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On the new lambda Bootis-type spectroscopic binary systems HD 84948 and HD 171948
We report the discovery of two new lambda Bootis-type spectroscopicbinary systems (HD 84948 and HD 171948). High resolution spectra wereanalyzed in order to derive abundances as well as astrophysicalquantities. It resulted in extreme (HD 171948) and moderate (HD 84948)underabundances of Na, Mg, Si, Ca, Sc, Ti, Cr and Fe with respect to theSun, thus establishing all four individual stars as true members of thelambda Bootis group. These two systems are therefore the firstspectroscopic binary systems where both components are lambdaBootis-type stars. Furthermore, we have discovered delta Scuti typepulsation for one component of HD 84948 making this star particularlyinteresting for further observations. Since many authors have used therotational velocity for discriminating lambda Bootis stars(vsini>50kms(-1) ), we note that both components of HD 171948 are theslowest (vsini<20kms(-1) ) rotating lambda Bootis-type stars known sofar. Using the Hipparcos data, we are able to rule out thediffusion/mass-loss mechanism as origin for these two systems. Pre- aswell as Main Sequence evolutionary tracks for the age determinationyield consistent results with the predictions of the diffusion/accretiontheory, establishing both systems as being very close to the MainSequence. A determination of the orbital elements and/or anasteroseismological investigation of HD 84948 could provide anindependent proof of our results. Based on observations obtained at theOsservatorio Astronomico di Padua-Asiago, McDonald Observatory, theSierra Nevada Observatory and with the Hipparcos satellite
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