E. Paunzen

Analysing the database for stars in open clusters - I. General methods and description of the data

We present an overview and statistical analysis of the data included in WEBDA. This database includes valuable information such as coordinates, rectangular positions, proper motions, photometric as well as spectroscopic data, radial and rotational velocities for objects of open clusters in our Milky Way. It also contains miscellaneous types of data like membership probabilities, orbital elements of spectroscopic binaries and periods of variability for different kinds of variable stars. Our final goal is to derive astrophysical parameters (reddening, distance and age) of open clusters based on the major photometric system which will be presented in a follow-up paper. For this purpose we have chosen the Johnson UB V, Cousins VRI and Stromgren uvbyβ photometric systems for a statistical analysis of published data sets included in WEBDA. Our final list contains photo- graphic, photoelectric and CCD data for 469 820 objects in 573 open clusters. We have checked the internal (data sets within one photometric system and the same detector technique) and external (different detector technique) accuracy and conclude that more than 97% of all investigated data exhibit a sufficient accuracy for our analysis. The way of weighting and averaging the data is described. In addition, we have compiled a list of deviating measurements which is available to the community through WEBDA.

On the current status of open-cluster parameters

We aim to characterize the current status of knowledge on the accuracy of open-cluster parameters such as the age, reddening and distance. These astrophysical quantities are often used to study the global characteristics of the Milky Way down to the very local stellar phenomena. In general, the errors of these quantities are neglected or set to some kind of heuristic standard value. We attempt to give some realistic estimates for the accuracy of available cluster parameters by using the independently derived values published in the literature. In total, 6437 individual estimates for 395 open clusters were used in our statistical analysis. We discuss the error sources depending on theoretical as well as observational methods and compare our results with those parameters listed in the widely used catalogue by Dias et al. In addition, we establish a list of 72 open clusters with the most accurate known parameters which should serve as a standard table in the future for testing isochrones and stellar models.

Chemically peculiar stars and their temperature calibration

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, Stromgren 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.

The status of Galactic field λ Bootis stars in the post-Hipparcos era

The λ Bootis stars are Population I, late B- to early F-type stars, with moderate to extreme (up to a factor 100) surface underabundances of most Fe-peak elements and solar abundances of lighter elements (C, N, O and S). To put constraints on the various existing theories that try to explain these peculiar stars, we investigate the observational properties of λ Bootis stars compared with a reference sample of normal stars. Using various photometric systems and Hipparcos data, we analyse the validity of standard photometric calibrations, elemental abundances, and Galactic space motions. There crystallizes a clear picture of a homogeneous group of Population I objects found at all stages of their main-sequence evolution, with a peak at about 1 Gyr. No correlation of astrophysical parameters such as the projected rotational velocities or elemental abundances with age is found, suggesting that the a priori unknown mechanism, which creates λ Bootis stars, works continuously for late B- to early F-type stars in all stages of main-sequence evolution. Surprisingly, the sodium abundances seem to indicate an interaction between the stars and their local environment.

The λ Bootis phenomenon: interaction between a star and a diffuse interstellar cloud

The λ Bootis stars, a group of late B- to early F-type Population I stars, have surface abundances that resemble the general metal depletion pattern found in the interstellar medium. Inspired by the recent result that the fundamental parameters of these peculiar stars differ in no respect from those of a comparison sample of normal stars, the hypothesis of an interaction between a star and a diffuse interstellar cloud is considered as a possible explanation of the peculiar abundance pattern. It is found that such a scenario is able to explain the selective accretion of interstellar gas depleted in condensable elements as well as the spectral range of the λ Bootis phenomenon.

A photometric study of chemically peculiar stars with the STEREO satellites - I. Magnetic chemically peculiar stars

About 10 per cent of upper main-sequence stars are characterized by the presence of chemical peculiarities, often found together with a structured magnetic field. The atmospheres of most of those chemically peculiar stars present surface spots, leading to photometric variability caused by rotational modulation. The study of the light curves of those stars therefore permits a precise measurement of their rotational period, which is important to study stellar evolution and to plan further detailed observations. We analysed the light curves of 1028 chemically peculiar stars obtained with the STEREO spacecraft. We present here the results obtained for the 337 magnetic chemically peculiar stars in our sample. Thanks to the cadence and stability of the photometry, STEREO data are perfectly suitable to study variability signals with a periodicity typical of magnetic chemically peculiar stars. Using a matched filter algorithm and then two different period searching algorithms, we compiled a list of 82 magnetic chemically peculiar stars for which we measured a reliable rotational period; for 48 of them, this is the first measurement of their rotational period. The remaining 255 stars are likely to be constant, although we cannot exclude the presence of long-period variability. In some cases, the presence of blending or systematic effects prevented us from detecting any reliable variability and in those cases we classified the star as constant. For each star we classified as variable, we determined temperature, luminosity, mass and fractional age, but the limited statistics, biased towards the shorter periods, prevented us from finding any evolutionary trend of the rotational period. For a few stars, the comparison between their projected rotational velocity and equatorial velocity let us believe that their real rotational period might be longer than that found here and previously obtained. For the 82 stars identified as variable, we give all necessary information needed to plan further phase-dependent observations.

On the detection of chemically peculiar stars using

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 interme- diate band photometric system samples the depth of the 5200 A flux depression by comparing the flux at the center with the adjacent regions with bandwidths of 110 A to 230 A. 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 per- centage 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 )a ndZ 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.

\mathsf{\Delta}

In the third paper of a series dedicated to the spectroscopic survey for new Bootis stars, we present all new and conrmed members of the group as well as a detailed analysis of the observed sample. The nature of this small group of chemically peculiar stars of the upper main sequence still challenges our understanding of processes like diusion, mass-loss and accretion. The typical abundances pattern (nearly solar values for C, N, O and S whereas the Fe-peak elements are moderate to strong underabundant) can still not be explained by any proposed theory. Hence, the signicant increase of new members gives the opportunity to investigate the group properties in more detail. We report the discovery of 26 new members of the group and the conrmation of 18 candidates from the literature. This almost triples the number of known Bootis stars. The existence of one member in the young open cluster NGC 2264 and four members in the Orion OB1 association proves that the Bootis phenomenon already works at very early stages of stellar evolution. Recent results from the Hipparcos mission have shown that the well established Bootis stars of the Galactic eld comprise the whole area from the Zero Age Main Sequence to the Terminal Age Main Sequence ( 10 9 yr for an A-type star). There is a continuous transition between very young and rather evolved evolutionary stages. We nd that the overall percentage of Bootis type among all normal type stars in the spectral range from B8 to F4 is 2% in the Galactic eld as well as in open clusters. Furthermore, 44 metal-weak objects are listed which might be connected with the Bootis phenomenon. Our biased sample (chosen by photometric boxes) is not distinguished from all A-type stars in the corresponding spectral region by the rotational velocity distribution. Only for the luminosity classes IV and III (especially for the cooler program stars) the determined mean v sini values are very high compared to those of the literature.

a photometry

Context. Metallicity is one of four free parameters typically considered when fitting isochrones to the cluster sequence. Unfortunately, this parameter is often ignored or assumed to be solar in most papers. Hence an unknown bias is introduced in the estimation of the other three cluster parameters (age, reddening and distance). Furthermore, studying the metallicity of open clusters allows us not only to derive the Galactic abundance gradient on a global scale, but also to trace the local solar environment in more detail. Aims. In a series of three papers, we investigate the current status of published metallicities for open clusters from widely different photometric and spectroscopic methods. A detailed comparison of the results allows us to establish more reliable photometric calibrations and corrections for isochrone fitting techniques. Well established databases such as WEBDA help us to perform a homogeneous analysis of available measurements for a significant number of open clusters. Methods. The literature was searched for [Fe/H] estimates on the basis of photometric calibrations in any available filter system. On the basis of results published by Tadross, we demonstrate the caveats of the calibration choice and its possible impact. In total, we find 406 individual metallicity values for 188 open clusters within 64 publications. The values were, finally, unweightedly averaged. Results. Our final sample includes [Fe/H] values for 188 open clusters. Tracing the solar environment within 4000 x 4000 pc(2) we identify a patchy metallicity distribution as an extension to the Local Bubble that significantly influences the estimation of the Galactic metallicity gradient, even on a global scale. In addition, further investigations of more distant open clusters are clearly needed to obtain a more profound picture at Galactocentric distances beyond 10 000 pc. Conclusions. Only a combination of all available photometric and spectroscopic data will shed more light on how the local and global Galactic properties are correlated with metallicity.

A spectroscopic survey for

Generally, chemical peculiarity found for stars on the upper main sequence excludes Scuti type pulsation (e.g. Ap and Am stars), but for the group of Bootis stars it is just the opposite. This makes them very interesting for asteroseismological investigations. The group of Bootis type stars comprises late B- to early F-type, Population I objects which are basically metal weak, in particular the Fe group elements, but with the clear exception of C, N, O and S. The present work is a continuation of the studies by Paunzen et al. (1997, 1998), who presented first results on the pulsational characteristics of the Bootis stars. Since then, we have observed 22 additional objects; we found eight new pulsators and confirmed another one. Furthermore, new spectroscopic data (Paunzen 2001) allowed us to sort out misidentified candidates and to add true members to the group. From 67 members of this group, only two are not photometrically investigated yet which makes our analysis highly representative. We have compared our results on the pulsational behaviour of the Bootis stars with those of a sample of Scuti type objects. We find that at least 70% of all Bootis type stars inside the classical instability strip pulsate, and they do so with high overtone modes (Q< 0.020 d). Only a few stars, if any, pulsate in the fundamental mode. Our photometric results are in excellent agreement with the spectroscopic work on high-degree nonradial pulsations by Bohlender et al. (1999). Compared to the Scuti stars, the cool and hot borders of the instability strip of the Bootis stars are shifted by about 25 mmag, towards smaller (b y)0. Using published abundances and the metallicity sensitive indices of the Geneva 7-colour and Stromgren uvby systems, we have derived (Z) values which describe the surface abundance of the heavier elements for the group members. We find that the Period- Luminosity-Colour relation for the group of Bootis stars is within the errors identical with that of the normal Scuti stars. No clear evidence for a statistically significant metallicity term was detected.