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M Wardle

Publications and source records attributed to M Wardle.

5 recordsLinked to original sources
PubMed2000

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 Galaxy contains five principal components that coexist within the central deep well of gravitational potential. These constituents are a black hole candidate (Sgr A*) with a mass equivalent to 2.6 +/- 0.2 x 10(6) solar masses, 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 us 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.

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PubMed1999

Thermal Stability of Cold Clouds in Galaxy Halos.

We consider the thermal properties of cold, dense clouds of molecular hydrogen and atomic helium. For cloud masses below 10-1.7 M middle dot in circle, the internal pressure is sufficient to permit the existence of particles of solid or liquid hydrogen at temperatures above the cosmic microwave background temperature. Optically thin thermal continuum emission by these particles can balance cosmic-ray heating of the cloud, leading to equilibria that are thermally stable even though the heating rate is independent of cloud temperature. For the Galaxy, the known heating rate in the disk sets a minimum mass of order 10-6 M middle dot in circle necessary for survival. Clouds of this type may in principle comprise most of the dark matter in the Galactic halo. However, we caution that the equilibria do not exist at redshifts z greater, similar1 when the temperature of the microwave background was substantially larger than its current value; therefore, the formation and the survival of such clouds to the present epoch remain open questions.

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PubMed1999

Enhanced OH in C-Type Shock Waves in Molecular Clouds.

Cosmic-ray and X-ray ionizations 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 50 K. The resulting column density of warm OH rises from 1015 to 1016 cm-2 as the ionization rate is increased from 10-17 s-1 (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 Infrared Space Observatory observations of emission from molecular shocks. For high ionization 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 18 cm and heterodyne measurements of emission in the far-infrared (e.g., from the Stratospheric Observatory for Infrared Astronomy).

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