Hans M. Maitzen

Ages and Metallicities of Fornax Dwarf Elliptical Galaxies

Narrowband 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 Gyr compared with 13 Gyr for bright Fornax ellipticals. Comparison of our metallicity color and Mg2 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 with 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.

On the detection of the first extragalactic classical chemically peculiar stars

We report on the detection of the first extragalactic, classical chemically peculiar (CP) stars more than 100 years after the discovery of this group. Using the tool of CCD

On the evolutionary status of chemically peculiar stars of the upper main sequence

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Photoelectric search for peculiar stars in open clusters XV. Feinstein 1, NGC 2168, NGC 2323, NGC 2437, NGC 2547 NGC 4103, NGC 6025, NGC 6633, Stock 2, and Trumpler 2

-photometry, eight definite positive detections in the globular-like stellar cluster NGC 1866 and its surrounding in the Large Magellanic Cloud were made. More than 1950 objects within a radius of 9.5′ of the cluster center down to 20.5 magnitude were observed in this three filter, narrow band photometric system. Averaging more than 70 individual frames for all three filters yielded a 3 σ detection limit of 0.015 mag for stars with

Chemically Peculiar Stars of the Upper Main Sequence

V=20

Galactic Rotation From Young Open Star Clusters

mag. Furthermore we found three objects with significantly negative

The Sun as a Variable Star

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New variable chemically peculiar stars identified in the Hipparcos archive

-values indicating either an Ae/shell or λ Bootis nature. The overall percentage of chemically peculiar stars for NGC 1866 (1.5% ) seems lower than for the majority of open clusters in our Milky Way whereas the incidence within the LMC field is decisively less (0.3% ).

An extensive

We present further evidence that the magnetic chemically peculiar stars (CP2) of the upper main sequence already occur at very early stages of the stellar evolution, significantly before they reach 30% of their life-time on the main sequence. This result is especially important for models dealing with dynamo theories, angular momentum loss during the pre- as well as main sequence and evolutionary calculations for CP2 stars. Results from the literature either derived for objects in the Hyades and the UMa cluster or from the Hipparcos mission contradict each other. A way out of this dilemma is to investigate young open clusters with known ages and accurate distances (error <10%), including CP2 members. Up to now, four open clusters fulfill these requirements: IC 2391, IC 2602, NGC 2451 A and NGC 2516. In total, 13 CP2 stars can be found within these clusters. We have used the measurements and calibrations of the Geneva 7-color photometric system to derive effective temperatures and luminosities. Taking into account the overall metallicity of the individual clusters, isochrones and evolutionary tracks were used to estimate ages and masses for the individual objects. The derived ages (between 10 and 140 Myr) are well in line with those of the corresponding clusters and further strengthen the membership of the investigated CP2 stars.

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The chemically peculiar (CP) stars of the upper main sequence are mainly characterized by strong overabundances of heavy elements. Two subgroups (CP2 and CP4) have strong local magnetic fields which make them interesting targets for astrophysical studies. This star group, in general, is often used for the analysis of stellar formation and evolution in the context of diffusion as well as meridional circulation. In continuation of a long term study of CP stars (initiated in the 1980ies), we present new results based on photoelectric measurements for ten open clusters that are, with one exception, younger than 235Myr. Observations in star clusters are favourable because they represent samples of stars of constant age and homogeneous chemical composition. The very efficient tool of Delta a photometry was applied. It samples the flux depression at 5200A typically for CP stars. In addition, it is able to trace emission line Be/Ae and lambda Bootis stars. Virtually all CP2 and CP4 stars can be detected via this tool. For all targets in the cluster areas, we performed a kinematic membership analysis. We obtained new photoelectric Delta a photometry of 304 stars from which 207 objects have a membership probability higher than 50%. Our search for chemically peculiar objects results in fifteen detections. The stars have masses between 1.7 M(Sun) and 7.7 M(Sun) and are between the zero- and terminal-age-main-sequence. We discuss the published spectral classifications in the light of our Delta a photometry and identify several misclassified CP stars. We are also able to establish and support the nature of known bona fide CP candidates. The new and confirmed CP stars are interesting targets for spectroscopic follow-up observations to put constraints on the formation and evolution of CP stars.