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First orbital elements for the lambda Bootis spectroscopic binary systems HD 84948 and HD 171948. Implications for the origin of the lambda Bootis stars
We present high-resolution spectroscopic observations of the two lambdaBootis type spectroscopic binary systems HD 84948 and HD 171948. Bothsystems consist of two true lambda Bootis stars, as has already beenproven by a detailed abundance analysis taking into account the binarynature. Nevertheless, we have obtained non-LTE abundances for carbon andoxygen in order to investigate those important elements. The abundancesfit excellently into the typical pattern for this group, leaving nodoubt that all four components are true lambda Bootis stars. With thehelp of the derived orbital elements it is possible to establish theages of the two systems. For the first time we can estimate theevolutionary status of Galactic field lambda Bootis stars. The origin ofthose nonmagnetic, metal-weak, Population I, late B- to early F-typestars is still controversial. The two widely discussed theoriesexplaining the lambda Bootis phenomenon (diffusion together with eitheraccretion of circumstellar matter or mass-loss) predict significantlydifferent evolutionary stages for this group of objects (close to eitherthe Zero-Age Main Sequence or Terminal-Age Main Sequence). It is alreadyknown that very young members of the group exist in the Orion OBIassociation and probably in NGC 2264. Hipparcos data established sixfurther Galactic field lambda Bootis stars which are close to theZero-Age Main Sequence, whereas the evolutionary status for the otherobjects remained undetermined. The Hipparcos data reveal that theGalactic space motions of both the systems that we discuss here aretypical of those of Population I objects. The photometric data from theHipparcos mission confirm the pulsation previously discovered for HD84948 with a period of about 110 min and a V-amplitude of about 14 mmag.For HD 84948, we estimate from the mass ratio an age of about 1 Gyr,ruling out a possible Pre-Main-Sequence status. HD 171948 has an age ofabout 0.01 to 0.1 Gyr which is close to the Zero-Age Main Sequence. Wetherefore conclude that the lambda Bootis phenomenon can be foundcontinuously from very early stages to the Terminal-Age Main Sequence,suggesting that different mechanisms might work at different stages ofstellar evolution producing the same abundance pattern. Based onobservations at BNAO Rozhen; SAAO and with the Hipparcos satellite.
| Nonvariability among lambda Bootis stars
With asteroseismic techniques it is possible to investigate the interiorand the evolutionary status of stars via their frequency spectrum. Bothinformation would be very much needed for lambda Bootis stars, a groupof metal-poor Population I, A-type stars, since no conclusive theoryexists explaining the observed abundance anomalies. Geneva and Stromgrenphotometry place these stars inside the classical instability strip orat least very close to it. We therefore have started an extensivephotometric survey for pulsation in lambda Bootis stars and havediscovered so far 13 new variables. In this paper we present results forstars which presumably are constant, because we are able to establishonly an upper level for possible variability. A typical noise level of 3mmag for Stromgren b was achieved in the relevant frequency domain up to100 d^{-1}. Considering the given noise level of our survey, we concludethat at least 50% of all investigated lambda Bootis stars inside theinstability strip are pulsating, making this group remarkable comparedto stars with similar spectral types. This may suggest that a low(surface) metallicity has an influence on the pulsation Based onobservations obtained at ESO-La Silla, CTIO, SAAO, McDonald Observatory,Instituto Astrofisica Andalucia Observatory.
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