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Astron. Astrophys. 356, 517-528 (2000)

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8. Conclusion

We have investigated several blue stragglers and main sequence stars from the following open clusters: NGC 3496, NGC 6475, NGC 6633 and IC 2602. For all of these stars a detailed spectroscopic study, based on the LTE assumption, has been performed for the first time. The main results are as follows:

  • 1). Our present knowledge about NGC 3496 does not allow a confident classification of the three stars S 43, S 115 and S 117 as blue stragglers.

  • 2). All investigated blue stragglers showed low projected rotational velocities, while investigated B-A main sequence stars appeared to be the fast rotators in accordance with their spectral types (the case of S 115 and S 117 is disputable).

  • 3). We have discovered two new He-weak stars (one is a blue straggler) and obtained a quantitative estimate of the helium content for the already known He-weak blue straggler HD 162374. For the quantitative analysis of the He-weak stars, atmosphere models were calculated with decreased helium content.

  • 4).All the investigated middle-to-late B stars showed a subsolar carbon abundance. These abundances have been checked by NLTE calculations.

  • 5). The severe helium deficiency found for some stars and the moderate-to-strong carbon underabundances could be explained as a result of atomic diffusion in the stellar atmospheres. However, three stars with apparent helium/carbon deficiencies possess rather high projected rotational velocities ([FORMULA] [FORMULA] 70 km s-1 - 135 km s-1).

  • 6). The silicon abundance in our program stars shows a clear dependence upon the helium content. In the helium deficient stars this element is increased, while its abundance is solar (or slightly subsolar) for the He-normal stars.

  • 7). The iron-group elements do not show any significant deviations from the solar abundances in either blue stragglers or main sequence stars.

  • 8). Generally, based on their chemical compositions, the blue stragglers stars are not different from main sequence stars. This conclusion is made for a very restricted sample of blue stragglers and requires further investigation.

To summarize, one can note that any theory which attempts an exhaustive explanation of the blue straggler phenomenon should be able to account for the two important features of these enigmatic stars: a) their small projected rotational velocities; b) the similarities of their chemical compositions with those of ordinary cluster and galactic field stars.

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© European Southern Observatory (ESO) 2000

Online publication: April 10, 2000