2014A&A...562A..65S

Many chemically peculiar (CP) stars, especially the magnetic CP2 stars, show a flux depression at 5200Å. The Δa photometric system takes advantage of this characteristic to detect these objects in an efficient way. In addition, it is capable of finding metal-weak, emission-type, and shell-type objects of the upper main sequence. To compare available observations and to detect new peculiar objects, we used a spectrophotometric catalogue consisting of 1159 stars. From this catalogue, we selected 1067 objects to synthesize three different a indices to find the most efficient one for further observations. In addition, we extended the analysis to stars cooler than F5. We employed classical Δa photometry described by Maitzen, using simulated filter curves, the spectrophotometric Δa index by Adelman, and a modified index. Even though the accuracy of the spectrophotometry used for this investigation is significantly lower than the photometric Δa measurements, we are able to confirm peculiarity for most of the known CP2 stars above a certain limit of Δa. We investigated 631 stars hotter than spectral type F5 to find additional that are not yet identified peculiar objects. We find that for very low mass stars (M0), the a index is independent of the colour (effective temperature). The Δa photometric system is very closely correlated with the effective temperature over a wide range of the main sequence. It is able to detect any kind of peculiarity connected to the 5200Å region. Especially for low-mass stars, this opens up a new possibility of detecting peculiar objects in an efficient way.