Microwave scattering and emission properties of large impact craters on the surface of Venus

Many of the impact craters on Venus imaged by the Magellan synthetic aperture radar (SAR) have interior floors with oblique incidence angle backscatter cross sections 2 to 16 times (3 dB to 12 dB) greater than the average scattering properties of the planet's surface. Such high backscatter cross sections are indicative of a high degree of wavelength-scale surface roughness and/or a high intrinsic reflectivity of the material forming the crater floors. Fifty-three of these (radar) bright floored craters are associated with 93 percent of the parabolic-shaped radar-dark features found in the Magellan SAR and emissivity data, features that are thought to be among the youngest on the surface of Venus. It was suggested by Campbell et al. that either the bright floors of the parabolic feature parent craters are indicative of a young impact and the floor properties are modified with time to a lower backscatter cross section or that they result from some property of the surface or subsurface material at the point of impact or from the properties of the impacting object. As a continuation of earlier work we have examined all craters with diameters greater than 30 km (except 6 that were outside the available data) so both the backscatter cross section and emissivity of the crater floors could be estimated from the Magellan data

Submillimeter observations of OH and CH in M42

The (sup 2) pi sub 1/2 (J = 3/2 to 1/2) transitions of OH at 163.12 and 163.40 micro m have been detected and upper limits have been obtained for the (sup 2) pi sub 3/2 (J = 3/2 to 1/2) transitions of CH at 149.09 and 149.39 micro m, in observations of the Kleinmann-Low Nebula of Orion. All four flux levels lie between 1 and 1.2 x 10 to the 17th power/sq.cm. The OH lines are bright when compared to the lower, (sup 2) pi sub 3/2 (J = 5/2 to 3/2) fluxes reported and imply that the 119 micro m emission observed is partially self-absorbed. The combined results provide strong constraints. Taken together with existing data on molecular hydrogen and CO and recent data on other OH transition, they suggest OH emission from post-shock regions at temperatures T approx 1000 k, densities approx. 7 x 10 to the 6th powr/cu cm N sub OH approx 80/cu cm optically thick for the (sup 2) pi sub 3/2 (J = 5/2 to 3/2), 119 micro m but only partially self-absorbing in the (J = 7/2 to 3/2), 84 micro m transitions over a Doppler velocity bandwidth of 30 km/sec. The OH column density is N sub OH approx 4 x 10 to the 16th powr/sq cm. in the emitting regions which occupy a fraction of approx 0.1 of a 1' x 1' field of view centered on the Becklin-Neugebauer source. The CO (J = 31 to 30), 84 micro m transition appears to lie sufficiently close to one of the 84 micro m OH line components to be partially absorbed as well, through a Bowen-type mechanism

COMPTEL Observations of AGN at MeV-Energies

The COMPTEL experiment aboard CGRO, exploring the previously unknown sky at MeV-energies, has so far detected 10 Active Galactic Nuclei (AGN): 9 blazars and the radio galaxy Centaurus A. No Seyfert galaxy has been found yet. With these results COMPTEL has opened the field of extragalactic Gamma-ray astronomy in the MeV-band.Comment: 4 pages, 2 figures including 1 color plot, to appear in the Proceedings of the 3rd INTEGRAL Workshop "The Extreme Universe", held in Taormina, Italy, 14-18 September 199

Mass Density Profiles of LSB Galaxies

We derive the mass density profiles of dark matter halos that are implied by high spatial resolution rotation curves of low surface brightness galaxies. We find that at small radii, the mass density distribution is dominated by a nearly constant density core with a core radius of a few kpc. For rho(r) ~ r^a, the distribution of inner slopes a is strongly peaked around a = -0.2. This is significantly shallower than the cuspy a < -1 halos found in CDM simulations. While the observed distribution of alpha does have a tail towards such extreme values, the derived value of alpha is found to depend on the spatial resolution of the rotation curves: a ~ -1 is found only for the least well resolved galaxies. Even for these galaxies, our data are also consistent with constant density cores (a = 0) of modest (~ 1 kpc) core radius, which can give the illusion of steep cusps when insufficiently resolved. Consequently, there is no clear evidence for a cuspy halo in any of the low surface brightness galaxies observed.Comment: To be published in ApJ Letters. 6 pages. Uses aastex and emulateapj5.sty Typo in Eq 1 fixe

Models of OH Maser Variations in U Her

Arecibo spectra of the mainline OH maser emission from U Her over more than a decade show variations of the OH emission over these time scales. These observations are combined with high spatial resolution VLBA maps to investigate the causes of the variations in the velocities of the maser components. Global properties of the dust shell, such as accelerations, variations in the pump and shell-wide magnetic field changes are examined as possibilities, and eliminated. A possible solution to the problem involving plasma turbulence and the local magnetic field is introduced, and the relevant time scales of the turbulence are calculated. The turbulent velocity field yields time scales of the turbulence are calculated. The turbulent velocity field yields time scales that are too long (of order centuries), while the turbulent magnetic field produces variations on appropriate time scales of a few years. A line-of-sight model of the turbulence is developed and investigated. The complete exploration of this solution requires extensive theoretical and observational work. Possible avenues of investigation of the plasma turbulence model are presented.Comment: 23 pages, 17 figures, ApJ: accepted Sept, 199

The formation of the first galaxies and the transition to low-mass star formation

The formation of the first galaxies at redshifts z ~ 10-15 signaled the transition from the simple initial state of the universe to one of ever increasing complexity. We here review recent progress in understanding their assembly process with numerical simulations, starting with cosmological initial conditions and modelling the detailed physics of star formation. In this context we emphasize the importance and influence of selecting appropriate initial conditions for the star formation process. We revisit the notion of a critical metallicity resulting in the transition from primordial to present-day initial mass functions and highlight its dependence on additional cooling mechanisms and the exact initial conditions. We also review recent work on the ability of dust cooling to provide the transition to present-day low-mass star formation. In particular, we highlight the extreme conditions under which this transition mechanism occurs, with violent fragmentation in dense gas resulting in tightly packed clusters.Comment: 16 pages, 7 figures, appeared in the conference proceedings for IAU Symposium 255: Low-Metallicity Star Formation: From the First Stars to Dwarf Galaxies, a high resolution version (highly recommended) can be found at http://www.ita.uni-heidelberg.de/~tgreif/files/greif08.pd

Open questions in the study of population III star formation

The first stars were key drivers of early cosmic evolution. We review the main physical elements of the current consensus view, positing that the first stars were predominantly very massive. We continue with a discussion of important open questions that confront the standard model. Among them are uncertainties in the atomic and molecular physics of the hydrogen and helium gas, the multiplicity of stars that form in minihalos, and the possible existence of two separate modes of metal-free star formation.Comment: 15 pages, 2 figures. To appear in the conference proceedings for IAU Symposium 255: Low-Metallicity Star Formation: From the First Stars to Dwarf Galaxie