Cl* NGC 6396 PPM 90 , the SIMBAD biblio

The time scale on which chemical peculiarity in upper main sequence stars begins is unknown.
Result. from the Hipparcos measurements indicate that the classical chemically peculiar (CP) stars occupy the whole area from the zero age main sequence to the terminal age main sequence (Gomez et al., 1998A&A...336..953G). However, Hubrig et al. (2000ApJ...539..352H) reported that the magnetic fields appear only in stars that have already completed at least approximately 30% of their main sequence life time. Previous observations in the very young Orion OB1 and Scorpio-Centaurus associations (Joncas & Borra, 1981A&A....94..134J; Borra et al., 1982A&A...111..117B) reveal a percentage of chemically peculiar stars that is significantly smaller (less than 5%) than that of the galactic field (up to 25%). This already indicates that the phenomenon needs at least several 10⁶yr to appear. Since no further observations have been published since then, we have observed the five very young (1Myr≤age≤25Myr) open clusters Collinder 272, Pismis 20, Lyngå 14, NGC 6396 and NGC 6611. Photometry from 250 CCD frames in the three filter Δa-system results in very low detection limits (0.008 to 0.015mag). We find no single photometrically chemically peculiar candidate in approximately 460 observed cluster members in the five program clusters. From the number of observed objects we deduce upper limits between 0.6% and 2.4% for the incidence of CP stars for the four observed aggregates.