On the period-luminosity-colour-metallicity relation and the pulsational characteristics of λ Bootis type stars

Abstract

Generally, chemical peculiarity found for stars on the upper main sequence excludes δ Scuti type pulsation (e.g. Ap and Am stars), but for the group of λ Bootis stars it is just the opposite. This makes them very interesting for asteroseismological investigations. The group of λ Bootis type stars comprises late B- to early F-type, Population I objects which are basically metal weak, in particular the Fe group elements, but with the clear exception of C, N, O and S. The present work is a continuation of the studies by Paunzen et al. (1997, 1998), who presented first results on the pulsational characteristics of the λ Bootis stars. Since then, we have observed 22 additional objects; we found eight new pulsators and confirmed another one. Furthermore, new spectroscopic data (Paunzen 2001) allowed us to sort out misidentified candidates and to add true members to the group. From 67 members of this group, only two are not photometrically investigated yet which makes our analysis highly representative. We have compared our results on the pulsational behaviour of the λ Bootis stars with those of a sample of δ Scuti type objects. We find that at least 70% of all λ Bootis type stars inside the classical instability strip pulsate, and they do so with high overtone modes (Q < 0.020 d). Only a few stars, if any, pulsate in the fundamental mode. Our photometric results are in excellent agreement with the spectroscopic work on high-degree nonradial pulsations by Bohlender et al. (1999). Compared to the δ Scuti stars, the cool and hot borders of the instability strip of the λ Bootis stars are shifted by about 25 mmag, towards smaller (b-y)0. Using published abundances and the metallicity sensitive indices of the Geneva 7-colour and Strömgren uvbyβ systems, we have derived [Z] values which describe the surface abundance of the heavier elements for the group members. We find that the Period-Luminosity-Colour relation for the group of λ Bootis stars is within the errors identical with that of the normal δ Scuti stars. No clear evidence for a statistically significant metallicity term was detected.