Aims. The determination of effective temperature for chemically peculiar (CP) stars by means of photometry is a sophisticated task due to their abnormal colours. Standard calibrations for normal stars lead to erroneous results and, in most cases corrections are necessary. Methods. In order to specify appropriate corrections, direct temperature determinations for 176 objects of the different subgroups were collected from the literature. This much larger sample than in previous studies therefore allows a more accurate investigation, mostly based on average temperatures. Results. For the three main photometric systems (UBV, Geneva, Strömgren uvbyβ), methods to determine effective temperature are presented together with a comparison with former results. Based on the compiled data we provide evidence that He (CP4) objects also need a considerable correction, not noticed in former investigations due to their small number. Additionally, a new relation for the bolometric correction and the capability of standard calibrations to deduce interstellar reddening for magnetic CP stars are shown.

Narrow band photometry is presented on 27 dwarf ellipticals in the Fornax cluster. Calibrated with Galactic globular cluster data and spectrophotometric population models, the colors indicated that dwarf ellipticals have a mean [Fe/H] of −1.00 ± 0.28 ranging from −1.6 to −0.4. The mean age of dwarf ellipticals, also determined photometrically, is estimated at 10±1 Gyrs compared to 13 Gyrs for bright Fornax ellipticals. Comparison of our metallicity color and Mg 2 indices demonstrates that the [Mg/Fe] ratio is lower in dwarf ellipticals than their more massive cousins, which is consistent with a longer duration of initial star formation to explain their younger ages. There is a increase in dwarf metallicity with distance from the Fornax cluster center where core galaxies are, on average, 0.5 dex more metal-poor than halo dwarfs. In addition, we find the halo dwarfs are younger in mean age compared to core dwarfs. One possible explanation is that the intracluster medium ram pressure strips the gas from dwarf ellipticals halting star formation (old age) and stopping enrichment (low metallicity) as they enter the core.In order to relink the uz, vz, bz, yz system to the metallicity system defined by globular clusters, a series of metal-rich and metal-poor systems were observed between 1997 and 1999 from both the northern and southern hemisphere. The northern cluster observations were made at Lowell Observatory using 1.1m Hall telescope. The imaging device was the SIT 2K CCD with 24 micron pixels binned 4x4. The field size covers 18x18 arc minutes of sky. The southern cluster observations were made at CTIO using the 0.91m Curtis Schmidt telescope plus STIS 2K CCD with 21 micron pixels. The plate scale of 2.1 arcsec/pixel covers 76x76 arcmins of sky. Reduction followed standard CCD procedures (bias, flatfield, etc). Standard

Abstract.We have summarized all ∆a measurements for galactic field stars (1474 objects) from the literature published over more than two decades. These measurements were, for the first time, compiled and homogeneously analyzed. The ∆a intermediate band photometric system samples the depth of the 5200 Å flux depression by comparing the flux at the center with the adjacent regions with bandwidths of 110 Å to 230 Å. Because it was slightly modified over the last three decades, we checked for systematic trends for the different measurements but found no correlations whatsoever. The ∆a photometric system is most suitable to detecting magnetic chemically peculiar (CP) stars with high efficiency, but is also capable of detecting a small percentage of non-magnetic CP objects. Furthermore, the groups of (metal-weak) λ Bootis, as well as classical Be/shell stars, can be successfully investigated. In addition, we also analyzed the behaviour of supergiants (luminosity class I and II). On the basis of apparent normal type objects, the correlation of the 3σ significance limit and the percentage of positive detection for all groups was derived. We compared the capability of the ∆a photometric system with the ∆(V1 − G) and Z indices of the Geneva 7-color system to detect peculiar objects. Both photometric systems show the same efficiency for the detection of CP and λ Bootis stars, while the indices in the Geneva system are even more efficient at detecting Be/shell objects. On the basis of this statistical analysis it is possible to derive the incidence of CP stars in galactic open cluster and extragalactic systems including the former unknown bias of undetected objects. This is especially important in order to make a sound statistical analysis of the correlation between the occurrence of these objects and astrophysical parameters such as the age, metallicity, and strength of global, as well as local, magnetic fields.

Abstract. Photoelectric photometry of 803 southern BS objects in the ∆a-system as detection tool for magnetic chemically peculiar (=CP2) stars has been carried out and compared to published spectral types. The statistical yield of such objects detected by both techniques is practically the same. We show that there are several factors which contaminate the search for these stars, but this contamination is only of the order of 10% in both techniques. We find a smooth transition from normal to peculiar stars.Our sample exhibits the largest fraction of CP2 stars at their bluest colour interval, i.e. 10% of all stars in the colour range −0.19 ≤ B − V < −0.10 or −0.10 ≤ b − y < −0.05. No peculiar stars based on the ∆a-criterion were found at bluer colours. Towards the red side the fraction of CP2 stars drops to about 3% for positive values of B − V or b − y with red limits roughly corresponding to normal stars of spectral type A5.The photometric behaviour of other peculiar stars: Am, HgMn, δ Del, λ Boo, He abnormal stars, as well as Be/shell stars and supergiants shows some slight, but definite deviations from normal stars. Spectroscopic and visual binaries are not distinguished from normal stars in their ∆a behaviour.The results of this work justify larger statistical work (e.g. in open clusters) employing more time-saving photometric methods (CCD).

The a photometric system provides an efficient observational method to identify and distinguish magnetic and several other types of chemically peculiar (CP) stars of spectral types B to F from other classes of stars in the same range of effective temperatures. We have developed a synthetic photometric system that can be used to explore the capability of model atmospheres with individual element abundances to predict photometric a magnitudes, which measure the extent of the flux depression around 5200Å found in different types of CP stars. In this first paper, we confirm the observed dependency of the a index as a function of various colour indices sensitive to the effective temperature of stars as well as its average scatter expected from surface gravity variations within the main-sequence band. The behaviour of the so-called 'normality line' of a systems used in photometric observations of CP stars is well reproduced. The metallicity dependence of the normality line of the a system was computed for several grids of model atmospheres where the abundances of elements heavier than He had been scaled ±0.5 dex with respect to the solar value. We estimate a lowering of a magnitudes for CP stars within the Magellanic Clouds by ∼ −3 mmag relative to those in the solar neighbourhood assuming an average metallicity of [Fe/H] = −0.5 dex. Using these results on the metallicity bias of the a system we find the observational systems in use suitable to identify CP stars in other galaxies or distant regions of our own Galaxy and capable to provide data samples on a statistically meaningful basis. In turn, the synthetic system is suitable to test the performance of model atmospheres for CP stars. This work will be presented in follow-up papers of this series.