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Abstract:

Two rival hypotheses have been proposed for the origin of the compact radio flux observed in radio-quiet quasars (RQQs). It has been suggested that the radio emission in these objects, typically some two or three orders of magnitude less powerful than in radio-loud quasars (RLQs), represents either emission from a circumnuclear starburst or is produced by radio jets with bulk kinetic powers 10^3 times lower than those of RLQs with similar luminosity ratios in other wavebands. We describe the results of high resolution (parsec-scale) radio-imaging observations of a sample of 12 RQQs using the Very Long Baseline Array (VLBA). We find strong evidence for jet-producing central engines in 8 members of our sample.

Abstract:

Differential carbon abundances (based on the C II doublet at 6580Ã…) are presented for eight early type stars, towards the Galactic anti-centre. All the stars have similar atmospheric parameters with effective temperatures in the range 25000 - 29000 K and surface gravities between logg = 3.9 - 4.3 dex. The derived photospheric abundances vary by up to 0.6 dex, and with the exception of one star, RLWT-41, the differential abundances are found to be closely correlated with those of nitrogen. This implies that both elements may have been formed by similar mechanisms and that the lack of correlation between the nitrogen and oxygen abundances previously found in this sample is not directly due to CNO-processed core material being mixed to the stellar surface.