Previous studies have shown clear, predictable successional trends in habitat characteristics and community structure in tubeworm aggregations at 3 similar hydrocarbon-seep sites on the central upper ...Louisiana slope of the Gulf of Mexico. In this study, we examine these trends in quantitative community collections from 7 additional hydrocarbon-seep sites widely distributed in the northern Gulf of Mexico. The relative proportions and sizes of
Lamellibrachia luymesi and
Seepiophila jonesi in tubeworm aggregations were similar at new and central sites, though
S. jonesi dominated some collections from new sites, a situation not previously observed at central sites. In general, sulfide declined with increasing aggregation age (average size of tubeworms), but there was more variability in this trend at the new sites. Tubeworm-associated community composition was similar at new and central sites, with only a few rare species collected at the new sites for the first time. The most significant differences in the communities at new sites were the lower relative abundance of various gastropod species, and the absence of gastropods from collections made at the Viosca Knoll site. This community type was largely restricted to young aggregations at new sites that were more isolated from other tubeworm aggregations and consisted of higher proportions of
S. jonesi. As succession proceeds from young to old aggregations, many of the previously described processes were apparent at the new sites including a reduction in biomass and a shift in trophic structure from endemic primary consumers to non-endemic higher order predators. Regardless of community composition in young aggregations, succession converges on similar late-stage community types.
Linear polarization has been measured in up to four SiO maser transitions for seven sources, including the Orion maser source associated with IRc2. Polarization variability characteristics are ...reported for some sources which were observed several times over a 6 month period in the v = 1, J = 2-1 transition. Stars with low envelope expansion velocities appear to have more highly polarized masers than those with high expansion velocities. Comparison of polarization of individual maser features in different transitions indicates that emission from the same rotational transition in different vibrational states often arises in the same volume of gas, but that masers of different rotational transitions within the same vibrational state arise in different regions. For a given source, the profile-averaged fractional polarizations show a uniformity between different transitions, while the profile-averaged position angles do not in general appear to be correlated between transitions.
Measurements of 87 GHz of the linear polarization and total flux density of 16 active extragalactic sources are presented. The observations were made over a 7 month period with time resolution of ...days to weeks for some sources. Two rapid and closely spaced outbursts with time scales of about 1 week were seen in OJ 287. These outbursts were characterized by both rapid rises and rapid falls in the flux density and as such differed from most previously observed rapid variations, which usually involve only dropouts in the flux density. For 3C 345, variations in the polarization may indicate the propagation of radiating particles along a jet in which the order and direction of the magnetic field changes with distance along the jet. No source was seen to undergo large-amplitude linear rotation of polarization position angle with time. Implications of these results, and the results for other sources, are discussed.
Recent improvements in data analysis and receiver techniques have allowed us to produce a map of the 100GHz emission from the compact radio source 3C273 with the unsurpassed resolution of 50-mu-as ...(microarcseconds). Our map shows that the structure within 300-mu-as (approximately 1.5.10(18).h-1 cm) has a position angle significantly different from the position angle of the jet observed at lower frequencies. There are also indications in our map that the inner structure has a more pronounced wiggling structure than has been observed on larger scales. The observations were made about 60 days from the start of the outburst of 1988. Most of the flux from the outburst is concentrated in a component which is elongated approximately (56 x 5).10(16).h-1 cm perpendicular to the overall jet-axis. The distance between this component and the core is approximately 128-mu-as, which corresponds to the distance expected from an apparent velocity of approximately 800-mu-as year-1.
A search has been made for circular polarization of HCN maser emission in the circumstellar envelope of the carbon star CIT 6, using the 14 m FCRAO telescope. The emission has been found to be ...circularly unpolarized, with a 3 sigma upper limit of about 3 percent. The absence of circular polarization, together with relatively strong linear polarization detected previously, suggests a situation with the stimulated emission rate far less than the magnetic splitting far less than the line width of maser emission. The intensity of the line, as well as the degree and position angle of linear polarization, have been found to be constant on a time scale of 1 yr. 19 refs.
A cryogenic 3-mm receiver has been developed which fully utilizes the low-noise potential of Schottky diodes by approaching the shot-noise limit within 10 percent. With a broad-band mixer design ...which properly terminates the input sidebands and reactively terminates the second harmonic of the local oscillator and its sidebands, the double sideband (DSB) mixer noise temperature is 35 K in the best case. This design has given an average DSB receiver noise temperature of 75 K over the 80 to 115-GHz band with a best noise temperature of 62 K.
