Pulsation in λ Bootis stars

In this paper we present a further step in applying asteroseismic techniques to the group of λ Bootis stars which can be characterized as nonmagnetic A to F-type Population I dwarfs with significant (surface) underabundances of Fe-peak elements. Since no conclusive theory explaining the origin of the observed abundance anomalies exists, an extensive photometric survey for pulsation in this group has been initiated. Knowledge about the pulsational properties (most members are located within the classical instability strip) could help to establish constrains about the overall abundance of these stars as well as on the evolutionary status. New photometric observations were carried out for eleven stars. Variability was detected in four stars (e.g. A Bootis itself) whereas the remaining seven objects are probably constant. In total, 52 members of this group have been photometrically investigated so far. With 22 pulsating and 30 'constant' stars, we derive a ratio of at least 50 % for variable to nonvariable members inside the classical instability strip. This result is based on high quality Hipparcos and new photometric data. The observed log ρ̄/ρ̄⊙ and log P values for the pulsating members are compatible with standard (solar abundant) δ Scuti models supporting the hypothesis that the found abundance anomalies are restricted to the surface only. Otherwise the pulsational properties of this group are not outstanding compared to 'normal' δ Scuti stars, indicating that the mechanism driving the pulsations is very similar.