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Massive star abundances in the Galaxy and the Magellanic Clouds
The use of photospheric abundances in OB-type main-sequence stars and Aand B-type supergiants as probes of rotation and evolutionary status isreviewed. The abundances of CNO and boron can be comparedquantitatively with stellar evolutionary calculations. In particularthe abundance ratios of N/O and N/C can be derived in blue supergiantsto determine if they are consistent with the stars having gone through ared supergiant phase and dredge-up of core material to the surface. Theresults from several different studies are reviewed and compared. Forblue supergiant stars in the mass-range 5 - 80 Msun thesituation appears consistent -- there is no evidence for stars of anymass having undergone blue-loops in the HR diagram. The stellar samplesshow significant signs of having N enriched atmospheres, and thequantitative values are consistent with stellar evolutionarycalculations which invoke turbulent diffusive mixing while massive starsare on the main-sequence. Results on the interesting blue supergiantSher 25 are presented, and linked to sk; the B3Ia progenitor of SN1987A. A spectacular ejection nebula surrounds Sher 25, much like thatwhich was ejected by sk during its final stages of evolution. Both ofthese were thought to have been formed during a mass-loss event when thestars were in the red supergiant phase. The CNO abundances derived inSher 25 suggest the nebula was ejected during the blue supergiant phase,and that rotation can explain the N-enrichment found in the stellarphotosphere.
| New periodic variables from the Hipparcos epoch photometry
Two selection statistics are used to extract new candidate periodicvariables from the epoch photometry of the Hipparcos catalogue. Theprimary selection criterion is a signal-to-noise ratio. The dependenceof this statistic on the number of observations is calibrated usingabout 30000 randomly permuted Hipparcos data sets. A significance levelof 0.1 per cent is used to extract a first batch of candidate variables.The second criterion requires that the optimal frequency be unaffectedif the data are de-trended by low-order polynomials. We find 2675 newcandidate periodic variables, of which the majority (2082) are from theHipparcos`unsolved' variables. Potential problems with theinterpretation of the data (e.g. aliasing) are discussed.
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