Searching for δ Scuti-type pulsation and characterising northern pre-main-sequence field stars

Context. Pre-main-sequence (PMS) stars objects evolving from the birthline to the zero age main sequence (ZAMS). Given a mass range near the ZAMS, the temperatures and luminosities of PMS and main-sequence stars are very similar. Moreover, their evolutionary tracks intersect one another causing some ambiguity in the determination of their evolutionary status. In this context, the detection and study of pulsations in PMS stars is crucial for differentiating between both types of stars by obtaining information of their interiors via asteroseismic techniques. Aims. A photometric variability study of a sample of northern field stars, which previously classified as either PMS or Herbig Ae/Be objects, has been undertaken with the purpose of detecting delta Scuti-type pulsations. Determination of physical parameters for these stars has also been carried out to locate them on the Hertzsprung-Russell diagram and check the instability strip for this type of pulsators. Methods. Multichannel photomultiplier and CCD time series photometry in the uvby Stromgren and BVI Johnson bands were obtained during four consecutive years from 2007 to 2010. The light curves have been analysed. and a variability criterion has been established. Among the objects classified as variable stars, we have selected those which present periodicities above 4 d(-1), which was established as the lowest limit for delta Scuti-type pulsations in this investigation. Finally, these variable stars have been placed in a colour magnitude diagram using the physical parameters derived with the collected uvby beta Stromgren-Crawford photometry. Results. Five PMS delta Scuti- and three probable beta Cephei-type stars have been detected. Two additional PMS delta Scuti stars are also confirmed in this work. Moreover, three new delta Scuti- and two gamma Doradus-type stars have been detected among the main-sequence objects used as comparison or check stars.