First orbital elements for the λ Bootis spectroscopic binary systems HD 84948 and HD 171948: Implications for the origin of the λ Bootis stars

We present high-resolution spectroscopic observations of the two λ Bootis type spectroscopic binary systems HD 84948 and HD 171948. Both systems consist of two true λ Bootis stars, as has already been proven by a detailed abundance analysis taking into account the binary nature. Nevertheless, we have obtained non-LTE abundances for carbon and oxygen in order to investigate those important elements. The abundances fit excellently into the typical pattern for this group, leaving no doubt that all four components are true λ Bootis stars. With the help of the derived orbital elements it is possible to establish the ages of the two systems. For the first time we can estimate the evolutionary status of Galactic field λ Bootis stars. The origin of those nonmagnetic, metal-weak, Population I, late B- to early F-type stars is still controversial. The two widely discussed theories explaining the λ Bootis phenomenon (diffusion together with either accretion of circumstellar matter or mass-loss) predict significantly different evolutionary stages for this group of objects (close to either the Zero-Age Main Sequence or Terminal-Age Main Sequence). It is already known that very young members of the group exist in the Orion OBI association and probably in NGC 2264. Hipparcos data established six further Galactic field λ Bootis stars which are close to the Zero-Age Main Sequence, whereas the evolutionary status for the other objects remained undetermined. The Hipparcos data reveal that the Galactic space motions of both the systems that we discuss here are typical of those of Population I objects. The photometric data from the Hipparcos mission confirm the pulsation previously discovered for HD 84948 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 of about 0.01 to 0.1 Gyr which is close to the Zero-Age Main Sequence. We therefore conclude that the λ Bootis phenomenon can be found continuously from very early stages to the Terminal-Age Main Sequence, suggesting that different mechanisms might work at different stages of stellar evolution producing the same abundance pattern.