CCD photometric search for peculiar stars in open clusters

Abstract

Abstract. The time scale on which chemical peculiarity in upper main sequence stars begins is unknown. Results 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 (Gómez et al. 1998). However, Hubrig et al. (2000 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 1981; Borra et al. 1982) 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 10 6 yr to appear. Since no further observations have been published since then, we have observed the five very young (1 Myr ≤ age ≤ 25 Myr) open clusters Collinder 272, Pismis 20, Lyngå 14, NGC 6396 and NGC 6611. Photometry from 250 CCD frames in the three filter ∆a-system results in very low detection limits (0.008 to 0.015 mag). 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.

On the detection of chemically peculiar stars using $\mathsf{\Delta}$a photometry

“…The only important difference is the filter set used by Maitzen et al (1997) together with the CCD equipment. This new CCD photometric ∆a system was successfully applied to open clusters of the Milky Way (Paunzen et al 2002b) and the Large Magellanic Cloud (Maitzen et al 2001). Although the bandwidths of g 1 and y were exchanged, the overall values of ∆a are in very good agreement with those of the photoelectric system.…”

“…This is especially important for observations in open clusters (Paunzen et al 2002b) and even in the Magellanic clouds (Maitzen et al 2001). The knowledge of how many peculiar stars for a certain detection limit are expected in comparison with the actually observed number, serves as important information for a statistical analysis, such as the incidence of CP stars that depend on different local environments.…”

CCD photometric search for peculiar stars in open clusters

“…Schöpfl (Austria) using the multimode instrument OEFOSC. The characteristic of the instruments and field-of-views can be found in Bayer et al (2000) and Paunzen et al (2002b). The observing log is listed in Table 1.…”

“…In continuation of our previous four papers dedicated to the search for chemically peculiar objects via CCD ∆a-photometry (Paunzen et al 2002b), another five open clusters in our Milky Way have been investigated.…”

Pismis 20: One or two clusters?

“…Most of the absorption is assumed to be produced in the Local and the Sagittarius-Carina arms. The cluster has been investigated by Paunzen et al (2002) in a search for chemically peculiar stars, with negative results. Nearly 2 from the central core of the cluster is WR 67 (LSS 3329, HD 134877), a WN6 + OB?…”