Depositional processes and gas pore pressure in pyroclastic flows: an experimental perspective

The depositional processes and gas pore pressure in pyroclastic flows are investigated through scaled experiments on transient, initially fluidized granular flows. The flow structure consists of a sliding head whose basal velocity decreases backwards from the front velocity (U _f) until onset of deposition occurs, which marks transition to the flow body where the basal deposit grows continuously. The flows propagate in a fluid-inertial regime despite formation of the deposit. Their head generates underpressure proportional to U _f ² whereas their body generates overpressure whose values suggest that pore pressure diffuses during emplacement. Complementary experiments on defluidizing static columns prove that the concept of pore pressure diffusion is relevant for gas-particle mixtures and allow characterization of the diffusion timescale (t _d) as a function of the material properties. Initial material expansion increases the diffusion time compared with the nonexpanded state, suggesting that pore pressure is self-generated during compaction. Application to pyroclastic flows gives minimum diffusion timescales of seconds to tens of minutes, depending principally on the flow height and permeability. This study also helps to reconcile the concepts of en masse and progressive deposition of pyroclastic flow units or discrete pulses. Onset of deposition, whose causes deserve further investigation, is the most critical parameter for determining the structure of the deposits. Even if sedimentation is fundamentally continuous, it is proposed that late onset of deposition and rapid aggradation in relatively thin flows can generate deposits that are almost snapshots of the flow structure. In this context, deposition can be considered as occurring en masse, though not strictly instantaneously.     

The distances to the X-ray binaries LSI +61° 303 and A0535+262

Chevalier & Ilovaisky use Hipparcos data to show that the X-ray binary systems LSI+61° 303 and A0535+262 are a factor of 10 closer (i.e. d  ∼ few hundred pc) than previously thought ( d  ∼ kpc). We present high-quality CCD spectra of the systems, and conclude that the spectral types, reddening and absolute magnitudes of these objects are strongly inconsistent with the closer distances. We propose that the Hipparcos distances to these two systems are incorrect owing to their relatively faint optical magnitudes.     

Near-infrared observations of the dust coma of Comet Kohoutek (1973f) with a tilting-filter Fabry-Perot photometer

By means of narrow-band Fabry-Perot filters, which exclude the interference from molecular line fluorescence, the brightness of Comet Kohoutek (1973f) has been measured at 8560 and 8748 Å. Data reduction on the basis of averaged Mie-scattering cross sections indicates that the dust production rate was different before and after perihelion at the same heliocentric distances. This asymmetry suggests that vaporization and dust entrainment were governed by fractionation of a multicomponent mixture of parent molecules in a comparatively porous cometary nucleus.     

Silicate absorption in heavily obscured galaxy nuclei

Spectroscopy at 8–13 μm with T-ReCS on Gemini-S is presented for three galaxies with substantial silicate absorption features, NGC 3094, NGC 7172 and NGC 5506. In the galaxies with the deepest absorption bands, the silicate profile towards the nuclei is well represented by the emissivity function derived from the circumstellar emission from the red supergiant, μ Cephei which is also representative of the mid-infrared absorption in the diffuse interstellar medium in the Galaxy. There is spectral structure near 11.2 μm in NGC 3094 which may be due to a component of crystalline silicates. In NGC 5506, the depth of the silicate absorption increases from north to south across the nucleus, suggestive of a dusty structure on scales of tens of parsecs. We discuss the profile of the silicate absorption band towards galaxy nuclei and the relationship between the 9.7-μm silicate and 3.4-μm hydrocarbon absorption bands.