical processes such as ionization and recombination are not followed. Both two-dimensional simulations of planar perpendicular and oblique C-type shocks, and fully three-dimensional simulation of a perpendicular shock are presented.Comment: 20 pages, 7 Postscript figures, LaTeX, accepted by Ap.

The Underluminous Nature of Sgr A*

In the last several years, a number of observing campaigns of the massive black hole Sgr A* has been carried out in order to address two important issues: one concerns the underluminous nature of Sgr A* with its bolometric luminosity being several orders of magnitude less than those of its more massive counterparts. It turns out that the angular momentum of the ionized stellar winds from orbiting stars in one or two disks orbiting Sgr A* could be a critical factor in estimating accurately the accretion rate unto Sgr A*. A net angular momentum of ionized gas feeding Sgr A* could lower the Bondi rate. Furthermore, the recent time delay picture of the peak flare emission can be understood in the context of adiabatic expansion of hot plasma. The expansion speed of the plasma is estimated to be sub-relativistic. However, relativistic bulk motion of the plasma could lead to outflow from Sgr A*. Significant outflow from Sgr A* could then act as a feedback which could then reduce Bondi accretion rate. These uncertain factors can in part explain the underluminous nature of Sgr A*. The other issue is related to the emission mechanism and the cause of flare activity in different wavelength bands. Modeling of X-ray and near-IR flares suggests that inverse Compton scattering (ICS) of IR flare photons by the energetic electrons responsible for the submm emission can account for the X-ray flares. A time delay of minutes to tens of minutes is predicted between the peak flaring in the near-IR and X-rays, NOT due to adiabatic expansion of optically thick hot plasma, but to the time taken for IR flare photons to cross the accretion flow before being upscattered.Comment: 4 pages, To appear in Proceedings of "X-ray Astronomy 2009: Present Status, Multi-Wavelength Approach and Future Perspectives", Bologna, Italy, September 7-11, 2009, AIP, eds. A. Comastri, M. Cappi, and L. Angelin

The Origin of Parsec-Scale Gaseous and Stellar Disks in the Galactic Center and AGNs

The Galactic center stellar disk and the circumnuclear ring provide a unique opportunity to study in detail the dynamics and physical conditions of distant molecular disks in the nuclei of galaxies. One of the key questions is how these disks form so close to their host black holes and under what condition they form stars in a tidally stressed environment. We argue that disk formation around a massive black hole is due to partial accretion of extended molecular clouds that temporarily pass through the central region of the Galaxy. The cancellation of angular momentum of the gravitationally focused gas naturally creates a compact gaseous disk. The disk can potentially become gravitationally unstable and form stars. We apply these ideas to explain the origin of sub-parsec megamaser disks found in the nuclei of Seyfert 2 galaxies. We show that an empirical scaling relation between the mass of the black hole and the size of the disk can be understood in the context of the cloud capture scenario. We conclude that the stellar and gas disks found in our Galactic center act as a bridge to further our understanding of more distant mega-maser disks in the nuclei of Seyfert 2 galaxies.Comment: 6 pages, 2 figures, to appear in "The Central Kiloparsec in Galactic Nuclei: Astronomy at High Angular Resolution 2011", open access Journal of Physics: Conference Series (JPCS), published by IOP Publishin

High Levels of Circularly Polarized Emission from the Radio Jet in NGC 1275 (3C 84)

We present multi-frequency, high resolution VLBA circular polarization images of the radio source 3C 84 in the center of NGC 1275. Our images reveal a complex distribution of circular polarization in the inner parsec of the radio jet, with local levels exceeding 3% polarization, the highest yet detected with VLBI techniques. The circular polarization changes sign along the jet, making 3C 84 also the first radio jet to show both signs of circular polarization simultaneously. The spectrum and changing sign of the circular polarization indicate that it is unlikely to be purely intrinsic to the emitted synchrotron radiation. The Faraday conversion process makes a significant and perhaps dominant contribution to the circular polarization, and the observed spectrum suggests the conversion process is near saturation. The sign change in the circular polarization along the jet may result from this saturation or may be due to a change in magnetic field order after an apparent bend in the jet. From the small spatial scales probed here, ~ 0.15 pc, and the comparably high levels of circular polarization inferred for the intra-day variable source PKS 1519-273, we suggest a connection between small spatial scales and efficient production of circular polarization.Comment: 4 pages, accepted in ApJ Letter

The influence of vegetation structure and composition on invasibility by Pinus radiata in the Blue Mountains, NSW

The exotic tree species Pinus radiata D. Don (in the family Pinaceae) has successfully spread from commercial plantations into adjacent vegetation in southeastern Australia. Identifying factors facilitating spread will aid the control of current invasions and the prediction of future invasion events. The structure and composition of vegetation can have an important role in determining community resilience to invasion. Two dry eucalypt sclerophyll woodlands in the Blue Mountains west of Sydney known to be invaded by Pinus radiata were surveyed to investigate the influence of eucalypt presence, species diversity, species composition and vegetation cover on the extent and density of invasion. Relationships between community characteristics and the level of pine invasion were weak and variable. Pines were found growing in plots with 0–70% understorey cover and 5–90% ground cover, and in areas of both high and low eucalypt diversity and presence, illustrating the high invasion potential of Pinus radiata

The Galactic Center: An Interacting System of Unusual Sources

The region bounded by the inner tens of light years at the center of the Milky Way contains five principal components that coexist within the central deep gravitational potential well. These constituents are a black hole candidate (Sgr~A*) with a mass equivalent to $2.6\pm0.2\times 10^6$ suns, a surrounding cluster of evolved stars, a complex of young stars, molecular and ionized gas clouds, and a powerful supernova-like remnant. The interaction of these components is responsible for many of the phenomena occurring in this complex and unique portion of the Galaxy. Developing a consistent picture of the primary interactions between the components at the Galactic Center will improve our understanding of the nature of galactic nuclei in general, and will provide with a better defined set of characteristics of black holes. For example, the accretion of stellar winds by Sgr A* appears to produce far less radiation than indicated by estimates based on models of galactic nuclei.Comment: 26 pages (text only); see the published article with ten figures under Science Online Article http://www.physics.nwu.edu/research/zadeh.htm

Angular momentum transport in protostellar discs

Angular momentum transport in protostellar discs can take place either radially, through turbulence induced by the magnetorotational instability (MRI), or vertically, through the torque exerted by a large-scale magnetic field that threads the disc. Using semi-analytic and numerical results, we construct a model of steady-state discs that includes vertical transport by a centrifugally driven wind as well as MRI-induced turbulence. We present approximate criteria for the occurrence of either one of these mechanisms in an ambipolar diffusion-dominated disc. We derive ``strong field'' solutions in which the angular momentum transport is purely vertical and ``weak field'' solutions that are the stratified-disc analogues of the previously studied MRI channel modes; the latter are transformed into accretion solutions with predominantly radial angular-momentum transport when we implement a turbulent-stress prescription based on published results of numerical simulations. We also analyze ``intermediate field strength'' solutions in which both modes of transport operate at the same radial location; we conclude, however, that significant spatial overlap of these two mechanisms is unlikely to occur in practice. To further advance this study, we have developed a general scheme that incorporates also the Hall and Ohm conductivity regimes in discs with a realistic ionization structure.Comment: 8 pages, 4 figures, 1 table; accepted for publication in MNRA

Proper Motion of the Irradiated Jet HH 399 in the Trifid Nebula

HH 399 is one of the first Herbig Haro flows recognized to be irradiated by the UV radiation of the massive O7.5 star in the Trifid nebula. We present the proper motion of the first irradiated jet based on two epochs of HST observations of HH 399 separated nearly by five years using H$\alpha$ and [SII] line filters. High proper motion with continuous velocities between 200$\pm$55 and 528$\pm24$ \kms are detected in both lines along the 18$''$ extent of the jet axis. The irradiated fully-ionized jet consists of numerous knots along the jet but also shows the evidence for a number of isolated blob-like structures running immediately outside the jet with lower transverse velocities. The transverse velocities combined with radial velocity measurements indicate that the jet axis lies away from the plane of the sky by only few degrees. We argue that the jet is fully ionized based on [SII]/H$\alpha$ line ratio as well as radio continuum emission detected from the full extent of the jet at 3.6cm wavelength. The stellar mass-loss rate producing HH 399 is estimated to be \approx 2\times10^{-6} \msol yr$^{-1}$.Comment: 14 pages, 6 figures, ApJ (in press

Cosmic-Ray Heating of Molecular Gas in the Nuclear Disk: Low Star Formation Efficiency

Understanding the processes occurring in the nuclear disk of our Galaxy is interesting in its own right, as part of the Milky Way Galaxy, but also because it is the closest galactic nucleus. It has been more than two decades since it was recognized that the general phenomenon of higher gas temperature in the inner few hundred parsecs by comparison with local clouds in the disk of the Galaxy. This is one of the least understood characteristics of giant molecular clouds having a much higher gas temperature than dust temperature in the inner few degrees of the Galactic center. We propose that an enhanced flux of cosmic-ray electrons, as evidenced recently by a number of studies, are responsible for directly heating the gas clouds in the nuclear disk, elevating the temperature of molecular gas ($\sim$ 75K) above the dust temperature ($\sim$ 20K). In addition we report the detection of nonthermal radio emission from Sgr B2-F based on low-frequency GMRT and VLA observations. The higher ionization fraction and thermal energy due to the impact of nonthermal electrons in star forming sites have important implications in slowing down star formation in the nuclear disk of our galaxy and nuclei of galaxies.Comment: 12 pages, one figure, ApJL (in press

Enhanced OH in C-type shock waves in molecular clouds

Cosmic-ray and X-ray ionisations in molecular gas produce a weak far-ultraviolet flux through the radiative decay of H2 molecules that have been excited by collisions with energetic electrons (the Prasad-Tarafdar mechanism). I consider the effect of this dissociating flux on the oxygen chemistry in C-type shocks. Typically a few percent of the water molecules produced within the shock front are dissociated before the gas has cooled to 50K. The resulting column density of warm OH rises from 10^15 to 10^16 cm^-2 as the ionisation rate is increased from 10^-17 (typical of dark clouds) to 10^-15 s^-1 (adjacent to supernova remnants). These column densities produce substantial emission in the far-infrared rotational transitions of OH, and are consistent with the OH/H2O ratios inferred from ISO observations of emission from molecular shocks. For high ionisation rates the column of warm OH is sufficient to explain the OH(1720 MHz) masers that occur where molecular clouds are being shocked by supernova remnants. The predicted abundance of OH throughout the shock front will enable C-type shocks to be examined with high spectral resolution through radio observations of the four hyperfine ground state transitions of OH at 18cm and heterodyne measurements of emission in the FIR (e.g. from SOFIA)Comment: 5 pp incl 3 figs, LaTeX, uses emulateapj.sty; ApJ Letters in press. Revised 2nd paragraph of discussio