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S Messenger

Publications and source records attributed to S Messenger.

4 recordsLinked to original sources
PubMed2002

Polytype distribution in circumstellar silicon carbide.

The inferred crystallographic class of circumstellar silicon carbide based on astronomical infrared spectra is controversial. We have directly determined the polytype distribution of circumstellar SiC from transmission electron microscopy of presolar silicon carbide from the Murchison carbonaceous meteorite. Only two polytypes (of a possible several hundred) were observed: cubic 3C and hexagonal 2H silicon carbide and their intergrowths. We conclude that this structural simplicity is a direct consequence of the low pressures in circumstellar outflows and the corresponding low silicon carbide condensation temperatures.

Astronomical Phenomena↗
PubMed2000

Identification of molecular-cloud material in interplanetary dust particles.

Interplanetary dust particles (IDPs) collected in the Earth's stratosphere and meteorites are fragments of comets and asteroids. These are 'primitive' meteorites in part because they have preserved materials which predate the formation of the Solar System. The most primitive (least altered) meteorites contain a few parts per million of micrometre-sized dust which formed in the atmospheres of giant stars. Some meteorites have elevated D/H and 15N/14N ratios that are attributed to surviving interstellar organic molecules which have probably been strongly diluted and altered by parent-body processes. Most IDPs are chemically, mineralogically, and texturally primitive in comparison to meteorites. Here I show that H and N isotopic anomalies among fragile 'cluster' IDPs are far larger, more common, and less equilibrated than those previously observed in other IDPs or meteorites. In some cases, the D/H ratios that we measure reach the values of interstellar molecules, suggesting that molecular-cloud material has survived intact. These observations indicate that cluster IDPs are the most primitive class of Solar System materials currently available for laboratory analysis.

Carbon Isotopes↗
PubMed2000

PCR amplification and phylogenetic analysis of groESL operon sequences from Ehrlichia ewingii and Ehrlichia muris.

Broad-range PCR primers were used to amplify part of the groESL operon of the canine pathogen Ehrlichia ewingii, recently recognized as a human pathogen, and the murine pathogen Ehrlichia muris. Phylogenetic analysis supported the relationships among Ehrlichia species previously determined by comparison of 16S rRNA gene sequences. These sequences provide additional PCR targets for species for which few gene sequences have been determined.

Animals↗