CCD photometric search for peculiar stars in open clusters

Abstract

Aims. The detection of magnetic chemically peculiar (CP2) stars in open clusters of the Milky Way can be used to study the influence of different galactic environments on the (non-)presence of peculiarities, which has to be taken into account in stellar evolution models. Furthermore it is still unknown if the CP2 phenomenon evolves, i.e. does the strength of the peculiarity feature at 5200 Å increase or decrease with age. Methods. We have observed eight young to intermediate age open clusters in the ∆a photometric system. This intermediate band photometric system samples the depth of the 5200 Å flux depression by comparing the flux at the center with the adjacent regions having bandwidths of 110 Å to 230 Å. The ∆a photometric system is most suitable to detect CP2 stars with high efficiency, but is also capable of detecting a small percentage of non-magnetic CP objects. Also, the groups of (metal-weak) λ Bootis, as well as classical Be/shell stars, can be successfully investigated. This photometric system allows one to determine the age, reddening and distance modulus by fitting isochrones. Results. Among the presented sample of eight galactic clusters, we have detected twenty three CP2, eight Be/Ae and eight metalweak stars. Another six objects show a peculiar behaviour which is most probably due to a non-membership, variability or duplicity. Fitting isochrones to ∆a photometry yields estimates of the age, reddening and distance that are in excellent agreement with published values.Key words. stars: chemically peculiar -stars: early-type -techniques: photometric -Galaxy: open clusters and associations: general IntroductionThe continuation of our CCD ∆a photometric survey to detect chemically peculiar (CP) stars of the upper main sequence currently comprises the largest sample of open clusters, including young and intermediate age clusters at a variety of galactic longitudes and galactocentric distances.The ∆a photometric system allows one to efficiently detect CP stars and related objects through the flux depression at 5200 Å (Kupka et al. 2004). The efficiency of this system was recently investigated by Paunzen et al. (2005a) surveying a large sample of published field star measurements, resulting in a probability of up to 95% of detecting all relevant magnetic CP stars. Furthermore the groups of λBootis and classical Be/shell systematically show negative ∆a values.Unambiguous detection or non-detection of CP stars in different galactic environments will help to understand their evolution and formation. The aim of this survey is to investigate the occurence of CP stars in open clusters depending on various parameters like age, metallicity and galactic location. Preliminary results have shown that the percentage of CP2 stars with ages between 30 and 100 Myr have two clear maxima, separated by a deep minimum. Therefore few of open clusters in this paper were selected within that age range to clarify this situation.Beside the detection of CP stars, we are able to determine cluster parameters like the age, ...

Optical/IR studies of Be stars in NGC 6834 with emphasis on two specific stars

“…The V mag measurements by Jerzykiewicz et al (2011) were also done during the same period (April-October 2001) and they report a value of 13.117 for the Be star. Paunzen et al (2006) also obtained a similar value (V = 13.22) from their observations conducted in August 2004. This presents a puzzle regarding the brighter magnitude reported for the Be star in speckle interferometric observations.…”

“…Similarly, one can find diverse age estimates; Miller et al (1996) estimated the age of the cluster to be around 50 Myr, while Moffat (1972) assigned a value of 80 Myr. Paunzen et al (2006) estimated a mean color excess E(B − V ) = 0.70±0.05 mag, and Jerzykiewicz et al (2011) found a range in E(B − V ) values between 0.61 and 0.82 mag. Jerzykiewicz et al (2011) identified 15 B-type variable stars in the cluster.…”

Does convective core overshooting depend on stellar mass?

“…Isochrone fits to the colour-magnitude diagrams of clusters, mainly the younger ones, also do not support such a high value of α ov . Several clusters were observed recently with the ∆a photometric system, which is designed to detect chemically peculiar stars, and it was found that α ov = 0.2 is sufficient to match the observations (Paunzen et al 2006, and references given therein). Moreover, as discussed by Claret (2003) the error bars in the effective temperatures of V380 Cyg, which were key to the determination of α ov = 0.6, seem to be optimistic.…”

The role of the stellar rotation on the internal constitution of PV Cassiopeiae

“…However, this conclusion is not suppported by several studies (see for example, Claret & Giménez 1993;Southworth et al 2004;Claret & Willems 2002;Claret 2007). Given that core overshooting appears necessary to reproduce the observations of DLEBS and the colour-magnitude diagram of clusters (VandenBerg et al 2006;Paunzen et al 2006, and references therein) we adopt for the models of PV Cas the value of α ov = 0.20. For PV Cas, we use (X, Z) = (0.70, 0.02), which represents the average chemical composition derived by using several DLEBS.…”