CCD photometric search for peculiar stars in open clusters. VII. Berkeley 11, Berkeley 94, Haffner 15, Lynga 1, NGC 6031, NGC 6405, NGC 6834 and Ruprecht 130

Abstract

The detection of magnetic chemically peculiar (CP2) stars in open clusters of the Milky Way can be used to study the influence of different galactic environments on the (non-)presence of peculiarities, which has to be taken into account in stellar evolution models. Furthermore it is still unknown if the CP2 phenomenon evolves, i.e. does the strength of the peculiarity feature at 5200A, increase or decrease with age. We have observed eight young to intermediate age open clusters in the Delta a photometric system. This intermediate band photometric system samples the depth of the 5200A, flux depression by comparing the flux at the center with the adjacent regions having bandwidths of 110A, to 230A. The Delta a photometric system is most suitable to detect CP2 stars with high efficiency, but is also capable of detecting a small percentage of non-magnetic CP objects. Also, the groups of (metal-weak) lambda Bootis, as well as classical Be/shell stars, can be successfully investigated. This photometric system allows one to determine the age, reddening and distance modulus by fitting isochrones. Among the presented sample of eight galactic clusters, we have detected twenty three CP2, eight Be/Ae and eight metal-weak stars. Another six objects show a peculiar behaviour which is most probably due to a non-membership,variability or duplicity. Fitting isochrones to Delta a photometry yields estimates of the age, reddening and distance that are in excellent agreement with published values.